Page 83 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

5. Coherent dacite-in situ hyaloclastite transition
Within this exposure there is a clear gradation from
comparatively coherent, flow-banded, hornblende-
phyric dacite (D) into matrix-poor in situ
hyaloclastite (Hy) that in turn grades into matrix-rich
hyaloclastite (Mh).

Green Tuff Belt, Late Miocene; Kariba, Hokkaido,
Japan.

6. In situ dacitic hyaloclastite
This example of blocky, matrix-poor, in situ
hyaloclastite in hornblende-phyric dacite shows
characteristic jigsaw-fit texture. The clasts have
curviplanar fracture surfaces, typical of quench
fragmentation of coherent glass.

Green Tuff Belt, Late Miocene; Shiroiwa, Hokkaido,
Japan.

7. In situ dacitic hyaloclastite
The hornblende-phyric dacite blocks and centimeter-
sized clasts in this moderately matrix-rich in situ
hyaloclastite are separated by a finely comminuted
matrix of the same composition. Jigsaw-fit texture,
indicative of in situ fragmentation, is well developed.

Green Tuff Belt, Late Miocene; Shiroiwa, Hokkaido,
Japan.

8. Resedimented andesitic hyaloclastite
Clasts in this thickly bedded, andesitic breccia were
formed by quench fragmentation. The beds have a
lenticular geometry, relatively steep primary dip, and
exhibit reverse grading. The section is typical of
resedimented hyaloclastite generated by grain-flow
redeposition and avalanching on the flanks of a
submarine lava-hyaloclastite pile. Notebook,
encircled, for scale.

Green Tuff Belt, Late Miocene; Kabuto Cape,
Shakotan Peninsula, Hokkaido, Japan.

75