Igneous petrology

Igneous petrology is the study of igneous rocks—those that are formed from magma. As a branch of geology, igneous petrology is closely related to volcanology, tectonophysics, and petrology in general. The modern study of igneous rocks utilizes a number of techniques, some of them developed in the fields of chemistry, physics, or other earth sciences. Petrography, crystallography, and isotopic studies are common methods used in igneous petrology.

Methods

Determination of chemical composition

The composition of igneous rocks and minerals can be determined via a variety of methods of varying ease, cost, and complexity. The simplest method is observation of hand samples with the naked eye and/or with a hand lens. This can be used to gauge the general mineralogical composition of the rock, which gives an insight into the composition. A more precise but still relatively inexpensive way to identify minerals (and thereby the bulk chemical composition of the rock) with a petrographic microscope. These microscopes have polarizing plates, filters, and a conoscopic lens that allow the user to measure a variety of crystallographic properties. Another method for determining mineralogy is to use X-ray diffraction, in which a powdered sample is bombarded by X-rays, and the resultant spectrum of crystallographic orientations is compared to a set of standards. One of the most precise ways of determining chemical composition is by the use of an electron microprobe, in which tiny spots of materials are sampled. Electron microprobe analyses can detect both bulk composition and trace element composition.

Dating methods

The dating of igneous rocks determines when magma solidified into rock. Radiogenic isotopes are frequently used to determine the age of igneous rocks.

Potassium–argon dating

In this dating method the amount of 40Ar trapped in a rock is compared to the amount of 40K in the rock to calculate the amount of time 40K must have been decaying in the solid rock to produce all 40Ar that would have otherwise not have been present there.

Rubidium–strontium dating

The rubidium–strontium dating is based on the natural decay of 87Rb to 87Sr and the different behaviour of these elements during fractional crystallization of magma. Both Sr and Rb are found in most magmas; however, as fractional crystallization occurs, Sr will tend to be concentrated in plagioclase[1] crystals while Rb will remain in the melt for a longer time. 87Rb decays in magma and elsewhere so that every 1.42×1011 years half of the amount has been converted into 87Sr. Knowing the decay constant and the amount of 87Rb and 87Sr in a rock it is possible to calculate the time that the 87Rb must have needed before the rock reached closure temperature to produce all 87Sr, yet considering that there was an initial 87Sr amount not produced by 87Rb in the magmatic body. Initial values of 87Sr, when the magma started fractional crystallization, might be estimated by knowing the amounts of 87Rb and 87Sr of two igneous rocks produced at different times by the same magmatic body.

Other methods

Stratigraphic principles may be useful to determine the relative age of volcanic rocks. Tephrochronology is the most common application of stratigraphic dating on volcanic rocks.

Thermobarometry methods

In petrology the mineral clinopyroxene is used for temperature and pressure calculations of the magma that produced igneous rock containing this mineral. Clinopyroxene thermobarometry is one of several geothermobarometers. Two things make this method especially useful: first, clinopyroxene is a common phenocryst in igneous rocks easy to identify; and secondly, the crystallization of the jadeite component of clinopyroxene implies a growth in molar volume being thus a good indicator of pressure.

Thermochronometry

Publications

Most contemporary ground breaking in igneous petrology has been published in prestigious American and British scientific journals of worldwide circulation such as Science and Nature. Study material, overviews of certain topics and older works are often found as books. Many works before the plate tectonics paradigm shift in the 1960s and 1970s contains inaccurate information regarding the origin of magmas.

Notable journals that publish igneous petrology studies
Name Publisher Scope
American Mineralogist Mineralogical Society of America Mineralogy, petrology, crystallography, geochemistry
Bulletin of Volcanology Springer Volcanology
Contributions to Mineralogy and Petrology Springer Mineralogy, petrology
Journal of Petrology Oxford University Press Igneous petrology, metamorphic petrology
Journal of Volcanology and Geothermal Research Elsevier Volcanology, geothermal research
Lithos Elsevier Igneous petrology, petrogenesis, metamorphic petrology

Notable igneous petrologists

References

  1. ^ Wilson, M. Igneous Petrogeneis. 1995 fifth edition (1989 first edition). Page 23.
Anorogenic magmatism

In geology, anorogenic magmatism is the formation, intrusion or eruption of magmas not directly connected with orogeny. This contrasts with orogenic magmatism that occurs at convergent plate boundaries where continental collision, subduction and orogeny are common.

