The field trips offered during the summer school are tied in with travel from the point of arrival in Iceland, Reykjavík, and the location of the summer school, Kirkjubæjarklaustur. The distance between these points is 230 km and the route lies across the two major volcanic zones in South-Iceland. Different routes will be taken on the way to and from Kirkjubæjarklaustur. One route follows the main highway along the coast while the second route will follow a mountain track through some of the volcanically most active areas in Central-South Iceland. Two full-day excursions will be offered. One excursion to the 20 km long crater row of the 1783 Laki eruption and the other to Öræfajökull and the Jökulsárlón.
The fieldguide provides basic information on the areas visited and introductory lectures will be offered ahead of each fieldtrip.
Reykjavík to Kirkjubæjarklaustur (Route no. 1)
The city of Reykjavík is located some 20 km to the west of the Reykjanes-Langjökull volcanic zone. It rests on lavas from a Pleistocene shield volcano, Mosfellsheiði, which has been slightly eroded by the glaciers of the last glaciation. The name of the city (lit. Bay of Smokes) bears witness of the geothermal activity within the city, which has been used since the 1920ies for space heating. Presently all houses in the Reykjavík area enjoy geothermal heating.
First stop: Þingvellir (Thingvellir)
Þingvellir (lit. The parliament plains) is a national park mainly for historical reasons but also due to its plate tectonic significance. Þingvellir is a rift segment with active central volcanoes to the North and South. Relatively little volcanic production in its immediate vicinity has left a spectacular graben structure between the Eurasian plate to the East and the American plate to the West. On either side of the 5 km wide graben are normal faults through Holocene lava flows, which cover the entire graben floor. Within the graben are open fissures demonstrating the tensional stress across the graben, while repeated levelling over the graben reveals ongoing subsidence on the order of 1 cm per year.
The Þingvellir graben was the site of the Icelandic parliament when the first republic was established in 930 AD. This location has ever since played a central role in the history of the nation.
Second stop: Þjórsá (Thjórsá)
The Þjórsá lavas are among the voluminous lava formations which where erupted in Iceland at the beginning of Holocene. With a total volume of about 19 km3. The Þjórsá lavas are of similar size as some of the major shield volcanoes in Iceland which were formed during the same time period. The production rate of basaltic magma at the beginning o Holocene appears to have exceeded the present rate by a factor of 30 to 40. This high production rate coincides with rapid crustal rebound after removal of the up to 2000 m thick ice shield of the last glaciation.
Third stop: Dyrhólaey
Dyrhólaey is a subaquatic volcano displaying some of the features of a submarine eruption as it emerges above the water surface and changes character into a regular subaerial effusion. The base of the volcano is made of hyaloclastite tuffs and breccias topped by regular subaerial lava flows. Not exposed but assumedly present at depth are pillow lava, which will be seen at other similar locations during the field trips. This sequence of pillow lavas, hyaloclatite tuffs and breccias covered by lavaflows is typical for Surtseyan eruptions, and if visibility allows Surtsey can be seen from this location as the southernmost island of the Westman island groups where two eruptions have occurred in 1963-1967 and 1973.
The Surtsey eruption started on 14 November 1963 with volcanic explosions merging from the surface of the ocean. The ocean depth at this location was 130 metres before the eruption. The eruption was explosive during the first months of activity producing fragmented hyaloclastite. In February 1964 the eruption had created a sizeable island with a crater that became sealed off from the ocean. As soon as the water no longer entered the crater the eruption changed from explosive to effusive. In the following years until 1967 the island was capped with lavas which increased its stability against the erosive forces of the ocean. Two smaller island were formed nearby during the eruption period, but both were washed away in a short time.
