Igneous Rocks Page
by Owen Borville
January 30, 2021
Learning, Geology, Science
Magma
Magma is molten rock inside the Earth which rises to the surface, cools, and solidifies. These rocks resulting from magma cooling are called igneous rocks. Magma which cools just underneath the surface produces intrusive igneous rocks including granite. Magma which reaches the Earth’s surface is called lava. Rocks which cool off after reaching the Earth’s surface are called extrusive igneous rocks, including basalt. Igneous, sedimentary, or metamorphic rocks can be melted inside the Earth to create magma. Rock which is thrown into the air during a volcanic eruption is called pyroclastic rock or material.
The composition of magma is classified based on the percentage content of silica. Magma containing more than 65 percent silica is called felsic magma. Magma containing 53 to 65 percent silica is called intermediate magma. Magma containing 45 to 52 percent silica is called mafic magma. Mafic magma also contains a large percentage of iron and magnesium.
Viscosity is a property which describes the resistance to flow of a liquid. The temperature of magma is a major factor of the viscosity of magma, as higher temperature would reduce the viscosity and allow the magma to flow easier. However, the composition of magma is the main factor of the viscosity of magma. The internal atomic structure of silica is large and complex, causing higher viscosity in felsic magma. Mafic magma with lower silica content has a lower viscosity and can travel farther distances. Felsic magma has higher silica content and higher viscosity, therefore this magma travels very slow and for short distances.
Bowen's Reaction Series
Bowen’s Reaction Series was introduced by N.L. Bowen, who developed a chart describing the order of crystallization of minerals inside magma as the temperature of the magma decreases (cools). The series also has two branches: the discontinuous branch and the continuous branch. The discontinuous branch begins with the crystallization of the mafic mineral olivine at the highest temperature. Then as temperature decreases pyroxene crystallizes. As temperature decreases further amphibole and finally biotite mica crystallizes. After further temperature decrease, the silica-rich minerals potassium feldspar, muscovite mica, and finally quartz crystallize. As the mafic minerals at the top of the series crystallize, the magma becomes progressively more silica rich.
The continuous branch begins with the crystallization of calcium-rich plagioclase at the highest temperature. As temperature decreases sodium rich plagioclase begins to crystallize. After the calcium and sodium rich plagioclase crystallize, the continuous branch merges with the discontinuous branch and potassium feldspar begins to crystallize as temperature decreases. As temperature decreases further, muscovite mica and finally quartz begin to crystallize. The magma becomes progressively richer in silica as minerals on the upper part of the continuous branch crystallize. The magma also becomes initially richer in sodium plagioclase and potassium feldspar as calcium plagioclase crystallizes at the top of the series.
Partial melting occurs when the magma is not fully melted and solid rock exists inside the melt. This partial melting occurs because of the different melting temperatures of different minerals inside the magma. When portions of the magma solidify, the remaining magma has a different composition. Since the mafic rocks crystallize before the silica rich rocks according to Bowen’s Reaction Series, a partially melted magma is usually more silica rich than the source rock or original magma. Exsolution occurs as different minerals in the magma solution separate as the temperature decreases and crystallization occurs, forming lamellae or layers of alternating minerals, such as plagioclase and potassium feldspar.
Magma can react with surrounding rock inside the Earth, which changes the composition of the magma. Magma can also react with a magma of different composition, which can change the composition of the magma.
Igneous Rock Texture
The texture of igneous rocks is based on the rate of cooling of magma. Extrusive igneous rocks cool quickly and produce small mineral crystals. Intrusive igneous rocks cool slower and are able to produce larger mineral crystals. Aphanitic texture commonly occurs in rapid cooling lava or magma and contains mineral crystals too small to see without a microscope. Phaneritic texture commonly occurs when magma cools slowly and contains visible coarse grained crystals.