Carbon Peak

Carbon Peak, elevation 12,088 ft (3,684 m), is a summit in the West Elk Mountains of Colorado. The peak is southwest of Crested Butte in the Gunnison National Forest. Carbon peak is a laccolith formed during the mid-Tertiary period, and is part of the “laccolith triangle” as described by the Colorado Geological Survey.

Dike (geology)

A dike or dyke, in geological usage, is a sheet of rock that is formed in a fracture in a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma flows into a crack then solidifies as a sheet intrusion, either cutting across layers of rock or through a contiguous mass of rock. Clastic dikes are formed when sediment fills a pre-existing crack.

Extrusive rock

Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. This is as opposed to intrusive rock formation, in which magma does not reach the surface.The main effect of extrusion is that the magma can cool much more quickly in the open air or under seawater, and there is little time for the growth of crystals. Sometimes, a residual portion of the matrix fails to crystallize at all, instead becoming a natural glass or obsidian.

If the magma contains abundant volatile components which are released as free gas, then it may cool with large or small vesicles (bubble-shaped cavities) such as in pumice, scoria, or vesicular basalt. Examples of extrusive rocks include basalt, rhyolite, andesite, obsidian and pumice, scoria, and feldspar.

Felsic

In geology, felsic refers to igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava.

Felsic rocks are usually light in color and have specific gravities less than 3. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite-rich).

Felsite

Felsite is a very fine-grained volcanic rock that may or may not contain larger crystals. Felsite is a field term for a light-colored rock that typically requires petrographic examination or chemical analysis for more precise definition. Color is generally white through light gray, or red to tan and may include any color except dark gray, green or black (the colors of trap rock). The mass of the rock consists of a fine-grained matrix of felsic materials, particularly quartz, sodium and potassium feldspar, and may be termed a quartz felsite or quartz porphyry if the quartz phenocrysts are present. This rock is typically of extrusive origin, formed by compaction of fine volcanic ash, and may be found in association with obsidian and rhyolite. In some cases, it is sufficiently fine-grained for use in making stone tools. Its fine texture and felsic components allow for good knapped pieces, much like working chert, producing conchoidal fracture.

Dendritic manganese oxides such as pyrolusite and/or iron oxides such as limonite may precipitate along rock crevices, giving some rock chunk surfaces multicolored or arborescent patterned textures.

Gabbro

Gabbro ( ) is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term "gabbro" may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic".

Mackenzie dike swarm

The Mackenzie dike swarm, also called the Mackenzie dikes, form a large igneous province in the western Canadian Shield of Canada. It is part of the larger Mackenzie Large Igneous Province and is one of more than three dozen dike swarms in various parts of the Canadian Shield.

The Mackenzie dike swarm is the largest dike swarm known on Earth, more than 500 km (310 mi) wide and 3,000 km (1,900 mi) long, extending in a northwesterly direction across the whole of Canada from the Arctic to the Great Lakes. The mafic dikes cut Archean and Proterozoic rocks, including those in the Athabasca Basin in Saskatchewan, Thelon Basin in Nunavut and the Baker Lake Basin in the Northwest Territories.

The source for the Mackenzie dike swarm is considered to have been a mantle plume center called the Mackenzie hotspot. About 1,268 million years ago, the Slave craton was partly uplifted and intruded by the giant Mackenzie dike swarm. This was the last major event to affect the core of the Slave craton, although later on some younger mafic magmatism registered along its edges.

Mafic

Mafic is an adjective describing a silicate mineral or igneous rock that is rich in magnesium and iron, and is thus a portmanteau of magnesium and ferric. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar.

Chemically, mafic rocks are enriched in iron, magnesium and calcium and typically dark in color. In contrast the felsic rocks are typically light in color and enriched in aluminium and silicon along with potassium and sodium. The mafic rocks also typically have a higher density than felsic rocks. The term roughly corresponds to the older basic rock class.

Mafic lava, before cooling, has a low viscosity, in comparison with felsic lava, due to the lower silica content in mafic magma. Water and other volatiles can more easily and gradually escape from mafic lava. As a result, eruptions of volcanoes made of mafic lavas are less explosively violent than felsic-lava eruptions. Most mafic-lava volcanoes are shield volcanoes, like those in Hawaii.

