Jökull


Jökull - 01.12.1989, Side 66

Jökull - 01.12.1989, Side 66
chronological methods, Thorarinsson demonstrated that eruptions in Hekla begin with an explosive phase producing the acid tephra. The tephra produc- tion phase is typically short, however, and studies of postglacial Hekla eruptions have shown that the tephra distribution is highly directional, being related to the wind direction at the outset of eruption (Larsen and Thorarinsson, 1978). Ten tephra sam- ples from Hekla were analyzed in the present study. Two samples are from the H5 layer (Sauðafell), a light coloured fraction (985) and a dark coloured one (986). The SiOz content in Table II of the light coloured part is based on Larsen and Thorarinsson (1978), the value for the dark coloured grains was estimated from refractive index under the micro- scope. Four samples from the H4 layer were meas- ured (988-991, Sigalda), each from a different phase of the eruption. These samples were analyzed chem- ically by Gudrún Sverrisdóttir who kindly supplied the unpublished Si02 data in Table II. The H5 and H4 layers were described by Larsen and Thorarins- son (Larsen and Thorarinsson, 1978). Three samples from different localities were analyzed from the 1104 AD eruption (H,, 1001 from Þóristungur, 1002 from Ófærugil, 1003 from Biskupstungnaafréttur). The Si02 value is from Thorarinsson (1967). A sin- gle sample from the 1693 eruption of Hekla was measured (941, the Si02 value is from Sigvaldason, 1974). Katla is a subglacial volcano under the Mýrdals- jökull glacier, South Iceland. The volcano may be related to the Eldgjá fissure system to the northeast of Mýrdalsjökull (see discussion in Einarsson and others, 1980). Tephra from Katla and Eldgjá is rich in Ti02 and total iron. The chemical composition of the erupted material from Katla has remained rela- tively constant from one eruption to another, which is in marked contrast to the variations observed in successive Hekla eruptions. Einarsson and others (1980) used grain size parameters (sorting vs. mean diameter) of the Katla 1357 tephra to classify it as a surtseyan type, and also published chemical data. One sample from the Katla 1625 eruption (940) was measured in the present study, and assuming that the eruption took place under the Mýrdalsjökull glacier, the eruption has probably been a phreatomagmatic one. An eruption began on the island of Heimaey on the 23rd of January 1973. The beginning of the eruption and its early development and chemistry was described by Thorarinsson and others (1973). The Heimaey eruption was of the strombolian- /hawaiian type (Self and others, 1974; Egilson, 1974). One sample from the Heimaey eruption was analyzed in the present study (769). The eruption in Surtsey which began in November 1963 ofif the south coast of Iceland, was the first submarine eruption monitored by scientists in Iceland. It was described by Einarsson (Einarsson, 1965) and Thorarinsson (1966, 1968). Phreato- magmatic explosions characterized the early stages of the eruption due to the contact with sea water, and tephra products resulting from such eruptive processes were given the term surtseyan by Walker and Croasdale (1972). The tephra sample analyzed in the present study (943) was taken from the sea bottom near Surtsey in February-March 1964, and the chemical data is from Steinthórsson 1966. The crater remnant Karl lies just off the tip of Reykjanes peninsula. Jónsson (1978) has associated Karl with the Vatnsfell tephra cone, which he con- siders to have been built up in a submarine eruption. The crater is built up of layered sideromelane glass with no signs of alteration. Our sample was col- lected from the crater remnant on land. Walker and Croasdale (1972) analyzed tephra from the Karl eruption and found that it was of the surtseyan type. No chemical analyses from tHe Karl tephra have been published, but a tephra layer in a core from Lake Thingvellir which probably correlates with Karl has the Si02 content given in Table II for the Karl tephra (Gudrún Larsen, pers. comm.). The volcano Askja in North Iceland is situated in the Dyngjufjöll massif which is largely built up of subglacial tuffs, pillow lavas and subaerial lava flows of upper Quaternary age (Sigvaldason, 1979; Sæmundsson, 1982). The most conspicuous part of the Dyngjufjöll central volcano is the Askja caldera and in 1875 a great plinian eruption took place within this caldera, leading to the formation of a 64 JÖKULL, No. 39, 1989
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