Other islands of the Westman island group are formed in a similar way as Surtsey except that several eruptions have contributed to the largest island, Heimaey. Before the fateful eruption of 1973 there was a prolonged period of 5000 years without any volcanic outbreak in Heimaey proper. There is however no information about possible submarine eruptions in the Westman island area during this period and it should be noted that the sea bottom is spotted with numerous hyaloclastite hills of various sizes. Volcanic activity in the area may therefore have been semi-continous during the past several thousand years. The 1973 Heimaey eruption started on 23 January it the opening of a 1600 m long fissure on the east side of the island, about 400 m east of the outskirts of the town. During the first hours lava issued from the entire fissure in 50-150 m high glowing fountains day later the eruption continued from two craters and on 6 February only one crater remained, where activity continued until 26 June, when the eruption came to an end. By that time 417 houses had been destroyed by lava and tephra and the remainder of the town was covered with millions of tons of tephra. The total volume of the eruption products was 250 million cubic meters (230 million m3 as lava and 20 million m3 as tephra). The chemical composition of the products is alkali basalt, murgearite to hawaiiite.
Fourth stop: Katla, stop at Múlakvísl
Katla is among the most frequently erupting volcanoes in Iceland, averaging about two eruptions each century. The volcanic massive is partly covered by the glacier Mýrdalsjökull which fills a caldera depression and covers the eruptive vents. In spite of the basaltic composition of the products (transalcalic FeTi basalts) the eruptions are explosive due to the subaquatic mode of extrusion. The eruptions are accompanied by enormous laharic floods which have formed a vast sandur plain which are widely distributed in Holocene while ocean sediments and ash particles in the Greenland ice core indicate strong activity over much longer time periods. A remarkable feature of this volcanism is the uniform composition of its products with time.
The last eruption in Katla occurred in 1918. The Southern coast was extended by 5 km by the laharic flood deposits. The present volcanic respose is among the longest known in historic times, but monitoring of ground deformation and seismicity does not reveal any signs of reawakening. Seismic unrest does occur from time to time and a precautionary measure the traffic across the sandur plain is then halted on both sides of the plain.
Fifth stop: Rootless craters at Skálm
Historic times in Iceland start with the settlement of the country in 874 AD. Settlement is considered to have been completed in 930 AD, marked by the establishment of the parliament at Þingvellir. Four years later, in 934 AD, a major volcanic eruption broke out in South Iceland origination in the Eldgjá fissure, which will be visited on the way back to Reykjavík. Volume estimates of the lava produced are of the same order of magnitude as the Laki lava of 1783 or about 14 cubickilometers. At this location the Eldgjá lava is mostly covered by later lahar deposits from the Katla volcano which has filled in all depressions around rootless craters in the lava, formed where the lava flowed over wet ground, possibly an outwash plain from the Mýrdalsjökull glacier. Further East the Elgfjá lava is partly exposed but partly covered by the 1783 Laki lava. The written historic record only refers to this eruption with one sentence , without providing the year of eruption. The exact date has, however emerged from tephrochronological studies and a strong acid layer in the Greenland ice core, comparable with that produced by the Laki eruption.
Sixth stop: The Laki lava
The 1873 Laki eruption will be the subject of a separate field trip during the Summer School. This stop on the main road through the middle of the Western branch of the lavafield is meant to provide a feeling for the size of the flow and to gain impression of the surface features of the lava. This is the third stop on this trip showing exceptionally large lava flows, the Þjórsá lava, the Eldgjá lava and now the Laki lava. The young age of these lavas and geological evidence for similarly large eruptive events in pre-Holocene times emphasises the high productivity of this part of the volcanic zones in Iceland and lends support to geochemical and geophysical evidence indicating that the North Atlantic mantle plume may be located in this general area.
Seventh stop: Hyaloclastite flows at Síða
Plio-Pleistocene strata of the Síða and Fljótshverfi districts of a 600 m thick, subglacial/subaqueous volcanic complex, dominated by voluminous (up to 30 km3) and extensive hyaloclastite sheets and interbedded sedimentary diamictite units, that overlies unconformably a succession of flatlying terrigenous lavas and sedimentary rocks.
Each hyaloclastite unit shows a regular structural sequence with basalt and/or pillow lava dominated facies at the base, and various hyaloclastite breccias and layered (sometimes large scale crossbedded) hyaloclastite tuff at the top. The lithological change appears in proximal-distal relationships. The breccia contains many large aligned and folded basalt bodies and lobes, attesting to emplacement by considerable flowage.
