Larry G Mastin
email: lgmastin@usgs.gov

Steven N Carey

Alexa R Van Eaton

Julia Eychenne

R S J Sparks

Understanding and modeling tephra transport: lessons learned from the 18 May 1980 eruption of Mount St. Helens

Keywords: Mount St. Helens, Tephra, Explosive volcanism, Particle aggregation, Volcanic plume, Umbrella cloud, Ash cloud

Discoveries made during the 18 May 1980 eruption of Mount St. Helens advanced our understanding of tephra transport and deposition in fundamental ways. The eruption enabled detailed, quantitative observations of downwind cloud movement and particle sedimentation, along with the dynamics of co-pyroclastic-density current (PDC) clouds lofted from groundhugging currents. The deposit was mapped and sampled over more than 150,000 km 2 within days of the event and remains among the most thoroughly documented tephra deposits in the world. Abundant observations were made possible by the large size of the eruption, its occurrence in good weather during daylight hours, cloud movement over a large, populated continent, and the availability of images from recently deployed satellites. These observations underpinned new, quantitative models for the rise and growth of volcanic plumes, the importance of umbrella clouds in dispersing ash, and the roles of particle aggrega-tion and gravitational instabilities in removing ash from the atmosphere. Exceptional detail in the eruption chronology and deposit characterization helped identify the eruptive phases contributing to deposition in different sectors of the distal deposit. The eruption was the first to significantly impact civil aviation, leading to the earliest documented case of in-flight engine damage. Continued eruptive activity in 1980 also motivated pioneering use of meteorological models to forecast ash-cloud movement. In this paper, we consider the most important discoveries and how they changed the science of tephra transport.

Introduction

A defining trend in volcanology since the 1960s has been the shift from qualitative descriptions of volcanic activity to measurement, calculation and ultimately models that forecast how far and how fast tephra moves through the atmosphere, and how severely tephra hazards affect people and property. A search on Google Scholar, for example, for papers containing all the words "volcanic ash," "deposit," and "numerical model" between 1900 and 1960 returns no results. The same search with end dates of 1970, 1979, 1990, and 2000 will return 3, 26, 102, and 290, respectively. This rate of increase is several times faster than the fifteen-year doubling time of scientific publications overall [START_REF] Larsen | The rate of growth in scientific publication and the decline in coverage provided by Science Citation Index[END_REF], reflecting an evolution from narrative description of phenomena to physics-based conceptual models and quantitative numerical models. In the science of tephra transport, the numbers increased dramatically around the time of the 1980 Mount St. Helens eruption.

The Mount St. Helens eruption occurred at a time of rapid advance in understanding of tephra transport and deposition. By the 1970s, tephra specialists began to integrate fieldbased observations with the physics of plume rise and particle settling. For example, [START_REF] Walker | Explosive volcanic eruptions-I the rate of fall of pyroclasts[END_REF] experimentally measured the fall velocity of pyroclasts, while [START_REF] Wilson | Explosive volcanic eruptions-II the atmospheric trajectories of pyroclasts[END_REF] calculated particle trajectories in the atmosphere. Observations from Heimaey and Stromboli in 1973 showed that debris was ejected upward through a gas thrust region [START_REF] Blackburn | Mechanisms and dynamics of strombolian activity[END_REF][START_REF] Sparks | A model for the formation of ignimbrite by gravitational column collapse[END_REF][START_REF] Wilson | Explosive volcanic eruptions III, Plinian eruption columns[END_REF], where it slowed due to drag and negative buoyancy, but then accelerated into a much higher convective thrust region if enough air could be entrained to make the mixture buoyant. With insufficient entrainment, the column would collapse, providing a mechanism for ignimbrite formation. [START_REF] Sparks | A model for the formation of ignimbrite by gravitational column collapse[END_REF] found that column collapse was favored by eroding vent walls, reducing magmatic gas content, or reducing exit velocity. They developed a pioneering model of Plinian column variations with height and used it to calculate particle trajectories by adapting established fluid dynamical models of plumes and jets. If plumes attain buoyancy, buoyant plume theory (Morton et al., 1956) suggested that H ∝ MER 1∕4 , where H is plume height and MER is mass eruption rate. This relationship was confirmed by observations of well-documented eruptions [START_REF] Settle | Volcanic eruption clouds and the thermal power output of explosive eruptions[END_REF][START_REF] Wilson | The control of volcanic column heights by eruption energetics and dynamics[END_REF].

Thus, the 1970s brought a new fundamental understanding of explosive eruption dynamics which set the stage for quantitative transport modeling. However, a large, wellobserved eruption where ideas could be tested was needed.

May 18: overview and key observations

By 1980, thanks to field studies in the previous 1-2 decades, Mount St. Helens was known to be the most active and explosive volcano in North America's Cascade Range.

In the late 1950s, volcanologists from the U.S. Geological Survey (USGS) began a systematic study of the eruptive history of Cascade volcanoes [START_REF] Crandell | A recent volcanic mudflow of exceptional dimensions from Mount Rainier, Washington[END_REF][START_REF] Crandell | Pyroclastic deposits of Recent age at Mount Rainier[END_REF][START_REF] Crandell | Paradise debris flow at Mount Rainier, Washington[END_REF]. Work started at Mount Rainier, which was assumed to be the most hazardous; but by the mid-1970s, investigators recognized that Mount St. Helens had produced more explosive, tephra-forming eruptions with more Holocene tephras than any other Cascade volcano, including three large deposits (> 0.1 km 3 ) since the 1400s C.E. [START_REF] Crandell | Potential hazards from future eruptions of Mount St. Helens volcano[END_REF].

Volcanologists were therefore not surprised when Mount St. Helens awakened in 1980 [START_REF] Christiansen | Chronology of the eruptive activity[END_REF][START_REF] Foxworthy | Volcanic eruptions of 1980 at Mount St. Helens: the first 100 days[END_REF]. Following two months of inflation, a M 5.2 earthquake at 08:32:11 Pacific Daylight Time (PDT = UTC-7: all times in this paper are local, 24-h time) on 18 May 1980 (Fig. 1). The earthquake caused the north flank to slide away in the largest subaerial landslide yet documented, uncorking a pressurized cryptodome that expanded north as a lateral blast, and then rose buoyantly to > 30 km above sea level (asl) as an umbrella cloud (Fig. 2), dropping branches, mud rain, and accretionary lapilli on nearby eyewitnesses [START_REF] Waitt | the path of destruction[END_REF]. At the summit, a vertical eruptive column rose to about 14 km asl shortly after 09:00 and fluctuated between 14 and 19 km asl until about 17:00 before subsiding (Fig. 1). The east-moving ash cloud (Fig. 3a) crossed the Idaho border (400 km downwind) by noon, the west edge of Yellowstone National Park (1000 km) by about 17:30, and continued towards the East Coast of the U.S. Impacts on the ground (Damby et al. (this volume)) included road closures [START_REF] Schuster | Effects of the eruptions on civil works and operations in the Pacific Northwest[END_REF], respiratory problems (Baxter [START_REF] Harris | Radar observations of ash eruptions[END_REF] are "near the volcano" and likely to reflect column heights above the vent, whereas satellite cloud-top heights reported by [START_REF] Holasek | GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption[END_REF] are not restricted to the vent area. During the morning of 18 May, the data from [START_REF] Holasek | GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption[END_REF] et al. 1983;[START_REF] Bernstein | Immediate public health concerns and actions in volcanic eruptions: lessons from the Mount St. Helens eruptions, May 18[END_REF], aircraft evacuations at Fairchild Air Force Base near Spokane, discharging of raw sewage into the Yakima River when ash clogged treatment filters [START_REF] Day | Mt. St. Helens: how a wastewater plant coped with its aftermath[END_REF], and damage to agricultural machinery and crops [START_REF] Cook | Impact on agriculture of the Mount St. Helens eruptions[END_REF].

At least three factors made this event a milestone in volcanology. One was the direct observation of phenomena such as the lateral blast. Second, emerging technology led to dramatic improvements in satellite imagery [START_REF] Menzel | Chapter 2 -history of geostationary weather satellites[END_REF] with unprecedented temporal and spatial resolution, enabling tracking of the volcanic plume [START_REF] Harris | Radar observations of ash eruptions[END_REF]. A third factor was clear weather. The eruption occurred on a Sunday in view of thousands of cameras and produced a deposit that fell over land, in an area where tephra could be quickly accessed and sampled before significant modification by wind and rain.

The 1980 Mount St. Helens eruption attracted scientists from around the globe. The data have been mined for decades for insights into the nature of ash hazards. The timing of the event, occurring when computer technology and ash transport calculations were nascent, jump-started the field of ash-transport modeling, which is now an operational mainstay of ash-hazards forecasting.

What we learned

An immense amount was quickly learned in the days and weeks after May 18 through vivid images, but it took many years to develop a fuller picture. Some of the most lasting discoveries are highlighted below.

