| The Earth is made of rock Most rocks at the Earth's surface are formed from eight elements oxygen, silicon, aluminum, iron, magnesium, calcium, potassium, and sodium. These eight elements are combined in a countless number of ways to make rocks that are very different. From the tallest mountains to the floor of the deepest ocean. Thousands of different types of rocks and minerals. The distribution of rocks is a unique and wonderful natural phenomenon. 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. |
| Types of Rocks: Igneous rocks, Sedimentary rocks, Sedimentary rocks. Geologists classify rocks in three groups, according to the major Earth processes that formed them. The three rock groups are igneous,sedimentary, and metamorphic rocks. Anyone who wishes to collect rocks should become familiar with the characteristics of these three rock groups. Knowing how a geologist classifies rocks is important if you want to transform a random group of rock specimens into a true collection. 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.... to Metamorphic rocks.... Metamorphic Rocks: 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. | Starting a Collection:
A good rock collection consists of selected,representative, properly labeled specimens. The collection can be as large or as small as its owner wishes. An active collection constantly improves as specimens are added or as poor specimens are replaced by better ones. A rock collection might begin with stones picked up from the ground near your home. These stones may have limited variety and can be replaced later by better specimens. Nevertheless, this first step is helpful in training the eye to see diagnostic features of rocks (features by which rocks can be differentiated). As you become more familiar with collecting methods and with geology, the collection will probably take one of two directions. You may try either to collect as many different types of igneous, sedimentary, and metamorphic rocks as possible or to collect all the related kinds of rocks from your own particular area. Identifying Rocks: Many books about geology explain the identification and classification of rocks and describe the underlying geologic principles. Almost any recent general book on geology would help a rock collector. Geologic maps, which are useful guides for collecting, are also excellent identification aids. They show the distribution and extent of particular rock types or groups of rock types. Depending on size and scale, the maps may cover large or small areas. Most have brief descriptions of the rock types. Some are issued as separate publications; others are included in books. Most geologic maps are issued by public or private scientific agencies. The U.S. Geological Survey (USGS), "Geologic and Water-Supply Reports and Maps" is a series of booklets published by the USGS, provides a ready reference to these publications for 13 States, (did you notice the number cool but don't ask me -OK!). |
Where to Find Rocks:
Collections usually differ depending on where the collector is able to search for rocks.In the great interior plains and lowlands of the U.S., a wide variety of sedimentary rocks are exposed. Igneous and metamorphic rocks are widespread in the mountains and Piedmont areas of New England, the Appalachians, the Western Cordillera, and scattered interior hill lands; Igneous rocks make up almost all the land of Hawaii. Atlantic and Gulf Coastal Plains, loose and unconsolidated rocks are widespread; Northern U.S., glaciers deposited many unconsolidated rocks. The best collecting sites are rock cliff edges where there is breakage, 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. I like to go after a storm landslide (if danger is past) or after some earth movement natural or other. Look for unusual rocks to study in large buildings or in cemeteries. Dimension stone blocks and monument stone are often transported long distances from where they are quarried. Polished stone sometimes looks different from unpolished rock. This provides good identification practice. Join a mineral club or subscribe to a mineral magazine. Collecting Equipment:
The beginning collector needs a few pieces of geologist's hammer, the head of a geologist's hammer has one blunt hammering end, magnification hand lens padded carry case to remove from the field fragile specimins, or containers of some sort ( just use foam packing scraps), you can use a shoe box to carry rocks out in, You many want to use chisel depending again on what your looking for. Heavy hunting Dilute hydrochloric acid helps in identifying limestone and dolomite. You have to decide for each expedition which tools are worth the weight. A chisel would be used mostly in soft sedimentary rocks and in some fossil collecting . Of course your knapsack, food; pen & notebook for keeping field notes, a packet knife, especially useful to test the hardness of mineral grains. Sledge hammers are 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. And a water spray bottle is useful as well. Always ask for permission to collect rocks on private property. Do not collect rocks in national parks and monuments or in State parks; it is illegal. Similar rocks commonly crop out on land nearby. Use proper caution depending on location. It's generally good to bring a friend, and more fun. Alert someone to where you are going and when you plan to return along with the planned route.
| | Housing and Enlarging a Collection:
The practical problems of cataloging and storing a collection must be considered and every collector will have their own way. Housing arrangements can be very simple because rocks are durable and do not require special treatment. I display mine in low teracotta dishes intended for gardening, crystal bowls and plates, and scatter them on the altars in my house. Others I put in wicker baskets. Each sorted by collection type or location of collection - sort of. There is no right or wrong way. | | |
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