Welcome to You Ask Andy

Arnold Duncan Jr., age 14, of Dotham, Ala., for his question:

HOW MANY KINDS OF ROCK DO WE HAVE ON EARTH?

Rock is the hard, solid part of the earth's crust. Soil itself is made up of tiny bits of rocks usually mixed with organic materials from plants and animals. Sand is rock that has been crushed fine. Rock can also be found under the oceans and the frozen polar icecaps.

There are only three main kinds of rocks found on earth: igneous rocks, sedimentary rocks and metamorphic rocks. Combinations of minerals, however, put the varieties of rocks into the thousands.

Igneous rocks formed when the molten rock materials deep within the earth cooled and solidified. Extrusive igneous rocks form when the magma, as the molten rock is called, is forced to the surface of a volcano and becomes lava. Intrusive igneous rocks form when magma hardens slowly beneath the surface of the earth.

Sedimentary rocks are made up of materials that once were part of older rocks or of plants and animals. These rocks were deposited millions of years ago as strata or layers of loose material. Most of the deposits settled on ocean floors.

Metamorphic rock is rock that has changed its appearance, and sometimes its mineral composition. These changes may be caused by hot magma, pressure and heat from mountain building movements in the earth or the chemical action of liquids and gases.

All kinds of rocks, including igneous and sedimentary, have gone through such metamorphism to produce metamorphic rocks.

Granite, for example, is an igneous rock that contains quartz, feldspar and mica. Metamorphism caused feldspar and quartz crystals to form layers between which mica crystals often lie in wavy bands.

Most rocks are aggregates, or combinations, of one or more minerals. Metals such as aluminum, iron, lead and tin come from rocks that we call ores. Ores also supply us with such radioactive elements as uranium and radium.

Some rocks contain nonmetallic minerals such as borax and graphite. All gems, except amber, coral and pearl, come from rocks.

Soft shales and clays harden to form slate, a rock that easily splits into smooth slabs. Impure sandstone, limestones and shales change into schists, a class of crystalline rocks.

Eight elements make up more than 98 percent of all the rocks in the world. These elements are found in about the following percentages: oxygen with 46.5, silicon with 27.6, aluminum with 8.0, iron with 5.0, calcium with 3.6, sodium with 2.8, potassium with 2.6 and magnesium with 2.0.

Although most rocks cannot be bent or squeezed out of shape, one kind, pumice, floats on water. Pumice once was volcanic lava filled with gases. When the gases left the rock, it left millions of holes that filled with air.

 

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