Insights from satellite, radar, photographs, and eyewitness accounts

The lateral blast and blast co-pyroclastic-density-current (co-PDC) plume

Ground-based photos were integrated with other observations to develop a detailed picture of the sequence of events (Fig. 1) [START_REF] Voight | Time scale for the first moments of the May 1981 eruption[END_REF][START_REF] Moore | Chronology and character of the May 18, 1980, explosive eruptions of Mount St. Helens[END_REF][START_REF] Pierson | Initiation and flow behavior of the 1980 Pine Creek and Muddy River lahars[END_REF][START_REF] Sparks | The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens[END_REF][START_REF] Hoblitt | Was the 18 May 1980 lateral blast at Mt St. Helens the product of two explosions?[END_REF]. Photo reconstruction showed that the north flank slid away as three separate blocks [START_REF] Moore | Topographic and structural changes, March-July 1980--Photogrammetric data[END_REF]. The lateral blast and plumes rising from it may have developed from two explosions [START_REF] Moore | Chronology and character of the May 18, 1980, explosive eruptions of Mount St. Helens[END_REF][START_REF] Hoblitt | Was the 18 May 1980 lateral blast at Mt St. Helens the product of two explosions?[END_REF]. From satellite and photo reanalysis, the first explosion, "cloud I," began as the mountain's north flank slid away, about a half minute after the M 5.2 earthquake [START_REF] Voight | Time scale for the first moments of the May 1981 eruption[END_REF]. The second, "cloud II," began at 1.5 min [START_REF] Moore | Chronology and character of the May 18, 1980, explosive eruptions of Mount St. Helens[END_REF], was manifested by intense infrared radiation in satellite images, and ascended into the atmosphere at speeds up to 70 m/s, centered on a location 12-14 km north of the volcano. [START_REF] Moore | Chronology and character of the May 18, 1980, explosive eruptions of Mount St. Helens[END_REF] attributed the second explosion to collision of blast/ avalanche with Johnston Ridge or perhaps steam generation in the North Fork Toutle River Valley. [START_REF] Hoblitt | Was the 18 May 1980 lateral blast at Mt St. Helens the product of two explosions?[END_REF] suggested that explosion 2 began with detachment of slide block II and that the cloud "exploded" when reaching rugged terrain north of the North Fork Toutle River. Clouds I and II reached about 7 km altitude.

Ascent of the blast co-PDC plume

Multiple satellite imagery sources, including restricted national sources and ground-based photos, were combined to give a detailed picture of the transition from lateral blast-a horizontally directed expulsion of pyroclastic currents-to a vertically rising plume [START_REF] Moore | Chronology and character of the May 18, 1980, explosive eruptions of Mount St. Helens[END_REF][START_REF] Sparks | The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens[END_REF]. The data from two classified U.S. Air Force infrared geostationary satellites [START_REF] Rice | Satellite observations of the Mt[END_REF][START_REF] Sparks | The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens[END_REF]) allowed rates of plume rise and umbrella expansion to be quantified with a time resolution of 5 min. These observations provided the first direct documentation of the buoyant liftoff process now recognized as a key aspect of co-PDC plumes.

These data showed that pyroclastic density currents from the blast expanded until about 08:36:00-08:37:30. Toward its outer edges, an overriding current of gas and particles began to separate from a dense, granular undercurrent [START_REF] Gardner | Liftoff of the 18 May 1980 surge of Mount St. Helens (USA) and the deposits left behind[END_REF]) while passing over ridges, leaving patches of standing trees before rejoining the main flow. Toward the center of the blast zone (the area where trees were toppled), a buoyant cloud began to rise. Shortly after lifting off, the ground-hugging PDC slowed its outward lateral movement and stalled. A photomosaic (Fig. 2) illustrates the rising plume at about 08:52, with a radius in the lowermost kilometer that decreases as air is drawn in and then spreads out at multiple levels at higher altitude. Satellite data [START_REF] Sparks | The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens[END_REF] showed ascent velocities up to 110 m/s. The plume reached a height of 25 km by 08:50 and then 31 km by about 08:54 following a late pulse. In map view, the radial growth of the umbrella cloud averaged 20-25 m/s between 08:50 and 09:10, and the westward side of the cloud pushed upwind 28 km until it stagnated as the radial expansion speed approached that of the ambient winds. Negatively buoyant material from the top of the plume collapsed into the neutrally buoyant umbrella with increased crosswind spreading and downwind dispersal (Bursik et al. 1992a;[START_REF] Holasek | GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption[END_REF].

Physics of plume rise and spreading

The dynamics of this cloud were clearly different from that of a steady Plinian column, which exits the vent as a negatively buoyant high-speed jet, decelerates, and then accelerates again after attaining buoyancy [START_REF] Sparks | A model for the formation of ignimbrite by gravitational column collapse[END_REF]. In contrast, this co-PDC cloud from Mount St. Helens lifted off the ground as a positively buoyant thermal with little initial upward velocity. Buoyant liftoff from a groundhugging PDC is attributed to reversal of buoyancy with respect to the overlying atmosphere by a combination of sedimentation, entrainment, and heating of entrained air [START_REF] Sparks | The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens[END_REF][START_REF] Woods | Dimensions and dynamics of co-ignimbrite eruption columns[END_REF][START_REF] Gardner | Liftoff of the 18 May 1980 surge of Mount St. Helens (USA) and the deposits left behind[END_REF]. The PDC decelerates rapidly and motion transitions from lateral to vertical. Buoyant liftoff was also demonstrated in PDCs from explosive activity later in the day on May 18 (Figs. 1,4d) and during later eruptions in 1980 [START_REF] Calder | Dynamics of co-ignimbrite plumes generated from pyroclastic flows of Mount St. Helens (7 August 1980)[END_REF], though not with the same intensity as from the lateral blast.

On reaching the height of neutral buoyancy in the atmosphere, both vent-derived and co-PDC plumes may spread radially as an umbrella cloud. A theoretical treatment of these two phenomena (Sparks et al. 1997, Ch. 11) found that umbrella clouds rising from sustained sources should grow at about R ∝ t 2∕3 (Sparks et al. 1997, Eq. 11.8), whereas those rising from instantaneous sources like the Mount St. Helens blast cloud should grow at R ∝ t 1∕3 (R = cloud radius, t = time). Observed umbrella growth rates at Pinatubo, Philippines, in 1991 follow the predictions for a steady plume [START_REF] Holasek | Satellite observations and interpretation of the 1991 Mount Pinatubo eruption plumes[END_REF]. The Mount St. Helens co-PDC plume fell between that of a steady and an instantaneous plume, reflecting perhaps that it was fed over a period of time comparable to the rise time [START_REF] Woods | Dimensions and dynamics of co-ignimbrite eruption columns[END_REF][START_REF] Sparks | Volcanic Plumes[END_REF].

Plinian phase and downwind plume spreading

USGS personnel made continuous airborne observations for more than 4.5 h (Krimmel and Post, 1981). From about 09:00 until 12:15, eyewitnesses and radio logs [START_REF] Criswell | Chronology and pyroclastic stratigraphy of the May 18, 1980 eruption of Mount St. Helens, Washington[END_REF] indicate that a steady Plinian column rose from the vent to 14-19 km asl (Figs. 1,4c) [START_REF] Christiansen | Chronology of the eruptive activity[END_REF][START_REF] Criswell | Chronology and pyroclastic stratigraphy of the May 18, 1980 eruption of Mount St. Helens, Washington[END_REF][START_REF] Holasek | GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption[END_REF]. Shortly after noon, lighter-toned material appeared discontinuously from the vent, eventually composing most of the column; then about 12:15, eruptive vigor increased and PDCs started to pour northward through the crater breach [START_REF] Foxworthy | Volcanic eruptions of 1980 at Mount St. Helens: the first 100 days[END_REF][START_REF] Criswell | Chronology and pyroclastic stratigraphy of the May 18, 1980 eruption of Mount St. Helens, Washington[END_REF]). An elongate ash plume from above the PDCs extended northeast, rising to more than 14 km asl [START_REF] Harris | Radar observations of ash eruptions[END_REF][START_REF] Holasek | GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption[END_REF]. From 13:30 to 15:00, PDCs erupted more episodically and then eventually abated by ~ 16:25, after which the Plinian plume reached its greatest height of ~ 19 km asl, before starting to decline around 17:30.

