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This 500 million year old trilobite fossil is about 1 inch long. It was found in Utah.
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mikeuk/iStockphoto
Rocks, Minerals, and Fossils
From Grolier's New Book of Popular Science
A geologist studies a rock outcrop.
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Scott Orr/iStockphoto
Rocks are subjected to a variety of physical and chemical conditions such as heat, weather, and erosion, which can change rocks from one kind to another.
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Grolier

Growing up during the 1950s, Jack Horner wandered the hills and gullies of Montana, head down and eyes open, looking at rocks. At age 8, he found his first dinosaur bone. Today Horner is the world's best-known "dinosaur hunter." He searches for their fossil remains in rock beds from Mexico to Mongolia.

Although few people will pursue rock or fossil hunting as a career, most everyone has, from time to time, stooped down to pick up an unusual rock, a pretty crystal, or a fossilized shell. This is hands-on geology, because the history of Earth is contained in its rocks, minerals, and fossils. Learning about their formation and forms can enhance one's appreciation of Earth's geologic history, and perhaps inspire a lifelong hobby.

ROCKS

Although almost everyone knows what a rock looks like, far fewer people can name its defining characteristics. A rock can be defined as a naturally formed solid that consists of particles of one or more natural substances such as mineral grains, glass, and fragments of plant debris. By the term "naturally formed," geologists exclude from this definition human-made solids such as concrete.

With the term "solid," most geologists also exclude loose substances such as silt, clay, and sand from their definition of a rock. But sometimes it can be difficult to distinguish a loose material such as sand from a solid rock such as sandstone. To simplify matters, many geologists follow a simple rule: if you need a hammer to break it, it is a rock. This is not to say that rocks always remain in a solid state. Under great pressure, rocks can melt. Indeed, natural glass—a common type of rock—is formed by supercooling molten rock, or magma.

Most rocks consist of a mixture of more than one substance, usually a variety of minerals and organic substances. Coal, for example, typically contains fragments of ancient plant debris, called macerals, mixed with assorted mineral grains. Most volcanic rocks contain a mixture of mineral grains and glass. Sometimes particles of a single mineral form the bulk of a rock, as calcite does in limestone.

Rocks form the outer layer, or crust, of Earth. The rocks that can be seen on the surface or reached by digging through the soil are called bedrock. Over the centuries, geologists have mapped the bedrock of regions large and small. They have found that rocks of similar types underlie many large regions of the world. From these, geologists have developed three basic categories of rock, distinguished by the processes that form them.

Igneous rock forms when molten rock below Earth's surface cools enough to solidify. The crust of the primitive Earth is believed to have once consisted entirely of igneous rock.

Sedimentary rock, as its name implies, consists of sediment—particles of dissolved and weathered rock, as well as the remains of decayed plants and animals. Typically, sedimentary rock forms when such particles collect in oceans, lake beds, and other places, where they become cemented together in layers over hundreds or thousands of years.

Metamorphic rock forms when preexisting rock sinks, without melting, deep into Earth, where it is twisted and deformed or chemically changed by heat, pressure, or chemicals.

The processes that can transform one type of rock into another comprise the rock cycle. Traditionally the cycle is said to begin with igneous rock, which is weathered to sedimentary rock, and then transformed into metamorphic rock. But shortcuts in this cycle often occur. For example, a metamorphic rock may be weathered away to sediment without passing through the igneous stage. Similarly, igneous rock deep in the ground may be deformed into metamorphic rock without passing through the sedimentary stage.

Igneous Rock. As magma cools, the elements flowing within it slow their random movement. Gradually, they begin to arrange themselves into orderly patterns. This process of crystallization forms igneous rock.

Most igneous rocks are hard and compact with little visible layering. They consist primarily of silicate minerals. Silicon, oxygen, aluminum, sodium, potassium, calcium iron, and magnesium are the primary elements found within igneous rocks.

The type of igneous rock that forms from these crystallized elements depends largely on the speed at which magma cools. When magma cools very rapidly, its elements have no time to form crystals. Instead, they solidify in an unordered, or amorphous, arrangement. The resulting rock is called a glass, which is quite similar in form to human-made glass. Obsidian is a common, and often beautiful, type of natural glass. More often, magma cools slowly enough to allow some degree of crystallization. In general, the slower the magma cools, the larger the size of its crystalline grains.