Porphyritic texture occurs in rocks with variable sized mineral crystals. In porphyritic texture large mineral crystals, or phenocrysts, are surrounded by significantly smaller mineral crystals or fine grained texture. Igneous rocks containing porphyritic texture are called porphyry. Groundmass or matrix of rock is the fine-grained mass of material in which larger grains or crystals are embedded. Glass is a type of rock formed when lava cools so fast that mineral crystals do not have time to form. Obsidian is a type of volcanic glass. Rocks with no crystalline structure are termed amorphous. Magma containing gases such as water vapor form rocks with holes or cavities called vesicles or vesicular texture. Pyroclastic rock from volcanic explosions produces fragmented texture with angular mineral crystals which can be seen under a microscope.
Igneous Rock Classification
Igneous rocks are classified based on composition and texture. Ultramafic rocks contain less than 45 percent silica and contain mostly iron and magnesium silicate minerals. Common minerals in ultramafic rocks are pyroxene, olivine, and plagioclase feldspar. Peridotite is an ultramafic igneous rock that has two varieties, harzburgite (a rock that contains more olivine than pyroxene) and dunite (a rock consisting almost entirely of olivine). Peridotite typically displays a coarse crystalline texture and is both very dark and very dense. Some peridotite originates from the mantle.
Two common classifications of igneous rocks are alkaline and basic rocks. Alkaline rocks contain a large percentage of sodium and potassium in proportion to silica. Basic rocks contain a large percentage of calcium rich feldspar minerals and no quartz.
Basalt is a hard, dark, fine grained volcanic rock with less than 52 weight percent silica (SiO2). Because of the low silica content of basalt, it has a low viscosity or low resistance to flow. Therefore, basaltic lava can flow quickly and easily move over 20 kilometers from a vent. The low viscosity typically allows volcanic gases to escape without generating enormous eruption columns. Basaltic lava fountains and fissure eruptions form explosive fountains hundreds of meters tall. Common minerals in basalt include olivine, pyroxene, calcium plagioclase feldspar, and amphibole. Basalt is erupted at temperatures between 1100 to 1250 degrees Celsius. Gabbro is a dark, coarse-grained intrusive igneous rock formed by slow cooling at some depth in the Earth's crust. Gabbro is made of calcium-rich plagioclase, amphibole and (or) pyroxene, olivine and is chemically equivalent to basalt.
Andesite is a gray to black colored volcanic rock containing 52 to 63 percent silica. Andesite contains crystals composed primarily of plagioclase feldspar, quartz and one or more of the minerals pyroxene, biotite, and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine. Andesite magma commonly erupts from stratovolcanoes as thick lava flows, some reaching several kilometers in length. Andesite magma can also generate strong explosive eruptions to form pyroclastic flows, surges, and enormous eruption columns. Andesites erupt at temperatures between 900 and 1100 degrees Celsius. Diorite is an intrusive igneous rock and the coarse-grained equivalent to andesite in composition. Diorite is composed of plagioclase feldspar, quartz, amphibole and/or pyroxene. Diorite is similar to gabbro only not as dark, and contains less iron and magnesium.
Granite is a common coarse-grained intrusive igneous rock with at least 65 percent silica. Granite contains quartz, plagioclase feldspar, potassium feldspar, and has a fairly light color. Granite contains more potassium feldspar than plagioclase feldspar. Granite commonly contains biotite and also may contain hornblende. The biotite is dark colored and is distinguished from the light colored feldspar and the gray colored quartz. Rhyolite is a volcanic rock chemically equivalent to granite and commonly light colored, very fine-grained or glassy-looking and can have tiny visible crystals of quartz and/or feldspar dispersed in a glassy white, green, or pink groundmass.
Dacite is a volcanic rock most often light gray, but can be dark gray to black. Dacite consists of 63 to 68 percent silica and includes the minerals plagioclase feldspar, quartz, pyroxene, amphibole and sometimes biotite.
Lamprophyre is a porphyritic potassium-rich igneous rock of extrusive origin commonly forming in dikes and contains feldspar, biotite, amphibole, and pyroxene. Syenite is a coarse-grained igneous rock similar to granite but contains very little or no quartz (less than 5 percent) and has more alkali feldspar than plagioclase feldspar. Syenite also contains dark minerals including biotite and hornblende or pyroxene. Trachyte is a silica-rich low-quartz volcanic rock consisting of high potassium feldspar. Trachyte is the volcanic equivalent of the rock syenite and contains biotite and hornblende or pyroxene.