The hyaloclastite flow units were produced by repeated voluminous extrusions of basaltic lava from subglacial (subaqueous) fissures, located near the present Lakagígar volcanic fissure. Fragmentation of the extruded lava by quenching, and avalanche of the hyaloclastite and associated basalts due to gravity collapse, followed the initial eruption stage. The composite mass moved like a high-concentration pyroclastic flow that filled topographic depressions and was gradually fragmented during the flowage.
The final emplacement of the hyaloclastite flow occurred in a fully subaqueous environment by the transportation into submarine valleys along the coast or alternatively in large dammed lakes. This model is supported by the sediment petrography of the interbedded diamictites which are layers of poorly sorted mud-pebble-clast rock with well-developed stratification, foreset cross beds and paleotopographically controlled thicknesses. The sedimentary material was probably supplied from inland glacier rivers and resedimented in valley and coastal shelf deltas, locally in the form of subaqueous sedimentary gravity flow deposits. Only minor parts were deposited directly from grounded or terminal ice.
Kirkjubæjarklaustur - Laki - Kirkjubæjarklaustur
- The 1783 Laki eruption
Stops will be made at strategic locations for an overview of the crater row and inspection of individual craters and flow channels. The selection of locations will largely depend on weather and visibility.
The course of the eruption
Strong earthquakes preceding the eruption began in late May and the eruption began in the morning of 8 June 1783. Four days later, on 12 June the lava reached inhabited areas when it came down the canyon of the river Skaftá and spread over vast areas of the lowland.
During the first 50 days of the eruption the average discharge calculated as solid lava may have been around 2000 m3/sek. The branch of the lava following the bed of north-eastern part of the Laki fissure flowed down the bed of the river Hverfisfljót and reached the lowland plain on 7 August. The eruption ended in early February 1784 after 8 months of activity.
Great damage was caused by the lava flow, which covers 565 km2, with a volume of approximately 12-13 km3. The damage caused by atmospheric pollution due to the volcanic fumes was even greater. The fumes lay over most of the country as a bluish haze during the summer of 1783. It studded and poisoned the grass crop and resulted in a disastrous famine still referred to as Móðuharðindin (haze famine). The effects on life in Iceland were catastrophic. Fifty percent of the cattle, 76 percent of the horses and 77 percent of the sheep perished and so did one-fifth of the human population. The Lakagígar lava is near aphyric, strongly evolved tholeiite. It is remarkable for its homogenous composition in particular for a low oxygen isotope ration of about 3.5d18O.
Kirkjubæjarklaustur - Jökulsárlón - Kirkjubæjarklaustur
This excursion will take you through areas displaying a variety of features of interest to many branches of the earth sciences. The route passes the southern edge of the Vatnajökull glacier which conceals beneath its ice shield a number of very active volcanoes. Some of the most impressive features on the way result from recent volcanic eruptions beneath the ice causing enormous flooding and sandur deposits (lahar) towards the coast. The route passes Öræfajökull volcano in the southern edge of Vatnajökull and several outlet glaciers from its slopes occasionally calving into marginal lakes.
During Pleistocene glaciations ice accumulation reached a maximum in this part of the country causing intensive erosion.
First stop: Skeiðarársandur
The building of a bridge across Skeiðará, the prototype of Iceland’s glacier rivers, in 1974 closed the autoroads around Iceland. The reason why an autoroad across Skeiðarársandur was not built earlier is the jökulhlaups which now and then flood the sandur. They are of two types:
The first type includes those caused by draining of lake Grænalón which is dammed by the west margin of Skeiðarárjökull. The second type is caused by draining of a caldera lake, Grímsvötn, in central Vatnajökull. This caldera has an area of ca 35 km2 and is covered by ice, at least 200 m thick. Between the jökulhlaups the water volume in the caldera increases by 0.66 to 0.7 km3/year. This volume increase is related to:
- Glaci-gen, caused by the sudden draining of an ice-dammed lake and
- Volcano-limnogen, caused by the draining of a subglacially dammed caldera lake.
