Ármann Höskuldsson (armh@hi.is)
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We shall start on the 27 of August early morning (about 0900). We go
from Reykjavik to cross over the highland planes of "Hellisheiði",
an active rift zone. Last eruption of Hellisheiði was in the year 1000
AD. From Hellisheiði we go to the South Iceland lowlands that are remnants
of a Pleistocene oceanic bottom. We cross Ölfusá (largest river
in Iceland) and from there on we shall be driving on the Þjórsár
lava flow. Þjórsár lava flow is the longest lava flow
in Iceland in Holocene time. It was erupted on surface along a fissure
in the center of Iceland (close to the Sigalda power plant) and flowed
about 150 km way down to the South Iceland coast. We shall be driving along
this lava flow up to the center of Iceland. On our right hand we shall
be admiring Hekla volcano, the great entrance to the bad one for centuries.
At the edge of the highlands we come to the largest center of hydrological
power plants in Iceland. These power plants are build in a Pleistocene
bedrock and produce about 600 MW. Some spectacular section through pillow
basalt piles are to be observed here. From the power plants we continue
over the highlands to the center of the Icelandic hot spot. The hot spot
is manifested on surface by unusually large amount of central volcanoes
and high volcanic production in the center. If weather is right we shall
see some of the largest Glaciers in Iceland, that are as well ice capped
differentiated volcanic centers. The center is marked be the glacier Tungnafellsjökull.
From there we shall go out of the volcanic zone, and cross Pleistocene
formations that have been eroded by the gentle Pleistocene ice shield,
that explains the soft land formes. Ones the highlands have been passed
we come into the valley of "Bárðardalur", into which has flowed
a lava flow from the "Fjallsendar" crater row, located in the Askja area
some 100 km inland. We shall follow this lava flow down to the "Goðafoss"
water fall. From Goðafoss we shall cross Tertiary formations all the
way to Mývatn, our destination for the day.
The figure below shows the route (yellow line) for our first day trip. |
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STOPS
Vífilsfell Our first stop is at the foot of a 655 m high hyaloclastite mountain. Here we shall look at some pillow basalt's. Vífilsfell is erupted under subglacial conditions. The ice was at the time of the eruption about 500-600 m thick. We shall look at some spectacular pillows that show commonly observed crystal sinking among other things. Kambar Is the name of the eastern edge of Hellisheiði. Kambar is a sharp cliff edge that decent from 400 m.a.s. down to 5 m. From the edge there is a picturesque view over the South Iceland low lands. At the horizon we observe the volcanoes Vestmann islands, Eyjafjallajökul, Katla, Tindfjöll and Hekla. Selfoss Selfoss is a small community that lives mainly on service for the agricultural community in the south. The village is standing on the South Iceland seismic zone. Several fractures are observed around the village. Earthquakes as large as 7 on Richter scale occurr in this zone once every century. Last large earthquake occurred 1912. The rive Ölvusá runs through the village. The river flows along the edge of the Þjórsárlava, a porphyritic lava flow that is originated some 100 km inland, from the Veiðivötn fissure swarm. The Þjórsárlavas have a total volume of some 19 km3 and erupted around 8000 BC (14C). They are believed to be the consequence of rapid crustal rebound after the removal of the 2000 m thick Pleistocene ice. Hekla 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 |
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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 3s-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. |
The figure shows the main routes of tephra erupted in historical Hekla eruptions
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Búrfell
Hyaloclastite mountain erupted under the Pleistocene ice. Búrfell is hosting a 150 MW power plant. The Torfajökull area and the Veiðivötn fissure 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 an anomaly. 1. It is located on a junction between an active axial rift zone, with its extensional tectonics and tholeiitic volcanism, and a nonrifting volcanic zone which produces alkaline basalts and mildly alkaline rhyolites. The rift zone propagates southwards at an estimated rate of 10-20 cm/year, causing renewed volcanic activity in the old crust. 2. It has a ratio of rhyolites to basalt of about 4/1, compared with the ratio of 1/1 for nonrifting volcanic zones and 1/200 for axial rift zones. 3. It is compositionally unique, with tholeiitic basalts (from the rift zone), 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. 4. The presence of a large (12-18 km diameter) caldera-like ring structure partly encircling the complex, as well as intense geothermal activity, indicates the existence of a magma chamber of considerable size under the volcano. The rhyolites at Torfajökull have been divided into three groups: 1. Interglacial rhyolites, older than 70,000 years. 2. Early-glacial rhyolites, 40-70,000 years old. 3. Late-glacial rhyolites, younger than 40,000 years. The early history of Torfajökull Some 3 million years ago a volcanic rift started propagating southwards. About 0.5-1 million years ago the first magmas were brought to the surface in the Torfajökull area. These magmas were alkaline basalts (now buried beneath younger volcanics) originating from the mantle-crust boundary. They were deposited on top of older basalts formed in the Reykjanes-Langjökull volcanic rift zone some 7-10 million years earlier. As rift propagation progressed southwards, crustal temperatures increased and rhyolitic melts were formed higher in the crust by partial melting of a hydrated basaltic crust. The last 100 000 years of the history of Torfajökull The lowest known stratigraphic units date back to the last interglacial period. These eruptive units 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 updoming 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. This is followed by the 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. Table 2: The Veiðivötn-Torfajökull rock series. L6: Laugahraun, SH9: Stútshraun, VE 1: Veiðivötn tephra,(M.B. Mörk 1982). |
| Rock no. | L6 | SH9 | VE 1 |
| SiO2 | 71.77 | 53.35 | 49.66 |
| Al2O3 | 13.66 | 13.99 | 13.43 |
| TiO2 | 0.28 | 1.25 | 1.86 |
| FeOtot | 2.64 | 9.50 | 12.68 |
| MnO | 0.08 | 0.18 | 0.22 |
| MgO | 0.21 | 4.93 | 6.33 |
| CaO | 1.00 | 9.52 | 11.71 |
| Na2O | 4.90 | 2.81 | 2.45 |
| K2O | 4.75 | 1.04 | 0.26 |
| P2O5 | 0.05 | 0.13 | 0.18 |
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Around the middle of the last glacation period rift propagation
had reached Torfajökull and intrusion of tholeiitic basalt began.
