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Raisin

A raisin is a dried grape. Raisins are produced in many regions of the world and may be eaten raw or used in cooking, baking, and brewing. In the United Kingdom, Ireland, New Zealand, Australia and South Africa, the word raisin is reserved for the dark-colored dried large grape, with sultana being a golden-colored dried grape, and currant being a dried small Black Corinth seedless grape.

Etymology
The word "raisin" dates back to Middle English and is a loanword from Old French; in modern French, raisin means "grape", while a dried grape is a raisin sec, or "dry grape". The Old French word, in turn, developed from the Latin word racemus, "a bunch of grapes."

Varieties

Raisin varieties depend on the type of grape and appear in a variety of sizes and colors including green, black, brown, purple, blue, and yellow. Seedless varieties include the sultana (the common American type is known as Thompson Seedless in the United States), the Zante currants (black Corinthian raisins, Vitis vinifera L. var. Apyrena) and Flame grapes. Raisins are traditionally sun-dried, but may also be artificially dehydrated.

"Golden raisins" are generally dried in dehydrators with controlled temperature and humidity, which allows them to retain a lighter color and more moisture. They are often treated with sulfur dioxide after drying.

Black Corinth or Zante currants are small, sometimes seedless raisins that are much darker and have a tart, tangy flavor. They are usually called currants. Muscat raisins are large compared to other varieties, and also sweeter.

Grapes used to produce raisins in the Middle East and Asia include the large black monukka (or manucca) grapes that produce large raisins.

Processing

Raisins are produced commercially by drying harvested grape berries. For a grape berry to dry, water inside the grape must be removed completely from the interior of the cells onto the surface of the grape where the water droplets can evaporate. However, this diffusion process is very difficult because the grape skin contains wax in its cuticle, which prevents the water from passing through. In addition to this, the physical and chemical mechanisms located on the outer layers of the grape are adapted to prevent water loss. The three steps to commercial raisin production include pre-treatment, drying, and post-drying processes.

Pre-treatment
Pre-treatment is a necessary step in raisin production to ensure the increased rate of water removal during the drying process. A faster water removal rate decreases the rate of browning and helps to produce more desirable raisins. The historical method of completing this process was developed in the Mediterranean and Asia Minor areas by using a dry emulsion cold dip made of potassium carbonate and ethyl esters of fatty acids. This dip was shown to increase the rate of water loss by two- to three-fold.

Recently, new methods have been developed such as exposing the grapes to oil emulsions or dilute alkaline solutions. These methods can encourage water transfer to the outer surface of grapes which helps to increase the efficiency of the drying process.

Drying
The three types of drying methods are: sun drying, shade drying, and mechanical drying. Sun drying is an inexpensive process; however, environmental contamination, insect infections, and microbial deterioration can occur and the resulting raisins are often of low quality. Additionally, sun drying is a slow process and may not produce the most desirable raisins.

Mechanical drying can be done in a safer and more controlled environment where rapid drying is guaranteed. One type of mechanical drying is to use microwave drying. Water molecules in the grapes absorb microwave energy resulting in rapid evaporation. Microwave drying produces puffed raisins.

Post-drying processes
After the drying process is complete, raisins are sent to processing plants where they are cleaned with water to remove any foreign objects that may have become embedded during the drying process. Stems and off-grade raisins are also removed. The washing process may cause rehydration, so another drying step is completed after washing to ensure that the added moisture has been removed.

All steps in the production of raisins are very important in determining the quality of raisins. Sometimes, sulfur dioxide is applied to raisins after the pre-treatment step and before drying to decrease the rate of browning caused by the reaction between polyphenol oxidase and phenolic compounds. Sulfur dioxide also helps to preserve flavor and prevent the loss of certain vitamins during the drying process.

Production
Global production of raisins in 2020–21 was 1.2 million tonnes, led by Turkey, the United States, Iran, and India as the largest producers.

Nutrition
Raisins are 15% water, 79% carbohydrates (including 4% fiber), 3% protein, and contain negligible fat. In a reference amount of 100 g, raisins supply 299 kilocalories and moderate amounts (10–19% DV) of the Daily Value for several dietary minerals, riboflavin, and vitamin B6.

Toxicity in animals
Raisins can cause kidney failure in both cats and dogs. The cause of this is not known.

title: Raisin


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World Network of Biosphere Reserves

The UNESCO World Network of Biosphere Reserves (WNBR) covers internationally designated protected areas, known as biosphere or nature reserves, which are meant to demonstrate a balanced relationship between people and nature (e.g. encourage sustainable development). They are created under the Man and the Biosphere Programme (MAB).

Mission
The World Network of Biosphere Reserves (WNBR) of the MAB Programme consists of a dynamic and interactive network of sites. It works to foster the harmonious integration of people and nature for sustainable development through participatory dialogue, knowledge sharing, poverty reduction, human well-being improvements, respect for cultural values and by improving society's ability to cope with climate change. It promotes north–south and South-South collaboration and represents a unique tool for international cooperation through the exchange of experiences and know-how, capacity-building and the promotion of best practices.

The network
total membership had reached 738 biosphere reserves in 134 countries (including 22 transboundary sites) occurring in all regions of the world. This already takes into account some biosphere reserves that have been withdrawn or revised through the years, as the program's focus has shifted from simple protection of nature to areas displaying close interaction between man and environment.

Criteria and periodic review process
Article 4 of the defines general criteria for an area to be qualified for designation as a biosphere reserve as follows:  It should encompass a mosaic of ecological systems representative of major biogeographic regions, including a gradation of human interventions. It should be of significance for biological diversity conservation. It should provide an opportunity to explore and demonstrate approaches to sustainable development on a regional scale. It should have an appropriate size to serve the three functions of biosphere reserves — conservation, development, logistic support. It should include these functions through appropriate zonation, recognizing core, buffer, and outer transition areas. Organizational arrangements should be provided for the involvement and participation of a suitable range of inter alia public authorities, local communities and private interests in the design and carrying out the functions of a biosphere reserve. In addition, provisions should be made for:  mechanisms to manage human use and activities in the buffer zone or zones;

a management policy or plan for the area as a biosphere reserve;

a designated authority or mechanism to implement this policy or plan;

programmes for research, monitoring, education and training.

Article 9 of the Statutory Framework states that "the status of each biosphere reserve should be subject to a periodic review every ten years, based on a report prepared by the concerned authority, on the basis of the criteria of Article 4". If a biosphere reserve no longer satisfies the criteria contained in Article 4, it may be recommended the state concerned take measures to ensure conformity. Should a biosphere reserve still does not satisfy the criteria contained in Article 4, within a reasonable period, the area will no longer be referred to as a biosphere reserve which is part of the network.

Withdrawals
Article 9 of the Statutory Framework gives a state the right to remove a biosphere reserve under its jurisdiction from the network. , a total of 45 sites had been withdrawn from the World Network of Biosphere Reserves by 9 countries. Some reserves have been withdrawn after they no longer met newer, stricter criteria for reserves, for example on zonation or area size.

In June 2017, during the International Coordinating Council of the Man and the Biosphere Programme (MAB ICC) meeting in Paris, the United States has withdrawn 17 sites (out of the country's previous total of 47 sites) from the program.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: World Network of Biosphere Reserves


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Hekla

Hekla, or Hecla, is an active stratovolcano in the south of Iceland with a height of 1491 m. Hekla is one of Iceland's most active volcanoes; over 20 eruptions have occurred in and around the volcano since the year 1210. During the Middle Ages, the Icelandic Norse called the volcano the "Gateway to Hell" and the idea spread over much of Europe.

The volcano's frequent large and often initially explosive eruptions have covered much of Iceland with tephra, and these layers can be used to date eruptions of Iceland's other volcanoes. Approximately 10% of the tephra created in Iceland in the last thousand years has come from Hekla, amounting to 5 km3. Cumulatively, the volcano has produced one of the largest volumes of lava of any in the world in the last millennium, around 8 km3.

Etymology
In Icelandic Hekla is the word for a short hooded cloak, which may relate to the frequent cloud cover on the summit. An early Latin source refers to the mountain as Mons Casule.