Igneous rocks with very fine grains are called aphanites. They typically form when magma cools quickly in the upper crust or at Earth's surface. (Magma is called lava once it is aboveground.) The individual mineral grains in aphanitic rock are too small to be seen by the naked eye. Basalt is a common dark-colored aphanite. Rarer examples of aphanites include rhyolites, which range in color from white to gray with pinkish and purple tints.

Large masses of magma deep beneath the surface of Earth tend to cool much more slowly—sometimes over tens of thousands of years. Such conditions often yield large-grained rocks called phanerites. The large, intergrown crystals of a phaneritic rock are often visible to the naked eye. Granite, a common phaneritic rock, generally contains large, visible crystals of quartz, feldspar, and mica.

All of the minerals within a large mass of magma do not necessarily crystallize at the same rate. As a result, it is common to find igneous rocks containing crystals of different sizes. A rock that has large crystals embedded in much smaller ones is called a porphyry.

Another type of igneous rock is made from ash and fragments expelled during a volcanic eruption. When these fragments cement together, they form pyroclastic rocks. Pyroclastic rock made primarily of fused ash is called tuff, while that composed of larger particles is called volcanic breccia.

Sedimentary Rock. Most of the rock that is exposed on Earth's surface is sedimentary. When cut or exposed, most sedimentary rock has obvious layering. Within these rock layers, one can find the remains of past environments, from rock fragments to fossils.

The process that transforms loose fragments into solid rock is called lithification. It can occur through compaction, the intense compression of sediment grains by the weight of overlying material. Shale is an example of compacted rock. In another common process called cementation, dissolved materials fill the open spaces between grains, and then solidify to cement them together. Calcite, silica, and iron oxide are common examples of such cementlike material. Many sedimentary rocks are formed by a combination of compaction and cementation.

There are two main classes of sedimentary rock—detrital and chemical. Detrital sedimentary rock consists of the cemented-together fragments of minerals and other rocks, primarily clay, quartz, feldspar, and mica. The most abundant detrital sedimentary rock is shale, a crumbly rock made of extremely fine, microscopic-sized particles. Also common is sandstone, which consists of coarser, "sand-sized" grains. A third type of detrital rock is called conglomerate; it consists of larger rock fragments, typically the size of gravel, but occasionally as large as boulders.

Chemical sedimentary rocks are formed from material that was once dissolved in lake or ocean water; limestone is the most common of this variety. It consists primarily of the mineral calcite, mostly from the broken-down fragments of marine shells and skeletons. Chalk is a type of limestone composed of the hard remains of microscopic sea creatures. Other types of limestone form inorganically, from dissolved calcium carbonate. Travertine, for example, is an inorganic limestone often seen in caves. Evaporites are a type of chemical sedimentary rock made of the solid compounds left behind when water evaporates. They include common salt, Epsom salts, and gypsum.

Metamorphic Rock. The term "metamorphic" means to "change form." Metamorphic rock arises from the transformation of preexisting rock that has sunk deep within Earth's crust. There the rock likely encounters heat, pressure, and active chemicals. Under these conditions, rock material becomes "plastic" and often bends into folds. An important characteristic of metamorphic rock—one that distinguishes it from igneous rock—is that it never actually melts during its transformation.

Metamorphism occurs when tectonic or volcanic forces warp and bake surrounding rock. Most metamorphic rock is created during the process of mountain building—when entire regions of Earth's crust are warped upward. On a smaller scale, hot, mineral-rich waters in the ground can transform the minerals in a rock into completely new substances.

Marble is a metamorphic rock made of transformed limestone or dolomite. Quartzite is an especially hard metamorphic rock whose parent rock was quartz sandstone. Slate, schist, and gneiss are transformed shales.

MINERALS

In poetic terms, a mineral is to a rock as a tree is to a forest, or as a rose is to a flower garden. In other words, minerals are pure substances, and rocks contain various mixtures of them. In its natural state, a mineral is called an ore. This ore may be mined, cut, and polished to make a gem.

