Rocks

Many artists and builderswork with rock becauseit lasts a very long time.

The statue picturedhere was carvedfrom rock. It hasstood for 2000years.

Minerals Make Up Rocks

About 3,000 minerals occur in the Earth.

Most of them are not found in a pure form.

They are mixed together in rocks.

A rock is a solid, natural material made ofone or more minerals.

Only about 20 minerals make up 95% ofthe Earth’s rocks.

Scientists Study RocksEarth scientists who study rocks are interested in which minerals make them up and how rocks are formed.

This information helps scientists and engineers locate valuable resources such as oil or metals.

Why Study Rocks

• Knowledge of rocks is necessary for undertaking construction projects.

• Rocks also help in understanding the environment.

• Rocks also provide clues about the history of the earth.

• Rocks tell us how the earth changes.

Three Types of Rocks

• Geologists group, or classify, rocks into three main types, depending on how they form.

Three Types of Rocks

• Some rock forms when hot, melted minerals cool and harden.

• This rock is igneous.

Three Types of Rocks

• Another type of rock forms when bits of other rocks and the remains of living things are pressed and cemented together.

• The result is sedimentary rock.

Three Types of Rocks

• Heat, pressure, and chemical reactions can change sedimentary or igneous rock into another type of rock.

• This type of rock is metamorphic rock.

Sedimentary Rock

How Sedimentary Rocks Form

• What happens if you fill a jar with river water and let it set for a while?

How Sedimentary Rocks Form

• Solid particles settle out on the bottom of the jar. These particles are sediment.

• It may include sand, soil, pebbles, and the remains of dead plants and animals.

• Sediment is the main ingredient of sedimentary rock.

How Sedimentary Rocks Form

• How do bits of sand and soil get turned into rock?

• Rivers carry sediment to a lake or ocean.

• The sediment settles to the bottom and accumulates.

• As layers of sediment accumulate, the weight of overlying sediment presses the bottom sediment together.

How Sedimentary Rocks Form

• In addition, the bottom sediment is cemented together by minerals, such as calcite, dissolved in water.

• The result is sedimentary rock.

How Sediment Becomes Rock

• Compaction– The sediment becomes buried deep in the

earth, and the increased pressure presses the particles together.

Cementation

-Water solutions filter through the sediment and the sediment particles are cemented together.

Sedimentary Rocks in the Earth’s Crust

• Sedimentary rocks form layers.

• The Grand Canyon is a striking example of layering in sedimentary rocks.

Three Types of Sedimentary Rocks

• Seventy-five percent of the rocks exposed at the earth’s surface are sedimentary.

• These rocks are classified into three main types, based on the kinds of sediments that form them.

Three Types of Sedimentary Rocks

• Clastic rock

• Chemical rock

• Organic rock

Clastic Rock

• Clastic Rock is the most common type of sedimentary rock.

• Clastic rock forms from fragments of other rocks.

Clastic Rock• Shale, for example, is made from fine

particles of clay, mica, and other tiny grains that form as rocks break down on the earth’s surface.

• The tiny particles form a muddy mixture of sediments on the bottom of a lake or ocean.

• Overlaying layers squeeze out the water and air, forming shale.

Clastic Rock

• This process is similar to squeezing a handful of mud.

• As you force out the water and air, the mud becomes more compact and “solid”.

Clastic Rock

• The sediments are clues to the environment in which the rock formed.

• Rounded pebbles of a conglomerate often form in rivers, where sediment in the moving water breaks off sharp edges of rocks, making them round

• The pebbles fall to the river bottom when the water slows down.

Clastic Rock

• So conglomerate might indicate the location of an ancient river.

• A conglomerate is a clastic rock made of pebbles cemented together.

Clastic Rock

• The tiny sediments that make up shale tend to settle out of ocean water far from where the river empties into the sea.

• So shale suggests the location of an ancient sea.

Clastic Rocks• Sandstone, as the name suggests, is

made of sand-sized grains.• What do these rocks tell you about the

environment in which they formed?• You might have correctly guessed that

sandstone formed from ancient beaches and sand dunes.

• These rocks also might indicate a delta-the land that builds up where a river empties into a body of water.

Chemical Rock

• Chemical rock forms from chemicals dissolved in water.

• Some limestone are chemical rocks.

• Chemical and temperature changes in ocean water can cause particles of calcium carbonate to form.

• These particles accumulate on the ocean floor and become limestone.

