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.
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 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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