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There are three main types of rocks: sedimentary, igneous, and metamorphic. Each of these rocks are formed by physical changes-such as melting, cooling, eroding, compacting, or deforming-that are part of the rock cycle.

Sedimentary rocks are formed from pieces of other existing rock or organic material. There are three different types of sedimentary rocks: clastic, organic (biological), and Sedimentary Rocks chemical. Clastic sedimentary rocks, like sandstone, form from clasts, or pieces of other rock. Organic sedimentary rocks, like coal, form from hard, biological materials like plants, shells, and bones that are compressed into rock.

The formation of clastic and organic rocks begins with the weathering, or breaking down, of the exposed rock into small fragments. Through the process of erosion, these fragments are removed from their source and transported by wind, water, ice, or biological activity to a new location. Once the sediment settles somewhere, and enough of it collects, the lowest layers become compacted so tightly that they form solid rock.

Chemical sedimentary rocks, like limestone, halite, and flint, form from chemical precipitation. A chemical precipitate is a chemical compound-for instance, calcium carbonate, salt, and silica-that forms when the solution it is dissolved in, usually water, evaporates and leaves the compound behind. This occurs as water travels through Earth's crust, weathering the rock and dissolving some of its minerals, transporting it elsewhere. These dissolved minerals are precipitated when the water evaporates.

Metamorphic rocks are rocks that have been changed from their original form by immense heat or pressure. Metamorphic rocks have two classes: foliated and nonfoliated. When a rock with flat or elongated minerals is put under immense pressure, the minerals line up in layers, creating foliation. Foliation is the aligning of elongated or platy minerals, like hornblende or mica, perpendicular to the direction of pressure that is applied. An example of this transformation can be seen with granite, an igneous rock. Granite contains long and platy minerals that are not initially aligned, but when enough pressure is added, those minerals shift to all point in the same direction while getting squeezed into flat sheets. When granite undergoes this process, like at a tectonic plate boundary, it turns into gneiss (pronounced "nice").

Nonfoliated rocks are formed the same way, but they do not contain the minerals that tend to line up under pressure and thus do not have the layered appearance of foliated rocks. Sedimentary rocks like bituminous coal, limestone, and sandstone, given enough heat and pressure, can turn into nonfoliated metamorphic rocks like anthracite coal, marble, and quartzite. Nonfoliated rocks can also form by metamorphism, which happens when magma comes in contact with the surrounding rock.

Igneous Rocks Metamorphic Rocks

Igneous rocks (derived from the Latin word for fire) are formed when molten hot material cools and solidifies. Igneous rocks can also be made a couple of different ways. When they are formed inside of the earth, they are called intrusive, or plutonic, igneous rocks. If they are formed outside or on top of Earth's crust, they are called extrusive, or volcanic, igneous rocks.

Granite and diorite are examples of common intrusive rocks. They have a coarse texture with large mineral grains, indicating that they spent thousands or millions of years cooling down inside the earth, a time course that allowed large mineral crystals to grow.

Alternatively, rocks like basalt and obsidian have very small grains and a relatively fine texture. This happens because when magma erupts into lava, it cools more quickly than it would if it stayed inside the earth, giving crystals less time to form. Obsidian cools into volcanic glass so quickly when ejected that the grains are impossible to see with the naked eye.

Extrusive igneous rocks can also have a vesicular, or "holey" texture. This happens when the ejected magma still has gases inside of it so when it cools, the gas bubbles are trapped and end up giving the rock a bubbly texture. An example of this would be pumice.

Director

Tyson Brown, National Geographic Society

Author

National Geographic Society

Production Managers

Gina Borgia, National Geographic Society

Jeanna Sullivan, National Geographic Society

Program Specialists

Sarah Appleton, National Geographic Society, National Geographic Society

Margot Willis, National Geographic Society

Last Updated

October 19, 2023

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The rock cycle is a series of processes that create and transform the types of rocks in Earth's crust.