Reputation
After the eruption of 1104, stories, probably spread deliberately through Europe by Cistercian monks, told that Hekla was the gateway to Hell. The Cistercian monk Herbert of Clairvaux wrote in his De Miraculis (without naming Hekla): "The renowned fiery cauldron of Sicily, which men call Hell's chimney ... that cauldron is affirmed to be like a small furnace compared to this enormous inferno." A poem by the monk Benedeit from c. 1120 about the voyages of Saint Brendan mentions Hekla as the prison of Judas. In the Flatey Book Annal it was recorded that during the 1341 eruption, people saw large and small birds flying in the mountain's fire which were taken to be souls. In the 16th century Caspar Peucer wrote that the Gates of Hell could be found in "the bottomless abyss of Hekla Fell". The belief that Hekla was the gate to Hell persisted until the 19th century. There is still a legend that witches gather on Hekla during Easter.

Geography
Hekla is part of a volcanic ridge, 40 km long. The most active part of this ridge, a fissure about 5.5 km long named Heklugjá, is considered to be within Hekla proper. Hekla looks rather like an overturned boat, with its keel being a series of craters, two of which are generally the most active.

Geology
Hekla has a morphological type between that of a fissure vent and stratovolcano (built from mixed lava and tephra eruptions) sited at a rift-transform junction in the area where the south Iceland seismic zone and eastern volcanic zone meet. The unusual form of Hekla is found on very few volcanoes around the world, notably Callaqui in Chile. The 5.5 km Heklugjá fissure opens along its entire length during major eruptions and is fed by a magma reservoir estimated to have a top 4 km below the surface with centroid 2.5 km lower. The chamber extends to an unusual depth of more than 10 km, and the more silicic lavas have matured at more than 9 km.

Many of the eruptions commence with thicker more explosive rhyolite, dacite or andesite eruptives which create tephra and have the potential for pyroclastic flows. Other or the later part of eruptions come from thinner basalt tending magma which forms lava fields.

The tephra produced by its eruptions is high in fluorine, which is poisonous to animals. Hekla's basaltic andesite lava generally has a SiO2 content of over 54%, compared to the 45–50% of other nearby transitional alkaline basalt eruptions (see TAS classification). It is the only Icelandic volcano to produce calc-alkaline lavas. Phenocrysts in Hekla's lava can contain plagioclase, pyroxene, titanomagnetite, olivine, and apatite.

When not erupting Hekla is often covered with snow and small glaciers; it is also unusually aseismic with activity only starting 30–80 minutes before an eruption. Hekla is located on the mid-ocean ridge, a diverging plate boundary. Hekla is closely studied today for parameters such as strain, tilt, deformation and other movement and seismic activity. Earthquakes in the volcano's vicinity are generally below magnitude 2 while it is dormant and magnitude 3 when erupting.

Eruption history
The earliest recorded eruption of Hekla took place in 1104. Since then there have been between twenty and thirty considerable eruptions, with the mountain sometimes remaining active for periods of six years with little pause. Eruptions in Hekla are varied and difficult to predict. Precursor seismic activity may only be for a couple of hours or less. Some are very short (a week to ten days) whereas others can stretch into months and years (the 1947 eruption started 29 March 1947 and ended April 1948). But there is a general correlation: the longer Hekla goes dormant, the larger and more catastrophic its opening eruption will be. The most recent eruption was on 26 February 2000.

Prehistoric eruptions
One of the largest Holocene eruptions in Iceland was the Hekla 3 (or H3) eruption c. 1000 BC, which threw about 7.3 km3 of volcanic rock into the atmosphere, placing its Volcanic Explosivity Index (VEI) at 5. This would have cooled temperatures in the northern parts of the globe for a few years afterwards. Traces of this eruption have been identified in Scottish peat bogs, and in Ireland a study of tree rings dating from this period has shown negligible tree ring growth for a decade. The dates were recently recalibrated of the major eruptions and a table is given below as the difference in dates could cause confusion. Unless otherwise stated eruption dates in Year (2019 values) column are from Global Volcanism Program and Catalogue of Icelandic Volcanoes, As other sources can disagree,  there has been a recent literature update. The values and range given in the Year (2024 values) column used IntCal20 for H-5 and H-3 and ice core data for H-4 which were not available in 2019.

Hekla 3, 4, and 5 produced huge amounts of rhyolitic ash and tephra, covering 80% of Iceland and providing useful date markers in soil profiles in other parts of Europe such as Orkney, Scandinavia, and elsewhere. H3 and H4 produced the largest layers of tephra in Iceland since the last ice age. During the last 7,000 years, one third of the volcanic ash deposited in Scandinavia, Germany, Ireland, and the United Kingdom originated from Hekla.

1104 to 1878
Hekla had been dormant for at least 250 years when it erupted explosively in 1104 (probably in the autumn), covering 55000 km2 which is over half of Iceland with 1.2 km3 / 2.5 km3 of rhyodacitic tephra. This was the second largest tephra eruption in the country in historical times with a VEI of 5. Farms upwind of the volcano 15 km in Þjórsárdalur valley, 50 km at Hrunamannaafréttur and 70 km at Lake Hvítárvatn were abandoned because of the damage. The eruption caused Hekla to become famous throughout Europe.
 * 1104 (H1)

A VEI-4 eruption began on 19 January 1158 producing over 0.15 km3 of lava and 0.2 km3 of tephra. It is likely to be the source of the Efrahvolshraun lava on Hekla's west.
 * 1158

The VEI-3 eruption began on 4 December.
 * 1206

The VEI-2 eruption and the 1206 eruption distributed around 0.24 km3 of tephra mainly to the northeast.
 * 1222

This VEI-4 eruption, which started on 11 July and lasted for a year, was the second largest tephra eruption of Hekla since Iceland was settled, covering 30000 km2 of land with 0.31 km3 of tephra. Over 0.5 km3 of lava was also expelled. The tephra caused significant damage to the settlements of Skagafjörður and Fljót, leading to over 500 deaths that winter. The material output from this eruption had SiO2 levels of between 56% and 64%, and apart from a slight abundance of olivine the lava, was typical of Hekla eruptions.
 * 1300–1301

A small eruption (VEI-3) started on 19 May and deposited around 5E7 km3 of tephra over the areas west and southwest of Hekla, leading to many cattle deaths, probably mainly from fluorosis.
 * 1341

In late 1389 Hekla erupted again (VEI-3), starting with a large ejection of tephra to the southeast. Later "the eruption fissure moved itself out of the mountain proper and into the woods a little above Skard". Skard and another nearby farm were destroyed by a large lava flow that now forms the 12.5 km2 Nordurhraun. In total around 0.3 km3 of lava and 5E7 m3 of tephra were produced.
 * 1389

An eruption may have occurred around 1440 at Raudölder; despite being close to Hekla this is not classed as an eruption of Hekla based on the SiO2 content of the lava.
 * 1440

Details of the 1510 eruption were not recorded until a century later. It started on 25 July and was particularly violent (VEI 4), firing volcanic bombs as far as Vördufell, 40 km west. Tephra was deposited over Rangárvellir, Holt and Landeyjar, 0.2 km3 in total. A man in Landsveit was killed.
 * 1510

A VEI-4 eruption began on 3 January and lasted for over 6 months, with 0.15 km3 of tephra being deposited to the south-southeast, damaging Mýrdalur.
 * 1597

A small (VEI-3) eruption began on 8 May 1636 and lasted for over a year. The 5E7 m3 of tephra from the eruption damaged pasture to the northeast causing death of livestock.
 * 1636–1637

Starting 13 January and lasting for over 7 months the eruption was one of Hekla's most destructive (VEI-4). Initially tephra was produced at 60,000 m3·s−1, 0.18 km3 during the entire eruption, which also caused lahars and tsunami. The tephra was deposited to the northwest, destroying and damaging farms and woodland in Þjórsárdalur, Land, Hreppar and Biskupstungur. Fine ash from the eruption reached Norway. There was damage to wildlife with significant numbers of trout, salmon, ptarmigan and farm animals dying.
 * 1693

A very small eruption, possibly only VEI-1, took place on 2 April 1725, producing flows of lava from locations around Hekla which have since been covered by later lava flows. These eruptions are not classed as of Hekla itself based on the SiO2 content of the lava.
 * 1725

The eruption of 1766 was large (VEI-4) and produced the second largest lava flow, 1.3 km3 covering 65 km2, and third largest tephra volume, 0.24 km3, of any Icelandic volcano during the inhabited era. The eruption started at around 3:30 am on 5 April 1766 and ceased in May 1768. Initially a 2–4 cm layer of tephra was deposited over Austur-Húnavatnssýsla and Skagafjördur, resulting in the deaths of both fish and livestock. Rangárvellir, Land and Hreppar also suffered damage. During the eruption up to 0.5 m lava bombs were thrown 15-20 km away, and flooding was caused by the sudden melting of snow and ice on Hekla's slopes.
 * 1766–1768