By definition, a mineral is a natural solid, generally formed by inorganic (or nonbiological) processes, with an ordered internal structure and a specific chemical composition. Some geologists expand this definition to include substances such as coal that are formed from the ancient remains of plants and animals.

A mineral's ordered internal structure derives from an orderly stacking and packing of atoms and elements, and it gives a mineral its characteristic crystal shape. Under different conditions, a given element can assume more than one crystalline form; a single element can thus form two or more minerals with vastly different qualities. Graphite and diamond, for example, both consist of pure carbon. Graphite—the material used in lead pencils—is dark and soft. By contrast, diamonds are the hardest mineral known, and their clear, sparkling beauty is world-renowned.

How Minerals Are Formed. Minerals arise in various ways and in vastly different environments. Quartz, mica, and feldspar form as crystals in cooling magma. The steam and mineral-rich water arising from such magma may deposit other minerals such as gold and wolframite. And volcanic gases and mineral water venting through the surface can condense to form such minerals as salts and sulfur.

The evaporation of a lake or sea leaves behind various deposits of minerals that had once been dissolved in the water. Vast amounts of familiar minerals—salts, sulfates, borates, carbonates, and nitrates—form in this way.

Certain very durable minerals are left behind when wind and water erode the softer material from a bed of sedimentary rock. These minerals include, among others, silica, clay, and aluminum ore (bauxite).

Most natural crystals—including the most beautiful and the most sought-after mineral ores—form where their individual atoms can exist for a time as a liquid or gas. Such a free-moving state enables the atoms to become arranged in orderly patterns as they undergo the solidification process.

Such conditions often exist where hot, mineral-rich water or steam circulates through deep cracks and cavities in the rock. The minerals condense out on the surfaces of these openings, where they grow into crystals. This type of mineral deposit is called a vein. Gold and quartz are commonly found in veins. Sometimes mineral crystals fill a hollow inside a boulder or stone, to form a geode.

Physical Properties of Minerals. The chemical composition and internal structure of a mineral is reflected in its unique array of physical properties. Mineralogists—both amateur and professional—use these observable qualities to identify and classify the minerals they study.

A mineral's characteristic crystal structure reflects the orderly arrangement of its atoms. Not all minerals exhibit an obvious crystal with well-formed faces. Only those minerals allowed to crystallize uninterrupted and without space restrictions form the distinctive crystals so highly valued by collectors. Usually minerals contain a multitude of tiny, intergrown crystals.

A mineral's hardness depends on the spacing between its atoms; this property is often described in terms of the mineral's resistance to scratching. When describing and identifying minerals, mineralogists compare them to 10 representative types, arranged from softest (1) to hardest (10) in Mohs' scale:

1) talc;  2) gypsum;  3) calcite;  4) fluorite;  5) apatite;  6) feldspar;  7) quartz;  8) topaz;  9) corundum; and  10) diamond.

Cleavage is the tendency of a mineral to split along one or more planes. Minerals that exhibit cleavage will break into similarly shaped pieces. Mica, for example, cleaves in one direction and commonly occurs in thin, flat sheets. A cubic fluorite crystal tends to break evenly along eight planes to produce octahedrons. Minerals that do not exhibit cleavage shatter into irregular pieces. Fracture is different from cleavage in that the break is unrelated to any plane or internal structure. Minerals that fracture when broken tend to produce dissimilarly shaped pieces, although some break into splinters or fibers.

Specific gravity represents the weight of a mineral as it compares to the weight of an equal volume of water. Experienced mineralogists can estimate the specific gravity of a mineral with surprising accuracy, simply by holding the substance in their hands. For comparison purposes, a chunk of common rock has a specific gravity of about 3, while gold has a specific gravity of about 20.

Color is an obvious mineral quality, but it can be misleading for identification purposes. Many minerals have characteristic, and often striking, colors. Sulfur, for example, is nearly always yellow, and malachite is bright green. But mineral color can be offset by various impurities. Quartz, for example, takes on a multitude of colors, from pink to purple to black, depending on the impurities present.

Luster refers to the way the surface of a mineral reflects light. A mineralogist may describe a particular specimen's luster as metallic, resinous, silky, pearly, diamondlike, or dull.