Subsequently all rhyolitic units were physically (and perhaps also chemically)
mixed with tholeiitic basalts. All eruptions during this period were probably
initiated by tholeiitic basalt periodically intruding the magma chamber.
All rhyolites (with basaltic inclusions) are erupted through northeast-southwest
oriented fissures reflecting the rift zone control over Torfajökull.
Hrauneyjarfoss - Sigalda Two power plants that use the water from Þjórsá to produce 360 MW electricity. The powerplant are build into Pleistocene formations and here we shall look at some spectacular pillow basalts. The curiosity here is that the pillows seams to have flowed as lava flows at the interface between the crust and glacier. Þjórsárver Short coffee stop. This area is one of the largest breading place of wild Gray goose in Europe. Nýidalur (Tungnafellsjökull) Sensation of being between two erupting giants, Tungnafellsjökull and Hofsjökull. The two central volcanoes have developed caldera. Aldeyjarfoss A water fall in the lava flow of Fjallsendi. Here we shall be able to observe the sharp discordance of Pleistocene rocks and Holocene lavas. Also there are some spectacular columnar formations in the Holocene lava flow that can be compared to columnar formations of the underlying Pleistocene pillows. This is an ideal example to observe the effect of external factors on cooling of lava flows. Goðafoss The great water fall of the old gods in Iceland. The water fall was named Goðafoss (or Water fall of the gods) when Þorgeir Ljósvetningagoði, threw his statues of the old gods into the river at around AD 1000, to show the surrounding peasants that he had changed to Christianity for good. Mývatn Good nights sleep!
Bibliography: Blake, S., 1982. Physical aspects of selected volcano-magmatic processes. Ph.D. Thesis. University of Lancaster Gudmundsson, A., et al., 1992. The 1991 eruption of Hekla, Iceland. Bull. Volcanol., 54, 238-246. Gunnarsson, B., 1987. Petrology and petrogenesis of silicic and intermediate lavas on a propagating oceanic rift. The Torfajökull and Hekla central volcanoes. Ph.D. Thesis. The Johns Hopkins University, Baltimore. Grönvold, K., Larsen, G., Einarsson, P., Thorarinsson, S. and Sæmundsson, K., 1983. The Hekla eruption 1980-1981. Bull. Volcanol., 46, 349-363. Ívarsson, G., 1992. Geology and petrochemistry of the Torfajökull central volcano in central South Iceland, in association with the Icelandic hot spot and rift zones. Ph.D. Thesis. The University of Hawaii. Larsen, G., 1984. Recent volcanic history of the Veidivötn fissure swarm, southern Iceland - An approach to volcanic risk assessment. J. Vol. Geoth. Res., 22, 33-58. MacDonald, R. et al., 1990. Petrogenetic evolution of the Torfajökull Volcanic Complex, Iceland, I. Relationship between the magma types. J. Petrol., 31, 429-459. McGarvie, D.W. et al., 1990. Petrogenetic Evolution of the Torfajökull Volcanic Complex, Iceland, II. The role of magma mixing. J. Petrol., 31, 461-482. Miller, J., 1989. The 10th century eruption of Eldgjá, southern Iceland. Nord. Volc. Res. Report 8903. Mörk, M.B.E., 1984. Magma mixing in the post-glacial Veiðivötn fissure eruption, southeast Iceland: a microprobe study of mineral and glass variations. Lithos, 17, 55-75. Sigvaldason, G.E., 1974. The petrology of Hekla and origin of silicic rocks in Iceland. In: Eruption of Hekla 1947-1948 5,1. Soc. Sci. Islandica. 44 s. Thorarinsson, S., 1967. The eruption of Hekla in historical times. A tephrochronological study. In: The Eruption of Hekla 1947-48. I. 183 s. (Soc. Sci. Isl.) Thorarinsson, S. and Sigvaldason, G.E., 1972. The Hekla eruption of 1970. Bull. Volanol., 36, 1-20. |