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There are three types of rock: igneous, sedimentary, and metamorphic. These names describe how each type of rock was formed.

Igneous rock is hardened magma. Magma may escape through cracks in Earth's crust. It may also come to Earth's surface when a volcano erupts. As the escaped magma cools, it hardens into solid rock. Granite and basalt are examples of igneous rock.

Sedimentary rock may form from pieces of other rock. In a process called erosion, wind and water slowly break rock into tiny pieces. Water washes the pieces into rivers. They settle along the river bottom in soft, loose layers called sediment. Over millions of years the sediment builds up, hardens, and becomes solid rock. Sandstone is a sedimentary rock that forms in this way. Sedimentary rock also may form from the remains of dead animals or plants. Limestone is a sedimentary rock that is made up of the shells and stony skeletons of certain kinds of living things.

Metamorphic rock forms from old igneous or sedimentary rock. Several forces can change old rock into new metamorphic rock. Great heat and pressure inside Earth's crust can shape old rock into metamorphic rock. Water can dissolve minerals in old rock or carry new minerals into it to form metamorphic rock. The heat of magma can also change old rock into metamorphic rock. Marble and slate are examples of metamorphic rock.

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There are three types of rock: igneous, sedimentary, and metamorphic. These names describe how each type of rock was formed. Igneous rock is hardened magma. Magma may escape through cracks in Earth's crust. It may also come to Earth's surface when a volcano erupts. As the escaped magma cools, it hardens into solid rock.

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Learning about rocks and minerals gives students a deeper appreciation of the story behind the scenery in our national parks. Noatak National Preserve, Alaska. NPS photo.

Introduction

Rocks are all around us. They make up the backbones of hills and mountains and the foundations of plains and valleys. Beneath the soil you walk on and the deep layers of soft mud that cover the ocean basins is a basement of hard rock.

Rocks are made up of different minerals, broken pieces of crystals, or broken pieces of rocks. Some rocks are made of the shells of once-living animals, or of compressed pieces of plants. What a rock is made of, the shapes of the grains, and how the grains fit together all provide valuable clues to help us unlock the rock's history hidden within.

Rocks are divided into three basic types depending on how they were formed: Igneous, Sedimentary, and Metamorphic.

Natural objects, such as rocks and minerals, contribute to the beauty and wonderment of the National Parks and should be left, as they were found, so that others can experience a sense of discovery.

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Rocks - Geology (U.S. National Park Service)

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Rocks are all around us. They make up the backbones of hills and mountains and the foundations of plains and valleys. Beneath the soil you walk on and the deep layers of soft mud that cover the ocean basins is a basement of hard rock.

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Sedimentary rock along the White Rim Road at Canyonlands National Park, Utah. NPS photo.

Introduction

Sedimentary rocks are formed from deposits of pre-existing rocks or pieces of once-living organism that accumulate on the Earth's surface. If sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock. These rocks often have distinctive layering or bedding and create many of the picturesque views of the desert southwest. Sedimentary rocks are classified into three groups: Clastic, Biologic, and Chemical.

Key Terms

Cementation: The process by which clastic sediments become lithified or consolidated into hard, compact rocks, usually through deposition or precipitation of minerals in the spaces among the individual grains of the sediment.

Compaction: The process of consolidating fine-grained sediments into rock.

Lithification: The conversion of loose sediment into solid sedimentary rock. Several processes, including compaction of grains, filling of spaces between grains with mineral cement, and crystallization act to solidify sediment.

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Sedimentary Rocks - Geology (U.S. National Park Service)

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Sedimentary rocks are formed from deposits of pre-existing rocks or pieces of once-living organism that accumulate on the Earth's surface. If sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock. These rocks often have distinctive layering or bedding and create many of the picturesque views of the desert southwest.

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Metamorphic rock creates a zebra striped pattern in the canyon walls of Marble Canyon. Death Valley National Park, California and Nevada. NPS photo by Dan Kish.

