This week, I decided to visit the Alturas and Susanville sheets from the Geologic Atlas of California. Upon my travels through the north-eastern part of California, these sheets (geologic counties) appear to contain great amounts of volcanic rock from previous volcanic activity. One of the key indicators that lead me to this observation was the amount of igneous rocks present. Igneous rocks are formed by the solidification of
magma, which is lava generated by partial melting of the upper mantle or the lower crust.
Typically, the rate of cooling of
magma is rapid, and crystal growth is inhibited; this was another valuable indicator that lead me to believe there was one volcanic activity present.
The specimens in this area are fine-grained and contain well-formed crystals visible to the naked eye, called
phenocrysts. This texture of igneous rock is very common and known as a
porphyritic texture It develops when
magma has been slowly cooling and crystallising within the Earth's crust, but then suddenly erupts at the surface,
causing the remaining uncrystallised
magma to cool rapidly. One of the final observations I made was the structural integrity of the rock formations in these sheets.
Volcanic rocks often exhibit structures caused by their eruption, such as vesicles (open cavities that represent escaped gasses) or flow banding (formed by shearing of
the lava as it flows). A vast majority of the specimens present in these geological sheets exhibit vesicles, but there are some major formations as well that demonstrate flow banding.
Vesicle traits from a specimen taken from Modoc National Forest, CA.
Source: http://www.maryhemmerlyhecker.org/sand-collection-archive/064-big-sand-butte-modoc-national-forest-ca/
An example of flow banding formations.
Source: https://www.usgs.gov/media/images/flow-banding-panum-pumice-and-obsidian
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