Pegmatite is a very coarse-grained igneous rock, often with a granitic composition and commonly forms from molten rock rich in water or other volatiles that facilitate the growth of large crystals. Stoping occurs when magma rising through the crust fractures the overlying country rock from the walls surrounding the igneous rock intrusion and bits of the fractured country rock sink into the magma chamber where they can melt or may be preserved upon cooling in the form of fragments called xenoliths. A xenolith is a piece of foreign rock enclosed within an igneous rock.
Kimberlite is an ultrabasic (very low silica content) igneous rock composed of at least 35 percent olivine, together with other minerals such as mica, serpentine, and calcite. Olivine, the main mineral constituent of the rock, is an olive-green, grayish green, or brown mineral made up of magnesium, iron, and silica. Kimberlite magma originates over 100 miles (150 kilometers) deep in the Earth’s mantle and travels in a matter of hours to the Earth’s surface where it forms small volcanic features. Kimberlites are known to contain diamonds.
Tuff is a volcanic rock made up of rock and mineral fragments in a volcanic ash matrix. Tuff is commonly composed of substantial shattered volcanic rock glass or chilled magma blown into the air and deposited. If volcanic particles fall to the ground at a very high temperature, they may fuse together, forming a welded tuff. Volcanic breccia is an igneous rock made of volcanic rock fragments, generally blown from a volcano or eroded from it. Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in color. Small crystals of iron oxide within the glass cause its dark color. Obsidian is often formed in rhyolite lava flows where the lava cools so fast that crystals do not have time to grow. Glass has no regular structure and therefore fractures in smooth conchoidal (curved) shapes.
Pumice is a light-colored, fine grained, frothy volcanic rock with many air pockets formed by rapidly expanding gas in erupting lava and has a rhyolitic composition. Scoria is a very bubbly, vesicular basalt or andesite. Scoria, along with pumice, develops bubbly textures when escaping gas is trapped as lava solidifies. Scoria is more dense and darker in color than pumice. Aerosol is fine liquid or solid particles suspended in the atmosphere. Aerosols resulting from volcanic eruptions are tiny droplets of sulfuric acid or sulfur dioxide that have picked up oxygen and water. Amygdules are the result of secondary minerals filling in gas bubbles or vesicles in volcanic lava.
Intrusive igneous rock bodies are formed by magma underneath the Earth's surface. These rocks include granite, diorite, and gabbro. A pluton is a large body of intrusive igneous rock that solidified within the crust. Batholiths and stocks are types of plutons. Plutonic rock is any igneous rock that cools beneath the surface. Plutons can be classified as shallow, intermediate, or deep. A concordant pluton is parallel to the sedimentary layering while a discordant pluton cuts across the layering. A dike is a sheet-like or tabular-shaped igneous intrusion that cuts across the sedimentary layering, metamorphic foliation, or other texture of a pre-existing rock. A sill is an igneous intrusion that intrudes between geologic layers forming sheets parallel to the strata. A laccolith is a body of igneous rock with a flat bottom and domed top and is parallel to the layers above and below it. A layered intrusion is a mass of intrusive igneous rock which has formed layers based on composition. A lopolith is an igneous intrusion that is commonly concordant and relatively thin in height but large in width, commonly as a bowl shape. Lopoliths commonly consist of mafic rock.
A volcanic neck or volcanic plug is the remaining part of the volcanic pipe which allows magma to flow from below to the surface and consists of erosion resistant rock. The neck is exposed after the less-erosion resistant outer section of the volcano has eroded away. Volcanic necks can be seen in the southwestern United States. A diatreme is another term for a volcanic vent or pipe.
A batholith is a large igneous intrusion of magma, covering areas greater than one hundred square kilometers (40 square miles). Batholiths typically contain many separate intrusions that form over a relatively long period of time. A stock is a relatively small globular or columnar-shaped pluton similar to a batholith only smaller.
