Once Colorado was 2/3’s covered
by volcanic igneous rocks but much of them eroded. Volcanic rocks do come in a variety of forms,
though the type of eruption depends. In
the San Juan’s volcanic field located southwest Colorado are were many of these
rocks formatted. The picture posted is
the Castles that consist of volcanic rock in the West Elk Wilderness.
What does the color of the rock say about
its composition?
The color of an igneous rock is a measure of
the ratio of dark colored, or mafic, minerals
to light colored, or felsic,
minerals.
What crystal sizes are present in the
rocks, and what does that say about the rock’s cooling rate?
Minerals crystallize at different
temperatures depending on their chemical composition. At high temperatures only
minerals that have structures stable under those conditions will be able to
crystallize. Typically those minerals are olivine, pyroxene, and plagioclase
feldspar. Therefore igneous rocks that form at high temperature would be composed
of olivine, pyroxene, and plagioclase feldspar and hence be dark in
color. Rocks that have formed
from magma at relatively low temperatures would be composed of the lighter
colored quartzes, potassium feldspar, and muscovite mica.
What type of environment would have formed
this rock?
Igneous rocks form as
molten rock cools and solidifies. Two environments are underground and on the
surface.
- underground - in which case the melt is called 'magma' and the rock that results from its solidification is described as 'intrusive'.
- on the surface - in which case the melt is called 'lava' and the rock that results is described as 'extrusive' or 'volcanic'.
References
Alden, A. (2004, January 18). What Do You Need to Know About Igneous Rocks? Retrieved from https://www.thoughtco.com/about-igneous-rocks-1438950
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