Igneous rock bodies

Igneous rock bodies are formations of rock created when magma generated within the earth’s mantle cools and solidifies into solid rock strata. Magma forms when a portion of the mantle is disrupted and decompressed, which causes pockets of the material to liquefy. Igneous rock is the most common type of rock in the crust and is a common component in construction and other industrial applications.

Origin of Igneous Rocks

Igneous rocks are crystalline or glassy rock bodies that form when superheated rock originating in the earth’s mantle rises toward the surface and cools to form solid structures. While it remains under the surface, this superheated rock is known as magma, and when it breaks through the crust to the surface of the earth, it becomes lava. The classification of igneous rocks depends largely on whether the rock solidified from magma or from lava.

Magma is a combination of solid, liquid, and gas molecules and is composed of a variety of elements, including oxygen, aluminum, silicon, sodium, potassium, magnesium, and iron. The two most important components of magma are water and silica, which is a molecule composed of silicon and oxygen. The proportion of silica and water within a deposit of magma determines its overall grain and texture and therefore determines the types of solids that will develop if the magma cools.

The surface of the earth, known as the crust, is formed of solid, low-density rock that floats on top of a layer of higher-density rock called the mantle. The mantle layer is partially solid, but because it is subject to greater pressure, it is more ductile. The mantle is constantly moving in response to heat rising from the inner layers of the earth.

Below the mantle is a layer of liquid rock known as the outer core, which remains liquefied because it is subjected to intense pressure and is continually heated by the radioactive decay of elements contained within. Heat rising from the inner core causes currents to develop within the mantle; the crust of the earth moves along the path of these currents.

The crust and the upper layers of the mantle are divided into a series of structures known as tectonic plates, each of which moves as a single unit. The movement of tectonic plates and mantle generates pressure and friction that cause portions of the mantle to liquefy into magma. Most magma forms at depths ranging from 10 to 200 kilometers (km), or 6 to 124 miles (mi) below the surface.

Some magmas form along subduction zones, which are areas in which two of the earth’s plates are driven together by convection currents. As the plates collide, a portion of one plate is driven under the leading portion of the opposing plate, which disrupts both the crust and the upper mantle. This leads to reduced density among the molecules of mantle rocks. Once freed from their dense compaction, the molecules of the mantle rock are excited by heat and pressure and begin to vibrate, causing a portion of the rock to morph into the liquid phase, becoming magma. Magma that forms along subduction zones is generally known as granitic or felsic magma because it is formed from melted rock derived from both the crust and the upper mantle.

Magma also forms in areas known as rifts, where two continental plates diverge. As the plates move apart, convection currents from deep within the mantle push upward, decompressing a portion of the upper mantle and leading the rock to become liquid. This type of magma is known as basaltic or mafic magma and is primarily composed of rock from the upper mantle.

Magma that forms away from the edges of tectonic plates is called intraplate magma. This type of magma results from what geologists call a hot spot, an area in which a portion of the mantle becomes destabilized from convection currents rising from the inner core, forming a plume of liquid rock that rises to the surface because it is less dense than the rock layers of the upper mantle or crust. The portion of the crust overlying the hot spot will continue to float along the mantle; through millions of years, this can lead to the formation of a chain of volcanoes breaking through the crust.

Classification of Igneous Rocks

Extrusive deposits are those that form when lava erupts through the crust and then solidifies into solid rock on the surface. Extrusive deposits typically involve volcanic eruptions and may occur either at areas where tectonic plates collide, at rift zones, or where hot spots form within the body of a tectonic plate.

By contrast, intrusive igneous rocks form under the surface of the earth, sometimes taking thousands or millions of years to solidify. Intrusive igneous formations are hidden within the crust until erosion or some other disruption of crustal rock exposes the igneous deposits at the surface.

Igneous rocks are classified according to both texture and chemical composition. The texture or grain of an igneous rock takes into account the thickness of individual grains, the presence of glass within the rock, and whether the rock contains hollow cavities or pores. In terms of chemical composition, geologists often use the proportion of silica as the basis for classifying igneous rocks.