A diapir is a forceful, upward intrusion of a rock mass causing deformation to the overlying rock. A diapir can be an igneous intrusion such as magma or sedimentary rock such as evaporites, both of which is less dense than the surrounding rock and contributes to the deformation.
by Owen Borville
January 30, 2021
Learning, Geology, Science
Magma
Magma is molten rock inside the Earth which rises to the surface, cools, and solidifies. These rocks resulting from magma cooling are called igneous rocks. Magma which cools just underneath the surface produces intrusive igneous rocks including granite. Magma which reaches the Earth’s surface is called lava. Rocks which cool off after reaching the Earth’s surface are called extrusive igneous rocks, including basalt. Igneous, sedimentary, or metamorphic rocks can be melted inside the Earth to create magma. Rock which is thrown into the air during a volcanic eruption is called pyroclastic rock or material.
The composition of magma is classified based on the percentage content of silica. Magma containing more than 65 percent silica is called felsic magma. Magma containing 53 to 65 percent silica is called intermediate magma. Magma containing 45 to 52 percent silica is called mafic magma. Mafic magma also contains a large percentage of iron and magnesium.
Viscosity is a property which describes the resistance to flow of a liquid. The temperature of magma is a major factor of the viscosity of magma, as higher temperature would reduce the viscosity and allow the magma to flow easier. However, the composition of magma is the main factor of the viscosity of magma. The internal atomic structure of silica is large and complex, causing higher viscosity in felsic magma. Mafic magma with lower silica content has a lower viscosity and can travel farther distances. Felsic magma has higher silica content and higher viscosity, therefore this magma travels very slow and for short distances.
Bowen's Reaction Series
Bowen’s Reaction Series was introduced by N.L. Bowen, who developed a chart describing the order of crystallization of minerals inside magma as the temperature of the magma decreases (cools). The series also has two branches: the discontinuous branch and the continuous branch. The discontinuous branch begins with the crystallization of the mafic mineral olivine at the highest temperature. Then as temperature decreases pyroxene crystallizes. As temperature decreases further amphibole and finally biotite mica crystallizes. After further temperature decrease, the silica-rich minerals potassium feldspar, muscovite mica, and finally quartz crystallize. As the mafic minerals at the top of the series crystallize, the magma becomes progressively more silica rich.
The continuous branch begins with the crystallization of calcium-rich plagioclase at the highest temperature. As temperature decreases sodium rich plagioclase begins to crystallize. After the calcium and sodium rich plagioclase crystallize, the continuous branch merges with the discontinuous branch and potassium feldspar begins to crystallize as temperature decreases. As temperature decreases further, muscovite mica and finally quartz begin to crystallize. The magma becomes progressively richer in silica as minerals on the upper part of the continuous branch crystallize. The magma also becomes initially richer in sodium plagioclase and potassium feldspar as calcium plagioclase crystallizes at the top of the series.
Partial melting occurs when the magma is not fully melted and solid rock exists inside the melt. This partial melting occurs because of the different melting temperatures of different minerals inside the magma. When portions of the magma solidify, the remaining magma has a different composition. Since the mafic rocks crystallize before the silica rich rocks according to Bowen’s Reaction Series, a partially melted magma is usually more silica rich than the source rock or original magma. Exsolution occurs as different minerals in the magma solution separate as the temperature decreases and crystallization occurs, forming lamellae or layers of alternating minerals, such as plagioclase and potassium feldspar.
Magma can react with surrounding rock inside the Earth, which changes the composition of the magma. Magma can also react with a magma of different composition, which can change the composition of the magma.
Igneous Rock Texture
The texture of igneous rocks is based on the rate of cooling of magma. Extrusive igneous rocks cool quickly and produce small mineral crystals. Intrusive igneous rocks cool slower and are able to produce larger mineral crystals. Aphanitic texture commonly occurs in rapid cooling lava or magma and contains mineral crystals too small to see without a microscope. Phaneritic texture commonly occurs when magma cools slowly and contains visible coarse grained crystals.
