Igneous Rocks

Igneous rocks are formed by the cooling and hardening of molten magma. The word igneous comes from the Latin word igneus, meaning fire, and there are two main types of igneous rocks: intrusive and extrusive.

Intrusive igneous rock forms within Earth’s crust; the molten material rises, filling voids or melting overlying rocks, and eventually hardens. Intrusive rocks are also called plutonic rocks, named after the Greek god Pluto, god of the underworld. Extrusive igneous rocks form when the magma, called lava once it reaches the surface, flows onto Earth’s surface. Extrusive rocks are also known as volcanic rocks.

Igneous rocks are classified according to their texture and mineral or chemical content. The texture of the rock is determined by the rate of cooling. The slower the rock cools, the larger the crystals form. Because the magma chamber is well insulated by the surrounding country rock, intrusive rocks cool very slowly and can form large, well developed crystals. Rapid cooling results in smaller, often microscopic, grains. Some extrusive rocks produced in explosive volcanic eruptions solidify in the air before they hit the ground. Sometimes the rock mass starts to cool slowly, forming large crystals, and then finishes cooling rapidly, resulting in rocks that have larger crystals surrounded by a fine-grained matrix. This is known as a porphyritic texture. Other extrusive rocks cool before the chemical constituents of the melt are able to arrange themselves into any crystalline form. These are said to have glassy texture and include the rocks obsidian and pumice.

The chemistry of the magma determines the minerals that will crystallize and their relative abundance in the rocks that form. Light-colored igneous rocks are likely to contain high proportions of light colored minerals, such as quartz and feldspars and are called felsic. Dark rocks will contain iron and magnesium-rich minerals like pyroxene and olivine and are known as mafic rocks. Those rocks with a color falling between the two are said to have an intermediate composition. The sequence of crystallization as temperatures decrease is represented by Bowen’s reaction series.

Once the composition and texture of the rock are determined, they are combined to establish the name of the rock. For example, a coarse-grained, felsic rock is called granite and a fine-grained felsic rock is called rhyo-lite. These two rocks are composed of the same minerals, but the slow cooling history of the granite has allowed its crystals to grow larger. These are some of the most familiar igneous rocks because continental portions of the crust are built largely of rock that is similar in composition to these felsic rocks. Coarse-grained and fine-grained mafic rocks are called gabbro and basalt, respectively. Each of these is easily recognized by their dark color. In general, oceanic crustal plates are primarily mafic in chemistry. Diorite and andesite are the respective names for coarse-and fine-grained rocks of intermediate composition. While geologists sometimes use more detailed classification systems, this basic method is used for preliminary differentiation of crystalline igneous rocks.

Certain igneous rocks are named on the basis of particular features. Fragmental rocks like tuff and volcanic breccia are named on the basis of the size of particles of volcanic material ejected during an eruption. Tuff is composed of fine particles of volcanic ash, whereas breccia includes larger pieces of broken rock. Obsidian, pumice, and scoria have a non-crystalline, glassy texture that can be distinguished on the basis of the quantity of trapped gas. Obsidian contains no such gas, but pumice has so many gas bubbles that it will float on water. Scoria is produced by the cooling of frothy mafic lava.

Earth’s tectonic plates are continually shifting and altered by earthquakes and volcanoes. As old plates are drawn downward into the mantle, rock is recycled through melting. New igneous material is continually added to the crust along plate margins and other locations through igneous intrusions and volcanic activity. Igneous rocks represent both the ancient history of the formation of the earth and modern episodes of regeneration. Associated igneous processes are evidence of the continuing activity of Earth’s interior and the form and composition of each of the igneous rocks give clues as to the conditions and processes under which they formed.

See also Plate tectonics; Volcano.

Igneous rocks

Igneous rocks are formed by the cooling and hardening of molten material called magma . The word igneous comes from the Latin word igneus, meaning fire. There are two types of igneous rocks: intrusive and extrusive. Intrusive igneous rock forms within Earth's crust; the molten material rises, filling voids in the crust, and eventually hardens. Intrusive rocks are also called plutonic rocks, named after the Greek god Pluto , god of the underworld. Extrusive igneous rocks form when the magma, called lava once it reaches the surface, pours out onto the earth's surface. Extrusive rocks are also known as volcanic rocks.

Igneous rocks are classified according to their texture and mineral or chemical content. The texture of the rock is determined by the rate of cooling. The slower the rock cools, the larger the crystals form. Because the magma chamber is well insulated by the surrounding country rock, intrusive rocks cool very slowly and can form large, well developed crystals. Rapid cooling results in smaller, often microscopic, grains. Some extrusive rocks solidify in the air, before they hit the ground. Sometimes the rock mass starts to cool slowly, forming large crystals, and then finishes cooling rapidly, resulting in rocks that have larger crystals surrounded by a fine-grained matrix . This is known as a porphyritic texture. Other extrusive rocks cool before the chemical constituents of the melt are able to arrange themselves into any crystalline form. These are said to have glassy texture and include the rocks obsidian and pumice.

The chemistry of the magma determines the minerals that will crystallize and their relative abundance in the rocks that form. Light-colored igneous rocks are likely to contain high proportions of light colored minerals, such as quartz and feldspars and are called felsic. Dark rocks will contain iron and magnesium-rich minerals like pyroxene and olivine and are known as mafic rocks. Those rocks with a color falling between the two are said to have an intermediate composition.

