A. Ignesious Rocks
How igneous rocks are formed: Igneous rock (derived from the Latin word ignis meaning fire) 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. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three 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 having formed beneath the surface of Earth's crust.
The answer to how are igneous rocks formed is pretty straight forward: Igneous rocks are called fire rocks and are formed either underground or above ground. Underground, they are formed when magma deep within the Earth becomes trapped in small pockets. As these pockets of magma cool slowly they become igneous rocks. Igneous rocks are also formed when volcanoes erupt. Igneous rocks are formed as the lava cools above ground. The upper 16 km of the Earth’s crust is composed of 95% igneous rock.
Intrusive igneous rocks are formed from magma that cools and solidifies underground. These rocks are coarse grained. The mineral grains in such rocks can generally be identified with the unaided eye. They can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Intrusive formations are batholiths, stocks, laccoliths, sills, and dikes. The central core (batholiths) of major mountain ranges consists of intrusive igneous rocks, usually granite and occupy huge areas of the Earth’s surface. Coarse grained intrusive igneous rocks which form deep in the crust are termed as abyssal and those that form near the surface are called hypabyssal.
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 and are fine grained.
Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body.
Two important variables used for the classification of igneous rocks are particle size and the mineral composition of the rock. Feldspar, quartz, olivines, micas, etc are all important minerals in the formation of igneous rocks, and are important to their classification. Types of igneous rocks with other essential minerals are very rare. In simplified classification, igneous rock types are separated by 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 are silica-oversaturated. Rocks with feldspathoids are silica-under saturated.
Igneous rocks which have crystals large enough to be seen with the unaided eye are called phaneritic. Those with crystals too small to be seen are called aphanitic. Usually phaneritic are intrusive rock and aphanitic are extrusive. An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is called porphyry. Porphyritic texture develops when some of the crystals grow before the main mass of the magma crystallizes as finer-grained material.
Igneous rocks cycle: The term igneous rock cycle is not really a term at all, but in terms of the rock cycle igneous rocks are the beginning place. All of the rocks on the surface of our planet were at one time molten rock. Beneath the thin rocky crust of the earth is the inferno of the mantle! This is the origin of igneous rocks.
The mantle: The mantle is the home of magma, the name of molten rock while it is still
beneath the surface of the earth.
Igneous Rock Formation: Igneous rocks are formed from this molten magma. These rocks form when the magma cools and crystallizes. This can happen above ground as with lava. It can also form below the surface when the molten rock rises in the crust but does not reach the surface.
The Cooling off Period: When the magma reaches the surface it cools quickly, a matter of days or weeks. When the magma forms pockets underground it cools much more slowly. This could take thousands or even millions of years. The rate at which the magma cools determines the kind of igneous rocks that are formed. Faster cooling surface lava creates rock that is fine grained or aphanites. The rapid cooling doesn’t allow large crystals to form. In addition most of the gasses are driven off into the atmosphere.
The slower cooling that takes place underground allows larger crystal formation. The Granite pictured above is an example of this type of rock formation.
Igneous Rocks Cycle: After igneous rocks have been formed they can be transformed into metamorphic or sedimentary rocks. They can even be re-melted to form new igneous rocks.
Igneous rocks can:
* Be glassy or frothy in appearance.
* Be felsic, intermediate, mafic, or ultramafic in nature.
* Be porous or full of bubbles.
* Have small crystal structure like basalts.
* Have a combination of large and small interlocked crystals.
* Be denser than other rock types.
* Have an interlocking crystalline structure.
* Appear to be peppered with black specks.
* Be light enough to float (pumice).
B. Sedimentary Rocks
Sedimentary rock is one of the three main rock groups (along with igneous and metamorphic rocks) and is formed in four main ways: by the deposition of the weathered remains of other rocks (known as 'clastic' sedimentary rocks); by the accumulation and the consolidation of sediments; by the deposition of the results of biogenic activity; and by precipitation from solution. Sedimentary rocks include common types such as chalk, limestone, sandstone, clay and shale. Sedimentary rocks cover 75% of the Earth's surface. Four basic processes are involved in the formation of a clastic sedimentary rock: weathering (erosion) caused mainly by friction of waves, transportation where the sediment is carried along by a current, deposition and compaction where the sediment is squashed together to form a rock of this kind.
