How to Recognize the Three Types of Rocks

Beaches make good rock hunting grounds.

There are three basic types of rocks found throughout the globe: igneous, metamorphic and sedimentary. With a little foreknowledge and a handheld magnifying glass, you can identify the three forms of rock. Igneous rocks and those that form from cooling magma, often in crystal structures. Metamorphic rocks are formed by pressure when the hard rocks are displaced and introduced to new minerals. Sedimentary rocks are the result of layers of minerals under pressure.

Things You'll Need

• Small pocket magnifying glass

Instructions

1. Pick up the rocks in question and hold it under the magnifying glass. Look for small crystal structures embedded in the rock itself, indicating the rock is igneous in nature. Look for small grains of crystal in the rock. This is known as aphanatic rock. Examples of the igneous aphanatic rock include basalt and granite. Look for igneous rocks around calderas and dormant volcano cones, such as around Mt. Rainer and Mt. St. Helens.

2. Look for defined striation lines going in the same direction in hard rocks. This is an indicator of sedimentary rocks. Sedimentary rocks form from pressure and layers of detritus. As the pressure exerts, the layers become fused from pressure and heat, making them solid rocks. Limestone and sandstone are examples of sedimentary rock. Locate these rocks in red rock canyon areas, or along river banks where the river eroded and made canyon walls.

3. Look for striations that swirl and are irregular. This is an indicator of metamorphic rock. Metamorphic rocks form as pressure dilutes heats and changes the rock into another form, hence the term "metamorphic," or change-in-shape. Foliation takes place in metamorphic rocks. Try to peel layers of the foliated leaves from the rock as another indicator of metamorphic rocks. This is one of the prime methods of differentiating between sedimentary and metamorphic rocks.

4. Use a knife blade to sheer off cleavage on rocks as another indicator of metamorphic rocks. Slate is one of the better examples of metamorphic rocks with cleavage and leaves.

References

• Ask Geo Man: The 3 Basic Rock Types
• University of California: Rocks, Dr. W.: The Three Basic Rock Types: PDF