Igneous Rock







Igneous rocks (etymology from latin ignis, fire) are Rock (geology)s formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as Intrusion (plutonic) rocks or on the surface as extrusive (geology) (Volcanic rock) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle (geology) or crust (geology). Typically, the melting is caused by one or more of the following processes — an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust (geology).

Geologic significance Igneous rocks make up approximately ninety-five percent of the upper part of the Earth's crust, but their great abundance is hidden on the Earth's surface by a relatively thin but widespread layer of Sedimentary rock and Metamorphic rock rocks.

Igneous rocks are geologically important because:



Morphology and setting In terms of modes of occurrence, igneous rocks can be either intrusion (plutonic) or extrusive (geology) (volcanic rock). Intrusive igneous rocks Intrusive igneous rocks are formed from magma that cools and solidifies within the earth. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are coarse grained. The mineral grains in such rocks can generally be identified with the naked eye. Intrusion (geology) rocks can also be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, stocks, laccoliths, Sill (geology)s and dike (geology). The extrusive rocks often produce lava flows.

The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite. When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earth's surface.

Coarse grained intrusive igneous rocks which form at depth within the earth are termed as abyssal; intrusive igneous rocks which form near the surface are termed hypabyssal.

Extrusive igneous rocks Extrusive igneous rocks are formed at the Earth's surface as a result of the partial melting of rocks within the mantle (geology) and crust.

The melt, with or without suspended crystals and gas bubbles, is called magma. Magma rises because it is less dense than the rock from which it was created. When it reaches the surface, magma extruded onto the surface either beneath water or air, is called lava. Eruptions of volcanoes into air are termed subaerial whereas those occurring underneath the ocean are termed submarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity.

The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions:Fisher, R. V. & Schmincke H.-U., (1984) Pyroclastic Rocks, Berlin, Springer-Verlag

Magma which erupts from a volcano behaves according to its viscosity, determined by temperature, composition, and crystal content. High-temperature magma, most of which is basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma such as andesite tends to form cinder cones of intermingled volcanic ash, tuff and lava, and may have viscosity similar to thick, cold molasses or even rubber when erupted. Felsic magma such as rhyolite is usually erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma commonly erupt explosively, and rhyolitic lava flows typically are of limited extent and have steep margins, because the magma is so viscous.

Felsic and intermediate magmas that erupt often do so violently, with explosions driven by release of dissolved gases — typically water but also carbon dioxide. Explosively erupted pyroclastic material is called tephra and includes tuff, agglomerate and ignimbrite. Fine volcanic ash is also erupted and forms ash tuff deposits which can often cover vast areas.

Because lava cools and crystallizes rapidly, it is fine grained. If the cooling has been so rapid as to prevent the formation of even small crystals after extrusion, the resulting rock may be mostly glass (such as the rock obsidian). If the cooling of the lava happened slowly, the rocks would be coarse-grained.

Because the minerals are mostly fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous rocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a optical microscope, so only an approximate classification can usually be made in the field.

Classification Igneous rock are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body.

The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. Two important variables used for the classification of igneous rocks are particle size, which largely depends upon the cooling history, and the mineral composition of the rock. Feldspars, quartz or feldspathoids, olivines, pyroxenes, amphiboles, and micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks. All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessory minerals. Types of igneous rocks with other essential minerals are very rare, and these rare rocks include those with essential carbonates.

In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence of quartz, and in rocks with no feldspar or quartz, the type of iron or magnesium minerals present. Rocks containing quartz (silica in composition) are silica-oversaturated. Rocks with feldspathoids are silica-undersaturated, because feldspathoids cannot coexist in a stable association with quartz.

Igneous rocks which have crystals large enough to be seen by the naked eye are called phaneritic; those with crystals too small to be seen are called aphanitic. Generally speaking, phaneritic implies an intrusive origin; aphanitic an extrusive one.

An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is termed porphyry (geology). Porphyritic texture develops when some of the crystals grow to considerable size before the main mass of the magma crystallizes as finer-grained, uniform material.

Texture Texture is an important criterion for the naming of volcanic rocks. The rock microstructure of volcanic rocks, including the size, shape, orientation, and distribution of mineral grains and the intergrain relationships, will determine whether the rock is termed a tuff, a pyroclastic lava or a simple lava.

However, the texture is only a subordinate part of classifying volcanic rocks, as most often there needs to be chemical information gleaned from rocks with extremely fine-grained groundmass or from airfall tuffs, which may be formed from volcanic ash.

Textural criteria are less critical in classifying intrusive rocks where the majority of minerals will be visible to the naked eye or at least using a hand lens, magnifying glass or microscope. Plutonic rocks tend also to be less texturally varied and less prone to gaining structural fabrics. Textural terms can be used to differentiate different intrusive phases of large plutons, for instance porphyritic margins to large intrusive bodies, Porphyry (geology) stocks and subvolcanic dike (geology)s (apophyses). Mineralogical classification is used most often to classify plutonic rocks. Chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite".



Chemical classification Igneous rocks can be classified according to chemical or mineralogical parameters:

Chemical: total alkali-silica content (TAS classification) for volcanic rock classification used when modal or mineralogic data is unavailable: Note: the acid-basic terminology is used more broadly in older (generally British) geological literature. In current literature felsic-mafic roughly substitutes for acid-basic.

Chemical classification also extends to differentiating rocks which are chemically similar according to the TAS diagram, for instance;