Until 1935 Grímsvötn was usually drained at 10-year intervals and the level of the caldera lake rose about 150 m between jökulhlaups. The total volume of the water drained out in these jökulhlaups was probably 6-7 km3 and their maximum discharge about 40.000 m3/sec. These jökulhlaups flooded the main part of Skeiðarársandur. They broke up the glacier front around the outlet and carried icebergs sometimes of enormous sizes, out over the sandur plain which as a result is in places dotted with kettleholes created by melting icebergs.
- Subglacial melting from solfatara areas within the north of the caldera,
- Ablation water from those 280 km2 of Vatnajökull that are drained to Grímsvötn and
- Volcanic eruptions within the caldera.
Second stop: Skaftafell
Skaftafell is a national park. If weather permits we will take a walk to the outlet glacier Skaftafellsjökull (about 1 hour).
Third stop: Öræfajökull
Öræfajökull is a huge cone volcano, 2119 m height, second in volume only to Etna among volcanoes in Europe. From the sandur plains near sea level in the south and west it rises, with an average angle of slopes of about 15 degrees to a crater rim with an average altitude of 1850 m. This crater rim surrounds an elliptical summit caldera with an area of 14 km2. The depth of the caldera is not known as it is, like the upper part of the cone, covered with an ice cap from which valley glaciers stretch down to the sandur plains. Rocks with reverse magnetic direction are found in the basement of the Öræfajökull massif, but the bulk is formed during the present magnetic epoch (Bruhnes) and thus less than 0.7 million years old. It is build up partly subaerially by basalt and andesite lavas and hyaloclastites. Rhyolite is also abundant. Hvannadalshnúkur, Iceland´s highest point , is a rhyolitic peak rising above the north-west rim of the caldera. The lower part of the massif is deeply dissected by glacial erosion. The upper, ice-covered part of the massif that is the present volcanic cone proper is younger than the second last glaciation. The postglacial activity is the summit area seems to have been almost exclusively explosive and tephrochronological studies in the neighbourhood of the volcano show that this activity was rather limited and did not add much to the height and volume of the volcano. Several postglacial radial fissures reaching below the present ice cover have also been active building up cinder cones and producing at least one lava flow reaching the lowland plain at Kvíárjökull on the south-east side of the volcanic massif.
The first historic eruption in Öræfajökull occurred in early June 1362. The most detailed, almost contemporary, description of this eruption is found in an annual fragment from the See of Skalholt believed to have been written in the monastery at Möðruvellir in North Iceland. It runs as follows:
Volcanic eruptions in three places in the south and kept burning from Flitting Days (=in early June) until the autumn with such monstrous fury as to lay waste the whole Litlahérað as well as a great deal of Hornafjörður and Lónshverfi districts, causing desolation for a distance of some 100 miles. At the same time there was a glacier burst from Knallafellsjökull into the sea carrying such quantities of rocks, gravel and sand as to form a sandur plain where there had previously been thirty fathoms of water. Two parishes, those of Hof and Rauðilækur, were entirely wiped out. On even ground one sank in the sand up to the middle of the legs and winds swept it into such drifts that buildings were almost obliterated. Ash was carried over the northern to this, pumice was floating off the north-west Iceland (Vestfirðir) in such masses that ships could hardly make their way through it.
Other more or less contemporary annals confirm this description. One of these annals states that the flood swept away all buildings of the Rauðilækur rectory except the church. A 16 century annual based on lost, much older annals, says that "no living creature survived except one old woman and a mare".
Tephrochronological studies have supplemented the scant information supplied by the old annals. The 1362 eruption preceded by a respose lasting at least 500 years. Presumably the eruption took place mainly, or wholly, within the caldera and was almost certainly purely explosive. The regular thickness distribution of the tephra on land indicates that the main tephra fall was of short duration. It lasted only one or two days. After that little or no tephra fell outside the immediate vicinity of the volcano. Probably the eruption as a whole did not last long. The passus: "kept burning until autumn" in the above-quoted annal fragment need not refer to this eruption. The tephra was carried mainly towards ESE and consequently the main past of it fell on sea. The tephra has been traced in peat bogs in Scandinavia. The volume on land and sea was probably at least 10 km3, corresponding to 2 km3 of solid rhyolitic rock.