Matachewan dike swarm

The Matachewan dike swarm is a large 2,500 to 2,450 million year old Paleoproterozoic dike swarm of Northern Ontario, Canada. It consists of basaltic dikes that were intruded in greenschist, granite-greenstone, and metamorphosed sedimentary terrains of the Superior craton of the Canadian Shield. With an area of 360,000 km2 (140,000 sq mi), the Matachewan dike swarm stands as a large igneous province.

Mistassini dike swarm

The Mistassini dike swarm is a 2.5 billion year old Paleoproterozoic dike swarm of western Quebec, Canada. It consists of mafic dikes that were intruded in the Superior craton of the Canadian Shield. With an area of 100,000 km2 (39,000 sq mi), the Mistassini dike swarm stands as a large igneous province.

Partial melting

Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the solid.

Partial melting occurs where the solidus and liquidus temperatures are different. For single minerals this can happen when they exhibit solid solution, for example in olivines between iron and magnesium. In rocks made up of several different minerals, some will melt at lower temperatures than others.

Partial melting is an important process in geology with respect to the chemical differentiation of crustal rocks. On the Earth, partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle and oceanic crust at subduction zones creates continental crust. In all these places partial melting is often associated with volcanism, although some melts do not make it to the surface. Partial melts are thought to play an important role in enriching old parts of the continental. In conclusion partial melting is not all that important and can be forgotten about. lithosphere in incompatible elements. Partial melts produced at depth move upwards due to the compaction of the surrounding matrix.

Peralkaline rock

Peralkaline rocks include those igneous rocks which have a deficiency of aluminium such that sodium and potassium are in excess of that needed for feldspar. The presence of aegerine (sodium pyroxene) and riebeckite (sodium amphibole) are indicative of peralkaline conditions. An example is the peralkaline granite that forms the islet of Rockall in the North Atlantic Ocean.Peralkaline rocks are indicative of continental rift basin-related volcanicity, for example the peralkaline rhyolite lavas of the East African Rift in central Kenya.

Pluton

In geology, a pluton is a body of intrusive igneous rock (called a plutonic rock) that is crystallized from magma slowly cooling below the surface of the Earth. While pluton is a general term to describe an intrusive igneous body, there has been some confusion around the world as to what is the definition of a pluton. Pluton has been used to describe any non-tabular intrusive body, and batholith has been used to describe systems of plutons. In other literature, batholith and pluton have been used interchangeably. In central Europe, smaller bodies are described as batholiths and larger bodies as plutons. In practice the term pluton most often means a non-tabular igneous intrusive body. The most common rock types in plutons are granite, granodiorite, tonalite, monzonite, and quartz diorite. Generally light colored, coarse-grained plutons of these compositions are referred to as granitoids. Examples of plutons include Denali (formerly Mount McKinley) in Alaska; Cuillin in Skye, Scotland; Cardinal Peak in Washington State; Mount Kinabalu in Malaysia; and Stone Mountain in the US state of Georgia.

Subvolcanic rock

A subvolcanic rock, also known as a hypabyssal rock, is an intrusive igneous rock that is emplaced at medium to shallow depths (>2 km) within the crust, and has intermediate grain size and often porphyritic texture between that of volcanic rocks and plutonic rocks. Subvolcanic rocks include diabase (also known as dolerite) and porphyry.

Common examples of subvolcanic rocks are diabase, quartz-dolerite, micro-granite and diorite.

Tephra

Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism.

Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they remain as tephra unless hot enough to fuse together into pyroclastic rock or tuff.

Ungava magmatic event

The Ungava magmatic event was a widespread magmatic event that began about 2.22 billion years ago during the Proterozoic Eon.

University Peak (Alaska)

University Peak is a high peak in the Saint Elias Mountains of Alaska. It is one of the twenty highest peaks in Alaska [1], and one of the fifty highest peaks in the United States[2]. It can be considered a southern outlier of the large massif of Mount Bona. However, it is a much steeper peak than Bona, and presents significant climbing challenges of its own.

The peak was named by Terris Moore during the first ascent of Mount Bona; the name refers to the University of Alaska.

Winagami sill complex

The Winagami sill complex, also called the Winagami sills, is a Paleoproterozoic large igneous province of northwestern Alberta, Canada. It consists of a series of related sills that were formed between 1.89 and 1.76 billion years ago. The Winagami sill complex covers an area of 120,000 km2 (46,000 sq mi).

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