ChapC2.2

Types of Rocks

Igneous Rock

• rock: a material made up of one or more minerals– rocks are not crystals - minerals are

crystals– some rocks form at or near Earth’s surface– some rocks form deep in the crust - in the

Earth’s middle layer (mantle)

• rocks are classified into 3 groups based on how they are formed

• igneous rock: rocks that forms when melted rock hardens– lava is melted rock that reaches Earth’s

surface– lava cools and hardens quickly

• large mineral pieces do not have time to form• rocks formed from lava have small mineral

pieces and are called fine-grained

• magma: melted rock below Earth’s surface– magma cools and hardens slowly– minerals can form large grains that are easy

to see– rocks formed from magma are called coarse-

grained

• basalt is the most common igneous rock– forms from lava at Earth’s surface– dark, greenish-black rock– made of minerals feldspar and pyroxene

• gabbro is an igneous rock made of feldspar and pyroxene– has larger mineral grains– forms inside Earth instead of at surface

basalt

gabbro

• pumice: another igneous rock– tiny holes in pumice caused from gases escaping

from lava as it cools– feels rough and scratchy

• obsidian: forms from lava– lava cools so quickly that rocks look like black glass– when it breaks sharp edges form

• granite: common igneous rock– forms when magma cools slowly beneath Earth’s

surface– made of large grains of feldspar, quartz, and mica

GRANITE

OBSIDIAN

PUMICE

Sedimentary Rock

• weathering: how rocks are broken down into smaller pieces– wind, water, ice, and plant roots cause

weathering

• rocks weathered into small pieces and moved to other places by wind, water, gravity, or slow moving glaciers– erosion: movement of weathered rock

pieces from one place to another

• erosion caused by water - over time rock pieces drop to lake, river, stream bottoms– this material is sediment

• layers of sediment form sedimentary rock because all the layers squeezed and get stuck together

• conglomerate is a type of sedimentary rock – pieces can be big as boulders or small as peas– pieces are round and smooth– form in shallow water

• smaller sediments are carried farther and dropped later– siltstone is a rock made of smaller

sediments

• limestone: fine-grained sedimentary rock– made of mostly mineral calcite– forms in oceans

• sometimes from seashells

– a few form in lakes

• sandstone another kind of sedimentary rock– made of bits of rock and minerals

the size of sand grains– nearly all sandstone is made of

mostly the mineral quartz– some sandstone are fine-grained

• feel smooth when you touch them

– some sandstone are coarse-grained

• feel rough against skin

– can form on land or in water

• shale: fine-grained sedimentary rock– made of small sediments– sediments so small that you can only see

them with a magnifying glass or microscope

Metamorphic Rocks

• metamorphic rocks: rocks that were changed due to high heat and pressure– texture (the way a rock looks and feels)

and the minerals that make up the rocks are changed

• metamorphic rocks can form from any rock– sedimentary, igneous, or other

metamorphic rocks

• some metamorphic rocks form when mountains built up– schist

• has wavy lines• splits into layers easily

– gneiss• forms when schist is heated and squeezed more• has bands of light and dark minerals

Schist

Gneiss

• marble is another metamorphic rock– forms when limestone is squeezed and

heated

• slate– forms when shale is under great pressure– has layers

• quartzite– forms from sandstone when heat melts the

sand grains together– milky color– other minerals can give it a gray or pink

color

QUARTZITESLATE

MARBLE

Summary

• rocks are made up of one or more minerals– classified by how they are formed

• igneous rocks form when magma or lava cools and hardens

• sedimentary rocks made of pieces of rock that have been squeezed and stuck together

• metamorphic rocks form when heat and pressure change rocks