Jökull


Jökull - 01.01.2015, Side 41

Jökull - 01.01.2015, Side 41
Grain characteristics of tephra from Katla and Hekla eruptions Table 2. Results of grain morphology measurements of the Hekla-1947 tephra. The parameters measured are Elongation (lower values - more elongated grains), Ruggedness (lower values - more rugged grains) and Cir- cularity (higher values - more circular grains). – Kornalögunarniðurstöður á Heklu-1947 gjósku. Mældar voru þrjár breytur, ílengd (lægri gildi - ílengri korn), hrjúfleiki (lægra gildi - hrjúfari korn) og hringlögun (hærri gildi - betri hringlögun). Unit Elongation Ruggedness Circularity Distance (km) Vestan Hafrafells 0.73 0.40 0.73 19 Hamragarðaheiði 0.73 0.41 0.75 42 (Figure 13). As in case with the Katla tephra the SEM imaging was carried out to visually demonstrate the grain shapes. More images can be seen in Þorsteins- dóttir (2015). DISCUSSION Changes with distance along axes of thickness There is prominent difference in the mean grain size and the proportion of the fine material in the tephra layers from these two volcanoes. At similar distances from their source the Hekla 1947 tephra is much coarser-grained than the Katla SILK-LN tephra (Fig- ure 14). The mean grain size of the SILK-LN tephra changes fairly regularly with distance but varies within bedded layers. The mean grain size of the finer bottom units is halved every 70–75 km and that of the coarser units every 20–22 km. The mean grain size of Hekla 1947 does however change much more with distance and is halved every 13–14 km, as expressed in the bulk samples. There is much more fine material in the Katla tephra than in the Hekla tephra (Figure 15). The range in proportion of fines ≤4 Φ (≤0.063 mm) is 24 to 43 Wt% at distances between 22 and 65 km in the SILK- LN tephra (samples from axis of thickness) but in the Hekla tephra this proportion is 1 to 33 Wt% at the same distance (excluding samples within 19 km dis- tance because no Katla tephra could be obtained at that distance and also potentially remobilized sam- ples). It is interesting that the changes in proportion of fine material ≤6.5 Φ (≤0.011 mm) with distance in the Katla tephra are little to none (Figure 8). Possible reasons for little to no change with distance in grains ≤6.5 Φ (≤0.011 mm) can be aggregation or scaveng- ing by or adhesion to larger grains. The main differences in grain size between the SILK-LN and the Hekla 1947 tephra can be summed up in the following way: The Hekla 1947 tephra has significantly higher mean grain size and much lower content of material ≤4 Φ (≤0.063 mm) at all distances than in the SILK-LN tephra. The mean grain size of Hekla 1947 decreases more rapidly with distance than that of the SILK-LN tephra. The higher mean grain size and lower content of fine material in the Hekla- 1947 tephra compared to SILK-LN tephra is thought to be connected to environmental conditions. Katla is covered in ice and Hekla is situated in a dry area. The interaction between water and magma in Katla eruptions can produce extremely fine grained tephra which explains the high amount of fines and lower mean grain size in the Katla tephra (e.g. Morrisey et al., 2000; White and Houghton, 2000; Zimanowski et al., 1997; Wohletz, 1983). Mean grain size will also decrease more slowly with distance in the case of SILK-LN due to high proportion of fines with lower terminal velocity and longer travel range, as compared to the more coarse grained Hekla 1947. Changes with time The grain size measurements of the SILK-LN tephra show an evolution from finer grained material in the beginning toward a coarser grained material in the latter part of the eruption (e.g. Figure 6). Where the tephra was bedded the proportion of fine ash de- creased from ca. 30–43 Wt% in the bottom unit to ca. 11–29 Wt% in the top unit. A possible reason for finer grained tephra in the beginning phase of the eruption could be due to in- creased fragmentation caused by optimal availability of external water and the fact that SILK-LN tephra layer is high in SiO2. The large amount of fine ash is JÖKULL No. 65, 2015 41
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