Once the basic composition and texture of the rock are determined, they are combined to establish the name of the rock. For example, a coarse-grained, felsic rock is called granite and a fine-grained felsic rock is called rhyolite. These two rocks are composed of the same minerals, but the slow cooling history of the granite has allowed its crystals to grow larger. These are some of the most familiar igneous rocks because continental portions of the crust are built largely of rock that is similar in composition to these felsic rocks. Coarse-grained and fine-grained mafic rocks are called gabbro and basalt, respectively. Each of these is easily recognized by their dark color. In general, oceanic crustal plates are primarily mafic in chemistry. Diorite and andesite are the respective names for coarse- and fine-grained rocks of intermediate composition. While geologists sometimes use more detailed classification systems, this basic method is used for preliminary differentiation of crystalline igneous rocks.

Certain igneous rocks are named on the basis of particular features. Fragmental rocks like tuff and volcanic

breccia are named on the basis of the size of particles of volcanic material ejected during an eruption. Tuff is composed of fine particles of volcanic ash, while breccia includes larger pieces. Obsidian, pumice, and often scoria have a non-crystalline, glassy texture that can be distinguished on the basis of the quantity of trapped gas. Obsidian contains no such gas and pumice has so many gas bubbles that it will sometimes actually float on water .

Earth's crustal plates are continually shifting, being torn open by faults, and altered by earthquakes and volcanoes. As old plates are drawn downward into the mantle, old rock material is recycled through melting. New igneous material is continually added to the crust along plate margins and other locations through igneous intrusions and volcanic activity. Igneous rocks represent both the ancient history of the formation of the earth and modern episodes of regeneration. Associated igneous processes are evidence of the continuing activity of Earth's interior and the form and composition of each of the igneous rocks give clues as to the conditions and processes under which they formed.

See also Plate tectonics; Volcano.

Igneous rocks

The first rocks on Earth were igneous rocks. Igneous rocks are formed by the cooling and hardening of molten material called magma . The word igneous comes from the Latin word ignis, meaning fire. There are two types of igneous rocks: intrusive and extrusive. Intrusive igneous rocks form within Earth's crust ; the molten material rises, filling any available crevices, into the crust, and eventually hardens. These rocks are not visible until the earth above them has eroded away. Intrusive rocks are also called plutonic rocks, named after the Greek god Pluto, god of the underworld. A good example of intrusive igneous rock is granite . Extrusive igneous rocks form when the magma or molten rock pours out onto the earth's surface or erupts at the earth's surface from a volcano . Extrusive rocks are also called volcanic rocks. Basalt , formed from hardened lava , is the most common extrusive rock. Obsidian , a black glassy rock, is also an extrusive rock.

Igneous rocks are classified according to their texture and mineral or chemical content. The texture of the rock is determined by the rate of cooling. The slower the cooling, the larger the crystal. Intrusive rock can take one million years or more to cool. Fast cooling results in smaller, often microscopic, grains. Some extrusive rocks solidify in the air, before they hit the ground. Sometimes the rock mass starts to cool slowly, forming larger crystals , and then finishes cooling rapidly, resulting in rocks that have crystals surrounded by a fine, grainy rock mass. This is known as a porphyritic texture.

Most of Earth's minerals are made up of a combination of up to ten elements. Over 99% of Earth's crust consists of only eight elements (oxygen , silicon , aluminum , iron , calcium, sodium, potassium, and magnesium). Most igneous rocks contain two or more minerals, which is why some rocks have more than one color. For example, the most common minerals in granite are quartz (white or gray), feldspar (white or pinks of varying shades), and mica (black). The amount of a specific element in a mineral can determine a color or intensity of color. Because of the way granite is formed, the different composition of minerals is easy to see. It is difficult to see the distinct composition of some extrusive rocks, like obsidian, due to their extremely fine texture. Igneous rocks contain mostly silicate minerals and are sometimes classified according to their silica content. Silica (SiO₂) is a white or colorless mineral compound. Rocks containing a high amount of silica, usually more than 50%, are considered acidic (sometimes the term felsic is used), and those with a low amount of silica are considered basic (or mafic ). Acidic rocks are light in color and basic rocks are dark in color.

Essentially, Earth's continents are slabs of granite sitting on top of molten rock. The crustal plates of Earth are continually shifting, being torn open by faults, and altered by earthquakes and volcanoes. New igneous material is continually added to the crust, while old crust falls back into the earth, sometimes deep enough to be remelted. Igneous rocks are the source of many important minerals, metals , and building materials.

See also Magma chamber; Volcanic eruptions

spilite A low-grade metamorphic rock composed of albite, chlorite, actinolite, sphene, and calcite, with or without epidote, prehnite, and laumonite, and formed by sea-floor metasomatism of mid-oceanic-ridge basalts. Sea water circulating through the oceanic crust is heated by the cooling basalt dykes and lavas and reacts with them, introducing sodium and water into the rock system and converting the basalt mineral assemblage into a typical spilite assemblage.

igneous rock One of three major classes of rock produced by the cooling and solidifying of molten magma. All igneous rocks are crystalline and most resist erosion. Extrusive, or volcanic, igneous rocks, such as basalt, form by the rapid cooling of molten material at the surface. Intrusive, or hyperbyssal, rocks, such as dolerite, form in sills or dykes at intermediate depth. Plutonic rocks, such as granite, form in batholiths at greater depth. The major chemical constituent is silica. Acid rocks contain high amounts of quartz, feldspar or mica, and are light coloured. Basic rocks are darker and contain 45–55% silica, including minerals such as hornblende. See also metamorphic rock; sedimentary rock