Introduction

Metamorphic rocks form when high temperatures and pressure act on a rock to alter its physical and chemical properties (metamorphism means 'to change form'). These conditions often stretch, twist and fold the rock as it cools. In metamorphic rocks some or all of the minerals in the original rock are replaced, atom by atom, to form new minerals. Types of metamorphic rocks include gneiss, quartzite, marble, schist, soapstone, and phyllite.

Parks with examples of metamorphic rocks include

Key Terms

Contact Metamorphism: A type of local, thermal metamorphism caused by the intrusion and extrusion of magmas; takes place in rocks at or near their contact with a body of igneous rock.

Parent Rock: Rock from which soil, sediment, or other rock is derived.

Regional Metamorphism: A type of metamorphism that affects an extensive region, as opposed to local metamorphism that is effective only in a relatively restricted area.

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Metamorphic Rocks - Geology (U.S. National Park Service)

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Metamorphic rocks form when high temperatures and pressure act on a rock to alter its physical and chemical properties (metamorphism means 'to change form'). These conditions often stretch, twist and fold the rock as it cools. In metamorphic rocks some or all of the minerals in the original rock are replaced, atom by atom, to form new minerals.

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Earth Science for Kids: Rocks, Rock Cycle, and Formation

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Kids learn about the science of rocks and the rock cycle. How different types such as igneous, sedimentary, and metamorphic form from minerals with the help of nature.

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Do you enjoy learning how things are made? For example, when you order a grilled cheese sandwich, do you like to know what kind of bread and cheese are going to be used? If you don't have a taste for Swiss cheese on rye, then you might be curious about the products.

But what about the bread and the cheese? Are you curious about how they make these items? Sometimes it's easy to see how things are made. For bread, it's easy to follow a recipe that shows how ground grains combine with other ingredients to make bread that comes hot and fresh out of the oven.

For other things, though, it's not as easy to figure out how they're made. Take rocks, for example. Can you look at a rock and how it came to be? Unless you're a , the answer is probably "no." So how are those hard lumps of matter made?

That's a bit of a trick question, though, because there are three main types of rocks: igneous, sedimentary, and metamorphic. And, of course, each type forms differently. Let's look at each type of rock and how it comes to be.

Igneous rocks form when melted rock cools into a . This can happen above or below the Earth's surface. When a volcano , the lava cools and becomes igneous rock. Some igneous rocks are granite, basalt, pumice, and obsidian.

Igneous rocks are made quickly, but sedimentary rocks take millions of years to form. When the ground wears away over time, it leaves layers of sediment. After a really long time, many layers of sediment build up and push down on the old layers. With lots of pressure and heat, the lower layers turn into rocks. Some types of sedimentary rocks are sandstone, limestone, and shale.

Metamorphic rocks are the products of rocks that have changed. You might know the word " metamorphosis" from when a caterpillar changes into a butterfly. The same idea applies to rocks. Metamorphic rocks change into a new form.

Metamorphic rocks might have started as igneous or sedimentary rocks. They might even have been a different metamorphic rock. When tectonic plates crash together or when there is a lot of heat and pressure from under the ground, these rocks can change. Some kinds of metamorphic rocks are marble, soapstone, slate, and gneiss.

Even though rocks seem like they'll stay the same forever, they actually change a lot over millions of years. Scientists call this the "rock cycle". One way this happens is when a volcano and hot lava cools down to become igneous rock.

As time goes by, the weather wears down the igneous rock into tiny pieces called sediment. The sediment pieces pile up and get compressed over millions of years to become sedimentary rock.

After millions more years, the sedimentary rock can get buried under other rocks and end up deep underground. There, it can get really hot and compressed by all the weight on top of it. This turns the sedimentary rock into metamorphic rock, and then the cycle can start again! Of course, not all rocks follow this exact pattern. There are many ways rocks can change from one type to another-and this happens over and over in a never-ending cycle.