Through a combination of eyewitness accounts, good timing, and new technology, we learned a great deal about the phenomena that dispersed tephra. The U.S. National Weather Service (NWS) tracked plume height and downwind movement continuously from radars in Portland, Seattle, and Spokane [START_REF] Harris | Radar observations of ash eruptions[END_REF]). In addition, the cloud was imaged every half hour, in both visible and thermal infrared (10-12 μm) [START_REF] Holasek | GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption[END_REF] by the GOES-3 satellite. (1981, Fig. 338). Blue circles represent sample locations [START_REF] Durant | Hydrometeor-enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens (USA)[END_REF]. Red triangle gives location of Mount St. Helens. Red dots in (b) and (c) give sample locations in Figs. 6a, b, d (Carey and Sigurdsson 1982). Green dots in (b) and (c) give sample locations in Fig. 7. Yellow star in (b) gives the location of aggregates photographed in Fig. 5 ◂ Modern techniques to identify ash clouds in infrared imagery (e.g., [START_REF] Prata | Infrared radiative transfer calculations for volcanic ash clouds[END_REF] had not yet been developed, and remote sensing analysts could track ash only during daylight hours based on grayscale tones in visible imagery. In GOES-3 visible imagery, the cloud moved eastward at about 100 km/h in the first 1000 km before night fell (Fig. 3a). The following day, the center of the increasingly diffuse cloud passed over the mid-continent, New England, maritime provinces of Canada, and into the North Atlantic [START_REF] Sarna-Wojcicki | Areal distribution, thickness, mass, volume, and grain size of air-fall ash from the six major eruptions of 1980[END_REF]. At downwind locations, the ash cloud arrived overhead before it started depositing at ground level [START_REF] Durant | Hydrometeor-enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens (USA)[END_REF]Eychenne et al. 2015) (Figs. 4c, 5). At Spokane, for example, the delay was about 1.75 h [START_REF] Sarna-Wojcicki | Areal distribution, thickness, mass, volume, and grain size of air-fall ash from the six major eruptions of 1980[END_REF][START_REF] Durant | Hydrometeor-enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens (USA)[END_REF].

Over years of post-eruption analysis, the scientific community learned many things about the factors controlling ash cloud transport. Some key findings are listed below and illustrated in Fig. 4: • Relationships between plume height and eruption rate generally followed established height-rate relationships at the time [START_REF] Settle | Volcanic eruption clouds and the thermal power output of explosive eruptions[END_REF][START_REF] Wilson | The control of volcanic column heights by eruption energetics and dynamics[END_REF]), but showed new complexities. [START_REF] Carey | Variations in column height and magma discharge during the May 18, 1980 eruption of Mount St. Helens[END_REF] found that the mass eruption rate inferred from deposit mass was a few times greater than predicted by plume height. The discrepancy was attributed either to partitioning of the erupted mass into PDCs (e.g., [START_REF] Carey | Variations in column height and magma discharge during the May 18, 1980 eruption of Mount St. Helens[END_REF] or bending of the plume by high winds [START_REF] Degruyter | Improving on mass flow rate estimates of volcanic eruptions[END_REF]. The addition of Mount St. Helens data into later compilations of eruption rate and plume height (e.g., [START_REF] Sparks | Volcanic Plumes[END_REF][START_REF] Mastin | A multidisciplinary effort to assign realistic source parameters to models of volcanic ash-cloud transport and dispersion during eruptions[END_REF][START_REF] Aubry | The Independent Volcanic Eruption Source Parameter Archive (IVESPA, version 1.0): A new observational database to support explosive eruptive column model validation and development[END_REF]) broadened the scatter and added to growing recognition of complexity.

• Both the initial umbrella cloud and later Plinian column were undercooled by about 15 °C (meaning the cloud top was colder than the atmosphere at that height) due to adiabatic expansion during plume ascent [START_REF] Holasek | GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption[END_REF]. The cloud equilibrated with surrounding temperatures over about 30-60 min. Undercooling is now a commonly recognized phenomenon in stratospheric eruptions (e.g., [START_REF] Woods | Thermal disequilibrium at the top of volcanic clouds and its effect on estimates of the column height[END_REF][START_REF] Holasek | Satellite observations and interpretation of the 1991 Mount Pinatubo eruption plumes[END_REF] and quantified in many plume models (e.g., [START_REF] Woods | Thermal disequilibrium at the top of volcanic clouds and its effect on estimates of the column height[END_REF][START_REF] Suzuki | Inter-comparison of threedimensional models of volcanic plumes[END_REF]. Undercooling complicates efforts to estimate stratospheric cloud height based on cloud-top temperature. [START_REF] Holasek | GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption[END_REF] overcame this complication by pioneering the use of cloud geometry to estimate plume height. • Although plume-top heights ranged from ~ 14 to 31 km, the distal cloud moved eastward at much lower levels, about 10-14 km asl, where the cloud was neutrally buoyant and wind speeds were the highest [START_REF] Sarna-Wojcicki | Areal distribution, thickness, mass, volume, and grain size of air-fall ash from the six major eruptions of 1980[END_REF][START_REF] Woods | Wind-driven dispersal of volcanic ash plumes and its control on the thermal structure of the plume-top[END_REF]) (Fig. 4c). It is now recognized that ash dispersal tends to occur at altitudes well below the overshooting top height (e.g., [START_REF] Holasek | Satellite observations and interpretation of the 1991 Mount Pinatubo eruption plumes[END_REF]). • Over distances of several hundred kilometers downwind, the width in plan view of the May 18 ash cloud increased with the square root of distance, suggesting that gravity current spreading was a more important cloud-widening process than turbulent diffusion (Bursik et al. 1992a).

This finding contradicts assumptions that cloud widening results primarily from turbulent diffusion (e.g., [START_REF] Folch | A review of tephra transport and dispersal models: Evolution, current status, and future perspectives[END_REF]. The detailed observations of [START_REF] Holasek | GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption[END_REF] were also used by [START_REF] Bonadonna | Sedimentation from strong volcanic plumes[END_REF] to quantify for the first time the relative effects of gravitational spreading and wind advection on plume widening and downwind transport.

• Gravitational instabilities at the base of the downwind cloud formed mammatus structures a few hundred kilometers downwind (Fig. 4c). Also, a region of high radar reflectivity appeared at a few hundred kilometers downwind, at about 2-3 h after the eruption start, and persisted for about 11 h [START_REF] Harris | Radar observations of ash eruptions[END_REF][START_REF] Durant | Hydrometeor-enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens (USA)[END_REF]). This high-reflectivity zone was interpreted by [START_REF] Durant | Hydrometeor-enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens (USA)[END_REF] to result either from the growth of aggregates in the cloud or from frozen hydrometeors falling through the melting level to form liquid water. The downwind distance, about 300 km, coincides with the location of a secondary thickness maximum described below (Fig. 4d).

Insights from the fall deposits

The 18 May 1980 eruption produced a widespread tephrafall deposit distributed over 150,000 km 2 with a total volume of about 0.25 km 3 DRE (dense rock equivalent, Sarna-Wojcicki et al. ( 1981)). Fallout occurred east of the volcano in the states of Washington, Idaho, and Montana [START_REF] Folsom | Glass geochemical compositions from widespread tephras erupted over the last 200 years from Mount St. Helens[END_REF][START_REF] Sarna-Wojcicki | Areal distribution, thickness, mass, volume, and grain size of air-fall ash from the six major eruptions of 1980[END_REF][START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF], although some fine ash was transported to the west and northwest at high levels in the atmosphere [START_REF] Danielsen | Trajectories of the Mount St[END_REF]. Volcanologists were able to sample and map out the distribution of a pristine fallout deposit prior to significant erosion and reworking. Observations of eruption column height [START_REF] Harris | Radar observations of ash eruptions[END_REF], atmospheric trajectories [START_REF] Rosenfeld | Observations on the Mount St. Helens eruption[END_REF][START_REF] Danielsen | Trajectories of the Mount St[END_REF], eruption chronology [START_REF] Christiansen | Chronology of the eruptive activity[END_REF][START_REF] Criswell | Chronology and pyroclastic stratigraphy of the May 18, 1980 eruption of Mount St. Helens, Washington[END_REF], and eyewitness accounts [START_REF] Rosenbaum | A summary of eyewitness accounts of the May 18 eruption[END_REF]) enabled quantitative assessment of factors controlling the formation of widespread tephra-fall deposits from a major explosive eruption. Additionally, the rich diversity of observations and complete documentation made this a benchmark event for subsequent studies and refinements of models for atmospheric dispersal of tephra (e.g., [START_REF] Armienti | A numerical model for simulation of tephra transport and deposition: applications to May 18, 1980, Mount St. Helens eruption[END_REF][START_REF] Fero | Simulation of the 1980 eruption of Mount St. Helens using the ash-tracking model PUFF[END_REF][START_REF] Folch | A model for wet aggregation of ash particles in volcanic plumes and clouds: 2 Model Application[END_REF][START_REF] Mastin | Adjusting particle-size distributions to account for aggregation in tephra-deposit model forecasts[END_REF].

