Tuesday, 29 November 2011

Volcanic activity ~ and the rocks and gems associated with them ~

Rare clear blue obsidian
Rocks from a volcanic source are termed igneous rocks.  In the month of November I've placed entries for all specimens of this sort as well as those that are associated with them.  Also included are those which have a relationship with hydrothermal activity.

The Wikipedia article linked to above provides an excellent introduction.  It states that:
The upper 16 kilometres (10 miles) of Earth's crust is composed of approximately 95% igneous rocks with only a thin, widespread covering of sedimentary and metamorphic rocks.[1]
Note: Obsidian is a form of volcanic glass and usually black.  You can find my brief article about it by clicking on the link below.  It includes a photograph of a snowflake obsidian necklace as well as a larger version of the above image:

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