All igneous rocks are composed primarily of silica, which is the main mineral component of the earth’s crust. Silica combined with calcium, potassium, and sodium may form feldspar. Feldspar occurs in many varieties and is generally classified according to the dominant types of minerals mixed in with the silica base. Therefore, feldspar types are calcium dominant, potassium dominant, and sodium dominant. Igneous rocks containing 50 to 70 percent silica blended with potassium-dominant feldspar are called felsic rocks, whereas mafic rocks contain only 40 to 50 percent silica and have a high proportion of iron or magnesium. Feldspar that occurs in mafic rocks is typically of the calcium-dominant variety, though mafic rocks contain much lower levels of feldspar than do felsic rocks.

Igneous rocks with large grains, visible without a microscope, are called phaneritic rocks, while aphanitic rocks have grains that typically cannot be seen except under a microscope. The texture of the rocks depends on the specifics of the cooling and solidification process. Phaneritic rocks result from slow cooling processes and are therefore characteristic of intrusive rock formations, in which the magma had cooled beneath the earth’s surface. Aphanitic rocks form from rapid cooling and are therefore characteristic of extrusive rock formations, such as when lava cools in contact with air or water after an eruption.

Under special conditions that lead to extremely rapid cooling, lava will form into glassy rocks that contain no minerals and no crystalline structure. The black glass known as obsidian is one example of a glassy igneous rock that forms when lava undergoes a rapid cooling process.

Another type of igneous rock, called porphyritic rock, combines aphanitic and phaneritic characteristics. Porphyritic rocks appear as fine-grained aphanitic rocks with large crystals embedded within their matrix. These crystals result when the rock experiences a mixed solidification regime including alternating periods of rapid and slow cooling.

Other igneous rocks are classified as vesicular because the rock contains large holes or pores that result from bubbles of gas that formed while the rock was cooling. After the rock solidifies, these pores remain in the rock’s structure.

Extrusive Igneous Bodies

Igneous formations that develop from lava are usually aphanitic in texture because the rapid cooling of the rock causes the formation of small crystals. Rapid cooling also may result in the formation of glass deposits, like obsidian.

Among the most common types of deposits resulting from lava flows are basalts, which are a type of aphanitic rock, rich in calcium and containing low levels of larger mineral deposits. Basalt is composed primarily of plagioclase, a type of feldspar that combines silica with aluminum, calcium, and sodium. Basalt is one of the most common types of igneous rocks and is a major component of the earth’s crust. Geologists use the term flood basalts to describe large deposits of basalt sediment that cover vast areas of terrestrial or submarine environments after large-scale volcanic eruptions.

Lava flows that occur in marine environments may result in pillow lavas, which are bulbous, spherical formations named for their resemblance to pillows. Lava flows that cool on the surface of the earth often form a variety of structures. Rapidly cooled surface lava may become frozen in the shape of a flowing liquid, leaving behind ropey structures called pahoehoe, a Hawaiian term for these formations, which are common on the Hawaiian Islands. In some cases, a lava flow may cool in such a way that the outer surface of the lava cools while the inner portion remains liquid. As the liquid lava flows from this solid shell, it leaves behind a hollow structure called a lava tube. Large lava tubes may combine to form a lava cave system with subterranean caverns stretching for miles.

Intrusive Igneous Bodies

Intrusive rock bodies occur when pockets of magma infiltrate preexisting rock, known as country rock. These formations are called plutons and are generally classified according to their position relative to the surrounding bed of country rock.

Tabular plutons form when magma is forced into cracks between layers of country rock. These formations are named for the sheet-like or table-like forms they take as they squeeze into the country rock. Tabular plutons that form parallel to the existing sediment are called sills, which tend to be thin, reaching less than 50 meters (164 feet) in thickness.

Many sills are connected to dikes, which are plutons that form in a concordant direction to the sediment of the country rock such that they extend at a horizontal angle through the bedrock. Dikes also tend to be relatively thin, rarely reaching more than 20 meters (66 feet). As the dikes extend horizontally, they may form one or more sills at different levels of the sediment. Dikes and sills often result from a flow of magma deeper in the mantle.

In some cases, a large sill may form such that the central portion of the sill bubbles up toward the surface. These formations, called laccoliths, may grow to such a size that they push the crust into a mountain-sized mound. The Henry Mountains in Utah were formed by an ancient laccolith that developed under the crust.

Larger, irregularly shaped plutons are sometimes grouped together as massive plutons. A stock is a type of massive pluton that results from an ancient magma chamber deep within the crust and upper mantle. Stocks are usually about the size of a large mountain and are often connected to one or more volcanoes at the surface. The magma body that forms the basis of the stock may feed active volcanoes and may also feed dikes and connected sills leading from the body of the magma chamber toward the surface. Geologists usually restrict stocks to formations that are less than 100 kilometers (62 miles) across.