Hekla was dormant for more than sixty years before 1845, when it suddenly burst forth on 2 September at 9 am: "After a violent storm on the night of the 2nd of September in that year, the surface of the ground in the Orkney Islands was found strown with volcanic dust. There was thus conveyed to the inhabitants of Great Britain an intimation that Hecla had been again at work. Accordingly, tidings soon after arrived of a great eruption of the mountain. On the night of 1 September, the dwellers in its neighbourhood were terrified by a fearful underground groaning, which continued till mid-day on the 2nd. Then, with a tremendous crash, there were formed in the sides of the cone two large openings, whence there gushed torrents of lava, which flowed down two gorges on the flanks of the mountain. The whole summit was enveloped in clouds of vapour and volcanic dust. The neighbouring rivers became so hot as to kill the fish, and the sheep fled in terror from the adjoining heaths, some being burnt before they could escape. On the night of 15 September, two new openings were formed — one on the eastern, and the other on the southern slope — from both of which lava was discharged for twenty-two hours. It flowed to a distance of upwards of twenty miles, killing many cattle and destroying a large tract of pasturage. Twelve miles from the crater, the lava-stream was between forty and fifty feet deep and nearly a mile in width. On 12 October a fresh torrent of lava burst forth, and heaped up another similar mass. The mountain continued in a state of activity up to April 1846; then it rested for a while, and began again in the following month of October. Since then, however, it has enjoyed repose. The effects of these eruptions were disastrous. The whole island was strewn with volcanic ash, which, where it did not smother the grass outright, gave it a poisonous taint. The cattle that ate of it were attacked by a murrain, of which great numbers died. The ice and snow, which had gathered about the mountain for a long period of time, were wholly melted by the heat. Masses of pumice weighing nearly half a ton were thrown to a distance of between four and five miles." The eruption ceased around 5 April 1846. Initially in this VEI-4 eruption tephra was produced at 20,000 m3·s−1. The tephra deposition of a total amount of 0.17 km3 was mainly to the east-southeast; immediately to the east of Hekla the layer was 20 - 40 cm deep. Fine ash was carried to the Faroes, Shetland and Orkney. Lava flows to the west and north-west covered an area of 25 km2 with a volume of 0.63 km3 of lava. Large quantities of dark ash were deposited over pasture in the same directions leading to many livestock deaths through fluorosis for the next two years.
 * 1845–1846

A small eruption (VEI-2) occurred between 27 February 1878 and April 1878, around 10 km east of Hekla, and produced 0.2 km3 of lava from two parallel fissures covering 15.5 km2.
 * 1878

1913 to 1948
A small eruption (VEI-2) occurred between 25 April 1913 and 18 May 1913, around 10 km east of Hekla, and caused large fissures at Mundafell and Lambafit which produced 3.8 and 6.3 km2 of lava respectively.
 * 1913

The VEI-4 eruption started on 29 March 1947 and ended on 21 April 1948. It is likely that this was both the second greatest lava eruption of Hekla whilst Iceland was inhabited and the second greatest lava eruption in the world in the period 1900–1970. A total lava volume 0.8 km3 was produced with 0.21 km3 of tephra. The height of Hekla was 1447 m before the eruption, increasing to a maximum of 1503 m, before dropping to 1491 m subsequently.
 * 1947–1948

The eruption occurred over a century after the last eruption of Hekla proper, the longest dormant period since 1104. Before the eruption the volcano had been visible from the surrounding area but nothing remarkable was noticed. The eruption occurred at 6:41 am ± 3 min with a loud roar; later eruptions could be heard throughout Iceland. An earthquake at 6:50 am measured 6 on the Mercalli intensity scale and increased the eruption intensity until it covered a 4 km fissure on the ridge. The cloud from the eruption had ascended to a height of 30 km by 7:08 am, the wind then carried it southwards towards Eyjafjallajökull, turning it black. Pumice first landed on Fljótshlíð at around 7:10 am, and tephra and ash continued falling until it formed a 3 - 10 cm layer. A lava bomb that landed 32 km from Hekla was 0.5 m across and weighed 20 kg. Between Vatnafjöll and Hekla, a layer of tephra up to 1 m thick was deposited, and this included bombs with a diameter larger than 0.5 m. Bombs with surface areas of 50 m2 were dropped onto the slopes of Hekla, for up to 1 km. 51 hours after the eruption had started, ash fell on Helsinki, Finland, having covered 2860 km in this time.

The initial tephra production rate in the first 30 minutes of the eruption was 75,000 m3·s−1, dropping to 22,000 m3·s−1 for the next half-hour. The initial phase produced 0.18 km3 of tephra, equating to 4.5E7 km3 of Dense-rock equivalent, covering 3130 km2 of land and sea. 98 farms were damaged by the eruption, but only 2 were no longer farmed in 1970. A large volunteer effort was mobilized to clear the tephra – around 1000 man-days by the end of July. The eruption produced around 3 ML of water (snowmelt and directly from the fissure) which caused flooding of the Ytri Rangá river.

In the first 20 hours of the eruption approximately 3,500 m3·s−1 of lava was produced from the fissure, dividing into various branches and covering 12 - 15 km2. On the second day, 8 distinct eruption columns were discernible. A crater formed at 860 m called the Lava Crater (Hraungígur), producing a constant flow of lava. Another crater named the Shoulder Crater (Axlargígur) produced a column of smoke every 10 seconds together with loud explosions that created visible compression waves in the smoke. By the fourth, fifth, and sixth days, the eruption was greatly diminished, and only the shoulder and summit craters were erupting explosively.

The explosive eruption increased in strength from 9–12 April and then from 28 April it reduced again. On 3 May, the volcano stopped throwing out lava in sudden explosions from its craters and changed to continuously ejecting tephra and ash for long periods, until early June when this reduced. On 2 September, the Shoulder Crater had a 960 m circumference at its top and the Summit Crater a 700 mcircumference at its highest point, 90 m above the ridge. Sandy tephra and ash fell over Iceland in May and June, sometimes making it dark in the daytime near Hekla. The tephra caused fluorine poisoning of grazing sheep, making them unable to walk. That winter more craters formed, building up cones. Explosive activity had ceased six months after the first eruption. Lava flowed from the Lava Crater continuously during the eruption, starting at a rate of over 100 m3·s−1, dropping to 5–10 m3·s−1 in April and early May at a speed of around 20 cm·s−1 before increasing, eventually reaching 150 m3·s−1 at the end of June and at similar levels until mid-July with a peak flow speed of 2–2.5 m·s−1. From there it gradually decreased to under 10 m3·s−1 in November. Initially the lava comprised 57–58% SiO2 and 11% Fe2O3, from the time of peak flow onwards this changed to 54% SiO2 and 13.5% Fe2O3.

The lava river sometimes ran through lava tubes before emerging again. The lava front had a height of up to 15 m. On 15 and 16 June, a branch of lava flow to the south of Melfell traveled over 1 km in 30 hours before slowing and stopping by 21 June, 7.8 km from the Lava Crater. The longest lava stream produced was 8 km long and stopped in Stóraskógsbotnar. A scientist filming one of the lava streams on 2 November was hit by a block of lava and was killed.

The lava flow stopped after 13 months on 21 April, having covered 40 km2 and with a maximum depth of 100 m. The lava beds produced were mainly the ʻAʻā lava type with Pāhoehoe and lava a budella (lava tubes) areas. In April and May 1948 CO2 emitted from cracks in the ground pooled in hollows near to Hekla, killing 15 sheep and some wild animals and birds. In total 24000 tonne of CO2 was emitted. Ditches were dug by farmers to drain these hollows, and the CO2 emission had stopped by the end of the year.

1970 to 1991
The 1970 eruption of Hekla started at 9:23 pm on 5 May 1970 and lasted until 5 July. It had a VEI of 3 and produced 0.2 km3 of lava covering an area of 18.5 km2 and 6.6×10 7 m³ of tephra, deposited over an area of 40000 km2, mainly to the northwest of the volcano.
 * 1970

The main Hekla fissure only erupted at its far southwest end, most of the eruption was from other fissures nearby. The eruption stopped in the south-southwest on 10 May and in Hlídargígar on 20 May, but a new fissure opened on the same day and lava flowed from this until 5 July. The lava was andesite containing olivine, similar to the lava produced later in the eruption of 1947.

