Rocks, Minerals, and Fossils Rocks, Minerals, and Fossils From Grolier's New Book of Popular Science A geologist studies a rock outcrop. 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. 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 Fossil and Fossilization - examples, body, used, water, process, Earth, plants, type, form Fossil and Fossilization Fossil and fossilization Photo by: Bastos A fossil is the remains or traces of a once-living plant or animal that was preserved in rock or other material before the beginning of recorded history. The term also is used to describe the fossil fuels (oil, coal, petroleum, and natural gas) that have been formed from the remains of ancient plants and animals. It is unusual for complete organisms to be preserved. Fossils usually represent the hard parts, such as bones or shells of animals and leaves, seeds, or woody parts of plants. Fossils occur on every continent and on the ocean floor. Through paleontology (the scientific study of fossils), it is possible to reconstruct ancient communities of living organisms and to trace the evolution of species. Fossils of single-celled organisms have been recovered from rocks as old as 3.5 billion years. Animal fossils first appear in rocks dating back about 1 billion years.

The occurrence of fossils in unusual places, such as dinosaur fossils in Antarctica and fish fossils on the Siberian steppes, is due to the shifting of the plates that make up Earth's crust and environmental changes (such as ice ages) over time. The best explanation for dinosaurs on Antarctica is not that they evolved there, but that Antarctica was once part of a much larger landmass with which it shared many lifeforms. Formation of fossils Most fossils are found in sedimentary rocks, those rocks produced by the accumulation of sediment such as sand or mud. Wind and other weathering conditions wash away sediment on land, depositing it in bodies of water. For this reason, fossils of sea creatures are more common than those of land creatures. Land animals and plants that have been preserved are found mostly in sediments in calm lakes, rivers, and estuaries. The likelihood that any living organism will become a fossil is quite low. The path from the organic, living world to the world of rock and mineral is long and indirect. In the best conditions, fossilization will occur if an animal or plant dies and is quickly covered over with moist sediment. This prevents the animal or plant from being eaten by other organisms or from undergoing natural decay through exposure to oxygen and bacteria.

The soft parts of an animal or plant decay more quickly than its hard parts. Teeth and bones are therefore more likely to be preserved than skin, tissues, and organs. Because of this fact, most fossils come from the time period dating to almost 600 million years ago, when organisms began to develop skeletons and hard parts. Words to Know Amber: Transparent golden-brown resin fossil formed from hardened pine tree sap. Cast: Fossil formed when a mold is later filled in by mud or mineral matter. Geologic time periods: Episodes in Earth's history marked by different climate factors, sea levels, and lifeforms. Index fossil: A distinctive fossil, common to a particular geological period, that is used to date rocks of that period. Mold: Fossil formed when acidic water dissolves a shell or bone around which sand or mud has already hardened. Paleontology: The scientific study of fossils. Petrifaction: Process of turning organic material into rock by the replacement of that material with minerals. Sediment: Sand, silt, clay, rock, gravel, mud, or other matter that has been transported by flowing water.

Successful fossilization or preservation of an organism can occur in several ways: (1) preservation without change; (2) complete replacement by a mineral; (3) filling in of a hollow space by a mineral; (4) formation of a thin film of carbon; and (5) formation of an imprint or the filling in of an imprint. Bones, shells, and teeth are examples of fossils preserved without change. The entire skeletal remains of animals that fell into ancient asphalt pits and quicksand have been preserved undamaged. Sometimes an entire organism may be preserved whole. An example of an almost perfectly preserved organism is an ancient insect trapped in pine tree sap. Over time, the pine sap hardened into a transparent golden-brown resin called amber, which contains the body of the insect. The remains of animals trapped in ice—such as woolly mammoths—have been found with skin and hair intact. The process by which an organism is completely replaced by minerals is called petrifaction. The best-known example of this process is petrified wood, as seen in the Petrified Forest National Park in Arizona. Trees in this area were buried in mud and sand that contained the mineral silica.