Kinds of Minerals. There are more than 3,000 kinds of known minerals in the world, with more still being discovered. But most are quite rare. In fact, 95 percent of Earth's crust is made up of about 20 different minerals.

Silicon and oxygen are far and away the most abundant elements in Earth's crust. In combination, they form the largest mineral group: the silicates. All silicates contain oxygen and silicon mixed with one or more other elements. The one exception is quartz, which is a pure oxygen-silicon combination.

Carbonates are another common mineral group. Carbonate minerals consist of carbon combined with oxygen and mixed with various metals. Calcite, which forms limestone, is the most abundant of the carbonates. Like limestone, carbonates are soft and dissolve easily in acidic water.

Compared with silicates and carbonates, other mineral groups are quite scarce. However, many of these less common minerals are commercially important or are treasured as gems. Among such valuable minerals are halides, or salts, in which various metals are combined with a halogen element like fluorine, chlorine, bromine, or iodine. Notable examples of halides include common salt (sodium chloride) and fluorite, which is used in steelmaking. Sulfates consist of sulfur and oxygen combined with various metals. Gypsum, used in making plaster, is a familiar sulfate. Oxides consist of oxygen mixed with various metals. Many important metal ores, including iron, copper, and titanium, exist in an oxide form.

FOSSILS

Fossils are the remains or impressions of plants and animals. Their appearance in exposed rock has fascinated humans throughout history. Today, scientists known as paleontologists study fossils and their placement in rock to construct a history of life on Earth.

Fossils are generally found in sedimentary rock. This occurs because the remains of plants and animals tend to accumulate with the inorganic debris that form such rock. In rare instances, fossils can form in igneous rock, as when volcanic ash or lava congeals around a leaf or insect wing, or fills a footprint. Fossils can also be found in metamorphic rock that has not been deformed enough to destroy the fossil.

Only a tiny fraction of the world's organisms end up preserved as fossils. Almost invariably, their soft parts are quickly decayed or eaten. Even bones and shells usually crumble within a few years. A host of special processes and circumstances must occur for a life-form to be preserved, or fossilized, for thousands or millions of years.

The Process of Fossilization. Fossilization is not an "equal-opportunity" process. The organisms most likely to be preserved have hard parts—such as teeth, bones, or shells—that will last long enough for fossilization to begin. The environment in which the organism dies is also important. On land, very cold and dry environments are more "preserving" than warm, damp places, where decay is rapid. But an underwater environment—especially an oceanic one—is the best of all, because an organism can quickly drop to the cold seabed and be covered with silt. As a consequence, the most common fossils are those of ancient seashells and other hard-bodied sea creatures.

The most widespread type of fossilization is called mineralization. This process, also known as petrification, occurs when an organism is left in sediment saturated with mineral-rich water. As a result, minerals fill in the pores and empty spaces of the organism's hard parts, or replace the original material entirely. The resulting fossil is like a plaster cast or mold of the original.

A common fossilization process for plants and soft-bodied animals such as jellyfish is carbonization. This occurs when the chemical decay of an organism's soft parts leaves a fine carbon film, or imprint, in the fine-grained sediment in which it was buried.

Another type of fossilization occurs when an organism in mineral-rich water becomes encrusted with a substance such as the mineral travertine. Although the organism itself dissolves, its imprint remains in the mineral crust.

The actual body of a plant or animal is seldom preserved in its unaltered state. One rare and fascinating exception occurs in a process called amberization. Occasionally an insect, pollen grain, or other small life-form becomes embedded in a glob of sticky tree resin. If the resin is then buried, it may be converted, over a period of millions of years, into a rock-hard, yellowish-clear substance known as amber. In recent years, scientists have been able to extract genetic material from ancient insects preserved in amber.

Another unusual type of "entombment" occurs when an animal or plant falls into natural tar or asphalt. The famous La Brea Tar Pits in Southern California is the most famous example. Two million years ago, mammoths, saber-toothed tigers, and other animals came to the tar pits to drink the water that collected over them. Stuck in the sticky tar and asphalt, the animals eventually sank to the bottom of the pools, where their bones became impregnated and preserved with the tar.