The eruption was accompanied b catastrophic jökulhlaups (glacier bursts) which emerged from underneath the outlet glaciers. Falljökull and Rótarfjallsjökull on the west side of the massif. Some farms were destroyed by these floods, but the tephra fall had the main share in the devastation caused by this eruption. The prosperous rural settlement along the foot of the volcanic massif, inclusive of at least 30 farms, were laid waste so thoroughly that they remained abandoned for decades. When a revival at last came, this district, which before the eruption was called Hérað, a name given on to extensive and important rural settlements, had got a new name, Öræfi, which means wasteland.
The tephra fall damaged rural settlements up to a distance of 70 km east of the volcano so that they were abandoned for several years.
The second and most recent historic eruption of Öræfajökull occurred on 3 August 1727 and lasted until April or May 1728.
On Sunday 3 August while people were attending divine service in the church of Sandfell in the Öræfi district, several earthquakes were felt. The seismic activity gradually increased. Early the following morning 4 August the earthquakes were so strong that everything standing uprights in the houses was thrown down. At 9 o’clock three particularly loud reports were heard and they were almost instantly followed by tremendous jökulhlaups from the glaciers Falljökull and Rótarfjallsjökull. The jökulhlaups destroyed two chalets and drowned three people. Later that day the farm on the west side of Öræfajökull were in complete darkness because of tephra fall which lasted for three days, but on the fourth day is cleared up and it seems that only a small amount of tephra fell after that. The farms of Svínafell and Skaptafell became inhabitable for a while. However the tephra production was small compared with that of the 1362 eruption and probably did not exceed 0.2 km3. The floods and the tephra fall killed about 600 sheep and 150 horses some of which were found completely mangled by the bomb fall. The great amount of water discharged by the jökulhlaups point to the eruption having started within the caldera, but already on 4 August a fissure with six or seven separate fires opened up on the outer west flank of the caldera, reaching down to about 1100 m a.s.l. and in this fissure fire and smoke was seen until late May fifth following year, but the production of lava was insignificant.
Fourth stop: Jökulsárlón -
Breiðamerkursandur is a collective name of the postglacial area of Hrútárjökull and Fjallsárjökull, both outlets from Öræfajökull and Breiðamerkurjökull, the broadest, south-eastern outlet of Vatnajökull proper. These glaciers reached their maximum postglacial extent during the 19 century and have on the whole been retreating since the middle 1890s. In 1894 the shortest distance between Breiðamerkurjökull´s front and the beach, 5 km east of Jökulsá river, was 256 m. At Jökulsá river it was about 1 km. In the period 1894-1968 the glacier front retreated up to 2.3 km, exposing 53 km2 of deglaciated area. During the same period Breiðamerkurjökull lost about 50 km3 in volume.
During the fist centuries after the settlement (A.D. 874) the front of Breiðamerkurjökull lay at least 10 km behind the 1894 moraine. Two farms were established on the western part of Breiðamerkursandur. One of them, named Fjall, at the south-east foot of Breiðamerkurfjall and the other, named Breiðá (with a church), somewhat farther east. Both were overrun by the advancing glacier between 1695 and 1720. The retreat of Breiðamerkurjökull since about 1930 has resulted in the formation off frontal lakes.
The biggest one, Jökulsárlón, reaches about 110 m below sea level.
Back to Kirkjubæjarklaustur
Kirkjubæjarklaustur - Reykjavík (Route 2)
Fist stop: Eldgjá
The 934 AD eruption of Eldgjá occurred on several fissure segments totalling 57 km in length. The most accessible part, Eldgjá proper is 8.2 km long. It has a width of about 600 m and a maximum dept of 140 m. The inner slopes of Eldgjá consist mainly of hyaloclastites and tillite-like breccias. The topmost past forms vertical walls, 10-15 m high, consisting mainly of scoria and lava lumps, partly fused together
into an agglutinate and geomorphic lavas. A thick layer of tephra extends towards SE. The shape of Eldgjá seems to be partly the result of explosive activity and partly dictated by the pre-existing topography. A very low amount of lithic fragments (<10%) in the initial tephra producing phase suggests that the main features of the present Eldgjá topography already exited before the 934 AD eruption.