Chemical Rock

• Other chemical rocks include salt and gypsum.

• These rocks form when sea water evaporates.

Organic Rock• Organic rock forms from the remains of

living things.

• Some limestone form from shells of sea animals.

• When shellfish, such as clams and mussels, die, their shells accumulate on the ocean floor.

• Layers of shells, which are made of calcite, are pressed and cemented together to form organic limestone.

Organic Rock

• One type of organic limestone is chalk.

• This soft rock is made of the shells of microscopic organisms.

• The White Cliffs of Dover in England are made of chalk.

• Coal is an organic rock that is an important fuel source.

Metamorphic Rock

How Metamorphic Rocks Form

• Metamorphic rocks can also form when liquids and gases escape from magma.

• The liquids and gases add new minerals to the surrounding rock.

Types of Metamorphic Rock

• Foliated rock—metamorphic rock in which minerals have been rearranged into visible bands.

• Nonfoliated rock—metamorphic rock that does not show bands

Foliated Rocks

• Foliated rocks form when heat and pressure have flattened the minerals into bands.

• Slate and gneiss are examples of foliated rocks.

Foliated Rocks• Slate forms from shale and has

very thin bands.

• Slate contains a lot of mica, which has good cleavage.

• Therefore, slate splits easily along its bands into sheets of rock.

• This property makes a good material for tiles.

Foliated Rocks• Gneiss forms from granite and other rocks

as mica and other minerals rearrange bands.

• Gneiss does not have as much mica as slate does and does not split as well.

Nonfoliated Rocks

• Rocks that are largely of only one material, such as limestone and sandstone, result in metamorphic rocks without bands, called nonfoliated rock.

• The crystals combine and interlock to form a harder rock.

• For example, marble is a nonfoliated rock.

Nonfoliated Rock

• Marble is a nonfoliated rock formed from limestone.

• Pure marble is white.

• Small amounts of various minerals produce the colorful streaks and swirls in some marbles.

How Igneous Rocks Form

• Deep below the earth’s surface, between 50 and 200 kilometers down, temperatures are about 1,400ºC.

• These temperatures are high enough to melt minerals, forming hot, liquid rock called magma.

• Magma sometimes rises toward the surface of the earth through openings in the rock.

• As the magma rises, it cools and hardens, becoming igneous rock.

• Igneous rocks can form on the earth’s surface or below it.

Intrusive and Coarse-Grained Rocks

• Many intrusive rocks have large crystals because magma deep below the surface cools slowly.

• The size of the mineral crystals in an igneous rock is called the rock’s texture.

• Rocks with large crystals are said to have a coarse-grained texture.

• Therefore, most intrusive rocks have a coarse-grained texture.

Intrusive and Coarse-Grained Rocks• One of the most common intrusive rocks is

granite.• The minerals in this coarse-grained rock are

large enough for you to see.• Find feldspar, quartz, and mica that make up the

granite in the photo.

Intrusive and Coarse-Grained Rocks

• Most of the rock that forms the foundations of the continents is granite.

• Granite might be gray, pink, or red, depending on the kind of feldspar in it.

• Granite is used for buildings and monuments because it is strong and can be highly polished.

Extrusive and Fine-Grained Rocks

• Extrusive rocks have small crystals because lava cools too quickly for larger crystals to form.

• Therefore, most extrusive rocks have a fine-grained texture.

Extrusive and Fine-Grained Rocks

• The most common extrusive rock is basalt.

• The Hawaiian Islands are made of basalt, resulting from volcanic eruptions.

• Most of the exposed rock of eastern Washington and Oregon is also basalt.

• It was formed during lava flows that spread away from large cracks in the ground.

Extrusive and Fine-Grained Rocks

• Sometimes lava cools so quickly that no crystals have time to form.

• Such is the case with obsidian.

• The lack of any crystals gives obsidian a glassy texture.

• Obsidian forms in lava flows or from clots of lava thrown from a volcano.

Extrusive and Fine-Grained Rocks

• Like glass, obsidian can be chipped to make a very sharp edge.

• Some native peoples used obsidian to make knives, arrowheads, and ornaments.

Igneous Rock Texture

Did you Know

• Gases sometimes escape from the top layer of lava so quickly that the lava swells into foam.

• The foam hardens into a rock that looks like a sponge.

• This rock is called pumice.• It also has so many holes that it floats in water.• Powdered pumice is used in making abrasive

soaps. Many toothpastes contain pumice.