As you know, scientist of all different types use their powers of observation. Geologists (scientists who study the forces that make and shape planet Earth) observe certain characteristics of rocks in order to classify them.
Three Classifications of Rocks
The three things geologists look at are:
Texture: the look and feel of the rock's surface
Mineral composition: the shape and size of crystals in the rocks
Rocks come in many different colors.
A rock's texture is the look and feel of the rock's surface. Some rocks are smooth and glassy; while others are rough and chalky. Most rocks are made up of particles of other rocks, which geologists call GRAINS. Grains give rocks their texture. Geologist looks at many characteristics to classify a rock. These characteristics are:
NO VISIBLE GRAIN
THERE ARE TWO CATEGORIES OF GRAIN SIZE:
COARSE-GRAINED: grains in the rock are large and easy to see
FINE-GRAINED: grains in the rock are so small that they can only be seen with a microscope.
The grains in a rock vary widely in shape. Some grains look like fine particles of sand; while others look like small seeds or stars. Some rocks contain fragments of other rocks. Sometimes the fragments can be smooth and rounded or sharp and jagged looking.
Many times grains in a rock form patterns. Some lie in flat layers that resemble a stack of pancakes Other grains form wavy, swirling patterns. Some rocks have grains that look like rows of multicolored beads or like stripes (banded). Other rocks have grains that occur randomly throughout the rock.
No Visible Grain
Some rocks have no grain, even when they are examined under a microscope. Some of these rocks have no grain because when they form, they cool very quickly. This quick cooling gives these rocks the smooth, shiny texture of a thick piece of glass.
To determine the mineral composition of a rock, sometimes geologists have to look at a small sliver of a rock under a microscope. In order to do this, geologists have to cut the rock very thin so light can shine through its crystals. Sometimes testing with chemicals can also help determine mineral composition.
Three major groups of rocks
The three major groups of rocks are:
If you predicted Igneous rock, you were right! Igneous rock forms from the cooling of molten rock, either magma below the surface or lava at the surface.
Sedimentary rock forms when particles of other rocks or the remains of plants and animals are pressed and cemented together. Sedimentary rock forms in layers below the surface.
Metamorphic rock is formed when an existing rock is changed by heat, pressure, or chemical reactions. Most metamorphic rock forms deep underground.
Test your skills as a geologist:
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