Jökull - 01.12.1994, Page 52

Lab number Sampling site Unit Type of eruption Type of deposit (AF = air fall, BS = base surge) Petrological classification Median, (þ / Sorting, a
1420 Reykjanes R-3 (above BL) Surtseyan BS Tholeiite not available
1418 Reykjanes R-3 (below BL) Surtseyan BS Tholeiite not available
1200 Reykjanes R-3 (BL) Surtseyan AF/BS Tholeiite 1,72/2,03
1203 Reykjanse R-3 (BL) Surtseyan AF/BS Tholeiite 2,47/1,81
1204 Reykjanes RG-2 V-cone, lower Surtseyan BS Tholeiite 1,47/1,80
1207 Reykjanes RG-2 V-cone, upper Surt./Stromb. AF/BS Tholeiite -2,40/2,60
1156 Reykjanes RG-3 K-cone Surtseyan BS, lower part Tholeiite 1,47/1,57
1157 Reykjanes RG-3 K-cone Surtseyan BS, upper part Tholeiite 2,21/1,26
1422 Reykjanes R-7 Surtseyan AF Tholeiite not available
1277 Reykjanes R-9 Surtseyan AF Tholeiite 2,36/1,64
1161 Surtsey Surtur Surtseyan BS Alk. ol. basalt 2,19/2,76
1162 Surtsey Surtur Surtseyan BS Alk. ol. basalt not available
1267 Lakagígar S3 Strombolian AF Tholeiite -2-4/<2,0
1270 Lakagígar P1 Phreatomagmatic BS/AF Tholeiite 2-2/> 2,0
1287 Heimaey Unitof 29/01/73 Strombolian AF Hawaiite -3,07/1,25
1153 Reykjanes RG-3 K-cone Surtseyan BS Tholeiite not available
1540 Yngri Stampar Crater Strombolian AF Tholeiite not available
1208 Reykjanes RG-2 V-cone, uppcr Surt./stromb. AF/BS Tholeiite not available
1273 Lakagígar S14 Strombolian AF Tholeiite not available
1268 Lakagígar S7 Strombolian AF Tholeiite not available
1419 Reykjanes R-3 (above BL) Surtseyan BS Tholeiite not available
1280 Reykjanes R-9 Surtseyan AF Tholeiite not available
Table 1. List of samples.
1. tafla. Sýnaskrá.
Unit Age
R-10 13. century
R-9 13. century
R-8 13. century
R-7 13. century
Cone complex 13. century
R-6 12. century
R-5 12. century
R-4 10. century
R-3 1500-2000 BP
R-2 1500-2000 BP
R-1 3500-4000 BP
Table 2. Tephra units from the
Reykjanes volcanic system.
2. tafla. Gjóskumyndanir í
Reykjaneskerfinu.
characterized by simple breakage of homogeneous
material. Such particles would display high values for
ruggedness, elongation, and circularity. This appears to
be bome out by the data obtained in the present study,
as the samples from Surtsey (samples 1161 and 1162)
and the tuff cones Vatnsfell (samples 1201 and 1207)
and Karl (samples 1156 and 1157) show quite high val-
ues for the shape parameters.
As the data set selected for the present study cov-
ers a narrow chemical range and emptions in similar
tectonic settings, the actual grain shape parameter val-
ues do not differ greatly. Strict statistical procedures
were therefore adopted to resolve uncertainties in the
interpretation of the results. Every sample was ob-
tained from a carefully mapped tephra unit, formed
under specific conditions, avoiding mixing of process-
es of formation. All the samples were sieved manually
in order to minimize modification of grain shape, and
subsamples of a fixed size range were taken randomly
from each one. Between 160 and 300 grains from each
sample were analyzed, and a subset of 160 grains was
subsequently selected at random.
Many powerful statistical procedures are available
to test for differences between samples. Several of
Sample 1156 1157 1161 1162 1200 1203 1204 1207 1267 1270 1277 1287 1418 1420 1422
Ruggedness
Mean 0.895 0.882 0.901 0.896 0.842 0.848 0.881 0.860 0.797 0.846 0.864 0.855 0.893 0.899 0.874
Standard deviation 0.029 0.044 0.023 0.021 0.062 0.045 0.036 0.043 0.066 0.064 0.054 0.043 0.040 0.022 0.052
Skewness -2.314 -1.779 -2.460 -1.384 -1.462 -1.254 -1.285 -1.126 -0.557 -1.035 -1.095 -0.982 -2.297 -1.156 -3.337
Elongation
Mean 0.761 0.736 0.753 0.737 0.718 0.705 0.745 0.745 0.656 0.689 0.694 0.709 0.729 0.750 0.736
Standard deviation 0.083 0.110 0.095 0.096 0.104 0.120 0.110 0.100 0.138 0.113 0.114 0.111 0.098 0.089 0.099
Skewness -0.759 -0.866 -0.527 -0.123 -0.633 -0.685 -0.740 -0.607 -0.792 -0.545 -0.358 -0.311 -0.616 -0.425 -0.422
Circularity
Mean 0.695 0.654 0.712 0.699 0.585 0.587 0.656 0.611 0.462 0.586 0.616 0.586 0.679 0.711 0.652
Standard deviation 0.068 0.110 0.058 0.060 0.120 0.102 0.100 0.092 0.113 0.128 0.116 0.095 0.105 0.060 0.103
Skewness -1.109 -1.404 -0.915 -0.900 -0.870 -1.076 -1.186 -0.305 -0.004 -0.518 -0.656 -0.425 -1.571 -0.863 -1.607
Table 3. Results of image analysis (mean, standard deviation, skewness).
3. tafla. Niðurstöður myndgreiningar (meðaltal, staðalfrávik og dreifingarskekkja).
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