Questions remain about the footprint of the ash deposit. Existing maps are not entirely consistent with each other (e.g., [START_REF] Folsom | Glass geochemical compositions from widespread tephras erupted over the last 200 years from Mount St. Helens[END_REF][START_REF] Sarna-Wojcicki | Areal distribution, thickness, and volume of downwind ash from the May 18, 1980 eruption of[END_REF] and later observations noted ashfall in new places (Jensen et al. 2019b). Jensen et al. (2019a, unpublished data) examined archival news reports and eyewitness observations that suggest that thicknesses of 0.5-1 mm extended well into southeastern British Columbia and southern Alberta. There Helens. Location given by yellow star in Fig. 3b are credible accounts of ash "dustings" much farther north, in Edmonton, Saskatchewan, and the U.S. east coast [START_REF] Tilling | Eruptions of Mount St. Helens: past, present, and future[END_REF]S. Moran, USGS, pers. commun. 2022). [START_REF] Cutler | How does tephra deposit thickness change over time? A calibration exercise based on the 1980 Mount St Helens tephra deposit[END_REF] revisited the May 18 tephra deposit and found good preservation where the original thickness exceeded 0.5 cm. Related studies performed decades after 1980 showed that winnowing of fine ash with time could modestly reduce fineash content [START_REF] Cutler | How do the grain size characteristics of a tephra deposit change over time?[END_REF] and that the quality of preservation depends on surface vegetation and development of biocrusts [START_REF] Cutler | Tephra transformations: variable preservation of tephra layers from two well-studied eruptions[END_REF].

Downwind and crosswind sedimentological variations

Detailed measurements of the 18 May 1980 tephra fall distribution included both thickness and mass per unit area (Figs. 3b,c) [START_REF] Folsom | Glass geochemical compositions from widespread tephras erupted over the last 200 years from Mount St. Helens[END_REF][START_REF] Sarna-Wojcicki | Areal distribution, thickness, and volume of downwind ash from the May 18, 1980 eruption of[END_REF][START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF], making this at the time the most thoroughly documented fall deposit out to its distal limits. Most previous studies lacked data on distal tephra. By combining mass loading and thickness measurements, investigators calculated bulk deposit density values ranging from > 1000 kg/m 3 in proximal areas to < 100 kg/m 3 distally (Fig. 6c) [START_REF] Folsom | Glass geochemical compositions from widespread tephras erupted over the last 200 years from Mount St. Helens[END_REF][START_REF] Sarna-Wojcicki | Areal distribution, thickness, and volume of downwind ash from the May 18, 1980 eruption of[END_REF]. Several distinct layers were identified at most sites, which represented tephra from the initial co-PDC blast cloud, the main Plinian column, and secondary plumes generated by PDC generation [START_REF] Sarna-Wojcicki | Areal distribution, thickness, mass, volume, and grain size of air-fall ash from the six major eruptions of 1980[END_REF]. A distinctive feature of the deposit is the occurrence of a secondary thickness maximum approximately 325 km east of Mount St. Helens in the vicinity of Ritzville, Washington (Figs. 3b,4c,d). This feature had been documented in only a few other fall deposits, most notably associated with the 1932 explosive eruption of Quizapu volcano, Chile [START_REF] Hildreth | Volcán Quizapu, Chilean Andes[END_REF], but its significance had not been examined in detail.

The May 18 tephra fall deposit consisted of a mixture of pumice, glass shards, accidental lithics, and crystals of plagioclase, orthopyroxene, amphibole, sparse clinopyroxene, and Fe-Ti oxides sourced from the primary dacitic magma, cryptodome, and older volcanic deposits of Mount St. Helens. These components varied systematically downwind along the main dispersal axis, controlled by the density and size of individual components [START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF]. The dominantly dacitic magma erupted on May 18 consisted of about 70% rhyodacitic melt and 30% pheno-and microphenocrysts [START_REF] Blundy | Ascent-driven crystallization of dacite magmas at Mount St. Helens, 1980-86[END_REF] [START_REF] Rutherford | The May 18, 1980, eruption of Mount St. Helens: 1, melt composition and experimental phase equilibria[END_REF] [START_REF] Kuntz | Petrography and particle-size distribution of pyroclastic-flow, ash-cloud, and surge deposits[END_REF]. The deposit consisted largely of pumice and lithics out to about 50 km [START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF]. With increasing distance from source, the percentage of crystals and glass shards increased significantly, with crystal content reaching a peak between about 150 and 200 km distance (Fig. 6d). At 175 km from source, the fall deposit was highly enriched in crystals of plagioclase and mafic phases, resembling beach sand. In the area of the secondary thickness maximum, the deposit was dominated by fine, glassy tephra. SEM imaging [START_REF] Eychenne | Impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 May 1980 Mount St. Helens fallout deposit[END_REF] and glass geochemistry [START_REF] Cutler | How does tephra deposit thickness change over time? A calibration exercise based on the 1980 Mount St Helens tephra deposit[END_REF][START_REF] Folsom | Glass geochemical compositions from widespread tephras erupted over the last 200 years from Mount St. Helens[END_REF]) discriminated components from the initial blast co-PDC cloud (fragments of dense cryptodome and older, altered volcanic deposits, higher glass SiO 2 ) and more vesicular magma from the later Plinian phase and associated PDCs. Surprisingly, beyond 150 km from vent, about half of the tephra by mass was derived from the initial co-PDC blast cloud even though that blast phase was much shorter than the later Plinian and PDC activity. These high-resolution componentry analyses demonstrated that significant lateral facies variations can occur in fall deposits from multi-phase eruptions.

As with other tephra-fall deposits, the grain size of the May 18 sequence decreased regularly with distance from source [START_REF] Fisher | Pyroclastic rocks[END_REF], but displayed a distinct bimodal character in the bulk deposit and within individual stratigraphic units (Fig. 6a) [START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF][START_REF] Durant | Hydrometeor-enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens (USA)[END_REF][START_REF] Eychenne | Impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 May 1980 Mount St. Helens fallout deposit[END_REF]. A coarse mode became systematically finer with increasing distance from source, whereas a fine mode remained relatively constant (Fig. 6b). At about 400 km from source, the grain size became unimodal (Fig. 6a). Subsequent work with more samples quantified the different modes and their areal distribution downwind and crosswind [START_REF] Eychenne | Impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 May 1980 Mount St. Helens fallout deposit[END_REF]. Transects perpendicular to the main dispersal axis (Fig. 7) showed the fine ash increasing in proportion to the north and the coarse mode increasing to the south [START_REF] Eychenne | Impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 May 1980 Mount St. Helens fallout deposit[END_REF]. The fine-ash mode is mostly attributable to the initial blast co-PDC cloud. This work demonstrated that the deposit was derived from at least two distinct plumes, initially at different altitudes, that eventually merged downwind. The initial blast co-PDC plume was injected to a height of > 30 km asl and was displaced to the north of Mount St. Helens, whereas the second, longer-lived plume was generated from a Plinian column and co-PDC clouds rising mostly over the summit and attaining heights of about 14-19 km asl [START_REF] Harris | Radar observations of ash eruptions[END_REF].

Proximal ash aggregation

Ash aggregation occurred within the May 18 ash plumes in distinct ways depending on the phase of the eruption and distance from source. Accretionary lapilli ranging from a few millimeters to about 1 cm in diameter (Fig. 8a) are preserved in the proximal deposits of the initial blast co-PDC plume. Eyewitnesses described falling mudballs by about 09:00 which variably felt "ice cold" or "a little warm" to the touch [START_REF] Rosenbaum | A summary of eyewitness accounts of the May 18 eruption[END_REF]. Some mudballs, about 1 cm in size, splatted on car windshields. [START_REF] Sisson | Blast ashfall deposit of May 18, 1980 at Mount St. Helens, Washington[END_REF] produced isopleth maps of ash aggregate diameters (Fig. 8a) and concluded that proximal sedimentation from the giant umbrella cloud was dominated by wet aggregation, although the origin of the water remains enigmatic. Magmatic water comprised only a few weight percent of the cryptodome magma, and the eruption occurred on a clear, dry day, limiting the amount of moisture entrainment from the surrounding atmosphere. External water could have been sourced from the hydrothermal system within the saturated edifice and glaciers/snowpack pulverized by the landslide [START_REF] Sparks | The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens[END_REF][START_REF] Hoblitt | Was the 18 May 1980 lateral blast at Mt St. Helens the product of two explosions?[END_REF]. These other sources of water increased the moisture content of the fragmenting cryptodome, portions of the blast surge, and the stratospheric co-PDC blast plume.

Other phases of the May 18 eruption contain evidence for aggregation on a smaller scale. Unit C1 of [START_REF] Waitt | proximal air-fall deposits from the May 18 eruption--stratigraphy and field sedimentology[END_REF] represents deposits up to 80 km distant, derived from co-PDC clouds on the afternoon of May 18. The ash contains concentrically structured, millimeterscale accretionary lapilli, which are only present near the margins of Spirit Lake (Fig. 8b). [START_REF] Waitt | proximal air-fall deposits from the May 18 eruption--stratigraphy and field sedimentology[END_REF] proposed that aggregation only took place when the PDCs entrained moisture as they swept across Spirit Lake.

These contrasting observations of pervasive, wet aggregation in the blast co-PDC ashfall and localized aggregation within the afternoon PDC's have influenced interpretations of the roles of external moisture on ash transport and fallout in other deposits worldwide (e.g., Oruanui (Van Eaton and Wilson 2013) and Tenerife [START_REF] Brown | Origin of accretionary lapilli within ground-hugging density currents: evidence from pyroclastic couplets on Tenerife[END_REF]). Aggregation processes also occurred in the downwind, distal cloud, as described below.