Before the eruption, a greater than normal amount of snow melting had occurred, indicating the volcano was heating up. Earth tremors began at 8:48 pm on the evening of the eruption; the largest had a magnitude of 4. The eruption started weakly at 9:23 pm IMT ± 2 min before increasing in power. The first pumice fell on Búrfell power station, 15 km away, at 9:35 pm causing people to evacuate. The eruption seems to have started in two locations at the same time – to the Shoulder crater's south-southwest and below the Lava Crater. At 10:30 pm a crater at 780 m was producing a lava column which reached an altitude of around 1 km. During the night a 700 m high lava fountain was thrown up from the main crater. A 500 m long fissure starting below the Lava Crater opened, and lava fountains and other lava flows emanated from it. One hour into the eruption, a new 400 m fissure opened to the northeast, producing two main lava fountains, and shortly after another adjoining fissure opened producing lava fountains to a height of 500 m. At around midnight, another fissure opened northwest of the Lava Crater, later hurling an over 300 m long lava fountain, 200 - 300 m into the air. By midnight lava had already covered over 1 km2 and this extended to 7.5 km2 by next morning implying a flow rate of around 1500 m³/s.

For the first two hours, tephra was produced at the rate of 10000 m³/s. The cloud from the eruption, which had reached 53000 ft by 10:10 pm, caused a lightning storm. The tephra was transported northwards by the wind, causing the sky to turn black in places – 190 km away at Blönduós tephra fell from midnight until 2 am, and ash fell on a trawler 330 km away at 2 am. Icelanders sampled the tephra fall in their locality by putting a plate outside to capture everything that fell onto it. This, and other measurements, showed the area covered was long and narrow with the 1 mm contour (an equivalent of 8 tonnes per hectare) extending to the north coast.

By 5:30 on 6 May, the lava flow measured 4 km long. Many lava bombs were found near the main crater, one had an area of 6 m2 and a likely weight of 12 tons. Xenoliths formed around 2% of the material produced by the craters. These were of rock types including basalt, andesite, ignimbrite and sedimentary rock.

The eruption became stronger at Skjólkvíar on 12 May, with columns of steam attaining a height of 2500 m. The eruption intensity then gradually reduced until it stopped on 20 May. The lava field then had an area of 5.8 km2. Later that day a 900 m long fissure opened 1 m21 km north of the main Hlídargígar crater. That night it contained 17 lava fountains, each 20 – in height. By the evening of the next day, 10–12 craters had formed, each throwing pieces of lava 50 – in the air. This row of craters was named Öldugígar. Gradually the number of active craters decreased, the most active of these built a cone 100 m higher than the level of the ridge. Lava flowed from its base until mid-June when the lava cut through the north crater wall. The larger cones produced more tephra, occasionally with lightning within the tephra cloud. By 5 July, the eruption had stopped.

During eruptions of Hekla, fluorine is produced and sticks to the surfaces of the grains of tephra. Fine grains can have a fluorine content of 350 ppm, and fluorine poisoning can start in sheep at a diet with fluorine content of 25 ppm. At 250 ppm, death can occur within a few days. In 1783, 79% of the Icelandic sheep stock were killed, probably as a result of fluorosis caused by the eruption of Lakagígar. Some of the ash produced in this eruption had a fluorine content of 0.2%, and two days after the eruption contaminated grass had a dry weight content of up to 0.4% fluorine. 450 farms and 95,000 sheep were affected by the eruption. Some sheep were kept inside and fed on hay or moved, but other farmers were forced to graze their flocks outside.

 * 1980 and 1981

This VEI-3 eruption started at 13:28 on 17 August 1980 and lasted until 20 August 1980. It was a mixed eruption producing a lava volume of 0.12 km3 and a tephra volume of 5.8E7 m3. The fissure opened along a 7 km length. Shortly before the eruption started a steam column was produced; eventually the eruption column reached a height of 15 km. The main tephra deposits were to the north-northeast and lasted for around 2 hours. Deposits were 20 cm thick 10 km from the summit, decreasing to less than 1 mm at the coast 230 km away. Lava was initially produced from close to the summit, spreading to other parts of the fissure and covering an area of 22 km2 in around 24 hours. The last scoria were seen on the morning of 20 August. This was an unusual eruption both in the short time since the previous eruption – the shortest since 1104, and the length – previous eruptions had lasted from 2 months to 2 years rather than just 3 days.

The 1981 eruption, which is regarded as being a continuation of the previous year's eruption, began at 3 am on 9 April 1981, had a VEI of 2 and produced 3E7 m3 of lava, lasting until 16 April 1981. The eruption threw ash to a height of 6.6 km, and a new crater formed at the summit from which 3 lava flows originated. These extended to a maximum of 4.5 km from the volcano, covering 5 - 6 km2.

A VEI-3 eruption occurred from 17 January 1991 to 11 March 1991, producing 0.15 km3 of lava and 2E7 m3 of tephra. The eruption, which was preceded by sulphurous smells and earthquakes, started as a Plinian eruption, producing an ash cloud reaching an altitude of 11.5 km within 10 minutes which had travelled over 200 km north-northeast to the coast within 3 hours. The eruption then began producing andesitic lava, the flows eventually covering an area of 23 km2 to an average depth of 6 - 7 m. Initially, part of the Heklugjá fissure and other fissures erupted with lava fountains reaching 300 m in height. By the second day, the activity stopped in all but one fissure where the main crater formed. During these 2 days, 800 m³/s of lava were produced, slowing to between 1 m³/s and 14 m³/s for most of the eruption. This low viscosity lava had a SiO2 content of approximately 54%.
 * 1991

2000
The most recent eruption was relatively short; it started at 18:18 on 26 February 2000 and lasted until 8 March. It was a VEI-3 eruption producing a lava volume of 0.189 km3, DRE 0.29 km3 and 1E7 m3m3 of tephra. The eruption went through four phases:
 * 1) initial explosive stage
 * 2) fire fountains
 * 3) bursts of Strombolian eruption
 * 4) effusion of lava

Eruption activity was at a maximum in the first hour, and by the first night the fissure on Hekla had opened to a length of 6 - 7 km. The steam column rose to a height of almost 15 km, and ash was transported to Grímsey on the North coast of Iceland. During this eruption, a NASA DC-8 aeroplane accidentally flew through the plume with all instruments switched on, resulting in unprecedented measurement of a young volcanic plume.

Up until this eruption, it had been assumed that Hekla was incapable of producing the most dangerous of volcanic phenomena, the pyroclastic flow. In January 2003, however, a team from the Norvol Institute in Reykjavík, under the leadership of Dr. Ármann Höskuldsson, reported that they found traces of a pyroclastic flow, roughly 5 km long, stretching down the side of the mountain. This will call for a reappraisal of volcanic eruptions of the basic rock type, which up to now were generally thought not to produce large pyroclastic flows. It will also require that the public and curious spectators who always rush to the scene at the start of a new outbreak, be kept much further away from the volcanic activity than was thought necessary during previous outbreaks.

Flora and fauna
The Hekla area was once forested. Forest and some grasses are much more resilient to ash and pumice fall than low vegetation, but the combined effect of human habitation and the volcanic activity has left an unstable surface very susceptible to erosion. Hekluskógar, a 90000 ha reforesting project is attempting to restore the birch and willow woodland to the slopes of Hekla, starting with soil fertilisation and grass sowing. This would stabilize the large areas of volcanic ash, help to reduce wind erosion of the frost heaved surface, slow drainage rates and hence water erosion, and ultimately increase biodiversity. It is the largest reforestation of its type in Europe. After an eruption, almost all of the 'safe sites' on new lava flows are colonised by mosses within 20 years expanding to a homogeneous layer up to 20 cm thick typically within 50 years.

Past eruptions have been associated with death of birds and live-stock related to either the high fluoride content of the tephra, carbon dioxide suffocation or toxic carbon monoxide gas release, and must have had local die back in the ecosystems. Local plant succession on the lava fields after eruptions has been better studied and there are four broad stages of natural succession with importantly the potential for regression: Local factors and other disturbances influence these rates but the 1991 laval flow first stage was completed in 24 years. The basaltic volcanic soil development is typical for Iceland. Vegetation height prior to a tephra fall is the single most important factor for vascular plant survival so the presence of a woodland before a further large tephra deposit enhances regrowth.
 * 1) In first 70 years colonization and cover coalescence of moss Racomitrium lanuginosum and the Stereocaulon mosses
 * 2) Moss secondary colonization to Racomitrium lanuginosum dominance which can take between 170 to 700 years
 * 3) After 600 years vascular plant dominance evolving towards the birch wood land climax ecosystem in Iceland if no disturbance
 * 4) Highland conditions/retrogression after tephra deposition which had occurred up to 860 years after the initial lava flow

Sport and recreation
Hekla is a popular destination for hiking. Following the most recent eruption the path goes most of the way to the summit; the walk takes 3 to 4 hours. In spring, skiing is possible on short routes around the rim of the crater. In summer, there are easy (F) mountaineering routes also around the crater rim, and it is possible to snowcat to the top in winter. The volcano can be reached using the buses to Landmannalaugar 30 km further east, and it is possible to stay or camp at farms in the area. A visitor centre, The Hekla Center at Leirubakki Farm, opened in 2007.