When you explore in your area, what types of rocks to you find? Can you classify them into sedimentary, igneous, and metamorphic categories? Based on what you know of your area, what types of rocks do you think you might find?

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We believe you'll think today's Wonder of the Day totally rocks!

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Rock is a naturally occurring solid material composed of one or more minerals. It is a basic component of Earth, providing the main substance of all but the innermost layers. The outer layer of Earth, called the crust, is made up of rigid rock. Between the crust and Earth's metallic core lie the layers of molten and rigid rock that make up the mantle.

Rocks are classified into three major groups according to how they were formed: igneous rock, which forms from molten material called magma; sedimentary rock, which forms from sand, clay, or other materials deposited by water, wind, or glacial action; and metamorphic rock, which forms from rocks that are modified by heat, pressure, or chemical activity. The three major classes of rock are further divided into different groups based on various factors, including chemical composition, texture, and mineral content.

Most rocks contain several types of minerals; for example, the igneous rock granite is largely composed of the minerals quartz, feldspar, and mica. In contrast, pure sandstone, a sedimentary rock, contains only the mineral quartz. Obsidian, a glassy igneous rock, is among the few rock types that contain no minerals.

Rocks form and change through a natural process called the rock cycle.The study of rocks and the processes that form and change them is called geology.

Geologists broadly classify Earth's rocks as igneous, sedimentary, or metamorphic rocks.

Igneous Rock
Igneous rock forms from magma when it cools and solidifies. Magma is a molten mixture of rock-forming minerals, gases, and steam produced at high temperatures deep within Earth, at underground depths of about 30 to 120 miles (50 to 200 kilometers). The word igneous comes from the Latin ignis, meaning "fire."

Magma is a hot, doughy material that rises along cracks in the Earth's crust. When a volcano erupts or a deep crack occurs in the Earth, the magma rises and overflows. When magma flows onto Earth's surface, it is called lava.

Igneous rocks are subdivided into two categories: intrusive and extrusive. Intrusive igneous rock forms if the magma cools and solidifies before it reaches the surface. If the molten material is expelled onto the surface (as lava), it forms extrusive igneous rock.

Sedimentary Rock
Sedimentary rock forms from rock fragments, sand, mud, and other materials that are compacted and cemented together at Earth's surface. The materials that form sedimentary rock generally are carried by water, wind, ice, or other means and are then deposited on land or on the bottoms of oceans, lakes, or rivers, forming layers. The materials in the lower layers are pressed and cemented into rock by the weight of the upper layers. The layers of sedimentary rocks are one of their chief characteristics and may be distinguished by differences in color, particle size, and other features. Fossils are found in sedimentary rock.

Because the bottom layers in a sedimentary rock sequence are deposited first, they are older than the upper layers. This follows the law of superposition, an important tenet in geology. This principle states that in an undisturbed sequence of rock layers, the layers at the bottom are older than those at the top. The principle of superposition allows scientists to determine the relative ages of the rocks in a sequence. ( See also stratigraphy.)

Metamorphic Rock
Metamorphic rock forms when environmental conditions change the composition or structure of preexisting rock. The term metamorphic comes from a Greek word meaning "change of form." Metamorphic rock can form from igneous rock, sedimentary rock, or from another metamorphic rock. Pressure and heat resulting from processes deep below Earth's surface, mechanical stresses, and chemical reactions all can produce metamorphic rocks. Probably the most common metamorphic rock is gneiss (pronounced "nice"), which is produced when intense heat and pressure rearrange the minerals in granite.

The conditions needed to form metamorphic rocks are common where tectonic plates come together. The plates are large rigid blocks of Earth's outer layer that slowly move across Earth's surface. The collision of two plates produces great temperatures and intense pressure. Plate collisions can cause new mountain ranges to form; most of the world's mountain belts are at least partially composed of metamorphic rocks.

