Igneous Rocks

Igneous rocks are one of three main types of rocks (along with sedimentary and metamorphic), and they include both intrusive and extrusive rocks.

Igneous rocks form when magma (molten rock) cools and crystallizes, either at volcanoes on the surface of the Earth or while the melted rock is still inside the crust. All magma develops underground, in the lower crust or upper mantle, because of the intense heat there.

Devil's Tower National Monument in Wyoming is an example of an igneous rock. Photograph by Koumlhn/ullstein bild courtesy of Getty Images.

Igneous rocks can have many different compositions, depending on the magma they cool from. They can also look different based on their cooling conditions. For example, two rocks from identical magma can become either rhyolite or granite, depending on whether they cool quickly or slowly. 

The two main categories of igneous rocks are extrusive and intrusive. Extrusive rocks are formed on the surface of the Earth from lava, which is magma that has emerged from underground. Intrusive rocks are formed from magma that cools and solidifies within the crust of the planet.

When lava comes out of a volcano and solidifies into extrusive igneous rock, also called volcanic, the rock cools very quickly. Crystals inside solid volcanic rocks are small because they do not have much time to form until the rock cools all the way, which stops the crystal growth. These fine-grained rocks are known as aphanitic—from a Greek word meaning “invisible.” They are given this name because the crystals that form within them are so small that they can be seen only with a microscope. If lava cools almost instantly, the rocks that form are glassy with no individual crystals, like obsidian. There are many other kinds of extrusive igneous rocks. For example, Pele’s hair is long, extremely thin strands of volcanic glass, while pahoehoe is smooth lava that forms shiny, rounded piles. 

Intrusive rocks, also called plutonic rocks, cool slowly without ever reaching the surface. They have large crystals that are usually visible without a microscope. This surface is known as a phaneritic texture. Perhaps the best-known phaneritic rock is granite. One extreme type of phaneritic rock is called pegmatite, found often in the U.S. state of Maine. Pegmatite can have a huge variety of crystal shapes and sizes, including some larger than a human hand.

Source: National Geographic Society.

Metamorphic Rocks

Metamorphic rocks start as one type of rock and—with pressure, heat, and time—gradually change into a new type of rock.

The term “metamorphosis” is most often used in reference to the process of a caterpillar changing into a butterfly. However, the word “metamorphosis” is a broad term that indicates a change from one thing to another. Even rocks, a seemingly constant substance, can change into a new type of rock. Rocks that undergo a change to form a new rock are referred to as metamorphic rocks.

In the rock cycle, there are three different types of rocks: sedimentary, igneous, and metamorphic. Sedimentary and igneous rocks began as something other than rock. Sedimentary rocks were originally sediments, which were compacted under high pressure. Igneous rocks formed when liquid magma or lava—magma that has emerged onto the surface of the Earth—cooled and hardened. A metamorphic rock, on the other hand, began as a rock—either a sedimentary, igneous, or even a different sort of metamorphic rock. Then, due to various conditions within the Earth, the existing rock was changed into a new kind of metamorphic rock.

The conditions required to form a metamorphic rock are very specific. The existing rock must be exposed to high heat, high pressure, or to a hot, mineral-rich fluid. Usually, all three of these circumstances are met. These conditions are most often found either deep in Earth’s crust or at plate boundaries where tectonic plates collide. In order to create metamorphic rock, it is vital that the existing rock remain solid and not melt. If there is too much heat or pressure, the rock will melt and become magma. This will result in the formation of an igneous rock, not a metamorphic rock.

Metamorphic Rock Isua Metamorphic rock, estimated to be as old as 3.8 billion years, located near Isua at Qorqut Sound, Greenland..

Consider how granite changes form. Granite is an igneous rock that forms when magma cools relatively slowly underground. It is usually composed primarily of the minerals quartz, feldspar, and mica. When granite is subjected to intense heat and pressure, it changes into a metamorphic rock called gneiss.

Slate is another common metamorphic rock that forms from shale. Limestone, a sedimentary rock, will change into the metamorphic rock marble if the right conditions are met.

Although metamorphic rocks typically form deep in the planet’s crust, they are often exposed on the surface of the Earth. This happens due to geologic uplift and the erosion of the rock and soil above them. At the surface, metamorphic rocks will be exposed to weathering processes and may break down into sediment. These sediments could then be compressed to form sedimentary rocks, which would start the entire cycle anew.

Source: National Geographic Society.

Sedimentary Rocks

Sedimentary rocks are one of three main types of rocks, along with igneous and metamorphic. They are formed on or near the Earth’s surface from the compression of ocean sediments or other processes.

Sedimentary Rocks An example of a sedimentary rock, which is, by definition, composed of many, smaller rocks. Photo courtesy of Alamy Stock Photo.

Sedimentary rocks are formed on or near the Earth’s surface, in contrast to metamorphic and igneous rocks, which are formed deep within the Earth. The most important geological processes that lead to the creation of sedimentary rocks are erosion, weathering, dissolution, precipitation, and lithification.

Erosion and weathering include the effects of wind and rain, which slowly break down large rocks into smaller ones. Erosion and weathering transform boulders and even mountains into sediments, such as sand or mud. Dissolution is a form of weathering—chemical weathering. With this process, water that is slightly acidic slowly wears away stone. These three processes create the raw materials for new, sedimentary rocks.

Precipitation and lithification are processes that build new rocks or minerals. Precipitation is the formation of rocks and minerals from chemicals that precipitate from water. For example, as a lake dries up over many thousands of years, it leaves behind mineral deposits; this is what happened in California’s Death Valley. Finally, lithification is the process by which clay, sand, and other sediments on the bottom of the ocean or other bodies of water are slowly compacted into rocks from the weight of overlying sediments.

Sedimentary rocks can be organized into two categories. The first is detrital rock, which comes from the erosion and accumulation of rock fragments, sediment, or other materials—categorized in total as detritus, or debris. The other is chemical rock, produced from the dissolution and precipitation of minerals.

Detritus can be either organic or inorganic. Organic detrital rocks form when parts of plants and animals decay in the ground, leaving behind biological material that is compressed and becomes rock. Coal is a sedimentary rock formed over millions of years from compressed plants. Inorganic detrital rocks, on the other hand, are formed from broken up pieces of other rocks, not from living things. These rocks are often called clastic sedimentary rocks. One of the best-known clastic sedimentary rocks is sandstone. Sandstone is formed from layers of sandy sediment that is compacted and lithified.

Chemical sedimentary rocks can be found in many places, from the ocean to deserts to caves. For instance, most limestone forms at the bottom of the ocean from the precipitation of calcium carbonate and the remains of marine animals with shells. If limestone is found on land, it can be assumed that the area used to be under water. Cave formations are also sedimentary rocks, but they are produced very differently. Stalagmites and stalactites form when water passes through bedrock and picks up calcium and carbonate ions. When the chemical-rich water makes its way into a cave, the water evaporates and leaves behind calcium carbonate on the ceiling, forming a stalactite, or on the floor of the cave, creating a stalagmite.

Source: National Geographic Society.

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