Secondary thickness maximum and enhanced fine ash sedimentation

Early simulations of the May 18 tephra fall using a simple particle trajectory model investigated the origin of the second thickness maximum in western Washington (Carey and [START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF]. Modeling inputs included ( 1) the proportions of different components, their sizes, densities, and settling velocities;

) ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Sources: Esri,
(2) transport elevations, plume spreading, and wind velocities; and (3) estimates of total source grainsize distribution. The results demonstrated that the second thickness maximum could not be reproduced if deposition was controlled by fallout of particles individually. Fine ash (< 63 microns) in the second thickness maximum area should have been transported much farther based on their settling velocities, wind strength, and elevation of transport. They concluded that formation of a second thickness maximum could only be generated if fine ash had formed aggregates and settled at a higher velocity than their constituent particles (Fig. 9a). Support for this inference came from the collection of loosely bound aggregates ("Sorem clusters") in Pullman, Washington, 390 km east of Mount St. Helens (Sorem 1982). Aggregates were typically 500-250 microns in diameter, consisted predominantly of glass shards with some crystals, and were likely held together by a combination of mechanical interlocking and/or electrostatic attraction (Sorem 1982). Dry aggregation alone cannot explain all the features of fine ash from the May 18 fall deposit, especially the proximal fine-ash deposition and the consistency of the fine-mode size over large horizontal distances. Consequently, enhanced fallout of fine ash is thought to be the combined result of dry and wet aggregation; detrainment instabilities of cloud base; hydrometeor formation, settling, and destruction; and entrainment of small particles by larger ones (Fig. 4d) [START_REF] Durant | Hydrometeor-enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens (USA)[END_REF][START_REF] Eychenne | Impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 May 1980 Mount St. Helens fallout deposit[END_REF].

Recognition of fine-ash aggregation at Mount St. Helens has implications for tephra-fall hazards. Premature fallout of fine ash significantly reduces the atmospheric residence time of particulates and thus partially mitigates in-flight hazards to aircraft. Aggregation can result in localized higher deposition of fine ash, leading to health impacts from respiratory problems or fouling of water supplies and other potential infrastructure damage [START_REF] Wilson | Impacts from volcanic ash fall[END_REF]. Premature fallout of fine ash causes bimodality of distal ash-fall deposits due to a combination of deposition of individual particles, usually coarser than 63 microns, and aggregates consisting fine ash [START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF][START_REF] Rose | Fine ash content of explosive eruptions[END_REF]. This result was supported by early modeling (Fig. 9c). Studies of other widespread tephra-fall layers demonstrate similar grain-size bimodality of distal deposits [START_REF] Sparks | The volcanological significance of deep-sea ash layers associated with ignimbrites[END_REF] and led to the recognition that aggregation and other enhanced sedimentation processes are common in tephra deposition [START_REF] Brazier | Bimodal grain size distribution and secondary thickening in airfall ash layers[END_REF][START_REF] Brown | A review of volcanic ash aggregation[END_REF].

The Mount St. Helens event set the standard for analyzing the total grain-size distribution of a large-magnitude explosive eruption and shed new light on how the fine-ash component constrains fragmentation processes. Based on an early granulometric application of the Coulter Counter Method [START_REF] Graham | The Coulter principle: foundation of an industry[END_REF], [START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF] showed that more than 60% of the May 18 deposit consisted of fragments 63 microns or smaller. A later compilation [START_REF] Rust | Permeability controls on expansion and size distributions of pyroclasts[END_REF] showed this to be among the most fine-enriched deposits yet sampled. It is now thought that the fine-ash component of the 1980 fall deposit was enhanced by the substantial addition of fragments milled during blast co-PDC activity [START_REF] Eychenne | Impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 May 1980 Mount St. Helens fallout deposit[END_REF], similar to other large, co-PDC deposits (e.g., [START_REF] Sparks | The volcanological significance of deep-sea ash layers associated with ignimbrites[END_REF][START_REF] Wilson | The 26.5ka Oruanui eruption, New Zealand: an introduction and overview[END_REF][START_REF] Wiesner | Grain size, areal thickness distribution and controls on sedimentation of the 1991 Mount Pinatubo tephra layer in the South China Sea[END_REF][START_REF] Engwell | Physical characteristics of tephra layers in the deep sea realm: the Campanian Ignimbrite eruption[END_REF][START_REF] Engwell | Chapter 4 -contribution of fine ash to the atmosphere from plumes associated with pyroclastic density currents[END_REF].

Other insights

The 1980 eruptions of Mount St. Helens also spawned two new fields of study associated with ashfall: ash resuspension and impacts to human health and agriculture. While these topics are outside the scope of this article, they are too important to completely ignore. Airborne measurements of resuspended ash in the atmosphere after 18 May for example [START_REF] Hobbs | Resuspension of volcanic ash from Mount St. Helens[END_REF] were among the first ever made, showing that wind speeds of only a few meters per second could pick up recently deposited ash and reduce visibility. Air quality monitoring stations set up throughout Washington State in 1980 provided some of the first-ever measurements of total suspended particulates (TSP) during an ashfall event. The 8-h average TSP in Yakima on 18 May, 33.8 mg/m 3 , remains the highest measured during any ashfall event anywhere [START_REF] Bernstein | Immediate public health concerns and actions in volcanic eruptions: lessons from the Mount St. Helens eruptions, May 18[END_REF]). From 18 through 25 May, resuspension kept daily TSP values in Yakima above the U.S. Environmental Protection Agency's Emergency Action Level of 0.8 mg/m 3 [START_REF] Bernstein | Immediate public health concerns and actions in volcanic eruptions: lessons from the Mount St. Helens eruptions, May 18[END_REF]. At Yakima's two major hospitals, visits per day to the Emergency Room due to asthma and/or bronchitis rose nearly an order of magnitude above normal on 19 May and remained above normal for more than two weeks. These observations motivated some of the first studies of effects of ash on human health (e.g., [START_REF] Baxter | Mount St Helens eruptions, May 18 to June 12, 1980: an overview of the acute health impact[END_REF]. Heavy ashfall over the agricultural college of Washington State University in Pullman prompted important first studies of the effects ash on agriculture [START_REF] Cook | Impact on agriculture of the Mount St. Helens eruptions[END_REF][START_REF] Johansen | Effects of ash from Mt. St. Helens on bees[END_REF][START_REF] Lyons | Agricultural impact and adjustment to Mount St. Helens ashfall: search for analogs[END_REF]) and on range and forest land [START_REF] Folsom | Mount St. Helens tephra on range and forest lands of eastern Washington: local erosion and redeposition[END_REF]). Since 1980, resuspension events at other volcanoes such as Eyjafjallajökull [START_REF] Leadbetter | Modeling the resuspension of ash deposited during the eruption of Eyjafjallajökull in spring 2010[END_REF][START_REF] Thorsteinsson | High levels of particulate matter in Iceland due to direct ash emissions by the Eyjafjallajökull eruption and resuspension of deposited ash[END_REF][START_REF] Liu | Ash mists and brown snow: remobilization of volcanic ash from recent Icelandic eruptions[END_REF], the Valley of Ten Thousand Smokes in Alaska [START_REF] Hadley | Resuspension of relic volcanic ash and dust from Katmai: still an aviation hazard[END_REF][START_REF] Wallace | Volcanic ash resuspension from the Katmai region[END_REF], and Cordòn Caulle volcano, Chile [START_REF] Folch | Modeling volcanic ash resuspension -application to the 14-18 October 2011 outbreak episode in central Patagonia, Argentina[END_REF][START_REF] Mingari | Volcanic ash resuspension in Patagonia: numerical simulations and observations[END_REF], demonstrate that ash-resuspension problems can be chronic and widespread. Similarly, areas of chronic ashfall such as at Montserrat [START_REF] Searl | Assessment of the exposure of islanders to ash from the Soufriere Hills volcano, Montserrat, British West Indies[END_REF][START_REF] Horwell | Characterization of respirable volcanic ash from the Soufrière Hills volcano, Montserrat, with implications for human health hazards[END_REF] or Merapi [START_REF] Damby | The respiratory health hazard of tephra from the 2010 Centennial eruption of Merapi with implications for occupational mining of deposits[END_REF]) have spawned extensive studies of volcanic ash health hazards [START_REF] Horwell | The respiratory health hazards of volcanic ash: a review for volcanic risk mitigation[END_REF].

How Mount St. Helens changed our understanding of ash transport and deposition

The discussions below summarize how Mount St. Helens improved our understanding of volcanic processes and our ability to anticipate and forecast hazards but are by no means comprehensive. Some aspects of what we learned, for example, about effects of ash resuspension and post-event impacts to health, are covered in greater detail by Damby et al. (this volume).