Porphyritic texture occurs in rocks with variable sized mineral crystals. In porphyritic texture large mineral crystals, or phenocrysts, are surrounded by significantly smaller mineral crystals or fine grained texture. Igneous rocks containing porphyritic texture are called porphyry. Groundmass or matrix of rock is the fine-grained mass of material in which larger grains or crystals are embedded. Glass is a type of rock formed when lava cools so fast that mineral crystals do not have time to form. Obsidian is a type of volcanic glass. Rocks with no crystalline structure are termed amorphous. Magma containing gases such as water vapor form rocks with holes or cavities called vesicles or vesicular texture. Pyroclastic rock from volcanic explosions produces fragmented texture with angular mineral crystals which can be seen under a microscope.
Igneous Rock Classification
Igneous rocks are classified based on composition and texture. Ultramafic rocks contain less than 45 percent silica and contain mostly iron and magnesium silicate minerals. Common minerals in ultramafic rocks are pyroxene, olivine, and plagioclase feldspar. Peridotite is an ultramafic igneous rock that has two varieties, harzburgite (a rock that contains more olivine than pyroxene) and dunite (a rock consisting almost entirely of olivine). Peridotite typically displays a coarse crystalline texture and is both very dark and very dense. Some peridotite originates from the mantle.
Two common classifications of igneous rocks are alkaline and basic rocks. Alkaline rocks contain a large percentage of sodium and potassium in proportion to silica. Basic rocks contain a large percentage of calcium rich feldspar minerals and no quartz.
Basalt is a hard, dark, fine grained volcanic rock with less than 52 weight percent silica (SiO2). Because of the low silica content of basalt, it has a low viscosity or low resistance to flow. Therefore, basaltic lava can flow quickly and easily move over 20 kilometers from a vent. The low viscosity typically allows volcanic gases to escape without generating enormous eruption columns. Basaltic lava fountains and fissure eruptions form explosive fountains hundreds of meters tall. Common minerals in basalt include olivine, pyroxene, calcium plagioclase feldspar, and amphibole. Basalt is erupted at temperatures between 1100 to 1250 degrees Celsius. Gabbro is a dark, coarse-grained intrusive igneous rock formed by slow cooling at some depth in the Earth's crust. Gabbro is made of calcium-rich plagioclase, amphibole and (or) pyroxene, olivine and is chemically equivalent to basalt.
Andesite is a gray to black colored volcanic rock containing 52 to 63 percent silica. Andesite contains crystals composed primarily of plagioclase feldspar, quartz and one or more of the minerals pyroxene, biotite, and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine. Andesite magma commonly erupts from stratovolcanoes as thick lava flows, some reaching several kilometers in length. Andesite magma can also generate strong explosive eruptions to form pyroclastic flows, surges, and enormous eruption columns. Andesites erupt at temperatures between 900 and 1100 degrees Celsius. Diorite is an intrusive igneous rock and the coarse-grained equivalent to andesite in composition. Diorite is composed of plagioclase feldspar, quartz, amphibole and/or pyroxene. Diorite is similar to gabbro only not as dark, and contains less iron and magnesium.
Granite is a common coarse-grained intrusive igneous rock with at least 65 percent silica. Granite contains quartz, plagioclase feldspar, potassium feldspar, and has a fairly light color. Granite contains more potassium feldspar than plagioclase feldspar. Granite commonly contains biotite and also may contain hornblende. The biotite is dark colored and is distinguished from the light colored feldspar and the gray colored quartz. Rhyolite is a volcanic rock chemically equivalent to granite and commonly light colored, very fine-grained or glassy-looking and can have tiny visible crystals of quartz and/or feldspar dispersed in a glassy white, green, or pink groundmass.
Dacite is a volcanic rock most often light gray, but can be dark gray to black. Dacite consists of 63 to 68 percent silica and includes the minerals plagioclase feldspar, quartz, pyroxene, amphibole and sometimes biotite.
Lamprophyre is a porphyritic potassium-rich igneous rock of extrusive origin commonly forming in dikes and contains feldspar, biotite, amphibole, and pyroxene. Syenite is a coarse-grained igneous rock similar to granite but contains very little or no quartz (less than 5 percent) and has more alkali feldspar than plagioclase feldspar. Syenite also contains dark minerals including biotite and hornblende or pyroxene. Trachyte is a silica-rich low-quartz volcanic rock consisting of high potassium feldspar. Trachyte is the volcanic equivalent of the rock syenite and contains biotite and hornblende or pyroxene.
