Jökull - 01.12.1977, Page 31

Alynd 1. Þylikt og útbreiðsla gjóskulagsins H\.
of the material produced was acid and
intermediate tephra, as here basaltic ande-
site lavas make up most of the volume.
Except for Hj (1104 A.D.) all historic Hekla
eruptions belong to this stage. Small basaltic
eruptions occurred on subparallel fissures
SE, E and NE of Hekla.
This paper describes some features of the five
acid tephra layers produced during stage 2. The
youngest one is Hi, produced by Hekla’s first
eruption in Historical Times, 1104 A.D., the
oldest one is Hr>, about 6200 C14 years old, its
absolute age probably about 7000 years (cf.
Table II). One of the tephra layers, H4, is
described in some detail, since it is a good
example of a large I’linian deposit formed dur-
ing stage 2. Changes in chemical composition
tluring the eruptions that produced H4 and H3
are reported.
AREA AND VOLUME OF H4
The area ancl thickness of H4 on land has
been measured in detail (Table I and Figs. 3,
5, 6). It covers ca. 78.000 km2 on land and the
corresponding area figure for land and sea is
roughly estimated as 250.000 km2. Traces of this
layer liave heen found in Norway, Sweden and,
possibly, on the Faroe Islands (Persson 1966a,
1966b, 1967, 1968; Waagstein and Johansen
1968). Its compressed volume on land is 4.1
km3. Assuming the average vol. weight of the
compressed layer to be 0.9 ancl that of the
freshly fallen one 0.55 (cf. Thorarinsson 1958),
the calculated volunte of the freshly fallen layer
on land is 6.7 km3, and, roughly estimated, 2.4
km3 fallen 011 sea. Total volunte thus about
9 km3, corresponding to 2 km3 of dense rock
of the same chemical composition. The area 011
land covered by H4 is the second largest cover-
JÖKULL 27. ÁR 29