NSSS5.1

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=5.1 Classifying Rocks=

Vocabulary

 * Grains**: Particles of minerals or other rocks
 * Igneous Rock**: It forms from the cooling of molten rock - either magma below the surface or lava at the surface
 * Sedimentary Rock**: It forms when particles of other rocks or the remains of plants and animals are pressed and cemented together
 * Metamorphic Rock**: It is formed when an existing rock is changed by heat, pressure, or chemical reactions

How Geologists Classify Rocks

 * Earth's crust is made of rocks and is used to make mountains, hills, valleys, and beaches
 * Geologist collect samples of rock and study them to classify the rock
 * In order to obtain a sample, geologists chisel a piece of rock out of the place where it was found
 * After the sample is obtained, geologist crack open the rock to see what is inside
 * Geologist look in the inside of the rock because the outside of a rock can be changed by weathering
 * While studying the sample, geologists note the rock's color and texture to find out its mineral composition
 * Using these characteristics, geologists can designate a rock based on its origin, or where the rock has and where it formed

Texture

 * Texture by itself doesn't classify a rock, but it does give some vital information to classify it
 * A rock's texture is how the rock looks and feels
 * Some rocks have a glassy or smooth texture while others have a rough or chalky texture
 * Rocks are made with tiny particles of other rocks and minerals called grains
 * The grains of the rock are what give the rock its texture
 * Geologist use a number of terms to describe the size, shape and pattern of the rock's grains
 * **Grain Size**
 * The grains in some rocks are so big that are very easy to see and are called coarse grained
 * However, there are rocks with grains so small that they are invisible
 * These rocks are fine grained
 * **Grain Shape**
 * The grains in a rock vary widely from the grains in other rocks
 * The grains could look like tiny particles of sand to small seeds and shooting stars
 * In other cases, the shape of the crystal could shape the grains
 * In other rocks, the grains result from fragments of other types of rock
 * **Grain Pattern**
 * The patterns that rocks often form, form from the grains in a rock
 * Some rocks look like stacks of pancakes or also like rows of multicolored beads while others form wavy, swirling patterns
 * In some rocks the grains occur randomly throughout the rock
 * **No Visible Grain**
 * Some types of rocks don't even have any grain even when observed under a microscope
 * These types of rocks have no grains because when they form they cool very quickly giving no time to form grains
 * Because they have no grains, these types of rocks are smooth and shiny, just like the texture of a thick piece of glass
 * Other rocks form from extremely small pieces of silica that settle out of the water
 * For example: the rock //flint// forms in this type of manner

Mineral Composition

 * Geologists often look very closely at a rock to find out its mineral composition
 * To identify a mineral, a geologist must do several things.
 * First the geologist has to cut the rock very thin so that light can shine through its crystals.
 * Then they do some tests to figure out the mineral's special properties.
 * One of the test that they do is that they test the rocks surface with acid so that they see whether a mineral is made up of compounds called carbonates.
 * Another test that they do is that they test the rock with a magnet to see if it has the minerals iron, nickel, or cobalt.

Origin

 * There are three major types of groups of rocks.
 * They are igneous rocks, sedimentary rocks, metamorphic rocks.
 * Igneous rocks form from the cooling of lava above the surface or molten rock below the surface.
 * Sedimentary rocks form when sediments pile up under pressure.
 * Metamorphic rocks form when pressure is applied to sedimentary or igneous rocks.
 * Metamorphic rocks often form deep below the surface.