Geology Reference
In-Depth Information
Ignimbrite flow unit
Fine ash deposit
3
Lapilli pipes
Reverse
coarse-tail
grading of
pumice
PCZ
2b
Normal
coarse-tail
grading of
lithics
LCZ
Reversely
graded ash
2a
1
Ground layer
Figure 5.16
Idealised flow unit from a ponded ignimbrite. Layer 1 can be
composed of lithic and/or pumice rich material, thought to deposit at the front
of the pyroclastic flow, they can show tractional structures consistent with low
concentration pyroclastic flows. Layer 2 can show a variety of structures and
grading and can also vary from lithic to pumice concentrated zones (LCZ, PCZ).
Layer 3 shows transition into low concentrated flows and fall deposits.
welding streaked out sheath folds can be present indicating extreme shear during
rheomorphism
. As with unconsolidated ignimbrites, welded examples are best
characterised by logging the vertical and horizontal variations recording the
major textures, grainsize and litholigical variations, and so on (for example,
Figure 5.18). An example of the welded part of an ignimbrite is shown in
Figure 5.19.
Debris avalanches
occur due to the collapse of volcanic material under grav-
ity. These can be from dome collapse or from sector collapse of a volcano and
