Náttúrufræðingurinn - 1975, Blaðsíða 46

170
NÁTTÚRUFRÆÐINGURTNN
Walker, G. P. L., 1966: Acid volcanic rocks in Iceland. Bull. Volc., 29, p. 375—
406.
Yoder, H. S. Jr., 1973: Contemporaneous basaltic and rliyolitic magmas. Ameri-
can Mineral. 58, p. 153—171.
ABSTRACT
The present article is an extract in Icelandic of a paper on the petrology of
Hekla and origin of silicic rocks in Iceland, which appeared in English in 1974
(See Ref.).
The volcanic history of Hekla is reviewed. It is thought that the first known
eruption in Hekla (6500 C14 years B. P.) marks the beginning of silic volcanic
activity in the mountain.
Hekla has produced two types of magma. Dacite was produced mainly in the
period 6600 to 2000 years B. P., while basaltic andesites have been produced in
bulk during the past 9 centuries. Although the production maxima of the two
liquids are separated in time, considerable overlap exists. At present botli
liquids coexist and mix at depth producing intermediate members which are
discharged at the beginning of each eruption of basaltic andesite. The extent
and amount of mixing is proportional to the repose between eruptions.
The derivation of the two liquids through a process of fractional crystal-
lizations is rejected on the basis of negative o inconclusive evidence. The
dacites and basaltis andesites are believed to result from successive partial
melting of subsided basalts, which were originally produced on the western
rift zone and llien carried by drift over tlie eastern volcanic zone.