rocks formation

Origin of Igneous Rocks:

Igneous rocks are called fire rocks and are formed either underground or above ground. Underground, they are formed when the melted rock, called magma, deep within the earth becomes trapped in small pockets. As these pockets of magma cool slowly underground, the magma becomes igneous rocks.

Igneous rocks are also formed when volcanoes erupt, causing the magma to rise above the earth's surface. When magma appears above the earth, it is called lava. Igneous rocks are formed as the lava cools above ground.

Igneous rocks begin as hot, fluid material, and the word "igneous" comes from the Latin for fire. This material may have been lava erupted at the Earth's surface, or magma (unerupted lava) at shallow depths, or magma in deep bodies (plutons). Rock formed of lava is called extrusive, rock from shallow magma is called intrusive and rock from deep magma is called plutonic.

Igneous rocks form in three main places: where lithospheric plates pull apart at mid-ocean ridges, where plates come together at subduction zones and where continental crust is pushed together, making it thicker and allowing it to heat to melting.

People commonly think of lava and magma as a liquid, like molten metal, but geologists find that magma is usually a mush — a liquid carrying a load of mineral crystals. Magma crystallizes into a collection of minerals, and some crystallize sooner than others. Not just that, but when they crystallize, they leave the remaining liquid

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with a changed chemical composition. Thus a body of magma, as it cools, evolves, and as it moves through the crust, interacting with other rocks, it evolves further. This makes igneous petrology a very complex field.

Geological significance :

The upper 16 kilometres (10 mi) of Earth's crust is composed of approximately 95% igneous rocks with only a thin, widespread covering of sedimentary and metamorphic rocks.

Igneous rocks are geologically important because:

their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted;

their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata, allowing a time sequence of events;

their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions

in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium are commonly associated with granites and diorites, whereas ores of chromium and platinum are commonly associated with gabbros.

What Are the Types of Igneous Rocks?

1)Intrusive igneous rocks:

Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet. 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. Intrusive 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, sills and dikes.

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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 crust are termed as abyssal; intrusive igneous rocks which form near the surface are termed hypabyssal.

2) Extrusive:

Extrusive igneous rocks are formed at the crust's surface as a result of the partial melting of rocks within the mantle and crust. Extrusive Igneous rocks cool and solidify quicker than intrusive igneous rocks. Since the rocks cool very quickly, they are fine grained.

The melted rock, with or without suspended crystals and gas bubbles, is called magma. It rises because it is less dense than the rock from which it was created. When magma reaches the surface from beneath water or air, it 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:[2]

divergent boundary: 73% convergent boundary (subduction zone): 15%

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hotspot: 12%.

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 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 microscope, so only an approximate classification can usually be made in the field.

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3)Hypabyssal igneous rocks :

Hypabyssal igneous rocks are formed at a depth in between the plutonic and volcanic rocks. These are formed due to cooling and resultant solidification of rising magma just beneath the earth surface. Hypabyssal rocks are less common than plutonic or volcanic rocks and often form dikes, sills, laccoliths, lopoliths , or phacoliths.

Texture :

Texture is an important criterion for the naming of volcanic rocks. The texture 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 stocks and subvolcanic dikes (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 ".

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Igneous textures :

Igneous textures include the rock textures occurring in igneous rocks. Igneous textures are used by geologists in determining the mode of origin igneous rocks and are used in rock classification. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic.

Aphanitic (a = not, phaner = visible) rocks in contrast to phaneritic rocks, typically form from lava which crystallize rapidly on or near the Earth' surface. Because extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals. The individual crystals in an aphanitic igneous rock are not distinguisable to the naked eye. Examples of aphanitic igneous rock include basalt, andesite and rhyolite.

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. Examples include obsidian and pumice.

Pegmatitic texture occurs during magma cooling when some minerals may grow so large that they become massive (the size ranges from a few centimetres to several metres). This is typical of pegmatites.

Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below the Earth's surface. As a magma cools slowly the minerals have time to grow and form large crystals. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. Examples of phaneritic igneous rocks are gabbro, diorite and granite.

Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained (aphanitic) matrix. The result is an aphanitic

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rock with some larger crystals (phenocrysts) imbedded within its matrix. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.

