Jökull - 01.01.2016, Page 76

Þorsteinsdóttir et al.
The ruggedness values are all quite similar ex-
cept for SILK-A11, and if SILK-A11 is excluded, the
ruggedness increases slightly with time (the rugged-
ness values get lower). The ruggedness value for
SILK-A11 is 0.35, so the grains in SILK-A11 have
more uneven outlines than in the other three layers.
The difference between A12, A8 and N1 is small,
from 0.42 in the oldest layer to average of 0.40 in
the youngest layer. The values of the N1 layer do
not change very much internally or from 0.40 in the
bottom unit to 0.41 in the middle unit. No systematic
change in ruggedness with time is seen even though
SILK-LN is included and SILK-A11 omitted.
The circularity value in the oldest layer A12 is
among the lowest measured but it increases signifi-
cantly in the A11 layer as observed in both elongation
and ruggedness. Then the values slowly trend away
from circular shape. The values for SILK-A8 and
SILK-N1 (average) are 0.74 and 0.70, respectively.
Within bedded layer circularity increases with time,
the value in the bottom unit is 0.66 and 0.73 in the
middle layer. Overall there is no systematic change
in circularity with time in the tephra layers investi-
gated even though SILK-LN is included and SILK-
A11 omitted.
In summary, the SILK-A11 tephra layer is very
different from the other three SILK layers in all pa-
rameters measured. The grains are not elongated but
rather circular in shape and have more uneven sur-
face. The other grains from the SILK layers including
the SILK-LN layer have much more elongated shape,
less circularity and smoother surface. This difference
is demonstrated in Figure 6. During the period in-
vestigated (between ∼3400 and ∼8100 years ago) no
systematic changes with time were seen in the grain
parameters of the SILK layers.
1a
2a
1b
2b
Figure 6. SEM (TM3000) and shadow (Morphocop) images showing differences between the two oldest SILK
tephra layers, A11 and A12. 1a: SEM image from SILK-A11. 1b: Shadow image from SILK-A11. 2a: SEM im-
age from SILK-A12. 2b: shadow image from SILK-A12. – SEM myndir (TM3000) og skuggamyndir (Morpho-
cop) sem sýna muninn á milli tveggja elstu SILK gjóskulaganna A12 og A11. SEM mynd (1a) og skuggamynd
(1b) af SILK-A11. SEM mynd (2a) og skuggamynd (2b) af SILK-A12.
76 JÖKULL No. 66, 2016