COLLECTING ROCKS, MINERALS, AND FOSSILS

A great way to learn about collecting rocks, minerals, and fossils is to get out into the field with an experienced collector. Natural-history museums, nature centers, and schools often organize such outings.

Whether one collects in a group or alone, it is important to keep common sense and consideration in mind. Irresponsible collecting has destroyed many of the world's most important collecting areas and robbed scientists of important specimens. When it comes to responsible collecting, two general rules apply:

 

  • Always get permission from landowners, quarry operators, or other authorities before collecting on their properties.
  • Take care to cause minimum impact on the environment. This means not hammering away at rock beds or veins when loose material is readily available. Take only what is needed, fill in any newly created holes, and clean up after collecting.

 

For safety's sake, remember that steep ground and cut slopes—although ideal for collecting—are often unstable. Collectors should be especially aware of the surroundings in situations where falling rock is likely.

Tools of the Trade

Fortunately, collecting equipment is minimal and relatively inexpensive. When hammering rock, it is important to use only a geologic hammer or pick (available at many hardware stores). Made of tempered steel, they are resistant to dangerous shattering and splintering. Other important safety gear includes protective goggles, work gloves, and hard-toe boots. Collectors should wear a hard hat wherever falling rock is possible.

Other useful items include a notebook for recording date, precise location, specimen description, and other details; collecting bag and wrapping material such as newspaper; hand lens for examining small specimens; shovel (for areas of soft sediment); collecting guidebooks; camera; and topographic or geologic maps of the area.

Where to Go and How to Look

Exposed rock is the collector's fruitful ground. Many interesting minerals and fossils can be found on weathered slopes, riverbanks, and cliff faces. Human-made excavations such as road cuts and construction or quarry sites can be excellent sites for mineral hunting. When working on a slope, amateur (and professional) geologists usually start by closely examining the rubble at its base.

Rocks and minerals.  The local geology of an area determines what rocks and minerals are likely to be found there. Topologic and geologic maps can greatly aid a search. A list of such maps is available from the U.S. Geological Survey (USGS). For areas east of the Mississippi, maps can be obtained from the USGS Distribution Section, 1200 South Eads Street, Arlington, VA 22202. For areas west of the Mississippi, such material can be acquired at the USGS Denver Distribution Section, Federal Center, Denver, CO 80225.

When looking for minerals, scan the face of a road cut or cliff for streaky seams and fissures and unusually colored pockets and cavities. Natural spaces in the rock provide an opportunity for minerals to grow into sizable specimens. Nodules of rock sometimes reveal pockets of crystals (geodes) when broken open. With experience, collectors learn to recognize likely geodes.

Fossils.  As mentioned, fossils are most abundant in sedimentary rock. Fossils are particularly common in limestone and shale, which are often uplifted beds of ancient seas that contain abundant shells. Look for unusual bumps and discolored impressions in the rock. When collecting in shale, try splitting the flat rock sheets apart to find out if fossil impressions are hidden between the layers.

Removing, Preserving, and Identifying Specimens

When extracting a rock-encased fossil specimen, always take a generous amount of surrounding material with it. This material should be carefully wrapped so the specimen's removal can be completed at home. Paleontologists use a variety of brushes, scalpels, and fine chisels to gently remove the dirt and rock encasing a fossil. Good "home" tools for the amateur paleontologist include toothbrushes, dental picks, and needles.

A simple cleaning procedure is to let a specimen soak overnight in a solution of soap and water. However, some specimens, such as pyritized fossils and chloride or sulfate minerals, decay or spoil when wet. They therefore should be carefully cleaned with dry brushes and picks, and then preserved in sealed plastic bags with a few grains of silica gel to absorb any unwanted moisture.

Once a rock, mineral, or fossil is clean enough, it can be identified using an appropriate field guide (widely available in libraries and bookstores). The previous sections on rocks and minerals can aid in identification by general attributes.

After identifying a specimen, collectors should label it with a name card or number, and enter a more complete description in a personal catalog. An entry should include the name of the specimen, its collection site, and other descriptive information.

Jessica Snyder Sachs