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is the high ratio between the acid material and the basic.
Even though chemical analyses of rather few lava flows are
available, this classification system gives us a rough idea
about the chemical composition of the bulk of the lavas and
the relative abundance of the fourth class to the other three.
This shows that the fourth class is much less abundant than
the other three so the greatest part of the Hekla volcanic
material is as acid or more acid than andesite (Si02-content
more than 54%). This great amount of acid material is diffi-
cult to explain by differentiation but assimilation would ex-
plain it.
I have suggested the following mechanism to explain the
chemical composition of the Hekla volcanic material: Firstly
a basic magma (Si02 45%) was erupted from great depth
(40—60 km) but at depths of several km the magma came
in contact with more silica rich material and created a new
magma by assimilation having a silica content of 54—55%.
In this assimilated magma the silica-content increased with
time through magmatic differentiation (gravitational diffe-
rentiation). (Thorarinsson 1954 and 1967).
My conclusion is that two magma chambers exist under
Hekla, one at a depth of 40—60 km with basic magma, but
another at depths of several km with more acid magma, the
typical Hekla magma. At times the deeper seated magma
may become mixed with the shallower magma. Therefore in
some instances the chemical evolution of Hekla’s magma is
more complex than can be accounted for by simple diffe-
rentiation and assimilation.
REFERENCES
Einarsson, Trausti, 1950: Chemical analyses and differentiation of Hekla’s
magma. The Eruption of Hekla 1947—1948 IV. 4, 30 pp.
Kjartansson, G., 1945: Hekla. Árbók Ferdafélags Islands, 167 pp. (In Ice-
landic).
Thorarinsson, S., 1954: Tephra fall from Hekla on March 29th 1947. The
Eruption of Hekla 1947-1948 II, 3: 1-68.