Jökull - 01.01.2016, Side 75

Grain characteristics of silicic Katla tephra layers
-1Φ whereas the largest grains from the finer grained
section all except SILK-A7 belong to grain size cate-
gory 0 Φ. The „change“ in grain size occurs between
5800 and 6000 years ago. The mean grain size, how-
ever, is more variable. The majority of tephra layers
younger than 5000 years have mean grain size of 2.2
to 1.1 Φ (0.2 to 0.45 mm) whereas the majority of
SILK layers older than 5000 years have mean grain
size of ∼3 to ∼4 Φ, or smaller than 0.125 mm. Exam-
ples are shown in Figure 5.
Table 3. Grain size4 and grain shape* analyzes of 12
SILK tephra layers. Age and associated references
are in Table 1. – Listi yfir 12 SILK gjóskulög sem
voru kornastærðar4- og kornalögunar*greind. Upp-
lýsingar um aldur gjóskulaganna og heimildir eru í
töflu 1.
Tephra layer Rounded age Largest Φ Mean Φ
SILK-UN4 ∼2800 -3 1.62
SILK-LN4* ∼3400 -3 2.24
SILK-N44 ∼3900 -1 3.96
SILK-N34 ∼4100 -3 1.12
SILK-N24 ∼5000 -1 2.01
SILK-N14* ∼5800 -1 3.03
SILK-A14 ∼6000 0 3.52
SILK-A54 ∼7100 0 3.47
SILK-A74 ∼7200 -2 1.61
SILK-A84* ∼7400 0 3.88
SILK-A11* ∼8000 – –
SILK-A124* ∼8100 0 2.96
Changes in grain shape
Grain shape analyses were performed on the SILK-
N1, SILK-A8, SILK-A11 and SILK-A12 tephra lay-
ers, sampled at about 20 km distance from source
at Einhyrningsflatir and SILK-LN (Thorsteinsdóttir et
al., 2015) sampled at Geldingasker about 33 km from
source (Table 4). SILK-A11 is significantly differ-
ent from the other layers with an elongation value of
0.78 (value of 1 is a perfect circle). SILK-A11 grains
are slightly less elongated than those from the Hekla-
1947 tephra layer (0.73) which was erupted subaeri-
ally. All other SILK layers have elongation values be-
tween 0.52 and 0.69 and more elongated grains. No
systematic change is observed with time, the grains
are most elongated in the oldest A12 layer with a
value of 0.52 and least elongated in the second old-
est A11 layer (0.78). Within the bedded layers SILK-
N1 and SILK-LN the values change towards less elon-
gated grains with time (Table 4).
Figure 5. Grain size distribution of two SILK layers.
A) SILK-A8 fine grained and B) SILK-UN coarser
grained. – Kornastærðardreifing (Φ) tveggja SILK
laga. SILK-A8 er fínkornóttara og SILK-UN gróf-
kornóttara.
Table 4. Results of grain shape measurements,
elongation (lower values represent more elongated
grains), ruggedness (R2) (lower values represent
more rugged grains) and circularity (higher values
for more circular grains). – Niðurstöður kornalögu-
nargreininga á ílengd (lægra gildi – ílengri korn),
hrjúfleika (lægri gildi – hrjúfara korn) og hringlögun
(hærra gildi – kringlóttara korn).
Layer/Unit Elong. R2 Circ. Age
SILK-LN middle unit 0.68 0.33 0.67 3400
SILK-LN bottom unit 0.55 0.36 0.63 3400
SILK-N1 middle unit 0.69 0.41 0.73 5800
SILK-N1 bottom unit 0.58 0.40 0.66 5800
SILK-N1 average 0.64 0.40 0.70 5800
SILK-A8 0.69 0.42 0.74 ∼7400
SILK-A11 0.78 0.35 0.78 ∼8000
SILK-A12 0.52 0.42 0.65 ∼8100
JÖKULL No. 66, 2016 75