In popular culture
Hekla has featured in artistic works since the time of its medieval infamy.

Films
In the Spanish apocalyptic science fiction film, Los Últimos Días (2013), some news reporters speculate that three recent eruptions of Mount Hekla could have caused the spreading form of agoraphobia that kills affected people who go outside.

The climax of Robert Eggers's 2022 film The Northman takes place on the slopes of Hekla.

Food
In the Boston, Massachusetts area, Hekla pastries can be found – large, upside-down cinnamon rolls with white sugar icing spooned over the top to look like the snow-topped volcano.

Literature
The British poet William Blake showed Winter being banished to Hekla in To Winter, one of the works from his Poetical Sketches.

In To Lie With Lions, by Dorothy Dunnett, a party of merchants visiting Iceland in the year 1471 witnesses the spectacular (fictional) eruption of both Hekla and Katla.

Mt. Hekla is referenced in the third chapter of Herman Melville's novel Moby Dick, in EE Ryan's The Odd Saga of the American and a Curious Icelandic Flock, and in the final chapters of Joan Aiken's novel Is.

The Hekla 3 eruption and the ensuing volcanic winter play a large role in Stephen Baxter's alternate-history novel Bronze Winter.

Music
The piece Hekla, Op 52 (1964) by Icelandic composer Jón Leifs, has been called the "loudest classical music of all time". The requirements for a performance of Hekla include four sets of rocks hit with hammers, steel plates, anvils, sirens, cannons, metal chains, choir, a large orchestra, and organ.

Transportation
A small Danish cruiser launched in 1890 was named Hekla; it was scrapped in 1955.

A Danish steamer called Hekla also saw combat in the First Schleswig War.

Icelandair named one of their aircraft after Hekla.

There have been several ships of the Royal Navy named HMS Hecla

Organizations
In October 2011 a German left-wing militant group called the Hekla-Empfangskomitee (Hekla Reception Committee) set at least 17 incendiary devices on railways in the Berlin area, with 2 of them going off.

The DBU Copenhagen football club Boldklubben Hekla play at Hekla Park.

title: Hekla


text:
Cædmon

Cædmon (fl. c. 657–684) is the earliest English poet whose name is known. A Northumbrian cowherd who cared for the animals at the double monastery of Streonæshalch (now known as Whitby Abbey) during the abbacy of St. Hilda, he was originally ignorant of "the art of song" but learned to compose one night in the course of a dream, according to the 8th-century historian Bede. He later became a zealous monk and an accomplished and inspirational Christian poet.

Cædmon is one of twelve Anglo-Saxon poets identified in mediaeval sources, and one of three of these for whom both roughly contemporary biographical information and examples of literary output have survived. His story is related in the Historia ecclesiastica gentis Anglorum ("Ecclesiastical History of the English People") by Bede who wrote, "[t]here was in the Monastery of this Abbess a certain brother particularly remarkable for the Grace of God, who was wont to make religious verses, so that whatever was interpreted to him out of scripture, he soon after put the same into poetical expressions of much sweetness and humility in Old English, which was his native language. By his verse the minds of many were often excited to despise the world, and to aspire to heaven."

Cædmon's only known surviving work is Cædmon's Hymn, a nine-line alliterative vernacular praise poem in honour of God. The poem is one of the early attested examples of Old English and is, with the runic Ruthwell Cross and Franks Casket inscriptions, one of three candidates for the earliest attested example of Old English poetry. It is also one of the early recorded examples of sustained poetry in a Germanic language. In 1898, Cædmon's Cross was erected in his honour in the graveyard of St Mary's Church in Whitby.

Bede's account
The sole source of original information about Cædmon's life and work is Bede's Historia ecclesiastica. According to Bede, Cædmon was a lay brother who cared for the animals at the monastery Streonæshalch (now known as Whitby Abbey). One evening, while the monks were feasting, singing, and playing a harp, Cædmon left early to sleep with the animals because he knew no songs. The impression clearly given by St. Bede is that he lacked the knowledge of how to compose the lyrics to songs. While asleep, he had a dream in which "someone" (quidam) approached him and asked him to sing principium creaturarum, "the beginning of created things." After first refusing to sing, Cædmon subsequently produced a short eulogistic poem praising God, the Creator of heaven and earth.

Upon awakening the next morning, Caedmon remembered everything he had sung and added additional lines to his poem. He told his foreman about his dream and gift and was taken immediately to see the abbess, believed to be St Hilda of Whitby. The abbess and her counsellors asked Cædmon about his vision and, satisfied that it was a gift from God, gave him a new commission, this time for a poem based on "a passage of sacred history or doctrine", by way of a test.

When Cædmon returned the next morning with the requested poem, he was invited to take monastic vows. The abbess ordered her scholars to teach Cædmon sacred history and doctrine, which after a night of thought, Bede records, Cædmon would turn into the most beautiful verse. According to Bede, Cædmon was responsible for a large number of splendid vernacular poetic texts on a variety of Christian topics.

After a long and zealously pious life, Cædmon died like a saint: receiving a premonition of death, he asked to be moved to the abbey's hospice for the terminally ill where, having gathered his friends around him, he died after receiving the Holy Eucharist, just before nocturns. Although he is often listed as a saint, this is not confirmed by Bede and it has been argued that such assertions are incorrect.

The details of Bede's story, and in particular of the miraculous nature of Cædmon's poetic inspiration, are not generally accepted by scholars as being entirely accurate, but there seems no good reason to doubt the existence of a poet named Cædmon. Bede's narrative has to be read in the context of the Christian belief in miracles, and it shows at the very least that Bede, an educated and intelligent man, believed Cædmon to be an important figure in the history of English intellectual and religious life.

Dates
Bede gives no specific dates in his story. Cædmon is said to have taken holy orders at an advanced age and it is implied that he lived at Streonæshalch at least in part during Hilda's abbacy (657–680). Book IV Chapter 25 of the Historia ecclesiastica appears to suggest that Cædmon's death occurred at about the same time as the fire at Coldingham Abbey, an event dated in the E text of the Anglo-Saxon Chronicle to 679, but after 681 by Bede.

The reference to his temporibus "at this time" in the opening lines of Chapter 25 may refer more generally to Cædmon's career as a poet. However, the next datable event in the Historia ecclesiastica is King Ecgfrith's raid on Ireland in 684 (Book IV, Chapter 26). Taken together, this evidence suggests an active period beginning between 657 and 680 and ending between 679 and 684.

Modern discoveries
The only biographical or historical information that modern scholarship has been able to add to Bede's account concerns the Brittonic origins of the poet's name. Although Bede specifically notes that English was Cædmon's "own" language, the poet's name is of Celtic origin: from Proto-Welsh *Cadṽan (from Brythonic *Catumandos). Several scholars have suggested that Cædmon himself may have been bilingual on the basis of this etymology, Hilda's close contact with Celtic political and religious hierarchies, and some (not very close) analogues to the Hymn in Old Irish poetry. Other scholars have noticed a possible onomastic allusion to 'Adam Kadmon' in the poet's name, perhaps suggesting that the entire story is allegorical.

Other medieval sources

No other independent accounts of Cædmon's life and work are known to exist. The only other reference to Cædmon in English sources before the 12th century is found in the 10th-century Old English translation of Bede's Latin Historia. Otherwise, no mention of Cædmon is found in the corpus of surviving Old English. The Old English translation of the Historia ecclesiastica does contain several minor details not found in Bede's Latin original account.

Of these, the most significant is that Cædmon felt "shame" for his inability to sing vernacular songs before his vision, and the suggestion that Hilda's scribes copied down his verse æt muðe "from his mouth". These differences are in keeping with the Old English translator's practice in reworking Bede's Latin original, however, and need not, as Wrenn argues, suggest the existence of an independent English tradition of the Cædmon story.

