Igneous rocks

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Igneous rocks are one of the three main rock types (igneous, sedimentary, and metamorphic), that constitute the earth’s external layer. Igneous rocks are formed from the cooling of magma. There are two types of igneous rocks: either intrusive (Plutonic) or extrusive(volcanic), the composition and conditions which are related to the magma whether it is the temperature or the location, are what determine the type of the resulting igneous rocks.

Introduction

The rock cycle

The origin of the term igneous comes from the Latin ‘ignis’ meaning fire. Igneous rocks and metamorphic rocks derived from igneous “parents” make up most of Earth’s crust and mantle. Thus, Earth can be described as a huge mass of igneous rock covered with a thin veneer of sedimentary rock and having a relatively small iron-rich core. The parent material for igneous rocks, called magma, is formed by partial melting that occurs at various levels within Earth’s crust and upper mantle to depths of about 250 km.[1]

Magma, molten rocks that form deep beneath the earth’s surface. Over time magma cools and solidifies, this process is called crystallization, the result from the crystallization process is igneous rocks. Igneous rocks mostly form at plate boundaries, but they can also form far from plate boundaries. Crystallization may occur beneath the surface of the earth and the resulting igneous rock would then be classified as intrusive(plutonic) igneous rocks, or at the surface of the earth following a volcanic eruption and then the resulting igneous rock would be classified as extrusive(volcanic) igneous rock.

Volcanic igneous rocks

Volcanic rocks (often shortened to volcanics in scientific contexts) are formed from lava (magma erupted onto the surface of the earth through volcanoes or fissures. In other words, it differs from other igneous rocks by being of volcanic origin. The term volcanic rock is taken to mean an igneous rock with an aphanitic texture, i.e. a relatively fine-grained (<1 mm) rock in which the individual crystals cannot be distinguished with the naked eye and which is presumed to have formed by relatively fast cooling. Such rocks often contain glass. Volcanics are usually light-colored. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks.

Plutonic igneous rocks

plutonic rocks (also known as Intrusive rocks) are formed from the crystallization of magma beneath the earth’s crust. The term plutonic rock is taken to mean an igneous rock with a phaneritic texture, i.e. a relatively coarse-grained (>3 mm) rock in which the individual crystals can be distinguished with the naked eye and which is presumed to have formed by slow cooling. When magma rises through the lithosphere, they forcefully displace preexisting rock (called host or country rock). After crystallization, the resulting magmatic structures are then called intrusions or plutons. These intrusions include:

  • Sill is a sheet-like body formed by the injection of magma between parallel layers of pre-existing bedded rock.
  • Dike is similar to sills in that it is a sheet-like intrusion, but dikes cut across bedding layers in country-rock, and so are discordant.
  • Vein represent deposits of minerals found within a rock fracture that are foreign to the host rock.
  • Laccolith is igneous rock forcibly injected between sedimentary layers, as a result, the beds above are arched, while those below are relatively flat.
  • Lopolith is a concordant body with a roughly flat top and a shallow convex base, may have a feeder dike or pipe below.
  • Phacolith is a concordant lens-shaped pluton that typically occupies the crest of an anticline or trough of a syncline.
  • Columnar joints are elongated pillar-like columns, which are formed as igneous rocks cool and develop shrinkage fractures.
  • Pipe or volcanic neck is a tubular roughly vertical body that may have been a feeder vent for a volcano.
  • Batholith is the largest intrusions. Batholiths occur as mammoth linear structures several hundreds of kilometers long and up to 100 kilometers wide. Batholiths are almost always made up of felsic and intermediate rock types and are often referred to as “granite batholiths.”
  • Stocks are smaller Batholiths, that would be called batholiths if they were fully exposed.

Composition

The composition of an igneous rock is determined by the minerals present. During the cooling and solidification of the magma, elements combine to form two major groups of silicate minerals. These two major groups of silicate minerals are:

  • The dark silicates: they are rich in magnesium (Mg) and/or iron (Fe), and comparatively low in silicates (SiO₂), hence they are also known as the ferromagnesium silicate. The dark silicate minerals include olivine, pyroxene, amphibole, and biotite mica.
  • The light silicates: they are rich in Potassium (K), Sodium (Na), and Calcium (Ca). They are low in Mg and Fe and rich in SiO₂. They are also known as the non-ferromagnesium silicate. The light silicate minerals include quartz, muscovite mica, and the most abundant mineral group, the feldspars. Feldspars make up at least 40 % of most igneous rocks.
The percent by volume of minerals in each classification of igneous rock. Some minerals are exclusive to one type and can, therefore, be used to identify these rocks (e.g. Potassium feldspar in felsic rocks and olivine in mafic rocks). The composition is also dependent on the temperature of melt

In terms of their mineral composition, igneous rocks can be classified as either Felsic, Intermediate, Mafic, or Ultramafic. -Igneous rocks in which silicates, quartz, and feldspars are the dominant minerals have a granitic composition (granitic rocks = felsic rocks). In addition to quartz and feldspar, most granitic rocks contain about 10 percent dark silicate minerals, usually biotite mica and amphibole. Granitic rocks are rich in silica (about 70 percent) and are major constituents of the continental crust.