Rocks are cycled through various forms in what is called the rock cycle, a set of natural processes that form, break down, change, and re-form rocks over time. Temperature, pressure, and changes in environmental conditions at and beneath Earth's surface drive the rock cycle. As a result of the cycle, each type of rock can change into other types. The events of the rock cycle take place over a very long time span, from thousands to millions of years.

Rocks can follow many paths through the rock cycle. For example, weathering and erosion at Earth's surface can reduce existing rocks to loose particles such as sand or clay that are deposited on Earth's surface. Over time, the particles are compacted and cemented together, forming sedimentary rocks: sand may become sandstone, and clay may become shale. With time, these rocks may become buried deep within Earth's crust. There, intense heat and pressure may change the composition or structure of the rocks, causing formation of metamorphic rocks. Sandstone may alter to the metamorphic rock quartzite, whereas shale may transform into the metamorphic rock slate. Further increases in temperature may cause these rocks to melt and form molten rock, or magma, beneath Earth's surface. Volcanic activity brings magma to the surface, where it may cool and form igneous rock.

It is important to note that the processes of the rock cycle can occur in any order. In the previous example, the sedimentary rock shale might melt under high temperatures, forming magma that may gradually cool and become the igneous rock basalt. Alternatively, weathering and erosion could reduce shale back to the clay particles that formed it. There is no single direct path through the rock cycle-any type of rock can alter to another type of rock under the right circumstances.

The history of Earth is held in rocks. Scientists can learn how parts of the Earth have changed over time by studying rocks and rock layers. For example, evidence in rocks in many parts of the world shows that vast areas were covered by thick layers of ice several times across history. The folds and breaks observed in rock layers provide evidence of ancient earthquakes that caused the ground to shift and the rock layers to move. Scientists can infer that some inland areas were once covered by water by the presence of rocks such as limestone (normally formed on ocean bottoms) or shale (generally found in muddy bottoms of lagoons, floodplains, and similar areas).

Rock is an important resource to humans. Soil itself is made up largely of rock that has been broken down by weathering. Crops are grown in soil, which is weathered rock. Rocks rich in the mineral halite are an important source of common table salt. Granite, limestone, sandstone, slate, and marble are rocks used as building stones. They are removed from the earth by quarrying.

Many rocks contain rich deposits of minerals, metals, and precious stones such as diamonds, emeralds, and rubies. Gold, silver, iron, copper, uranium, and thorium are among the minerals highly valued for their utility in industry. Many of these minerals are removed from rocks by mining.

Collecting rocks and minerals is a popular and worthwhile hobby. It combines outdoor exercise with indoor study. Learning to identify the minerals and fossils in rocks may lead to a career in such rewarding sciences as chemistry, physics, geology, physical geography, or paleontology.

Amateur collectors call themselves rockhounds. With a little inexpensive equipment, such as a hammer and chisel, a notebook, a magnifying glass, and a rock hunter's guidebook, a collector is ready to gather specimens.

Rocks and minerals can be picked up almost anywhere-quarries and mines, road cuts, building excavations, beaches, streambeds, rock outcroppings, and exposed cliffs. Most rockhounds specialize, rather than try to collect all of the known minerals. Colorful and beautifully shaped crystals are prized by all collectors. Petrified wood and fossils are interesting specialties.

Collections of semiprecious stones such as agates, zircons, garnets, and amethysts may lead to gem cutting and jewelry making. This offshoot of mineral collecting has become a major hobby in itself.

Some collectors make dramatic exhibits with fluorescent minerals. In normal light the specimens appear dull and unattractive. In a dark room, exhibited under purple light, they glow in spectacular colors. Argon electric bulbs may be used to illuminate a box of specimens.

Many rockhounds band together in a mineral or gem society. In the United States local and state societies belong to regional federations, which in turn are affiliated with the American Federation of Mineralogical Societies. Many groups hold conventions at which individuals exhibit their collections, exchange specimens, and discuss every aspect of their hobby.