Heliand
A second, possibly pre-12th-century allusion to the Cædmon story is found in two Latin texts associated with the Old Saxon Heliand poem. These texts, the Praefatio (Preface) and Versus de Poeta (Lines about the poet), explain the origins of an Old Saxon biblical translation (for which the Heliand is the only known candidate) in language strongly reminiscent of, and indeed at times identical to, Bede's account of Cædmon's career. According to the prose Praefatio, the Old Saxon poem was composed by a renowned vernacular poet at the command of the emperor Louis the Pious. The text then adds that this poet had known nothing of vernacular composition until he was ordered to translate the precepts of sacred law into vernacular song in a dream.

The Versus de Poeta contain an expanded account of the dream itself, adding that the poet had been a herdsman before his inspiration and that the inspiration itself had come through the medium of a heavenly voice when he fell asleep after pasturing his cattle. While our knowledge of these texts is based entirely on a 16th-century edition by Flacius Illyricus, both are usually assumed on semantic and grammatical grounds to be of medieval composition. This apparent debt to the Cædmon story agrees with semantic evidence attested to by Green demonstrating the influence of Old English biblical poetry and terminology on early continental Germanic literatures.

Sources and analogues
In contrast to his usual practice elsewhere in the Historia ecclesiastica, Bede provides no information about his sources for the Cædmon story. Since a similar paucity of sources is also characteristic of other stories from Whitby Abbey in his work, this may indicate that his knowledge of Cædmon's life was based on tradition current at his home monastery in (relatively) nearby Wearmouth-Jarrow.

Perhaps as a result of this lack of documentation, scholars have devoted considerable attention since the 1830s to tracking down possible sources or analogues to Bede's account. These parallels have been drawn from all around the world, including biblical and classical literature, stories told by the aboriginal peoples of Australia, North America and the Fiji Islands, mission-age accounts of the conversion of the Xhosa in Southern Africa, the lives of English romantic poets, and various elements of Hindu and Muslim scripture and tradition.

Although the search was begun by scholars such as Sir Francis Palgrave, who hoped either to find Bede's source for the Cædmon story or to demonstrate that its details were so commonplace as to hardly merit consideration as legitimate historiography, subsequent research has instead ended up demonstrating the uniqueness of Bede's version: as Lester shows, no "analogue" to the Cædmon story found before 1974 mirrors Bede's chapter in more than about half its main properties; the same observation can be extended to cover all analogues since identified.

General corpus
Bede's account indicates that Cædmon was responsible for the composition of a large oeuvre of vernacular religious poetry. In contrast to Saints Aldhelm and Dunstan, Cædmon's poetry is said to have been exclusively religious. Bede reports that Cædmon "could never compose any foolish or trivial poem, but only those which were concerned with devotion", and his list of Cædmon's output includes work on religious subjects only: accounts of creation, translations from the Old and New Testaments, and songs about the "terrors of future judgment, horrors of hell, ... joys of the heavenly kingdom, ... and divine mercies and judgments."

Of this corpus, only his first poem survives. While vernacular poems matching Bede's description of several of Cædmon's later works are found in London, British Library, Junius 11, traditionally referred to as the "Junius" or "Cædmon" manuscript, the older traditional attribution of these texts to Cædmon or Cædmon's influence cannot stand. The poems show significant stylistic differences both internally and with Cædmon's original Hymn, and there is nothing about their order or content to suggest that they could not have been composed and anthologised without any influence from Bede's discussion of Cædmon's oeuvre.

The first three Junius poems are in their biblical order and, while Christ and Satan could be understood as partially fitting Bede's description of Cædmon's work on future judgment, pains of hell and joys of the heavenly kingdom, the match is not exact enough to preclude independent composition. As Fritz and Day have shown, Bede's list itself may owe less to direct knowledge of Cædmon's actual output than to traditional ideas about the subjects fit for Christian poetry or the order of the catechism. Similar influences may, of course, also have affected the makeup of the Junius volume.

Cædmon's Hymn
The only known survivor from Cædmon's oeuvre is his Hymn (audio version ). The poem is known from 21 manuscript copies, making it the best-attested Old English poem after Bede's Death Song (with 35 witnesses) and the best attested in the poetic corpus in manuscripts copied or owned in the British Isles during the Anglo-Saxon period. The Hymn also has by far the most complicated known textual history of any surviving Old English poem.

It is found in two dialects and five distinct recensions (Northumbrian aelda, Northumbrian eordu, West-Saxon eorðan, West-Saxon ylda, and West-Saxon eorðe), all but one of which are known from three or more witnesses. It is one of the early attested examples of written Old English and one of the early recorded examples of sustained poetry in a Germanic language. Together with the runic Ruthwell Cross and Franks Casket inscriptions, Cædmon's Hymn is one of three candidates for the early attested example of Old English poetry.

There is continuing critical debate about the status of the poem as it is now available to us. While some scholars accept the texts of the Hymn as more or less accurate transmissions of Cædmon's original, others argue that they originated as a back-translation from Bede's Latin, and that there is no surviving witness to the original text.

Manuscript evidence
All copies of Hymn are found in manuscripts of the Historia ecclesiastica or its translation, where they serve as either a gloss to Bede's Latin translation of the Old English poem, or, in the case of the Old English version, a replacement for Bede's translation in the main text of the History. Despite this close connection with Bede's work, the Hymn does not appear to have been transmitted with the Historia ecclesiastica regularly until relatively late in its textual history. Scribes other than those responsible for the main text often copy the vernacular text of the Hymn in manuscripts of the Latin Historia. In three cases, Oxford, Bodleian Library, Laud Misc. 243, Oxford, Bodleian Library, Hatton 43, and Winchester, Cathedral I, the poem is copied by scribes working a quarter-century or more after the main text was first set down.

Even when the poem is in the same hand as the manuscript's main text, there is little evidence to suggest that it was copied from the same exemplar as the Latin Historia: nearly identical versions of the Old English poem are found in manuscripts belonging to different recensions of the Latin text; closely related copies of the Latin Historia sometimes contain very different versions of the Old English poem. With the exception of the Old English translation, no single recension of the Historia ecclesiastica is characterised by the presence of a particular recension of the vernacular poem.

Earliest text
The oldest known version of the poem is the Northumbrian aelda recension. The surviving witnesses to this text, Cambridge, University Library, Kk. 5. 16 (M) and St. Petersburg, National Library of Russia, lat. Q. v. I. 18 (P), date to at least the mid-8th century. M in particular is traditionally ascribed to Bede's own monastery and lifetime, though there is little evidence to suggest it was copied much before the mid-8th century.

The following text, first column on the left below, has been transcribed from M (mid-8th century; Northumbria). The text has been normalised to show a line-break between each line and modern word-division. A transcription of the likely pronunciation of the text in the early 8th-century Northumbrian dialect in which the text is written is included, along with a modern English translation.

Bede's Latin version runs as follows:
 * Nunc laudare debemus auctorem regni caelestis, potentiam creatoris, et consilium illius facta Patris gloriae: quomodo ille, cum sit aeternus Deus, omnium miraculorum auctor exstitit; qui primo filiis hominum caelum pro culmine tecti dehinc terram custos humani generis omnipotens creavit.
 * "Now we must praise the author of the heavenly realm, the might of the creator, and his purpose, the work of the father of glory: as he, who, the almighty guardian of the human race, is the eternal God, is the author of all miracles; who first created the heavens as highest roof for the children men, then the earth."

title: Cædmon


text:
Geothermal energy

Geothermal energy is thermal energy extracted from the Earth's crust. It combines energy from the formation of the planet and from radioactive decay. Geothermal energy has been exploited as a source of heat and/or electric power for millennia.

Geothermal heating, using water from hot springs, for example, has been used for bathing since Paleolithic times and for space heating since Roman times. Geothermal power, (generation of electricity from geothermal energy), has been used since the 20th century. Unlike wind and solar energy, geothermal plants produce power at a constant rate, without regard to weather conditions. Geothermal resources are theoretically more than adequate to supply humanity's energy needs. Most extraction occurs in areas near tectonic plate boundaries.

The cost of generating geothermal power decreased by 25% during the 1980s and 1990s. Technological advances continued to reduce costs and thereby expand the amount of viable resources. In 2021, the US Department of Energy estimated that power from a plant "built today" costs about $0.05/kWh.

In 2019, 13,900 megawatts (MW) of geothermal power was available worldwide. An additional 28 gigawatts provided heat for district heating, space heating, spas, industrial processes, desalination, and agricultural applications as of 2010. As of 2019 the industry employed about one hundred thousand people.

The adjective geothermal originates from the Greek roots γῆ (gê), meaning Earth, and θερμός (thermós), meaning hot.

