Rock
Rock is a hard, solid material that makes up part of Earth. In many places, the rock is covered by a layer of soil in which plants or trees may grow. Soil is made up of tiny bits of rocks mixed with materials from dead plants and animals. Rock also lies under the oceans.
Where highways cut through hills, you can often see layers of rock in the hillsides. Many rivers cut through rock to form canyons. Great cliffs of rock line the seashore in such places as the state of Maine and the country of Norway. In desert regions, cliffs and towers of rock may rise high above the sandy plains.
Types of rocks
Most rocks are made up of one or more minerals. The three main kinds of rocks are igneous <<IHG nee uhs>> rocks, sedimentary <<SEHD uh MEHN tuh ree>> rocks, and metamorphic <<meht uh MAWRF ihk>> rocks.
Igneous rocks start out as melted rock, called magma, deep within Earth. Earthquakes and other movements of Earth's crust sometimes force the magma to the surface. Igneous rocks form when the magma cools and hardens. A dark greenish-gray or black rock called basalt, a black glassy rock called obsidian, and granite are examples of igneous rocks.
Sedimentary rocks are made of layers of loose materials that were once part of older rocks or dead plants and animals. Most of these layers form on ocean floors, but some form on land and in fresh water. Over time, the loose materials harden into solid rock. Sedimentary rocks include limestone, sandstone, and shale.
Metamorphic rock is igneous and sedimentary rock that has been changed by pressure and heat deep within Earth's crust. This pressure and heat change the way the rock looks. In many cases, new minerals form in the rock. Marble and slate are metamorphic rocks.
Uses of rock
People use rocks and minerals in many ways. Builders use granite, marble, and other rocks. Cement made from limestone and other rocks is mixed with crushed stone to make long-lasting concrete used in buildings, dams, and highways.
Metals such as aluminum, iron, lead, and tin come from rocks called ores <<ohrz>>. Ore may lie close to Earth's surface or deep underground. In some places, large amounts of iron or copper ores make up entire mountains. Almost all gems come from rocks. Gems are beautiful stones such as diamonds and emeralds.
Rocks also provide information about Earth and its history. Scientists called geologists <<jee AHL uh jihsts>> learn the history of Earth by studying rocks. They figure out where to find oil by studying rock layers. Other scientists study fossils to learn about life on Earth millions of years ago. Fossils are the remains of plants and animals found in rock.
Rock collecting
Thousands of young people and grown-ups collect rocks and minerals as a hobby. Interesting rocks and minerals can be found in many places. Good places to hunt for rocks include mines, quarries, building sites, ocean cliffs and beaches, riverbanks, and the rocky sides of roads cut through mountains.
Rock collectors label each rock with the name of the place it was found, the date, and what kind of rock or mineral it is. Many rock collectors keep their rocks in shallow cardboard trays with small compartments, or sections. Many also list the names of all their rocks in a record book.
For more information, see these articles:
AluminumChalk
Coal
Copper
Coral
Fossil
Gem
Geology
Gold
Iron and steel
Lead
Marble
Silver
Soil
Tin
How to cite this article:
To cite this article, World Book recommends the following format:
External Web Sites specially selected by World Book Editors:
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Pebble Pups
http://rockhoundingar.com/pebblepups.html
Rockhounding Arkansas’s Web site has a special section for children to help you get started in collecting and identifying rocks.
Rockology 101
http://www.rogersgroupinc.com/ourcommunities/rockology/index.htm
Rogers Group, a company that produces crushed stone, has created this Web site of information and activities for classroom use, related especially to commercial rocks.
Rocks and the Rock Cycle
http://www.bbc.co.uk/schools/gcsebitesize/chemistry/
A diagram and simple text explain how rocks are formed.