Rock collectors use a number of simple tests based on the physical properties of minerals to help identify their finds. Among the most basic tests are hardness, color, luster, streak, and the way the rock breaks when struck with a hammer or other force. For a detailed description of these tests, see mineral.

Rocks can also be identified using a dichotomous key, a guide that uses a series of paired clues to identify one or more unknown samples. As the user works through the key, the clues help narrow down the possible identities of the samples. Some keys are structured as paired statements; others resemble a branching flowchart. Dichotomous keys may also be used to identify plants, animals, and other living things.

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Rock is a naturally occurring solid material composed of one or more minerals. It is a basic component of Earth, providing the main substance of all but the innermost layers....

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Igneous Rocks - Geology (U.S. National Park Service)

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Igneous rocks are “fire-born,” meaning that they are formed from the cooling and solidification of molten (melted) rock. The word igneous derives from ignis, the Latin word for “fire.” Molten rock material is known as magma until it is erupted onto the surface when it then is termed lava. There are two broad types of igneous rocks:

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Igneous rocks are one form of rocks commonly found on the Earth. They are created when hot magma from deep within the Earth cools and hardens. The magma can cool below the Earth’s crust, or erupt as lava and cool on the Earth’s surface.

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IGNEOUS ROCKS
Igneous rocks form when molten rock (magma) originating from deep within the Earth solidifies. The chemical composition of the magma and its cooling rate determine the final rock type.
INTRUSIVE IGNEOUS (PLUTONIC)
Intrusive igneous rocks are formed from magma that cools and solidifies deep beneath the Earth"s surface. The insulating effect of the surrounding rock allows the magma to solidify very slowly. Slow cooling means the individual mineral grains have a long time to grow, so they grow to a relatively large size. Intrusive rocks have a characteristically coarse grain size.

EXTRUSIVE IGNEOUS (VOLCANIC)
Extrusive igneous rocks are formed from magma that cools and solidifies at or near the Earth's surface. Exposure to the relatively cool temperature of the atmosphere or water makes the erupted magma solidify very quickly. Rapid cooling means the individual mineral grains have only a short time to grow, so their final size is very tiny, or fine-grained. Sometimes the magma is quenched so rapidly that individual minerals have no time to grow. This is how volcanic gtass forms.

SEDIMENTARY ROCKS
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's
surface.
CLASTIC
Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering, then transported by erosion to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock.
Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their grain size.

CHEMICAL
Chemical sedimentary rocks are formed by chemical precipitation. This process begins when water traveling through rock dissolves some of the minerals, carrying them away from their source. Eventually, these minerals are redeposited when the water evaporates away or when the water becomes over- saturated.

BIOLOGIC
Biologic sedimentary rocks form from once-living organisms. They may form from accumulated carbon-rich pfant material or from deposits of animal shells.

Metamorphic rocks are rocks that have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rooks are subjected to high heat, high pressure, hot, mineral-rich fluids or, more commonly, some combination of these factors.
FOLIATED
Foliation forms when pressure squeezes the flat or elongate minerals within a rock so they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.

NONFOLIATED
Non-foliated metamorphic rocks do not have a platy or sheet-like structure. There are several ways that nonfoliated rocks can be produced. Some rocks, such as limestone are made of minerals that are not flat or elongate. No matter how much pressure you apply, the grains will not align! Another type of metamorphism, contact metamorphism, occurs when hot Igneous rock intrudes into some pre-existing rock. The pre-existing rock is essentially baked by the heat, changing the mineral structure of the rock without addition of pressure.

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How Rocks are Classified - Geology (U.S. National Park Service)

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IGNEOUS ROCKS Igneous rocks form when molten rock (magma) originating from deep within the Earth solidifies. The chemical composition of the magma and its cooling rate determine the final rock type. INTRUSIVE IGNEOUS (PLUTONIC) Intrusive igneous rocks are formed from magma that cools and solidifies deep beneath the Earth"s surface.