History
Hot springs have been used for bathing since at least Paleolithic times. The oldest known spa is at the site of the Huaqing Chi palace. In the first century CE, Romans conquered Aquae Sulis, now Bath, Somerset, England, and used the hot springs there to supply public baths and underfloor heating. The admission fees for these baths probably represent the first commercial use of geothermal energy. The world's oldest geothermal district heating system, in Chaudes-Aigues, France, has been operating since the 15th century. The earliest industrial exploitation began in 1827 with the use of geyser steam to extract boric acid from volcanic mud in Larderello, Italy.

In 1892, the US's first district heating system in Boise, Idaho was powered by geothermal energy. It was copied in Klamath Falls, Oregon, in 1900. The world's first known building to utilize geothermal energy as its primary heat source was the Hot Lake Hotel in Union County, Oregon, beginning in 1907. A geothermal well was used to heat greenhouses in Boise in 1926, and geysers were used to heat greenhouses in Iceland and Tuscany at about the same time. Charles Lieb developed the first downhole heat exchanger in 1930 to heat his house. Geyser steam and water began heating homes in Iceland in 1943.

In the 20th century, geothermal energy came into use as a generating source. Prince Piero Ginori Conti tested the first geothermal power generator on 4 July 1904, at the Larderello steam field. It successfully lit four light bulbs. In 1911, the world's first commercial geothermal power plant was built there. It was the only industrial producer of geothermal power until New Zealand built a plant in 1958. In 2012, it produced some 594 megawatts.

In 1960, Pacific Gas and Electric began operation of the first US geothermal power plant at The Geysers in California. The original turbine lasted for more than 30 years and produced 11 MW net power.

A binary cycle power plant was first demonstrated in 1967 in the USSR and introduced to the US in 1981. This technology allows the generation of electricity from much lower temperature resources than previously. In 2006, a binary cycle plant in Chena Hot Springs, Alaska, came on-line, producing electricity from a record low temperature of 57 C.

Resources

The Earth has an internal heat content of 1031 joules (3·1015 TWh), About 20% of this is residual heat from planetary accretion; the remainder is attributed to past and current radioactive decay of naturally occurring isotopes. For example, a 5275 m deep borehole in United Downs Deep Geothermal Power Project in Cornwall, England, found granite with very high thorium content, whose radioactive decay is believed to power the high temperature of the rock.

Earth's interior temperature and pressure are high enough to cause some rock to melt and the solid mantle to behave plastically. Parts of the mantle convect upward since it is lighter than the surrounding rock. Temperatures at the core–mantle boundary can reach over 4000 °C.

The Earth's internal thermal energy flows to the surface by conduction at a rate of 44.2 terawatts (TW), and is replenished by radioactive decay of minerals at a rate of 30 TW. These power rates are more than double humanity's current energy consumption from all primary sources, but most of this energy flux is not recoverable. In addition to the internal heat flows, the top layer of the surface to a depth of 10 m is heated by solar energy during the summer, and cools during the winter.

Outside of the seasonal variations, the geothermal gradient of temperatures through the crust is 25 – per km of depth in most of the world. The conductive heat flux averages 0.1 MW/km2. These values are much higher near tectonic plate boundaries where the crust is thinner. They may be further augmented by combinations of fluid circulation, either through magma conduits, hot springs, hydrothermal circulation.

The thermal efficiency and profitability of electricity generation is particularly sensitive to temperature. Applications receive the greatest benefit from a high natural heat flux most easily from a hot spring. The next best option is to drill a well into a hot aquifer. An artificial hot water reservoir may be built by injecting water to hydraulically fracture bedrock. The systems in this last approach are called enhanced geothermal systems.

2010 estimates of the potential for electricity generation from geothermal energy vary sixfold, from 0.035 to  2  TW depending on the scale of investments. Upper estimates of geothermal resources assume wells as deep as 10 km, although 20th century wells rarely reached more than 3 km deep. Wells of this depth are common in the petroleum industry.

Geothermal power
Geothermal power is electrical power generated from geothermal energy. Dry steam, flash steam, and binary cycle power stations have been used for this purpose. As of 2010 geothermal electricity was generated in 26 countries.

As of 2019, worldwide geothermal power capacity amounted to 15.4 gigawatts (GW), of which 23.86 percent or 3.68 GW were in the United States.

Geothermal energy supplies a significant share of the electrical power in Iceland, El Salvador, Kenya, the Philippines and New Zealand.

Geothermal power is considered to be a renewable energy because heat extraction rates are insignificant compared to the Earth's heat content. The greenhouse gas emissions of geothermal electric stations are on average 45 grams of carbon dioxide per kilowatt-hour of electricity, or less than 5 percent of that of coal-fired plants.

Geothermal electric plants were traditionally built on the edges of tectonic plates where high-temperature geothermal resources approach the surface. The development of binary cycle power plants and improvements in drilling and extraction technology enable enhanced geothermal systems over a greater geographical range. Demonstration projects are operational in Landau-Pfalz, Germany, and Soultz-sous-Forêts, France, while an earlier effort in Basel, Switzerland, was shut down after it triggered earthquakes. Other demonstration projects are under construction in Australia, the United Kingdom, and the US. In Myanmar over 39 locations are capable of geothermal power production, some of which are near Yangon.

Geothermal heating
Geothermal heating is the use of geothermal energy to heat buildings and water for human use. Humans have done this since the Paleolithic era. Approximately seventy countries made direct use of a total of 270 PJ of geothermal heating in 2004. As of 2007, 28 GW of geothermal heating satisfied 0.07% of global primary energy consumption. Thermal efficiency is high since no energy conversion is needed, but capacity factors tend to be low (around 20%) since the heat is mostly needed in the winter.

Even cold ground contains heat: below 6 m the undisturbed ground temperature is consistently at the Mean Annual Air Temperature that may be extracted with a ground source heat pump.

Hydrothermal systems
Hydrothermal systems produce geothermal energy by accessing naturally-occurring hydrothermal reservoirs. Hydrothermal systems come in either vapor-dominated or liquid-dominated forms.

Vapor-dominated plants
Larderello and The Geysers are vapor-dominated. Vapor-dominated sites offer temperatures from 240 to 300 °C that produce superheated steam.

Liquid-dominated plants
Liquid-dominated reservoirs (LDRs) are more common with temperatures greater than 200 C and are found near volcanoes in/around the Pacific Ocean and in rift zones and hot spots. Flash plants are the common way to generate electricity from these sources. Steam from the well is sufficient to power the plant. Most wells generate 2–10 MW of electricity. Steam is separated from liquid via cyclone separators and drives electric generators. Condensed liquid returns down the well for reheating/reuse. As of 2013, the largest liquid system was Cerro Prieto in Mexico, which generates 750 MW of electricity from temperatures reaching 350 C.

Lower-temperature LDRs (120–200 °C) require pumping. They are common in extensional terrains, where heating takes place via deep circulation along faults, such as in the Western US and Turkey. Water passes through a heat exchanger in a Rankine cycle binary plant. The water vaporizes an organic working fluid that drives a turbine. These binary plants originated in the Soviet Union in the late 1960s and predominate in new plants. Binary plants have no emissions.

Engineered geothermal systems
An engineered geothermal system is a geothermal system that engineers have artificially created or improved. Engineered geothermal systems are used in a variety of geothermal reservoirs that have hot rocks but insufficient natural reservoir quality, for example, insufficient geofluid quantity or insufficient rock permeability or porosity, to operate as natural hydrothermal systems. Types of engineered geothermal systems include enhanced geothermal systems, closed-loop or advanced geothermal systems, and some superhot rock geothermal systems.

Enhanced geothermal systems
Enhanced geothermal systems (EGS) actively inject water into wells to be heated and pumped back out. The water is injected under high pressure to expand existing rock fissures to enable the water to flow freely. The technique was adapted from oil and gas fracking techniques. The geologic formations are deeper and no toxic chemicals are used, reducing the possibility of environmental damage. Instead proppants such as sand or ceramic particles are used to keep the cracks open and producing optimal flow rates. Drillers can employ directional drilling to expand the reservoir size.

Small-scale EGS have been installed in the Rhine Graben at Soultz-sous-Forêts in France and at Landau and Insheim in Germany.

