2005-04-29 / Columnists

Drawing On Science


by Stephen Yaeger

Our planet is composed of three types of substances: water, air, and rocks. Rocks can be found most anywhere in one form or another, and range in size from microscopic flakes of clay to sand to pebbles to large boulders. They are made of rock-forming minerals, which are common minerals that make up most of the Earth’s crust. Some rocks are made of one type of mineral and others composed of two or more minerals. Minerals are chemical substances consisting of one or more elements.

There are three major types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks are formed from hot, molten rock known as magma from deep inside the Earth. When this magma comes to the surface it is called lava. When the lava cools and hardens it forms igneous rock. Some examples of igneous rocks are obsidian, rhyolite, and granite.

The process of erosion (=the physical breakup of material due to water or wind) breaks down the igneous rock into smaller particles such as sand. When small rock particles are carried by water into a lake or ocean they settle to the bottom—heavier particles first, then the lighter particles.

Over time the particles become cemented together. Additional rock particles then settle on top and the process is repeated. In time layers of sedimentary rock are formed. Examples of sedimentary rock include coal from organic materal, sandstone from sand grains, shale from clay, and limestone from lime.

Metamorphic rocks such as slate, marble, and gneiss (NICE) are formed when rock masses come under extremely high heat and pressure. The heat and pressure changes the atomic or molecular arrangement of the original rock. Most of the metamorphic rock in the Earth’s crust is formed during regional metamorphism associated with mountain building processes. It is at this time that the high temperatures and pressures occur. Heat is built up due to the friction of moving rock layers and pressure is increased due to the weight and squeezing patterns of overlying rock masses.

Metamorphic rock may also be formed due to contact metamorphism. Magma moves upward into overlying rock. The magma’s heat, coupled with hot liquids and gases from the magma, results in changing the rock’s physical and chemical structure. The high temperature bakes the rock and the hot liquids and gases react with the rock’s minerals.

Some sedimentary/metamorphic rock relationships are: sandstone changes into quartzite, shale changes into slate, and limestone changes into marble.

Igneous rocks may be thought of as the primary rock of the Earth’s crust. Weathering (=chemical reactions) and erosion (=physical changes) form sediments, which either settle to the bottom of lakes or oceans where sedimentary rock is formed or become buried deep within the Earth’s crust to melt due to high temperatures. Thus the sedimentary rock becomes magma. Magma may become igneous rock or be changed into metamorphic rock directly. Igneous and metamorphic rock may be weathered and eroded to form sedimentary rock again (getting dizzy?) or re-enter the earth’s crust, and the process goes on and on. This is known as the rock cycle.

So the next time you’re strolling along the beach or a wooded path and pick up a rock, think about what it went through to get where it is.

Questions/comments?

E-mail Steve: Drawingonscience @aol.com

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