The 934 AD lava flows from Eldgjá cover an area of about 780 km2, with an estimated volume of 14 km3, similar to the 1783 AD Laki lavas. The age of this event is bracketed between a tephra layer which fell in the year 871 AD and a layer from Katla formed in the year 1000 AD. A strong acid peak in the Greenland ice core, identical with the acid peak produced in the ice by Laki eruption, occurs in the year 934 AD. This date is in good agreement with the historic record although it does not mention the exact year of eruption.
Second stop: Torfajökull at Landmannalaugar
The Torfajökull central volcano in South Iceland is by far the largest rhyolitic region in Iceland, with rhyolites covering 350 km2. When compared with the predominantly basaltic composition of volcanic rocks in Iceland Torfajökull is anomaly:
The lowest known stratigraphic units within the complex date back to the last interglacial period. These are thick metaluminous to slightly peralkaline rhyolitic lavas and welded air fall tuffs. Following their extrusion during the early part of the last glacial period regional upcoming took place and the magma chamber developed a substantial peralkaline top. This is reflected in the peralkaline nature of the feldspars (high FeO K2O and low Al2O3) and their restricted compositional range in the volcanic products of this period.
- It is located on a junction between an active axial rift zone, with its extensional tectonics and tholeiitic volcanism and non-rifting volcanic zone which produces alkaline basalts and mildly alkaline rhylolites.
- It has a ratio of rhyolites to basalt of about 4/1 compared with the ratio of 1/1 for non-rifting volcanic zones and 1/200 for axial rift zones.
- It is compositionally diverse with tholeiitic basalts, transitional alkali basalts (hy-normative) and alkali basalts (ne-normative) all well represented. Also present is a variety of rhyolites ranging from metaluminous ones, through mildly peralkaline comendites to pantellerites. A compositional gap (Daly gap) is present in the extrusives between the values of 59%-66% silica.
This was followed by a large ring-fracture eruption forming the prominent pantelleritic ring structure partly encircling the volcano. The ring structure was later filled with decreasingly peralkaline magmas (both comendites and pantellerites). During this period many alkali basaltic and transitional alkali basaltic dykes, pillows and hyaloclastites were erupted.
Around the middle of the last glaciation period rift propagation had reached Torfajökull and intrusion of tholeiitic basalt began. Subsequently all rhyolitic units were mixed with tholeiitic basalt. All eruptions during this period were probably initiated by tholeiitic basalts periodically intruding the magma chamber. All rhyolites (with basaltic inclusions) are erupted through northeast-southwest oriented fissures reflecting the rift zone control over Torfajökull.
Third stop: Hekla, lava of 1970
Iceland's most famous volcano is built up on a WSW-ENE trending fissure by repeated fissure eruptions, forming a vaulted ridge about 5 km long and split lengthwise in major eruptions. The present height of the volcano is 1491 m (1447 before the 1947 eruption). Morphologically Hekla represents an intermediary stage between a crater row and a stratovolcano. Seen in the direction of the fissure it has the concave outline typical of a stratovolcano. Hekla erupts a magma type which is unique for Iceland. It resembles the calcalkaline products of subduction volcanism. The postglacial products of Hekla can be described as two end members of a series, one highly silicic, the other andesitic (icelandite). Intermediate magmas between these end members may result from magma mixing. After the 1980 eruption it was possible, by measurements of surface deformation, to determine the depth to the magma reservoir which is at about 8 km. Hekla has had a number of large postglacial eruptions, producing vast amounts of tephra which repeatedly covered up to two thirds of the country with light-coloured tephra (i.e. 7000 B.P., 4500 B.P., 2900 B.P., A.D. 1104 and A.D. 1158). During historical time the first eruption (A.D. 1104) was a tremendous explosive eruption which destroyed the Þjórsárdalur valley. This eruption produced about 2.5 km3 of rhyodacitic tephra, which was carried towards NNW. The following eruptions in Hekla, producing both lava and tephra, occurred in 1158, 1206, 1222, 1300, 1341, 1389, 1510, 1597, 1636, 1693, 1766, 1845, 1947, 1970, 1980 and 1991. Some of these eruptions caused great damage, especially the eruptions in 1510, 1693 and 1766. The total volume of lava produced by Hekla in historical times is about 8 km3, and the total volume of tephra about 7 km3.