Ground water carried this dissolved mineral into the trees, where it replaced the wood cells so completely the trees became hardened opal. Sometimes only small open spaces or holes in a shell or bone are filled in by dissolved minerals. The added mineral matter makes the shell or bone more compact and stonelike, preventing it from decaying. A mosquito in amber, 35 million years old. (Reproduced by permission of JLM Visuals .) Another type of fossil is the darkened carbon imprint of a buried plant or, more rare, animal. When an organism dies and is buried, pressure underneath Earth's surface compresses the organism between rock faces. The organism decomposes, leaving a thin film of carbon on the rock face in the form of the organism. (All organic matter is made of carbon.) Leaves, insects, and fish are often found preserved this way. An imprint in rock or the filling in of that imprint are other very common fossils. When an organism trapped in hardening sedimentary rock was dissolved by acidic water percolating through the rock, only the shape or form of the organism remained as an imprint. The cavity remaining in the rock is called a mold.

If the mold were later filled in by mud or mineral matter, the resulting fossil is called a cast. The fossil clock The principal use of fossils by geologists has been to date rock layers (called strata) that have been deposited on the surface of Earth over millions of years. Different episodes in Earth's history (known as geologic time periods) are marked by different climate factors, sea levels, and lifeforms. Distinctive fossilized lifeforms that are typically associated with a specific geologic time period are known as index fossils. These are fossils The bottom view of a fossil. (Reproduced by permission of JLM Visuals .) of organisms that lived for a short period of time in many areas. Since fossils and the rock they are found in are considered to be the same age, specific index fossils found in different rock layers in different areas indicate that the rock layers are the same age. Fossils - Window to the past Fossils Permineralization What is permineralization ? One of the common types of fossils is permineralization. This occurs when the pores of the plant materials, bones, and shells are impregnated by mineral matter from the ground, lakes, or ocean.

In some cases, the wood fibers and cellulose dissolve and some minerals replace them. Sometimes the mineral substance of the fossils will completely dissolve and some other minerals replace them. The common minerals that form this kind of fossils are calcite, iron, and silica. Since the pores of the organic tissues are filled with minerals or the organic matter is replaced with minerals, the fossils are formed in the original shape of the tissue or organism, but the composition of the fossils will be different and they will be heavier. Petrification (petros means stone) occurs when the organic matter is completely replaced by minerals and the fossil is turned to stone. This generally occurs by filling the pores of the tissue, and inter and intra cellular spaces with minerals, then dissolving the organic matter and replacing it with minerals. This method reproduces the original tissue in every detail. This kind of fossilization occurs in both hard and soft tissues. An example of this kind of fossilization is petrified wood. The process of permineralization The ground water generally do not contain pure water molecules alone. It is hard to some degree meaning it contains some minerals. The degree of hardness varies.

The different minerals are found in the ground, and water dissolves them until saturation at which point water will not hold any additional mineral matter. This process is enhanced by the acidification of the water. For example, the rain water when pure in the beginning picks up carbon dioxide from air and becomes a weak carbonic acid. The organic matter in the ground, and other decaying materials also will make ground water more acidic. This acidic water dissolves more minerals. Organic tissues like wood, bone, and shell contain pores and spaces. The mineralized water fills the pores of the organic tissues and moves through the cellular spaces. During this process the saturated water evaporates, and the excess minerals are deposited on the cells and tissues. This process creates many layers of mineral deposits creating hard fossilized record. What can we tell from permineralization ? Since permineralizations of organisms are three-dimensional fossils with organic matter replaced by minerals, what they mainly tell us are the about the internal structures of the organisms. The mineralization process itself helps to prevent tissue compaction, which could distort the actual size proportions of the various organs.

Permineralizations are also not "limited" to hard body parts (such as bones or shells), but can also be found preserving soft body parts. This could be very important to researchers who wish to look at what life was like in the past in relation to what it is now in the present. An example are the fragile reproductive structures of many plants. Depending on the conditions for the fossilization process and the specific mineral that was used for the fossilization, however, varying degrees of detail do exist. Sometimes, only very differentiated cell types can be distinguished (such as between vascular tissue for conducting water and nutrients and ground tissue in plants), while in other fossils, the detail can be so fine as to distinguish between the different organelles within the various cells. There are three subgroups of permineralizations: silicification, pyritization, and carbonate mineralizations. As with almost all fossilization processes, the specific type of permineralization, silicification (because of its conditions for fossilization), tells us a much about what type of environment the organism most likely lived in. This is because specific fossil types occur in environments with certain features.

