Rocks are continually changing. Wind and water wear them down and carry bits of rock away; the tiny particles accumulate in a lake or ocean and harden into rock again. The oldest rock that has ever been found is more than 3.9 billion years old. The Earth itself is at least 4.5 billion years old, but rocks from the beginning of Earth's history have changed so much from their original form that they have become new kinds of rock. By studying how rocks form and change, scientists have built a solid understanding of the Earth we live on and its long history.
Igneous rocks are formed from melted rock that has cooled and solidified. When rocks are buried deep within the Earth, they melt because of the high pressure and temperature; the molten rock (called magma) can then flow upward or even be erupted from a volcano onto the Earth's surface. When magma cools slowly, usually at depths of thousands of feet, crystals grow from the molten liquid, and a coarse-grained rock forms. When magma cools rapidly, usually at or near the Earth's surface, the crystals are extremely small, and a fine-grained rock results. A wide variety of rocks are formed by different cooling rates and different chemical compositions of the original magma. Obsidian (volcanic glass), granite, basalt, and andesite porphyry are four of the many types of igneous rock.
Sedimentary rocks are formed at the surface of the Earth, either in water or on land. They are layered accumulations of sediments-fragments of rocks, minerals, or animal or plant material. Temperatures and pressures are low at the Earth's surface, and sedimentary rocks show this fact by their appearance and the minerals they contain. Most sedimentary rocks become cemented together by minerals and chemicals or are held together by electrical attraction; some, however, remain loose and unconsolidated. The layers are normally parallel or nearly parallel to the Earth's surface; if they are at high angles to the surface or are twisted or broken, some kind of Earth movement has occurred since the rock was formed. Sedimentary rocks are forming around us all the time. Sand and gravel on beaches or in river bars look like the sandstone and conglomerate they will become. Compacted and dried mud flats harden into shale. Scuba divers who have seen mud and shells settling on the floors of lagoons find it easy to understand how sedimentary rocks form.
Sometimes sedimentary and igneous rocks are subjected to pressures so intense or heat so high that they are completely changed. They become metamorphic rocks, which form while deeply buried within the Earth's crust. The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks. New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks. Some kinds of metamorphic rocks--granite gneiss and biotite schist are two examples--are strongly banded or foliated. (Foliated means the parallel arrangement of certain mineral grains that gives the rock a striped appearance.) Pressure or temperature can even change previously metamorphosed rocks into new types.
Rock-forming and rock-destroying processes have been active for billions of years. Today, in the Guadalupe Mountains of western Texas, one can stand on limestone, a sedimentary rock, that was a coral reef in a tropical sea about 250 million years ago. In Vermont's Green Mountains one can see schist, a metamorphic rock, that was once mud in a shallow sea. Half Dome in Yosemite Valley, Calif., which now stands nearly 8,800 feet above sea level, is composed of quartz monzonite, an igneous rock that solidified several thousand feet within the Earth. In a simple rock collection of a few dozen samples, one can capture an enormous sweep of the history of our planet and the processes that formed it.
Most geologic maps are issued by public or private scientific agencies. The most prolific publisher of geologic maps in the United States is the U.S. Geological Survey (USGS). "Geologic and Water-Supply Reports and Maps, (State)," a series of booklets published by the USGS, provides a ready reference to these publications for 13 States. The booklets also list libraries in the subject State where USGS reports and maps may be consulted. These booklets are available for less than $5.00 and may be obtained from:
USGS Information Services
US Geological Survey
Comparing one's own specimens with those in a museum collection can help in identifying them. Most large rock collections are well labeled. Small rock collections abound in libraries, schools, public buildings, small museums, and private homes.
The best collecting sites are quarries, road cuts or natural cliffs, and outcrops. Open fields and level country are poor places to find rock exposures. Hills and steep slopes are better sites. Almost any exposure of rock provides some collection opportunities, but fresh, unweathered outcrops or manmade excavations offer the best locations. If possible, visit several exposures of the same rock to be sure a representative sample is selected.
The hand lens, sometimes called a pocket magnifier, is used to identify mineral grains. Hand lenses can be purchased in jewelry stores, optical shops, or scientific supply houses. Six-power to ten-power magnification is best. Optically uncorrected hand lenses are inexpensive and quite satisfactory, but the advanced collector will want an optically corrected lens.
Other pieces of necessary equipment are inexpensive and easy to find: a knapsack to carry specimens, equipment, and food; bags and paper in which to wrap individual specimens; a notebook for keeping field notes until more permanent records can be made; and a pocket knife, helpful in many ways, especially to test the hardness of mineral grains.
On some collecting trips, additional equipment is needed. Sledge hammers can be used to break especially hard ledges of rock. Cold chisels often make it possible to loosen specimens. Dilute hydrochloric acid helps in identifying limestone and dolomite. A long list could be made of such equipment; the collector must decide for each expedition which tools are really worth the weight.
It is important to have a careful system of permanent labeling so that specimens do not get mixed up. Many people paint a small oblong of white lacquer on a corner of each specimen and paint a black number on the white oblong. The number, rock name, collector's name, date collected, description of collection site, geologic formation, geologic age, and other pertinent data are entered in a small notebook. If rocks are kept on separate trays, a small card containing some data is usually placed in the tray.
Extra specimens are sometimes used for trading with other collectors. Few people have the opportunity to obtain all varieties of rock types, and exchanging can fill gaps in a collection. Collectors interested in trading are usually located by word of mouth. No nationwide organization of rock collectors exists, though local clubs and individual collectors are found throughout the United States. It may be necessary to buy some specimens, but good specimens are expensive.
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Last modified 06-07-12 (jmw)