Co-PDC clouds

Observations made during the Mount St. Helens eruption provided critical insights into the origin, and behavior, of buoyant clouds formed from ground-hugging PDCs. Before the Mount St. Helens eruption, evidence for co-PDC clouds came from the analyses of ancient ignimbrite and fall deposits, whose componentry (enrichment in crystals in ignimbrites), grain size, and stratigraphy (homogeneous fine ashfall deposits) suggested that, during emplacement of PDCs, significant elutriated clouds formed (e.g., Sparks and Walker, 1977). The giant umbrella cloud formed by the liftoff of the blast PDC during the Mount St. Helens eruption provided the first direct observation of these processes from a PDC [START_REF] Sparks | The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens[END_REF]. It also demonstrated that elutriated co-PDC clouds reached similar (or even greater) heights than vent-derived columns, contributing significantly to the fallout volume and influencing grain size and componentry characteristics of fallout deposits [START_REF] Eychenne | Impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 May 1980 Mount St. Helens fallout deposit[END_REF]. The early interpretations of the Mount St. Helens plume dynamics (both the early blast co-PDC cloud and co-PDC plumes from later phases) informed the interpretation of other deposits, including from the Pinatubo eruption in 1991 [START_REF] Holasek | Satellite observations and interpretation of the 1991 Mount Pinatubo eruption plumes[END_REF], and eruptions in the geological record that lack direct evidence for co-ignimbrite clouds [START_REF] Engwell | Chapter 4 -contribution of fine ash to the atmosphere from plumes associated with pyroclastic density currents[END_REF].

Umbrella cloud growth

Umbrella growth relationships derived from the Mount St. Helens observations [START_REF] Sparks | The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens[END_REF]) have been generalized and combined with theoretical relations for steady growth derived from Pinatubo (e.g., Sparks et al. 1997, ch. 11). The theory has been applied to estimate mass eruption rate of eruptions [START_REF] Pouget | Estimation of eruption source parameters from umbrella cloud or downwind plume growth rate[END_REF][START_REF] Van Eaton | Volcanic lightning and plume behavior reveal evolving hazards during the April 2015 eruption of Calbuco volcano[END_REF][START_REF] Hargie | Globally detected volcanic lightning and umbrella dynamics during the 2014 eruption of Kelud, Indonesia[END_REF], a key parameter used to model ash-cloud concentration for aviation safety (e.g., [START_REF] Beckett | Atmospheric dispersion modelling at the London VAAC: a review of developments since the 2010 Eyjafjallajökull volcano ash cloud[END_REF]. The advection of ash in the upwind [START_REF] Sparks | The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens[END_REF]) and crosswind directions (Bursik et al. 1992a) inferred in 1980 has been considered for much larger eruptions since the Pinatubo event [START_REF] Koyaguchi | Origin of the giant eruption cloud of Pinatubo, June 15, 1991[END_REF], with the recognition that eruptions of size VEI 6 or larger on the Volcanic Explosivity Index [START_REF] Newhall | The volcanic explosivity index/VEI/-an estimate of explosive magnitude for historical volcanism[END_REF] are likely to produce large, circular clouds and deposits that differ qualitatively from the oblong and fan-shaped features associated with weak plumes. Such giant umbrella clouds may be subject to Coriolis forces that increase areal dispersion and reduce the sensitivity of dispersion to prevailing wind directions or plume height [START_REF] Baines | Dynamics of giant volcanic ash clouds from supervolcanic eruptions[END_REF][START_REF] Mastin | Modeling ash fall distribution from a Yellowstone supereruption[END_REF]. For example, umbrella growth may account for Yellowstone tephra deposits that lie 2000 km upstream of prevailing winds [START_REF] Mastin | Modeling ash fall distribution from a Yellowstone supereruption[END_REF].

Ash aggregation

The correlation between a secondary thickness maximum and aggregation was first made at Mount St. Helens [START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF]. Observations of ash aggregates in the proximal and distal May 18 deposit have motivated laboratory and field-based studies of aggregation for several decades (Sorem 1982;[START_REF] Gilbert | The origin of accretionary lapilli[END_REF][START_REF] Schumacher | A reappraisal of Mount St. Helens' ash clusters -depositional model from experimental observation[END_REF][START_REF] Schumacher | Models for the origin of accretionary lapilli[END_REF][START_REF] Veitch | Particle aggregation in volcanic eruption columns[END_REF][START_REF] James | Experimental investigation of volcanic particle aggregation in the absence of a liquid phase[END_REF][START_REF] James | Density, construction, and drag coefficient of electrostatic volcanic ash aggregates[END_REF]Textor et al. 2006aTextor et al. , 2006b;;[START_REF] Durant | Hydrometeor-enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens (USA)[END_REF][START_REF] Rose | Fate of volcanic ash: aggregation and fallout[END_REF][START_REF] Brown | A review of volcanic ash aggregation[END_REF][START_REF] Telling | An experimental evaluation of ash aggregation in explosive volcanic eruptions[END_REF][START_REF] Telling | Ash aggregation in explosive volcanic eruptions[END_REF][START_REF] Durant | RESEARCH FOCUS: toward a realistic formulation of fine-ash lifetime in volcanic clouds[END_REF][START_REF] Bagheri | Timing and nature of volcanic particle clusters based on field and numerical investigations[END_REF][START_REF] Durant | Chapter 3 -ash aggregation in volcanic clouds[END_REF][START_REF] Mueller | Stability of volcanic ash aggregates and break-up processes[END_REF][START_REF] Pollastri | Modelling the effect of electrification on volcanic ash aggregation[END_REF]. For example, [START_REF] Schumacher | A reappraisal of Mount St. Helens' ash clusters -depositional model from experimental observation[END_REF] sought to reproduce the "Sorem clusters" that fell in eastern Washington using laboratory experiments of charged ash particles, and the secondary thickness maximum described by [START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF] is often cited to demonstrate how aggregation can impact the large-scale structure of fall deposits.

Influence on operational tephra forecasting

Throughout 1980 and beyond, multiple explosive eruptions of Mount St. Helens sent ash in many different directions [START_REF] Sarna-Wojcicki | Areal distribution, thickness, mass, volume, and grain size of air-fall ash from the six major eruptions of 1980[END_REF]. Portland, Oregon, located southwest of the volcano, was assumed to be mostly safe from ashfall in the pre-1980 hazard assessment [START_REF] Crandell | Potential hazards from future eruptions of Mount St. Helens volcano[END_REF], but was affected on 12 June and 17 October. The 12 June event required removal of 5300 m 3 of ash from more than 2400 km of city streets [START_REF] Schuster | Effects of the eruptions on civil works and operations in the Pacific Northwest[END_REF]. Many communities inundated by ash on 18 May and subsequent events were caught by surprise, as State emergency managers and the USGS were concentrating communication efforts on local counties where impacts were greater [START_REF] Warrick | Operational modelling of umbrella cloud growth in a Lagrangian volcanic ash transport and dispersion model[END_REF].

The widespread impact of ash raised the importance of producing daily forecasts that could show areas at risk when an eruption occurred. Thus, from late March 1980 through to the end of 1981, the USGS ran an early and pioneering NOAA forecast model [START_REF] Heffter | A regional-continental scale transport, diffusion, and deposition model[END_REF]) that would interpolate wind trajectories at different elevations (Fig. 10a) [START_REF] Miller | Hazards assessments at Mount St. Helens[END_REF]). These efforts heralded major modeling advances in the next few decades.

From the 1980s through the 1990s, models that calculate tephra sedimentation became common in volcanology (e.g., [START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF][START_REF] Suzuki | A theoretical model for dispersion of tephra[END_REF][START_REF] Hopkins | A volcanic ash transport model and analysis of Mount St. Helens ashfall[END_REF][START_REF] Carey | Variations in column height and magma discharge during the May 18, 1980 eruption of Mount St. Helens[END_REF][START_REF] Armienti | A numerical model for simulation of tephra transport and deposition: applications to May 18, 1980, Mount St. Helens eruption[END_REF][START_REF] Glaze | Ashfall dispersal for the 16 September 1986, eruption of Lascar, Chile, calculated by a turbulent diffusion model[END_REF]Bursik et al. 1992b;[START_REF] Sparks | Sedimentation of tephra by volcanic plumes. Part 2: controls on thickness and grain-size variations of tephra fall deposits[END_REF][START_REF] Hurst | ASHFALL--A computer program for estimating volcanic ash fallout: report and users guide[END_REF][START_REF] Bonadonna | Thickness variations and volume estimates of tephra fall deposits: the importance of particle Reynolds number[END_REF]). Most such models solved for advection and diffusion semi-analytically (e.g., [START_REF] Hurst | ASHFALL--A computer program for estimating volcanic ash fallout: report and users guide[END_REF] or, in weak plumes, by calculating air entrainment and particle settling (e.g., [START_REF] Bonadonna | Modeling tephra sedimentation from a Ruapehu weak plume eruption[END_REF]. Given a specified plume height, erupted volume, grain-size distribution, and one-dimensional wind sounding, they can produce theoretical isopach maps. Many of these models were validated by comparing with the Mount St. Helens deposit data [START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF][START_REF] Hopkins | A volcanic ash transport model and analysis of Mount St. Helens ashfall[END_REF][START_REF] Armienti | A numerical model for simulation of tephra transport and deposition: applications to May 18, 1980, Mount St. Helens eruption[END_REF][START_REF] Glaze | Ashfall dispersal for the 16 September 1986, eruption of Lascar, Chile, calculated by a turbulent diffusion model[END_REF][START_REF] Bonadonna | Sedimentation from strong volcanic plumes[END_REF]. During the next phase at Mount St. Helens from 2004 to 2008, the USGS employed the ashfall model [START_REF] Hurst | ASHFALL--A computer program for estimating volcanic ash fallout: report and users guide[END_REF] for daily forecasting (Fig. 10b).