Pegmatite is a very coarse-grained igneous rock, often with a granitic composition and commonly forms from molten rock rich in water or other volatiles that facilitate the growth of large crystals. Stoping occurs when magma rising through the crust fractures the overlying country rock from the walls surrounding the igneous rock intrusion and bits of the fractured country rock sink into the magma chamber where they can melt or may be preserved upon cooling in the form of fragments called xenoliths. A xenolith is a piece of foreign rock enclosed within an igneous rock.
Kimberlite is an ultrabasic (very low silica content) igneous rock composed of at least 35 percent olivine, together with other minerals such as mica, serpentine, and calcite. Olivine, the main mineral constituent of the rock, is an olive-green, grayish green, or brown mineral made up of magnesium, iron, and silica. Kimberlite magma originates over 100 miles (150 kilometers) deep in the Earth’s mantle and travels in a matter of hours to the Earth’s surface where it forms small volcanic features. Kimberlites are known to contain diamonds.
Tuff is a volcanic rock made up of rock and mineral fragments in a volcanic ash matrix. Tuff is commonly composed of substantial shattered volcanic rock glass or chilled magma blown into the air and deposited. If volcanic particles fall to the ground at a very high temperature, they may fuse together, forming a welded tuff. Volcanic breccia is an igneous rock made of volcanic rock fragments, generally blown from a volcano or eroded from it. Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in color. Small crystals of iron oxide within the glass cause its dark color. Obsidian is often formed in rhyolite lava flows where the lava cools so fast that crystals do not have time to grow. Glass has no regular structure and therefore fractures in smooth conchoidal (curved) shapes.
Pumice is a light-colored, fine grained, frothy volcanic rock with many air pockets formed by rapidly expanding gas in erupting lava and has a rhyolitic composition. Scoria is a very bubbly, vesicular basalt or andesite. Scoria, along with pumice, develops bubbly textures when escaping gas is trapped as lava solidifies. Scoria is more dense and darker in color than pumice. Aerosol is fine liquid or solid particles suspended in the atmosphere. Aerosols resulting from volcanic eruptions are tiny droplets of sulfuric acid or sulfur dioxide that have picked up oxygen and water. Amygdules are the result of secondary minerals filling in gas bubbles or vesicles in volcanic lava.
Intrusive igneous rock bodies are formed by magma underneath the Earth's surface. These rocks include granite, diorite, and gabbro. A pluton is a large body of intrusive igneous rock that solidified within the crust. Batholiths and stocks are types of plutons. Plutonic rock is any igneous rock that cools beneath the surface. Plutons can be classified as shallow, intermediate, or deep. A concordant pluton is parallel to the sedimentary layering while a discordant pluton cuts across the layering. A dike is a sheet-like or tabular-shaped igneous intrusion that cuts across the sedimentary layering, metamorphic foliation, or other texture of a pre-existing rock. A sill is an igneous intrusion that intrudes between geologic layers forming sheets parallel to the strata. A laccolith is a body of igneous rock with a flat bottom and domed top and is parallel to the layers above and below it. A layered intrusion is a mass of intrusive igneous rock which has formed layers based on composition. A lopolith is an igneous intrusion that is commonly concordant and relatively thin in height but large in width, commonly as a bowl shape. Lopoliths commonly consist of mafic rock.
A volcanic neck or volcanic plug is the remaining part of the volcanic pipe which allows magma to flow from below to the surface and consists of erosion resistant rock. The neck is exposed after the less-erosion resistant outer section of the volcano has eroded away. Volcanic necks can be seen in the southwestern United States. A diatreme is another term for a volcanic vent or pipe.
A batholith is a large igneous intrusion of magma, covering areas greater than one hundred square kilometers (40 square miles). Batholiths typically contain many separate intrusions that form over a relatively long period of time. A stock is a relatively small globular or columnar-shaped pluton similar to a batholith only smaller.
A diapir is a forceful, upward intrusion of a rock mass causing deformation to the overlying rock. A diapir can be an igneous intrusion such as magma or sedimentary rock such as evaporites, both of which is less dense than the surrounding rock and contributes to the deformation.
