Jökull - 01.12.1989, Síða 65

Table II. Mean values and variance of shape parameters, and Si02 content of tephra layers. Elong., elonga-
tion; Spher., sphericity; OP-ind., OP - index; El. var., elongation variance; Sph. var., sphericity variance; OP.
var., OP - index variance; Range, see explanation in text.
Taflall. Meðaltalsgildi og fervik formþátta ásamt kísilsýruinnihaldi sýnanna. Elong., ílengd; Spher., kýlni;
OP - ind., OP - þáttur; El. var.,fervik ílengdar; Sph. var.,fervik kýlni; OP - var.,fervik OP - þáttar; Range,
sjá skýringar í texta.
Sample Elong. Spher. OP-ind. El. var. Sph. var. OP. var. SÍO^
769 0.755 0.725 0.592 0.011 0.006 15.390 50.29
940 0.796 0.709 -1.189 0.015 0.011 22.705 46.00
941 0.803 0.731 -0.931 0.015 0.010 18.550 55.23
942 0.628 0.641 3.875 0.035 0.016 61.748 71.46
943 0.787 0.714 -0.798 0.011 0.011 17.239 46.50
985 0.674 0.668 2.988 0.029 0.017 42.602 73.76
986 0.769 0.706 -0.259 0.017 0.010 22.896 60.00
987 0.748 0.725 1.065 0.022 0.013 31.047 49.06
988 0.779 0.708 -0.699 0.013 0.012 24.473 57.82
989 0.726 0.706 1.321 0.019 0.014 24.315 62.87
990 0.701 0.723 2.716 0.021 0.011 23.175 65.25
991 0.745 0.714 0.657 0.015 0.010 23.581 71.64
998 0.645 0.642 3.325 0.033 0.022 79.888 70.00
999 0.438 0.575 13.424 0.038 0.013 166.926 67.00
1000 0.774 0.740 0.306 0.017 0.014 22.062 63.13
1001 0.661 0.655 2.603 0.019 0.016 34.024 66.84
1002 0.686 0.660 1.667 0.020 0.009 28.516 66.84
1003 0.744 0.712 0.696 0.019 0.012 28.154 66.84
1004 0.661 0.684 3.328 0.027 0.010 35.438 73.72
5933 0.740 0.738 1.452 0.013 0.010 16.777 66.22
5838/1 0.691 0.681 1.969 0.020 0.012 29.246 57.45
5838/2 0.712 0.665 0.722 0.016 0.013 24.681 57.45
Range 0.081 0.063 3.412
The second volcano, Snæfellsjökull, shares
several physiographical features with Öræfajökull.
It lies outside the spreading zones of Iceland (Fig. 3)
and the volcanic massif, which is built up of lava
flows and tuff layers as well as rhyolitic intrusives, is
covered by an ice cap. Samples from two light
coloured ash layers that have been related to Snæ-
fellsjökull, Sn-1 (sample 5838) and Sn-2 (sample
5933), were included in the present study. Stein-
thórsson (1967) has published l4C dates for these
layers, and he kindly supplied the unpublished
chemical data in Table II. Jóhannesson and others
(1981) studied these two ash layers and came to the
conclusion that they may have been redeposited
because some rounding of ash particles was
observed and because their thickness varies consid-
erably.
The volcano Hekla lies at the westem margin of
the volcanic zone of South Iceland. Several of the
Hekla eruptions have been of rhyolitic composition,
but the mountain itself is mainly built up of basalts
and andesites (Thorarinsson, 1967). Using tephro-
JÖKULL, No. 39, 1989 63