The largest igneous formations are called batholiths, which result from several magma chambers that have joined; batholiths can extend for thousands of square kilometers just under the surface of the crust. Batholiths may take millions of years to cool and solidify and may be renewed by infusions of newly generated magma from within the mantle.

Batholiths are often connected to collections of stocks, dikes, and sills in massive networks that extend toward the surface of the crust. The Sierra Nevada in California are formed from a massive batholith. The interior of many mountains contain batholith material that cooled during the mountains’ formation. Because of a batholith’s size, erosion does not extend to its lower level, making it impossible to know exactly where the lower limit of the batholith resides. Some of the larger batholiths may extend through most of the crust into the upper mantle.

In some cases, intrusive rock bodies may form close to the surface, leading to a formation that has characteristics between those of rocks that form on the surface and those that form at deeper depths. These types of igneous formations are called hypabyssal and result in rocks that cannot be classified as either plutons or volcanic rocks. Hypabyssal rocks are usually porphyritic in structure, containing a medium-grained sediment punctuated by inclusions of various minerals and other types of rock. Hypabyssal rocks are rocks that did not cool as quickly as those rocks that reached the surface; hypabyssal rocks took less time to cool than rocks characteristic of batholiths, stocks, and other deep plutons. This intermediate cooling period leads to the structural variations observed in the rocks.

Uses for Igneous Rock

Because igneous rocks vary widely in texture, color, and other characteristics, they are essential to many industrial processes. One of the most common types of igneous rock is granite, which is the main component of many intrusive deposits, including batholiths and stocks found within mountains and under much of the earth’s crust.

Granite is one of the most common types of basement rock, which is the layer of rock that occurs under the uppermost surface layer of the earth’s crust. Because of its thickness and stability, granite is commonly used as a sculptural and building material, especially in the manufacture of headstones and statues.

Basalt is another common type of igneous rock that typically forms from extrusive lava flows. Large basalt deposits are formed in deep-sea rifts where the continents diverge and magma is forced to the surface. Because it is rapidly cooled at the surface, basalt is a finely grained stone, largely devoid of intrusive minerals. The mineral composition of basalt tends to result in relatively dark colored stone.

Basalt underlies more of the earth’s surface than any other type of rock, and its abundance has made it a popular stone for construction. Basalt is often ground and used as a base for concrete and other types of composite stones in building and sculpture.

Pumice is another kind of volcanic rock that forms when lava is rapidly ejected from the mouth of a volcano. As the rock cools, minerals within the lava begin to depressurize, leaving hollow pores and pockets within the rock. Pumice is an important component of insulating stone used in many construction projects. The open pores trap air, which creates a buffer zone mediating the loss of heat from the internal environment. In addition, ground pumice is commonly used as an abrasive additive to cleaners and hand soaps. Shaped pumice stones are sometimes used as a cosmetic aid to rub away callused and dead skin from the body.

Some types of crystallized igneous rocks are used in the manufacture of jewelry because of their attractive appearance. Igneous glass formations, like obsidian and basaltic glass, are often polished and used to make jewelry. A number of gemstones are found within igneous rock formations, including both diamonds and emeralds. For this reason, mining of igneous formations is important to the jewelry industry. Many types of precious metals are found within igneous deposits, too, and the deposits from dikes and stocks can become an important source of precious metals.

Principal Terms

aphanitic: igneous rocks with fine grains that can be distinguished only with visual aid

crystalline: solid structure composed of atoms arranged in a regular, repeating pattern to form a repeating three-dimensional structure

extrusion: igneous rocks that are formed after liquid lava cools on the surface of the earth

feldspar: family of silicon-based minerals that occur in igneous rocks; formed from silica combined with calcium, potassium, and sodium in various concentrations

felsic: igneous rocks mainly comprising silica blended with feldspar and composed largely of such lighter elements as oxygen, sodium, potassium, and aluminum

intrusion: liquid rock that forms beneath the earth’s surface

mafic: igneous rocks that are rich in magnesium and iron and contain less feldspar

pahoehoe: rope-like structures formed when a flow of lava cools on the earth’s surface

phaneritic: igneous rocks with large grains that can be seen without a microscope

pluton: a body of igneous rock formed by intrusion

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