The compositional evolution of the Hekla magma system is roughly a linear function of the length of the repose periods between eruptions. Thus the silica and alkali content of the initial product of each eruption increases with the length of the preceding repose. Also, the longer the repose the greater the force of the initial outbreak and the volume of the products. After the initial explosive outbreak there follows a less violent eruption of lavas which can last for many months. The composition of the products changes from the initial silicic towards an intermediate icelandite (54-55% SiO2) at the end of the eruption.
Table 1: 012: Lambafit lava, 1913, 240: Öxi lava, 4: Lava erupted at the end of the 1947 Hekla eruption, 5: Lava erupted at the end of the 1970 Hekla eruption, 1: Tephra from the Hekla eruption A.D. 1104, 2: Tephra from the first phase of the 1947 Hekla eruption.
Rock No. 012 240 4 5 1 2 SiO2 46.50 46.28 54.25 54.5 66.84 61.88 Al2O3 13.80 13.88 16.34 15.8 14.75 16.11 TiO2 3.96 3.97 1.54 1.9 0.30 1.03 Fe2O3 2.74 2.77 2.24 2.8 1.75 2.11 FeO 12.04 13.39 10.25 8.5 3.88 6.47 MnO 0.23 0.24 0.26 0.3 0.20 0.26 MgO 6.38 5.64 3.39 2.8 0.96 1.76 CaO 10.41 9.32 7.09 6.4 3.24 4.93 Na2O 2.84 2.90 3.41 3.9 2.84 4.21 K2O 0.54 0.59 0.95 1.4 3.13 1.16 P2O5 0.48 0.49 0.35 1.0 0.38 0.44 H2O 0.24 0.55 0.42 0.3 1.48 0.34
A pattern of compositional change is found in products from all known postglacial eruptions of Hekla. In the large silicic eruptions, however, the known endproducts are not as basic as the lavas produced in historical time. Detailed studies of this compositional pattern indicate a compositional zoning in the Hekla magma system, which cannot be explained by any single evolutionary process such as fractional crystallization. A complex pattern of processes including both fractional crystallization, partial melting and various diffusion phenomena are implied by the available data. In addition to 17 summit eruptions in Hekla itself, 5 eruptions are known to have occurred in the its immediate vicinity in historical time. Some of these eruptions, such as the Rauðubjallar eruption in 1554 and the eruption in Lambafit in 1913, produced lavas of alkali olivine basalt, distinctly different from the material produced by Hekla proper. These basaltic lavas are probably derived from the margine of the compositionally stratified Hekla reservoir.
