Igneous Rocks

Porphyry

Porphyry is a textural term for an igneous rock consisting of large-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate rich, generally aphanitic matrix or groundmass. The larger crystals are called phenocrysts.

Magmatic differentiation

magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption.

Intrusive

ntrusive rock, also called plutonic rock, igneous rock formed from magma forced into older rocks at depths within the Earth's crust, which then slowly solidifies below the Earth's surface, though it may later be exposed by erosion.

Extrusive

relating to or denoting rock that has been extruded at the earth's surface as lava or other volcanic deposits.

Volcanic

relating to or produced by a volcano or volcanoes.

Pegmatite

a coarsely crystalline granite or other igneous rock with crystals several centimeters to several meters in length.

Andesitic

A gray, fine-grained volcanic rock. Andesite consists mainly of sodium-rich plagioclase and one or more mafic minerals such as biotite, hornblende, or pyroxene. It often contains small, visible crystals (phenocrysts) of plagioclase. It is the fine-grained equivalent of diorite. Intermediate composition between granitic and basaltic rocks

Pegmatite texture

A pegmatitic texture is one in which the mineral grains are exceptionally large. ... This texture is found in intrusive rocks. The extra large size does not mean that they cooled extra slowly. Instead, the large crystals of a pegmatite formed in a magma that was extra rich in dissolved water.

Pyroclastic texture

A pyroclastic texture shows a mixture of rock fragments, pumice, and volcanic ash. The ash is very fine grained, so only the rock fragments and pumice are identifiable.

Vesicular texture

A pyroclastic texture shows a mixture of rock fragments, pumice, and volcanic ash. The ash is very fine grained, so only the rock fragments and pumice are identifiable.

Assimilation

Assimilation is that process of magmatic differentiation whereby ascending magmas evolve chemically by recruiting easily melted or dissolved components (fusibles) from the walls of their conduits.

Basaltic

Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a composition similar to gabbro.

Bowen's Reaction Series

Bowen's Reaction Series is the order of crystallization of the common silicate minerals from a magma. The idealized progression which in which Bowen determined is still accepted as the general model for the evolution of magmas during the cooling process.

Crystal settling

During the crystallization of magma, the earlier-formed minerals are denser than the liquid portion and settle to the bottom of the magma chamber.

Geothermal gradient

Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior. Away from tectonic plate boundaries, it is about 25-30 °C/km (72-87 °F/mi) of depth near the surface in most of the world.

Glassy texture

Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals.

Granite

Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below Earth's surface. Granite is composed mainly of quartz and feldspar with minor amounts of mica, amphiboles, and other minerals.

Plutonic

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. Plutons include batholiths, stocks, dikes, sills, laccoliths, lopoliths, and other igneous formations.

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. ... The most common felsic rock is granite.

Intermediate (andesitic)

Intermediate rocks are roughly even mixtures of felsic minerals (mainly plagioclase) and mafic minerals (mainly hornblende, pyroxene, and/or biotite). There is little or no quartz.

Crystallization

It is one of the main processes of magmatic differentiation. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the composition of the magma.

Lava

Lava is molten rock generated by geothermal energy and expelled through fractures in planetary crust or in an eruption, usually at temperatures from 700 to 1,200 °C (1,292 to 2,192 °F). The resulting structures after solidification and cooling are also sometimes described as lava.

Mafic

Mafic rocks are dominated by plagioclase and pyroxene (even if you can't see them with the naked eye) and smaller amounts of olivine.

Volatiles

Magma in the mantle and lower crust have a lot of volatiles within and water and carbon dioxide are not the only volatiles that volcanoes release. Also they leak hydrogen sulfide and sulfur dioxide. Sulfur dioxide is usually possible to find in basaltic and rhyolite rocks.

Magma mixing

Magma mixing is a common process in volcanic magma chambers, which are open-system chambers where magmas enter the chamber, undergo some form of assimilation, fractional crystallisation and partial melt extraction (via eruption of lava), and are replenished.

Magma

Molten rock usually located deep within the mantle of the Earth that occasionally comes to the surface through cracks in the mantle or through the eruption of volcanoes. Note: When magma cools and solidifies, it forms igneous rock, of which lava is one type.

Aphanitic texture

Of or relating to an igneous rock in which the crystals are so fine that individual minerals cannot be distinguished with the naked eye. Aphanitic rocks are extrusive rocks that cooled so quickly that crystal growth was inhibited.

Partial melting

Partial melting is an important consideration in geology with respect to the chemical differentiation of crustal rocks. Virtually all rocks on Earth derive from material from the interior of the earth that has been partially melted.

Phaneretic texture

Phaneritic: any coarse-grained igneous rock, often intrusive, usually formed as a result of a longer cooling history (ex. granite, gabbro). Porphyritic: an igneous rock with one mineral (called the phenocryst) exhibiting a grain size larger than the remainder of the minerals (called the groundmass).

Texture

Texture (or rock microstructure) in geology refers to the relationship between the materials of which a rock is composed. ... Porhyritic texture is one in which larger pieces (phenocrysts) are embedded in a background mass made of much finer grains.

Decompression melting

The process of decompression melting involves the upward movement of the earth's mantle to an area of lower pressure. The reduction in overlying pressure enables the rock to melt, leading to magma formation. Magma is formed by the melting of the earth's matle

Porphyritic texture

This porphyritic texture indicates that the magma sat and cooled a bit below the Earth's surface, thus giving time for the large crystals to grow, before erupting onto the surface and cooling very quickly. The large crystals are termed phenocrysts while the aphanitic rest of rock is called the groundmass.

Ultramafic

Ultramafic rocks are dominated by olivine and/or pyroxene.

Phenocryst

a large or conspicuous crystal in a porphyritic rock, distinct from the groundmass.

Fragmental texture

broken rock

Igneous rocks

gneous rock (derived from the Latin word ignis meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava.

Melt

make or become liquefied by heat.

Glass (obsidian)

olcanic glass can refer to the interstitial, or matrix, material in an aphanitic (fine grained) volcanic rock or can refer to any of several types of vitreous igneous rocks. Most commonly, it refers to obsidian, a rhyolitic glass with high silica content. ... Apache tears, a kind of nodular obsidian.

Groundmass

the compact, finer-grained material in which the crystals are embedded in a porphyritic rock.