In parallel efforts, from the 1980s into the 2000s, the meteorological community developed three-dimensional dispersal models to track the path of pollutant clouds [START_REF] Draxler | An overview of the Hysplit-4 modeling system for trajectories, dispersion, and deposition[END_REF][START_REF] Jones | Atmospheric dispersion modelling at the Met Office[END_REF][START_REF] Daniele | Applications of the PUFF model to forecasts of volcanic clouds dispersal from Etna and Vesuvio[END_REF][START_REF] Folch | FALL3D: A computational model for transport and deposition of volcanic ash[END_REF][START_REF] Stein | NOAA's HYSPLIT atmospheric transport and dispersion modeling system[END_REF]. These models differ from the above-described models in that they use a threedimensional time-varying wind field, are fully numerical, slower than one-dimensional semi-analytical models (running over minutes to hours versus seconds for the latter). These models can forecast cloud movement as well as ashfall and can simulate very large or long-lived eruptions in changing wind fields. Today, these models are the core tools used to forecast ash-cloud movement for aviation safety. Many of them consider key processes highlighted in the Mount St. Helens eruption such as particle aggregation [START_REF] Costa | A model for wet aggregation of ash particles in volcanic plumes and clouds: 1. Theoretical formulation[END_REF][START_REF] Folch | A model for wet aggregation of ash particles in volcanic plumes and clouds: 2 Model Application[END_REF][START_REF] Beckett | Modelling the size distribution of aggregated volcanic ash and implications for operational atmospheric dispersion modelling[END_REF]) and umbrella growth [START_REF] Costa | Density-driven transport in the umbrella region of volcanic clouds: implications for tephra dispersion models[END_REF][START_REF] Mastin | Modeling ash fall distribution from a Yellowstone supereruption[END_REF][START_REF] Beckett | Atmospheric dispersion modelling at the London VAAC: a review of developments since the 2010 Eyjafjallajökull volcano ash cloud[END_REF].

The literature on plume dynamics, tephra transport, and deposition has grown hugely, and a complete up-to-date review is out of the scope here. Some publications that summarize developments over the four decades since 1980 include [START_REF] Sparks | Volcanic Plumes[END_REF], [START_REF] Bonadonna | Future developments in modelling and monitoring of volcanic ash clouds: outcomes from the first IAVCEI-WMO workshop on Ash Dispersal Forecast and Civil Aviation[END_REF][START_REF] Folch | A review of tephra transport and dispersal models: Evolution, current status, and future perspectives[END_REF]. [START_REF] Schwaiger | Ash3d: a finite-volume, conservative numerical model for ash transport and tephra deposition[END_REF]; https:// vsc-ash. wr. usgs. gov/ Ash3d). Results of this type are generated daily and posted online for instructional purposes (https:// www. usgs. gov/ volca noes/ mount-st.-helens/ ash-cloud-simul ationswhat-if-mount-st-helens-produ ced-explo sive). Similar results are automatically generated twice daily for all U.S. volcanoes in a state of unrest. Results of each simulation also include animations of cloud movement, maps of cloud height, arrival time, and tables of ash arrival time, ashfall thickness, and fall duration at regional airports cancellation of nearly 1400 flights and damaged nine aircraft in flight-among the most of any historical eruption behind Eyjafjallajökull in 2010 [START_REF] Guffanti | Encounters of aircraft with volcanic ash clouds: a compilation of known incidents[END_REF][START_REF] Christmann | STO-MP-AVT-272: Impact of Volcanic Ash Clouds on Military Operations[END_REF]. The first recorded case of in-flight engine damage occurred on 25 May when an L-100 aircraft flew into a plume over the volcano [START_REF] Gabbard | Dust-cloud effects on aircraft engines--emerging issues and new damage mechanisms[END_REF][START_REF] Guffanti | Encounters of aircraft with volcanic ash clouds: a compilation of known incidents[END_REF].

Influence on ash and aviation hazards

In 1980, the NWS Forecast Office in Seattle forecast ashcloud movement using interpolated NOAA-based wind trajectories, similar to the USGS forecasts [START_REF] Miller | Hazards assessments at Mount St. Helens[END_REF]. Nearly two decades later, as Volcanic Ash Advisory Centers (VAACs) were established [START_REF] Servranckx | Volcanic ash advisory centers--roles and challenges[END_REF], national meteorological agencies adapted their three-dimensional dispersion models to track the movement of volcanic ash clouds. Examples include the Met Office NAME model [START_REF] Jones | Atmospheric dispersion modelling at the Met Office[END_REF], the NOAA Hysplit model [START_REF] Stein | NOAA's HYSPLIT atmospheric transport and dispersion modeling system[END_REF], the Canadian CANERM model (now MLDP0, D'Amours et al. ( 2015)), and the Meteo France model MOCAGE Accident [START_REF] Sič | Modelling of primary aerosols in the chemical transport model MOCAGE: development and evaluation of aerosol physical parameterizations[END_REF]. In the late 2000s and early 2010s, two additional 3D models, Fall3d [START_REF] Folch | FALL3D: A computational model for transport and deposition of volcanic ash[END_REF]) and Ash3d [START_REF] Schwaiger | Ash3d: a finite-volume, conservative numerical model for ash transport and tephra deposition[END_REF], were developed specifically for ash deposition and movement of volcanic clouds. They too were validated by comparison with the Mount St. Helens 1980 deposit data [START_REF] Folch | A model for wet aggregation of ash particles in volcanic plumes and clouds: 2 Model Application[END_REF][START_REF] Mastin | Adjusting particle-size distributions to account for aggregation in tephra-deposit model forecasts[END_REF].

Today, tephra dispersal models are used by both volcano observatories and VAACs. User interfaces [START_REF] Bear-Crozier | Development of python-FALL3D: a modified procedure for modelling volcanic ash dispersal in the Asia-Pacific region[END_REF][START_REF] Mastin | User's guide and reference to Ash3d: a three-dimensional model for atmospheric tephra transport and deposition[END_REF][START_REF] Palma | Vhub: a knowledge management system to facilitate online collaborative volcano modeling and research[END_REF]) make them usable to all volcano observatory staff during unrest [START_REF] Coombs | Overview, chronology, and impacts of the 2016-2017 eruption of Bogoslof volcano, Alaska[END_REF] or as instructional tools (e.g., Fig. 10c). In 1980, it took about an hour for an operator to connect remotely to a NOAA server, run the model, and generate a map. Today, daily Ash3d simulations for all Alaska volcanoes at a state of unrest are generated and posted automatically (https:// www. avo. alaska. edu/).

Discussion and remaining questions

Based on both plume height and volume, the 18 May 1980 eruption ranked 5 on the VEI scale. On average, VEI 5 + eruptions occur once or twice per decade [START_REF] Deligne | Recurrence rates of large explosive volcanic eruptions[END_REF]. The growth of umbrella clouds, climatic effects, societal impacts of tephra fallout and reworking, physics of shock waves and tsunamis are just a few examples of phenomena associated with such large eruptions. In addition, large eruptions attract widespread attention and observations that help science surge forward. Thus, the size of the Mount St. Helens eruption and the attention it garnered partially explain why we learned so much. But even among eruptions this size, its impact was large, and it seems appropriate to ask why.