The three last eruptions of Hekla occurred in 1970, 1980 and 1991. The 1970 eruption started on 5 May. Fissures opened nearly simultaneously NE, S and SW of the Hekla ridge. During the tephra-producing phase, which lasted about 2 hours, about 30 million m3 of tephra were produced and carried towards NNW. The maximum thickness of the tephra layer in the vicinity of Hekla is 18 cm. About 170 km from Hekla it is 4 mm. The tephra sector within the 0.1 mm isopachyte covers nearly one-tenth of the country. The tephra was high in fluorine (exceeding 2000 ppm F in some places), which poisoned and killed grazing animals, especially in North Iceland. On 20 May a new fissure, 1 km in length, opened up about 1 km north of the northernmost fissure from 5 May. That fissure in turn became inactive just before the new fissure opened. During the first days 8 to 10 craters were active on the new fissure, but gradually the activity became localised to one crater. That crater had built a 100 meter high cone of scoria by the end of June. The second last Hekla eruption occurred in 1980-1981. The first stage of the eruption began in August 1980 with emission of tephra and lava from a 7 km long fissure along Hekla's main ridge. This stage lasted for three days and produced circa 60 million m3 of tephra and a lava flow about 22.5 km2 in size (average thickness 5m). After a repose which lasted for several months the second stage started in April 1981 and lasted for a week. Only the top crater was active, producing lava which covered 6 km2. The last Hekla eruption started on 17 January 1991, came to an end on 11 March, and produced mainly andesitic lava. This lava covers 23 km2 and has an estimated volume of 0.15 km3. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02 km3 of tephra. The eruption cloud attained 11.5 km in height in only 10 min, but it became detached from the volcano a few hours later. By the second day, however, the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m3sec-1. After this peak, the effusion rate declined rapidly to 10-20 m3s-1, then more slowly to 1 m3sec-1, and remained at 1-12 m3sec-1 until the end of the eruption. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vicinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. The shape of this reservoir, its location at the west margin of a propagating rift, and its association with a crustal weakness, all contribute to the high eruption frequency of Hekla.
Fourth stop: The Great Geysir
Geothermal activity as expressed in hot aqueous solutions is abundant and conspicuous in Iceland. It is practical to divide the surface expressions of the geothermal areas into low temperature and high temperature areas. The distinction between the two is based on the maximum temperature attained during the circulation of water within the system. If the temperature is below 150°C the is a low temperature area while high temperature areas have a base temperature above this value. Obviously there is a continuous gradation in temperature so there may be unclear boardercases. However the distribution of the geothermal activity in relation to the plate boundary and rift zone volcanism gives a logical background to this empirical grouping. All high temperature areas occur in geologically older formations that have drifted out of the rift zone. The low temperature areas are associated with active volcanic centres in the rift zone. The low temperature areas occur in geologically older formation that have drifted out of the rift zone. In addition one finds cold or lukewarm carbonated springs, especially in the Snæfellsnes flank zone, share the volcanism is not intense and probably of deep origin. In the field the distinction between high and low temperature areas is mostly clear. The low temperature areas are manifested in water springs with a surface temperature in the range from above ambient to boiling. The water is low in dissolved solids and precipitation of encrustation’s at the surface are inconspicuous. In fact the amount of dissolved solids and especially the amount of dissolved silica is a reliable geothermometer to assess the base temperature. The surface manifestation of the high temperature areas is more varied depending on the position of the ground-water level relative to the surface. A geothermal system is a converting body of water with a draw-down of heavy cold meteoric water which is heated and then rises as light hot solution which frequently boils on the way to the surface. If the ground-water level is at some depth below the surface the steam produced b the subsurface boiling appears in fumaroles at the surface. If the ground-water level coincides with the surface as it does in an artesian spring the water emerges as a boiling solution at the temperature of boiling for that particular location. Since the base temperature may have been in excess of 300°C the water may boil explosively before reaching the surface. Such explosive boiling is the basic reason for geyser activity.
The heat source of a high temperature area is generally a shallow magma chamber or a crustal section with many recent dykes. The low temperature areas derive their heat from contact with the cooling crustal plate as it drifts away from the rift zone. The high temperature areas take their energy more or less directly from the magmatic sources, either from dense dyke swarms or shallow magma chambers. The dissolved chemicals in the low temperature waters are derived from reaction with the wall rock and volatile element of the chemical load are mainly nitrogen and argon of atmospheric origin (the oxygen is consumed in reactions with the wall rock). The dissolved chemicals of the high temperature waters are also derived from the wall rock, but the volatile content comes partly from the degassing magmatic body. The volatile composition is therefore dominated by CO2 with varying amount of H2S and H2. As a result of subsurface boiling the H2S is released and oxidised upon contact with air. The resulting condensed solution is therefore dilute sulphuric acid which attacks the wall rock and causes extensive alteration above the ground-water level where the boiling occurs.
Geothermal energy plays an important economic role in Iceland. It is mainly used for domestic heating and 80% of the population enjoys this facility. District schools have been preferentially located near geothermal areas to utilise this source of energy for heating and for swimming pools.