- Rocks that contain at least 45 % dark silicate minerals and calcium-rich plagioclase feldspar (but no quartz) are said to have a basaltic composition (basaltic rocks = mafic rocks. Basaltic rocks contain a high percentage of ferromagnesian minerals (mafic minerals).

-Rocks with a composition between granitic and basaltic rocks are said to have an intermediate composition or andesitic composition after the common volcanic rock andesite. Intermediate rocks contain at least 25 % dark silicate minerals, mainly amphibole, pyroxene, and biotite mica with the other dominant mineral being plagioclase feldspar.

- Rocks that contain mostly olivine and pyroxene have an ultramafic composition, a common example is peridotite. Ultramafic rocks are rare so they are sometimes excluded from these groups

Bowen’s reaction series

Bowen's reaction series

Bowen's reaction series explains why certain types of minerals tend to be found together while others are rarely associated with one another, also that the range of igneous rocks, from ultramafic to felsic, can be produced by the same original mafic magma. From the minerals present in a rock the relative conditions under which the material had formed can be known. The minerals at the top of the series are first to crystallize and so the temperature gradient is from high at the top to low at the bottom. Since the surface of the Earth is a low-temperature environment compared to the zones of rock formation, the series also shows the stability of minerals with the ones at the bottom being most stable and the ones at the top being quickest to weather. This is because minerals are most stable in the conditions closest to those under which they were formed. The series has two branches, the continuous reaction series, and the discontinuous.

-The continuous (right) reaction series, involves the plagioclase feldspars. They exhibit gradations in chemical and physical properties. Chemically, this series consists of two "end members": albite or Na plagioclase (NaAlSi3O8) (the sodium) "end-member", and anorthite or Ca plagioclase (CaAlSi2O8) (the calcium) "end-member". There are continuous chemical and physical gradations between the two end-members (Various plagioclase mineral names are given, based on the percentages of calcium and sodium present, including anorthite, bytownite, labradorite, andesine, oligoclase, and albite). This series of plagioclase minerals is called continuous because all of the plagioclase minerals have the same crystal structure. The minerals differ primarily in the proportions of calcium and sodium present.

-The discontinuous (left) reaction series, involves the dark-colored ferromagnesian minerals: olivine pyroxene, amphibole, and biotite. As magma cools, olivine crystallizes first. The olivine crystals react with the remaining magma to form pyroxene. Pyroxene reacts with the magma to form amphibole. Amphibole reacts with the magma to form biotite. Each successive mineral, from olivine to biotite, has a different composition and a different silicate crystal structure. As crystallization proceeds, the crystal structures become more complex (olivine has an isolated tetrahedral structure, pyroxene has a single chain structure, amphibole has a double chain structure, and biotite has a sheet structure). The series of minerals is called discontinuous because a series of different minerals are formed, each with a different crystal structure.

Classification and nomenclature

Igneous rocks cannot all be classified sensibly by using only one system. The primary classification of igneous rocks is based on their mineral composition. If the mineral composition is impossible to determine, because of the presence of glass, or because of the fine-grained nature of the rock, then other criteria may be used, e.g. chemical composition, as in the TAS classification[2]

Modal classification

The primary modal classification or QAPF classification for plutonic and volcanic rocks, which is based on the modal mineral proportions of five minerals. The modal classification consists of two stacked ternary diagrams. This is possible because the minerals at the opposing vertices, Quartz, and Feldspathoids do not occur together within the composition of any igneous rock.

Classification, and nomenclature of plutonic rocks(on the left) and volcanic rocks(on the right), according to their modal mineral contents using the QAPF diagram (based on Streckeisen, 1976)

The QAPF classification classifies plutonic and volcanic igneous rocks (mafic minerals must be less than 90%) on the relative percentage of the five minerals they may contain: Q = quartz A = alkali feldspar (orthoclase, but including albite [sodium plagioclase] if anorthite [calcium plagioclase] content does not exceed 5%) P = plagioclase F = Feldspathoids (silica poor minerals) M = mafic minerals (such as pyroxene, amphibole, olivine, and mica) In the QAPF classification, each corner represents a different mineral. A particular rock sample plots on the diagram as a point within the triangle. The samples fall within specific sub-areas of the triangle, creating a particular rock classification scheme.

  • Rocks with mafic content >90% have their own ultramafic classification.
  • If the mineral mode cannot be determined as is often the case for volcanic rocks, then a chemical classification of total alkalis versus silica (TAS) is used.

The TAS classification

Chemical classification of volcanic rocks using TAS (total alkali–silica diagram) (after Le Bas et al., 1986)

The TAS (Total Alkali-Silica) classification should be used only if:

  1. The rock is considered to be volcanic
  2. A mineral modal classification cannot be determined, because of the presence of glass or because of the fine-grained nature of the rock
  3. A chemical analysis of the rock is available

To use this classification, the values of Na₂O, K₂O, and SiO₂ of the rock must be known.

