Geoscience Reference
In-Depth Information
7
Figure 7.20 Part of a fi eld
notebook, containing a log with
preliminary interpretation of a
near-vent section of the 11.4 Ma
Wilson Creek Ignimbrite, Snake
River Plain, Idaho, USA. The scale
(given elsewhere in the notebook)
is 1 m per ruled division on the
page. (Notebook of Janet Sumner,
The Open University, UK.)
Relationships between ash and large fragments. Is the
'ash' made of the same stuff as the large bits? Are the
largest clasts supporting one another (rather than fl oating
in an ashy matrix)? If the answer to both is 'yes', you
should think along the lines of a 'block-and-ash-fl ow'
produced by collapse or explosive fragmentation of a lava
dome.
Another discriminator between fall deposits and pyroclastic
fl ows is that fall deposits mantle the topography, whereas
pyroclastic fl ows are much thicker in, or entirely confi ned to,
valleys. Thus, you should try to document any thickness
variations and their relationship with palaeotopography.
7.3 Mineralogy and small-scale textures of
igneous rocks
Here we consider diagnostic mineralogical and textural
evidence that can be seen in the fi eld with the unaided eye for
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