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| Matthew Nyman, TERC |
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Is the rock dark? If yes, your sample is probably basalt.
Basalts may also contain vesicles (holes where volcanic gases were trapped) and
small, light-colored, rectangular-shaped feldspar crystals.
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| Matthew Nyman, TERC |
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Is the rock light in color (cream-colored or tan,
or with an orange tint)? If yes, your sample is probably rhyolite. Rhyolites
may also show banding that formed as viscous lava flowed. Small crystals of
quartz may be present.
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| Matthew Nyman, TERC |
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Is the rock medium in color, between dark
and light (gray, pink or green colored)? If yes, your sample is
probably andesite. Andesite may contain small visible crystals
embedded within fine-grained material.
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If you have identified your sample, return to the beginning
of the rock identification
key, select another rock sample, and begin the process
over again.
If your rock sample does not have any of the features
described on this page, reexamine the rock with a hand
lens or magnifying glass. Look at the sample from all
angles. Concentrate on fresh surfaces where you might
be able to better determine the texture. Return to the
beginning of the rock identification
key and start over again.
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