PERSPECTIVE
THREE KINDS OF ROCKS
By Michael Ellis
As an educator I try to reduce the complicated to simple components and then build from that base of knowledge. Geology is a very complex subject and to make it a bit easier all one has to remember is there are only three kinds of rocks on our planet. Yes only three – igneous, sedimentary and metamorphic.
Igneous from the Latin “lgni” meaning fire. When you turn on your Ignition, you fire up your car. Basically any rocks created directly from the pools of hot magma – from the Earth’s mantle, which lies 5 to 70 miles below the surface – are igneous. Hot liquid rock that flows up in plumes but does NOT make it all the way up to the surface but cools at depth is called granite. This simple name drives geologists nuts because there actually are many different kinds of granite depending upon the parent material of the magma. Too bad.
However, if the magma manages to make it all the way up to the earth’s surface, it is then called volcanic rock. It can be the same exact magma but simply cools more quickly and at the surface. Now wait a minute you say. Granite is all over the Sierra and it is not buried at depth. True, but that rock originally formed far below and was later uplifted and is now exposed to view.
Sedimentary rocks are formed from -duh -sediment. Sediment that is derived from rocks that are weathered eroded and deposited. This cemented together material can vary in composition from small boulders to very fine silt. Sandstone, shale, siltstone, and conglomerate are some common sedimentary rocks. Biological action can also result in sedimentary rocks. Seashells of trillions of ancient marine invertebrates create huge layered beds of calcium carbonate known as limestone. And coal is the accumulation of organic matter, mainly trees, from 300 million years ago. .
The final type is metamorphic – literally to change shape. This change is brought about by tremendous heat and pressure driven by the Earth’s constant tectonic activity, as surface rocks are recycled back into the mantle. The parent material can be igneous, sedimentary or even older metamorphic rock. Sandstone under heat and pressure becomes quartzite. Limestone becomes marble.
See how simple that is? This is Michael Ellis with a Perspective.

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January 4, 2013