Pyroclastic (pyro = igneous, clastic = fragment) textures occur when explosive eruptions blast the lava into the air resulting in fragmental, typically glassy material which fall as volcanic ash, lapilli and volcanic bombs.

Chemical Composition:

Chemical composition of igneous rock can often be estimated just from looking at the rock. Geologists look at the proportions of light-colored and dark-colored minerals in an igneous rock to estimate the chemical makeup of rock. Light-colored or felsic, minerals have more silica in them. Silica is one of the most abundant elements on Earth and is the chief component of quartz. Felsic minerals are most often colorless, white, gray or pink but can be any number of colors. The dark, or mafic, minerals are richer in iron and magnesium. Mafic minerals are chiefly black, brown, dark gray and sometimes green.

The mineral proportions of the rock are what allows geologists to classify rocks chemically. Depending on the proportion of light minerals to dark minerals, igneous rocks can be broken into four main types: felsic, intermediate, mafic and ultramafic. The following list gives more information about igneous chemical categories.

Felsic rocks are high in silica (65% +). They are usually light-colored. Some examples are: Rhyolite (extrusive) and granite (intrusive).

Intermediate rocks have lower silica content (55-65%). They are darker than felsic rocks but lighter than mafic rocks. Some examples are: Andesite/dacite (extrusive) and diorite/granodiorite (intrusive).

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Mafic rocks have low silica content (45-55%). They are usually dark-colored and contain iron and magnesium. Some examples are: Basalt (extrusive) and gabbro (intrusive). Basalt is the rock that is produced at spreading ridges and makes up the sea floor.

Ultramafic rocks have extremely low silica content (less than 45%) and contain large amounts of iron and magnesium. They are usually dark-colored, but high olivine content can lend green shades to the rock. Other rare colors can be found. An example of ultramafic rock is Peridotite (intrusive).

Igneous rocks Examples:

1-Andesite

Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite. The specimen shown is about two inches (five centimeters) across.

2-Basalt

Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene. The specimen shown is about two inches (five centimeters) across.

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3-Gabbro

Gabbro is a coarse-grained, dark colored, intrusive igneous rock that contains feldspar, augite and sometimes olivine. The specimen shown above is about two inches (five centimeters) across.

4-Diorite

Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende and sometimes quartz. The specimen shown above is about two inches (five centimeters) across.

5-Granite

Granite is a coarse-grained, light colored, intrusive igneous rock that contains mainly quartz and feldspar minerals. The specimen above is about two inches (five centimeters) across.

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6-Obsidian

Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. The specimen shown above is about two inches (five centimeters) across.

7-Pegmatitie

Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock. It forms near the margins of a magma chamber during the final phases of magma chamber crystallization. It often contains rare minerals that are not found in other parts of the

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magma chamber. The specimen shown above is about two inches (five centimeters) across.

8-Peridotite

Peridotite is a coarse-grained intrusive igneous rock that is composed almost entirely of olivine. It may contain small amounts of amphibole, feldspar, quartz or pyroxene. The specimen shown above is about two inches (five centimeters) across.

Peridotite is a coarse-grained intrusive igneous rock that is composed almost entirely of olivine. It may contain small amounts of amphibole, feldspar, quartz or pyroxene. The specimen shown above is about two inches (five centimeters) across.

9-Pumice

Pumice is a light-colored vesicular igneous rock. It forms through very rapid solidification of a melt. The vesicular texture is a result of gas trapped in the melt at the time of solidification. The specimen shown above is about two inches (five centimeters) across.

10-Rhyolite

Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar minerals. The specimen shown above is about two inches (five centimeters) across.

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11-Scoria

Scoria is a dark-colored, vesicular, extrusive igneous rock. The vesicles are a result of trapped gas within the melt at the time of solidification. It often forms as a frothy crust on the top of a lava flow or as material ejected from a volcanic vent and solidifying while airborne. The specimen shown above is about two inches (five centimeters) across.

12-Tuff

Welded Tuff is a rock that is composed of materials that were ejected from a volcano, fell to Earth, and then lithified into a rock. It is usually composed mainly of volcanic ash and sometimes contains larger size particles such as cinders. The specimen shown above is about two inches (five centimeters) across.

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