Silicification is a fossilization process whereby the organism is penetrated by minerals that form on the cells and cell structures. In this case, the mineral is silica, and because the mineral "follows" the internal structures of the organism during mineralization, this accounts for the amazing amount of detail found in permineralizations. For example, (for silicification) fluids in volcanic terrain often contain silica that could be absorbed by the plants themselves. This would indicate that a volcano was near the plant in the past. An interesting point that this example presents is that the plant was already beginning its fossilization process when it was still living. The silica that is taken up by the plants become embedded within them and when they die, the material (silica) is already present within them to quickly mineralize the organism and fossilize it. The silicification process can often show very fine detail in this way. Pyritization involves the mineral sulfur. Many of the plants are thus pyritized when they are in marine sediments since they often contain a large amount of sulfur.

This could have been their natural habitat in the past or they could have been near enough to a marine environment to end up there to be pyritized (after being carried down by a river, flood, or some other method). Some plants are also pyritized when they are in a clay terrain, but to a lesser extent than in a marine environment. Carbonate mineralizations occur both in marine and nonmarine environments. The most popular forms of carbonate mineralizations that are cited in biology are what are called "coal balls." Coal balls (which are often found in a round ball shape, which gives them their name) are often a fossilization of many different plants and their tissues. Often, they occur in the presence of seawater or acidic peat. Acetate peels can also usually be made to study the various organic material trapped within a coal ball. These peels may sometimes be fairly revealing of cellular detail. Index Division of Geology and Mineral Resources - Fossils DIVISION OF GEOLOGY AND MINERAL RESOURCES Rocks, Minerals, and Fossils Common Fossils of Virginia Fossils are the preserved remains or imprints of once living plants or animals. These relics hold the key to understanding portions of Virginia’s geologic past.

Click here if you would like more details about fossil collecting in Virginia. Click on each image below for a more detailed view. Pelecypods Pelecypods, also called bivalves, are soft-bodied animals that secrete a hard shell with two sides, such as an oyster or clam. Pelecypods are abundant in the Cenozoic sedimentary deposits of Virginia’s Coastal Plain. Virginia’s state fossil is the bivalve Chesapecten jeffersonius, pictured here. Brachiopods A variety of marine invertebrate, brachiopods are fossils that once littered the ocean floor. Fossil brachiopods can be found in the Paleozoic rocks of Virginia’s Valley and Ridge Province. Ferns Ferns are vascular non-flowering plants. Impressions of ferns can be found in Late Paleozoic rocks of the Valley and Ridge Province as well as the Appalachian Plateau, where these fossils are associated with coal beds. Coral Coral is the hard calcareous skeleton formed by the polyps of bottom-dwelling marine organisms. Some type of corals are formed by colonies, others occur as single individuals.

Colonial corals can be found in the Cenozoic sediments of the Coastal Plain; individual and colonial corals can be found in the Paleozoic sedimentary rocks of the Valley and Ridge Province. Gastropods Gastropods are mollusks such as snails that secrete a single shell, usually spiraled. Cenozoic gastropods, such as the high-spired Turritella, are found on the Coastal Plain, while Paleozoic gastropods such as the low-spired Maclurites, are found in the Valley and Ridge Province. Shark Teeth Teeth from a wide variety of sharks are found in the Cenozoic sedimentary deposits of Virginia’s Coastal Plain. Several times during the Cenozoic Era this area was covered by a shallow sea. Skolithos Trace Fossils The oldest common fossils in Virginia, these are traces of the tubes inhabited by primitive worms living along a Paleozoic shoreline. Found along the western flank of the Blue Ridge Mountains. Trilobites Trilobites are members of an extinct group of Arthropods that scuttled across the sea floor throughout the Paleozoic Era.

Characterized by a three-lobed skeleton, a variety of trilobite fossils can be found in the rocks of the Valley and Ridge Province. Whale Bones Whale bones are frequently found in the Cenozoic sedimentary deposits of Virginia’s Coastal Plain. Several times during the Cenozoic this area was covered by a shallow sea. Whale bones are often associated with shark teeth. DMME Headquarters

Question: What common mineral is formed by the fossilization of vegetation?
Answer: Coal