Since 1980, there have been only four or five eruptions ranked VEI 5 or larger by the Smithsonian Institution's Global Volcanism Program (https:// volca no. si. edu/): El Chichón (1982) [START_REF] Sigurdsson | The 1982 eruptions of El Chichón Volcano, Mexico: stratigraphy of pyroclastic deposits[END_REF], Hudson (1991) [START_REF] Scasso | Sedimentological analysis of the tephra from the 12-15 August 1991 eruption of Hudson volcano[END_REF]), Pinatubo (1991) [START_REF] Wiesner | Grain size, areal thickness distribution and controls on sedimentation of the 1991 Mount Pinatubo tephra layer in the South China Sea[END_REF]), Puyahue-Cordòn Caulle (2011) [START_REF] Pistolesi | Complex dynamics of small-moderate volcanic events: the example of the 2011 rhyolitic Cordón Caulle eruption[END_REF], and Hunga-Tonga Hunga Ha'apai (HTHH, 2022) [START_REF] Terry | Tonga volcanic eruption and tsunami, January 2022: globally the most significant opportunity to observe an explosive and tsunamigenic submarine eruption since AD 1883 Krakatau[END_REF]. Each eruption taught important lessons about the nature and consequences of large eruptions. Pinatubo for example demonstrated the power of large umbrella clouds in dispersing tephra [START_REF] Koyaguchi | Origin of the giant eruption cloud of Pinatubo, June 15, 1991[END_REF]. HTHH provided the first modern data on global volcanogenic tsunamis [START_REF] Carvajal | Worldwide signature of the 2022 Tonga volcanic tsunami[END_REF]) and atmospheric waves [START_REF] Matoza | Atmospheric waves and global seismoacoustic observations of the January 2022 Hunga eruption[END_REF]. Sulfur dioxide emissions could be measured with increasing accuracy during each eruption and used to calibrate the cooling effect of SO 2 on climate [START_REF] Carn | Multi-decadal satellite measurements of global volcanic degassing[END_REF]). Yet none of these subsequent eruptions could be directly observed as well as Mount St. Helens in 1980. Hudson and HTHH were too remote. Puyahue Cordòn Caulle was also remote and is ranked VEI 5 only because its several VEI 3-4 events are aggregated into a single eruption [START_REF] Pistolesi | Complex dynamics of small-moderate volcanic events: the example of the 2011 rhyolitic Cordón Caulle eruption[END_REF]. Major phases of El Chichón occurred at night and destroyed nearby villages of observers [START_REF] Sigurdsson | The 1982 eruptions of El Chichón Volcano, Mexico: stratigraphy of pyroclastic deposits[END_REF]. Pinatubo erupted during a major typhoon. Large eruptions are infrequent; and the well-observed ones are rarer still.

Many questions raised by the 1980 Mount St. Helens eruption remain. For example, what controlled the size and density of aggregates? How has the tephra-fall deposit degraded over time? How many of the tephra eruptions of 1980 could we identify in the stratigraphic record, using only exposures available today? These questions await further study.

Conclusions

The 18 May 1980 Mount St. Helens eruption taught us more about tephra transport and sedimentation than just about any other in the past half-century. The rich observations launched a new era of research on tephra transport and forecasting of tephra hazards. Many phenomena were highlighted, such as the growth of umbrella clouds, aggregation processes, and hydrometeor formation. These are now regarded as first-order processes essential to consider in forecasting and modeling tephra hazards. The pioneering use of dispersion models has evolved into a core component of tephra-hazard forecasting. The disruptive effects of ash on flight operations helped motivate the development of a global infrastructure to forecast ash-cloud movement for aviation safety. Every eruption teaches us something new. But the abundant observations and significant impacts of 18 May 1980 made it a singular teaching event.

Fig. 1

 1 Fig. 1 (a) Chronology of eruptive activity on 18 May 1980, including plume height with time, and time of occurrence of PDCs. Radar heights reported by[START_REF] Harris | Radar observations of ash eruptions[END_REF] are "near the volcano" and likely to reflect column heights above the vent, whereas satellite cloud-top heights reported by[START_REF] Holasek | GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption[END_REF] are not restricted to the vent area. During the morning of 18 May, the data from Holasek and Self (1995) likely reflect the height of the drifting umbrella cloud

Fig. 2

 2 Fig. 2 Photomosaic of the growing Mount St. Helens umbrella cloud, taken by Rocky Kolberg from about 3.2 km NNE of downtown Toledo, Washington, USA (46.4686°N, -122.8413°W), at about 08:52. Annotations indicate key processes inferred. Inset shows successive outlines of the growing umbrella cloud, taken at 5-min intervals from 08:40 to 09:10, from a military satellite as reported in Sparks et al. (1986, Fig. 8). Wikimedia Commons photo (https:// commo ns. wikim edia. org/ wiki/ File: MtStH elens_ Mushr oom_ Cloud. jpg)

Fig. 3

 3 Fig. 3 (a) Map showing outlines of the ash cloud at different times on 18 May from Sarna-Wojcicki et al. (1981, Fig. 332); (b) isopach map of the deposit (mm), from Sarna-Wojcicki et al. (1981, Fig. 336); (c) isomass map of the deposit (kg/m 2 ), from Sarna-Wojcicki et al.(1981, Fig. 338). Blue circles represent sample locations[START_REF] Durant | Hydrometeor-enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens (USA)[END_REF]. Red triangle gives location of Mount St. Helens. Red dots in (b) and (c) give sample locations inFigs. 6a, b, d (Carey and Sigurdsson 1982). Green dots in (b) and (c) give sample locations in Fig.7. Yellow star in (b) gives the location of aggregates photographed in Fig.5

Fig. 4

 4 Fig. 4 Key processes and features of the plume and downwind cloud at three times on 18 May 1980 eruption. (a, b) Rise and expansion of the coignimbrite plume, 08:40-09:00 local time. (c) Movement of the downwind cloud shortly before noon, as it was being fed by a Plinian column from the vent.(d) Downwind movement of the cloud in late afternoon, as it was being fed by ash from both the Plinian column and from ash elutriating from PDCs near the vent. Some of the key processes described in this paper are labeled

aggregatesFig. 5

 5 Fig. 5 Ash aggregates on pavement, photographed by Spencer Wood on the morning of May 19, 1980, on Whitebird Grade near Grangeville, Idaho (45.8289°N, 116.2465°W), 470 km ESE of Mount St.Helens. Location given by yellow star in Fig.3b

Fig. 6

 6 Fig.6(a) Grain-size distribution of the bulk fall deposit at different distances downwind along the main dispersal axis. Data from[START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF] (Fig.3). (b) Plot of the size of the coarse mode, fine mode, and mean grain size of the bulk fall deposit as a function of distance downwind. Numbers on the blue and yellow curves indicate the weight percent of the total deposit that each mode represents at various distances downwind. After[START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF]. (c) Bulk deposit density versus distance downwind from Mount St. Helens. After[START_REF] Sarna-Wojcicki | Areal distribution, thickness, mass, volume, and grain size of air-fall ash from the six major eruptions of 1980[END_REF]. (d) Variations in componentry with distance. Data from[START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF] (Fig.7). Sample locations given as red dots in Figs.3b, c. Data used in Figs.6a, b, d provided in TableS1

Fig. 7

 7 Fig.7Grain-size distributions along north-south transects across the deposit in central Washington. sample locations given as green dots in Figs.3b, c. Data from[START_REF] Durant | Hydrometeor-enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens (USA)[END_REF]

Fig. 8

 8 Fig. 8 (a) Isopleth map of diameter of aggregates in the blast ashfall deposit. From Sisson (1995). (b) Thin section of accretionary lapilli collected from the blast ashfall. Photo by Alexa Van Eaton

Fig. 9

 9 Fig. 9 (a) Predicted mass loading of the Mount St. Helens fall deposit using the tephra dispersal model of Carey and Sigurdsson (1982), with and without particle aggregation. (b) Observed mass loading of the Mount St. Helens fall deposit along the main dispersal axis from Sarna-Wojcicki et al.(1981). (c) Predicted (solid lines) and observed (dashed lines) values of the coarse and fine modes of the Mount St. Helens deposit along the main dispersal axis. After[START_REF] Carey | Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano[END_REF]

Fig. 10

 10 Fig. 10 Three generations of models used at the USGS Cascades Volcano Observatory. (a) Example wind trajectory forecast of the type generated daily in 1980 (from Miller et al. (1981)). This forecast uses the wind field available for 18 May 1980, 11:00 PDT. The first two symbols on each altitude track show positions of airborne ash 3 h apart; symbols farther along each track show positions at 6-h intervals. (b) Hypothetical eruption forecast using the semi-analytical ashfall model (Hurst 1994), assuming a tephra volume of 1 million cubic meters and plume height of 7 km. Forecasts of this type were produced daily during the 2004-2008 eruption of Mount St. Helens. (c) Modern model forecast of the hypothetical cloud from a 18 May-

Acknowledgements

We owe a particular debt to the few scientists who were involved in the 1980 response and were still available to share their recollections during preparation of this report. Among these are Carolyn Driedger, Michael Doukas, Richard Waitt, Thomas Murray, Ed Brown, and Tom Casadevall of the USGS. Spencer Wood (Boise State Univ., retired) drove the north-south transect just east of the Idaho-Washington border in 1980 and kindly sent us the photo of aggregates on the roadway in Fig. 5. Tom Murray and Ed Brown shared their experiences in preparing daily model forecasts in 1980 that are illustrated in Fig. 10a. Carolyn Driedger, Tom Casadevall, Mike Doukas, and Richard Waitt shared their experiences collecting tephra. We also thank reviewers Kristi Wallace Costanza Bonadonna, and Britta Jensen.

Supplementary Information

The online version contains supplementary material available at https:// doi. org/ 10. 1007/ s00445-022-01613-0.

Declarations

Disclaimer Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.