Closed-loop geothermal systems
Closed-loop geothermal systems, sometimes colloquially referred to as Advanced Geothermal Systems (AGS), are engineered geothermal systems containing subsurface working fluid that is heated in the hot rock reservoir without direct contact with rock pores and fractures. Instead, the subsurface working fluid stays inside a closed loop of deeply buried pipes that conduct Earth's heat. The advantages of a deep, closed-loop geothermal circuit include: (1) no need for a geofluid, (2) no need for the hot rock to be permeable or porous, and (3) all the introduced working fluid can be recirculated with zero loss. Eavortm, a Canadian-based geothermal startup, piloted their closed-loop system in shallow soft rock formations in Alberta, Canada. Situated within a sedimentary basin, the geothermal gradient proved to be insufficient for electrical power generation. However, the system successfully produced approximately 11,000 MWh of thermal energy during its initial two years of operation."

Economics
As with wind and solar energy, geothermal power has minimal operating costs; capital costs dominate. Drilling accounts for over half the costs, and not all wells produce exploitable resources. For example, a typical well pair (one for extraction and one for injection) in Nevada can produce 4.5 megawatts (MW) and costs about $10 million to drill, with a 20% failure rate, making the average cost of a successful well $50 million.

Drilling geothermal wells is more expensive than drilling oil and gas wells of comparable depth for several reasons:
 * Geothermal reservoirs are usually in igneous or metamorphic rock, which is harder to penetrate than the sedimentary rock of typical hydrocarbon reservoirs.
 * The rock is often fractured, which causes vibrations that damage bits and other drilling tools.
 * The rock is often abrasive, with high quartz content, and sometimes contains highly corrosive fluids.
 * The rock is hot, which limits use of downhole electronics.
 * Well casing must be cemented from top to bottom, to resist the casing's tendency to expand and contract with temperature changes. Oil and gas wells are usually cemented only at the bottom.
 * Well diameters are considerably larger than typical oil and gas wells.

As of 2007 plant construction and well drilling cost about €2–5 million per MW of electrical capacity, while the break-even price was 0.04–0.10 € per kW·h. Enhanced geothermal systems tend to be on the high side of these ranges, with capital costs above $4 million per MW and break-even above $0.054 per kW·h.

Between 2013 and 2020, private investments were the main source of funding for renewable energy, comprising approximately 75% of total financing. The mix between private and public funding varies among different renewable energy technologies, influenced by their market appeal and readiness. In 2020, geothermal energy received just 32% of its investment from private sources.

Socioeconomic benefits
In January 2024, the Energy Sector Management Assistance Program (ESMAP) report "Socioeconomic Impacts of Geothermal Energy Development" was published, highlighting the substantial socioeconomic benefits of geothermal energy development, which notably exceeds those of wind and solar by generating an estimated 34 jobs per megawatt across various sectors. The report details how geothermal projects contribute to skill development through practical on-the-job training and formal education, thereby strengthening the local workforce and expanding employment opportunities. It also underscores the collaborative nature of geothermal development with local communities, which leads to improved infrastructure, skill-building programs, and revenue-sharing models, thereby enhancing access to reliable electricity and heat. These improvements have the potential to boost agricultural productivity and food security. The report further addresses the commitment to advancing gender equality and social inclusion by offering job opportunities, education, and training to underrepresented groups, ensuring fair access to the benefits of geothermal development. Collectively, these efforts are instrumental in driving domestic economic growth, increasing fiscal revenues, and contributing to more stable and diverse national economies, while also offering significant social benefits such as better health, education, and community cohesion.

Development
Geothermal projects have several stages of development. Each phase has associated risks. Many projects are canceled during the stages of reconnaissance and geophysical surveys, which are unsuitable for traditional lending. At later stages can often be equity-financed.

Precipitate scaling
A common issue encountered in geothermal systems arises when the system is situated in carbonate-rich formations. In such cases, the fluids extracting heat from the subsurface often dissolve fragments of the rock during their ascent towards the surface, where they subsequently cool. As the fluids cool, dissolved cations precipitate out of solution, leading to the formation of calcium scale, a phenomenon known as calcite scaling. This calcite scaling has the potential to decrease flow rates and necessitate system downtime for maintenance purposes.

Sustainability
Geothermal energy is considered to be sustainable because the heat extracted is so small compared to the Earth's heat content, which is approximately 100 billion times 2010 worldwide annual energy consumption. Earth's heat flows are not in equilibrium; the planet is cooling on geologic timescales. Anthropic heat extraction typically does not accelerate the cooling process.

Wells can further be considered renewable because they return the extracted water to the borehole for reheating and re-extraction, albeit at a lower temperature.

Replacing material use with energy has reduced the human environmental footprint in many applications. Geothermal has the potential to allow further reductions. For example, Iceland has sufficient geothermal energy to eliminate fossil fuels for electricity production and to heat Reykjavik sidewalks and eliminate the need for gritting.

However, local effects of heat extraction must be considered. Over the course of decades, individual wells draw down local temperatures and water levels. The three oldest sites, at Larderello, Wairakei, and the Geysers experienced reduced output because of local depletion. Heat and water, in uncertain proportions, were extracted faster than they were replenished. Reducing production and injecting additional water could allow these wells to recover their original capacity. Such strategies have been implemented at some sites. These sites continue to provide significant energy.

The Wairakei power station was commissioned in November 1958, and it attained its peak generation of 173 MW in 1965, but already the supply of high-pressure steam was faltering. In 1982 it was down-rated to intermediate pressure and the output to 157 MW. In 2005 two 8 MW isopentane systems were added, boosting output by about 14 MW. Detailed data were lost due to re-organisations.

Environmental effects

Fluids drawn from underground carry a mixture of gasses, notably carbon dioxide, hydrogen sulfide , methane and ammonia. These pollutants contribute to global warming, acid rain and noxious smells if released. Existing geothermal electric plants emit an average of 122 kg of per megawatt-hour (MW·h) of electricity, a small fraction of the emission intensity of fossil fuel plants. A few plants emit more pollutants than gas-fired power, at least in the first few years, such as some geothermal power in Turkey. Plants that experience high levels of acids and volatile chemicals are typically equipped with emission-control systems to reduce the exhaust. New emerging closed looped technologies developed by Eavor have the potential to reduce these emissions to zero.

Water from geothermal sources may hold in solution trace amounts of toxic elements such as mercury, arsenic, boron, and antimony. These chemicals precipitate as the water cools, and can damage surroundings if released. The modern practice of returning geothermal fluids into the Earth to stimulate production has the side benefit of reducing this environmental impact.

Construction can adversely affect land stability. Subsidence occurred in the Wairakei field. In Staufen im Breisgau, Germany, tectonic uplift occurred instead. A previously isolated anhydrite layer came in contact with water and turned it into gypsum, doubling its volume. Enhanced geothermal systems can trigger earthquakes as part of hydraulic fracturing. A project in Basel, Switzerland was suspended because more than 10,000 seismic events measuring up to 3.4 on the Richter Scale occurred over the first 6 days of water injection.

Geothermal power production has minimal land and freshwater requirements. Geothermal plants use 3.5 km2 per gigawatt of electrical production (not capacity) versus 32 km2 and 12 km2 for coal facilities and wind farms respectively. They use 20 L of freshwater per MW·h versus over 1000 L per MW·h for nuclear, coal, or oil.

Philippines
The Philippines began geothermal research in 1962 when the Philippine Institute of Volcanology and Seismology inspected the geothermal region in Tiwi, Albay. The first geothermal power plant in the Philippines was built in 1977, located in Tongonan, Leyte. The New Zealand government contracted with the Philippines to build the plant in 1972. The Tongonan Geothermal Field (TGF) added the Upper Mahiao, Matlibog, and South Sambaloran plants, which resulted in a 508 MV capacity.

The first geothermal power plant in the Tiwi region opened in 1979, while two other plants followed in 1980 and 1982. The Tiwi geothermal field is located about 450 km from Manila. The three geothermal power plants in the Tiwi region produce 330 MWe, putting the Philippines behind the United States and Mexico in geothermal growth. The Philippines has 7 geothermal fields and continues to exploit geothermal energy by creating the Philippine Energy Plan 2012–2030 that aims to produce 70% of the country's energy by 2030.

United States
According to the Geothermal Energy Association (GEA) installed geothermal capacity in the United States grew by 5%, or 147.05 MW, in 2013. This increase came from seven geothermal projects that began production in 2012. GEA revised its 2011 estimate of installed capacity upward by 128 MW, bringing installed US geothermal capacity to 3,386 MW.

Hungary
The municipal government of Szeged is trying to cut down its gas consumption by 50 percent by utilizing geothermal energy for its district heating system. The Szeged geothermal power station has 27 wells, 16 heating plants, and 250 kilometres of distribution pipes.

title: Geothermal energy