  • Felsic rocks generally contain >75% felsic minerals. The term ‘acidic’, although sometimes used as a synonym, in current usage refers to high silica content (greater than 63% SiO₂ by weight). The felsic volcanic, Rhyolite (extrusive, also pictured). The most common igneous felsic rock is granite (intrusive, pictured).
  • Intermediate rocks (or andesitic) are those igneous rocks that contain between 52 and 63% silica. Diorite (intrusive) and Andesite (extrusive) are the two most common types of intermediate rock.
  • Mafic rocks have 45-52% silica content. Common mafic rocks include basalt (extrusive) and gabbro (intrusive) as pictured.
  • Ultramafic rocks have low silica content, less than 45%. An example of an ultramafic rock is the intrusive peridotite and extrusive komatiite.

Igneous rock texture

The igneous rock texture helps in identifying the rock origin, mode of crystallization, and classification of the rock. The texture of igneous rocks depends on the factors affecting the crystal size, which are: the amount of silica present, the amount of dissolved gases in the magma, and the most dominant factor which is the rate of cooling. There are six main types of textures:

  • Aphanitic texture is evenly fine-grained texture, because of relatively rapid cooling.
  • Glassy texture very rapid cooling of lava resulting in few or no crystals (example: Obsidian)
  • Pegmatitic texture is a very coarse-grained igneous rock that has a grain size of 20 mm or more.
  • Phaneritic texture evenly coarse-grained texture with large and visible crystals, due to slow cooling.
  • Porphyritic texture is where there are large crystals embedded in a finer-grained groundmass. This forms as a result of a two-stage cooling and crystallization process.
  • Pyroclastic texture formed from the consolidation of individual rock fragments that are ejected during a violent volcanic eruption. The ejected particles might be very fine ash, molten blobs, or large angular blocks torn from the walls of the vent during the eruption.[3]

Magma

Magma is a completely or partially hot molten rock, which on cooling crystallize and solidifies forming igneous rock composed of silicate minerals. As temperature rises, the rock expands and occupies more space. When the temperature is high enough, melting occurs. Melting converts a solid consisting of tight, uniformly packed ions into a liquid composed of unordered ions. As the temperature of the melt drops (cooling reverses the melting process), ions become closely packed together once more. When cooled sufficiently, chemical bonds will form an orderly crystalline arrangement. When magma cools, the Silicon and oxygen atoms will link together first forming silicon-oxygen tetrahedra (basic building blocks of the silicate minerals). As magma continues to lose heat to its surroundings, the tetrahedra join with each other and with other ions. Eventually, all of the melt is transformed into a solid mass of interlocking silicate minerals that we call an igneous rock[4]. The amount of silica present in magma strongly influences its behavior. Granitic magma, which has a high silica content, is quite viscous (“thick”) and may erupt at temperatures as low as 650° C. On the other hand, basaltic magmas are low in silica and are generally more fluid. Basaltic magmas also erupt at higher temperatures than granitic magmas—usually at temperatures between 1050° and 1250°.

Origin of magma

Rocks melt forming magma due to one of three factors or the combination of them, these factors are a decrease in pressure or addition of water or an increase in temperature. Decrease in pressure (decompression melting) Pressure increases with depth affecting the melting temperature of the rocks. Melting, which is accompanied by an increase in volume, occurs at progressively higher temperatures with increased depth. This is the result of the steady increase in confining pressure exerted by the weight of overlying rocks. Reducing confining pressure lowers a rock’s melting temperature. When confining pressure drops sufficiently (for example because of convection currents) decompression melting occurs. Decompression melting occurs at divergent plate boundaries and generates magma from peridotite in the mantle at sea-floor spreading centers.

Addition of water The addition of water to any rocks results in lower melting temperatures. This process occurs mainly at convergent plate boundaries, where cool oceanic plates descend into the mantle.

Increase in temperature (melting crustal rocks) In continental setting, when hot basaltic magma from the mantle rises towards the crust, it may heat the overlaying crustal rocks to a degree that would result in the generation of a secondary, silica-rich magma. If these secondary magmas reach the surface, they tend to produce explosive eruptions that we associate with convergent plate boundaries. Also, during a continental collision (which results in the formation of a large mountain belt), the crust becomes very thick and some crustal rocks are buried at a depths with temperatures high enough to cause partial melting.

Gallery

Igneous rocks texture

Intrusions

References

  1. E.J. Tarbuck and F.K. Lutgens. 2002. Earth An introduction to physical geology. seventh edition, Prentice Hall. 9780134283159
  2. Igneous rocks a classification and glossary of Terms. second edition, 2002. 978-0-511-06651-1
  3. E.J. Tarbuck and F.K. Lutgens. 2002. Earth An introduction to physical geology. seventh edition, Prentice Hall. 9780134283159
  4. E.J. Tarbuck and F.K. Lutgens. 2002. Earth An introduction to physical geology. seventh edition, Prentice Hall. 9780134283159

See also

Further reading

  • Robin Gill (2010) Igneous Rocks and Processes A Practical Guide
  • W.S. Mackenzie, C.H. Donaldson & C. Guilford (1982) Atlas of igneous rocks and their textures

External links