Edward Gottschald, age 15, of Baltimore, Md., for his question:
JUST WHAT IS ROCK?
Rock is the solid material that makes up the earth's crust. It can be as hard as granite or as soft as soapstone. All rocks are combinations of various minerals in different amounts.
Some of the common rock making minerals are feldspar, quartz, mica and iron. A number of rocks contain such metallic minerals as gold, silver, copper or iron.
Ore is rock that holds large amounts of useful metals.
Rocks are put into groups according to the way they were formed: sedimentary, igneous and metamorphic.
Sedimentary rocks are those that were formed from the breakdown of older rocks. Rock changes when it is attacked by the forces of nature. Wind, frost, glaciers, water and even air break up rock, wear it way, move it and dissolve it. The weathering or decaying of rock is an ongoing thing.
Some sedimentary rocks are made of the organic deposits, such as coal and diatomite, which were plants, and certain limestones, which were animal.
Sedimentary rocks can also be made from the mineral grains in rocks that were dissolved by rain. In time, the minerals can harden into rock.
Igneous rocks were once molten masses, or magma, located deep within the earth's crust. While under pressure, hot rock is a solid. But when there is a lessening of pressure, the hot rock melts and becomes magma. It can then flow to the earth's surface.
The word igneous comes from the Latin word that means "born of fire."
Metamorphic rock is either igneous or sedimentary rock that has been changed from its original form to another. The changes happen deep in the earth without having the rock melt. These changes can be caused by great temperature changes, pressure or hot water.
Under such conditions a rock may change from one crystal form to another. The rock may even change its mineral composition.
Scientists tell us that rocks form over great periods of time. Some rocks may take millions, hundreds of millions or even billions of years to form.
Through the centuries, sedimentary, igneous and metamorphic rock masses have been built up and then broken down time after time. The action of rock making and rock decay goes on and on is continuing cycles.
Because rocks have been going through these cycles of change since the beginning of time, billions of years ago, the earth's present crust is made up of rocks of all kinds and of all ages.
Extrusive rocks are found in or near volcanoes. Lava rocks are hard and fine grained, or glassy. Volcanic ash is usually soft and porous.
Intrusive rocks contain crystals big enough to be seen with the naked eye. The grains show crystal faces and are often evenly sized. Granite is an example.