CLASSIFYING ROCKS Coulter

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