CLASSIFYING ROCKSCoulter

MINERAL COMPOSITION AND COLOR

When studying a rock sample, geologists observe the rocks minerals composition, color, and texture

Rocks are made of mixtures of minerals and other materials.

About 20 minerals make up most of the rocks of Earth’s crust, these minerals are known as rock-forming minerals

Geologist observe the shape and color of crystals in a rock to identify the minerals that the rock contains.

TEXTURE

Grains are particles of minerals or other rocks that make up rocks.

Texture is the look and feel of the rock’s surface

Grain size:

often grains in a rock are large and easy to see, these are called coarse-grained.

Grains sizes are so small that they can only be seen by microscope, fine-grained.

Some have no visible grain even under a microscope.

Grain shape:

Some grains look like tiny particles of sand, small seeds, or exploding stars.

In some rocks grain shape results from fragments, they can be smooth and rounded or they can be jagged

Grain pattern:

Some grains lie in flat layers that look like a stack of pancakes

Others make a swirling pattern

Others the grains occur randomly throughout.

HOW ROCKS FORM

Geologists classify rocks into three major groups Igneous rock: forms from cooling of magma or

lava Sedimentary rock: forms when particles of other

rocks or the remains of plants and animals are pressed and cemented together.

Metamorphic rock: forms when an existing rock is changed by heat, pressure, or chemical reaction.

CLASSIFYING IGNEOUS ROCKS

Igneous rocks are classified according to their origin, texture, and mineral composition.

Origin: Igneous rock may form on or beneath earth’s

surface Extrusive rock is igneous rock formed from lava

that erupted onto Earth’s surface. Intrusive rock is igneous rock that formed when

magma hardened beneath Earth’s surface.

IGNEOUS ROCK CLASSIFICATION: TEXTURE

Texture: Texture of an igneous rock depends on the size and shape

of its mineral crystals. Rapidly cooling lava forms fine-grained igneous rocks with

small crystals. Slowly cooling magma forms coarse-grained rocks with

large crystals. Intrusive and extrusive rocks usually have different textures

Intrusive rocks have larger crystals than extrusive rock. Extrusive rocks have a fine-grained or glassy texture.

Obsidian is an extrusive rock that cooled very rapidly without forming crystals.

IGNEOUS ROCK CLASSIFICATION: MINERAL COMP.

Most of Earth’s minerals contain silica, a material formed from oxygen and silica.

The amount of silica in magma and lava affect the type of rock they form.

Lava that is low in silica usually forms dark-colored rocks.

Magma that is high in silica usually forms light-colored rocks.

SEDIMENTARY ROCKS

Most sedimentary rocks are formed through a series of processes: erosion, deposition, compaction, and cementation.

Erosion: running water, wind or ice loosen and carry away fragments of rocks.

Deposition: is the process by which sediment settles out of the water or wind carrying it.

Compaction: process that press sediment together.

Cementation: is the process in which dissolved minerals crystallize and glue particles of sediment together.

TYPES OF SEDIMENTARY ROCK

There are three major groups of sedimentary rocks: clastic rocks, organic rocks, and chemical rocks.

CLASTIC ROCK

Is a sedimentary rock that forms when rock fragments are squeezed together.

These fragments can range in size from clay that are to small to be seen with a microscope to large boulders that are too heavy for you to lift.

Clastic rocks are grouped by the size of fragments of which they are made.

ORGANIC ROCK

Organic rocks forms where the remains of plants and animals are deposited in thick layers.

Organic means once part of a living thing.

Two important organic sediment is coal and limestone

CHEMICAL ROCK

Limestone can form when calcite that is dissolved in lakes, seas, or underground water comes out of solution and forms crystals. This kind of limestone is considered a chemical rock.

Chemical rocks can also form from mineral deposits left when seas or lakes evaporate.

METAMORPHIC ROCK

Heat and pressure deep beneath Earth’s surface can change any rock into metamorphic rock.

Types of metamorphic rock: geologists classify metamorphic rocks according to the arrangement of the grains that make up the rocks.

FOLIATED ROCKS

Metamorphic rocks that have their grains arranged in parallel layers or bands are said to be foliated.

One common foliated rock is slate. Heat and pressure change the sedimentary rock shale into slate

NON-FOLIATED

Mineral grains in these rocks are arranged randomly.

Metamorphic rocks that are nonfoliated do not split into layers.

ROCK CYCLE