Sedimentary rocks are formed from overburden pressure as particles of sediment are deposited out of air, ice, or water flows carrying the particles in suspension. As sediment deposition builds up, the overburden (or 'litho static') pressure squeezes the sediment into layered solids in a process known as lithification ('rock formation') and the original connate fluids are expelled. The term digenesis is used to describe all the chemical, physical, and biological changes, including cementation, undergone by sediment after its initial deposition and during and after its lithification, exclusive of surface weathering.
Sedimentary rocks are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle and accumulate or minerals to precipitate from a solution.
Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers which are called agents of denudation.
The sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of and metamorphic rocks. Sedimentary rocks are deposited in layers as strata, forming a structure called bedding. The study of sedimentary rocks and rock strata provides information about the subsurface that is useful for civil engineering, for example in the construction of roads, houses, tunnels, canals or other constructions. Sedimentary rocks are also important sources of natural resources like coal, fossil fuels, drinking water or ores.
The study of the sequence of sedimentary rock strata is the main source for scientific knowledge about the Earth's history, including palaeogeography, pale climatology and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology.
Sedimentology is both part of geology and physical and overlaps partly with other disciplines in the Earth sciences, such as pedology, geomorphology, geochemistry or structural geology.
Sedimentary Rocks Facts: Rocks on Earth are often broken down by the process of weathering. These broken particles, called sediments, are carried away by streams and rivers. When these particles collect and become compacted together over time, they stick together and form a larger structure. These larger structures are referred to as sedimentary rocks.
Interesting Sedimentary Rocks Facts
* Sedimentary rocks are extremely important resources that give us clues about the Earth's past.
* Sedimentary rock has layers of sediments that are arranged according to their density.
* Compaction is the process by which sediments form sedimentary rock.
* Cementation is the process of minerals dissolving and then holding the particles together in a sedimentary rock.
* The process by which sediments harden to form sedimentary rock is called lithification.
* Fossils of organisms become buried over time and are often found in sedimentary rock.
* Sedimentary rocks are divided among three different groups based on their composition and texture.
* Classic sedimentary rock forms when older rocks break down and become compacted together.
* Examples of classic sedimentary rock include sand-stone and breccias.
* Sand-stone is a sedimentary rock that has been used to make housewares since prehistoric times.
* Organic sedimentary rock has a high amount of organic material and can be used as a source of petroleum and tar.
* Examples of organic sedimentary rock include limestone and coal.
* Chemical sedimentary rock forms when minerals come out of a solution and crystallize.
* The majority of chemical sedimentary rock comes from minerals left behind by evaporated water.
* An example of chemical sedimentary rock includes halite which is also known as table salt.
C. Metamorphic Rocks
Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200° C) and pressure (1500 bars), causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion. They are also formed when rock is heated up by the intrusion of hot molten rock called magma from the Earth's interior. The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides information about the temperatures and pressures that occur at great depths within the Earth's crust. Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.
A contact metamorphic rock made of inters layered calcite and serpentine from the Precambrian of Canada. Once thought to be a fossil called Eozoöncanadense. Scale in mm.
Contact metamorphism is the name given to the changes that take place when magma is injected into the surrounding solid rock (country rock). The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it. Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a contact metamorphism aureole? Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed (unchanged) country rock some distance away. The formation of important ore minerals may occur by the process of metasomatism at or near the contact zone.
When a rock is contact altered by an igneous intrusion it very frequently becomes more indurated, and more coarsely crystalline. Many altered rocks of this type were formerly called hornstones, and the term hornfels is often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism. A shale may become a dark argillaceous hornfels, full of tiny plates of brown is biotitic; a marl or impure limestone may change to a grey, yellow or greenish lime-silicate-hornfels or siliceous marble, tough and splintery, with abundant augite, garnet, wollastonite and other minerals in which calcite is an important component. A diabase or andesite may become a diabase hornfels or andesite hornfels with development of new hornblende and biotite and a partial recrystallization of the original feldspar.
Chert or flint may become a finely crystalline quartz rock; sandstones lose their clastic structure and are converted into a mosaic of small close-fitting grains of quartz in a metamorphic rock called quartzite.
If the rock was originally banded or foliated (as, for example, a laminated sandstone or a foliated calc-schist) this character may not be obliterated, and a banded hornfels is the
product; fossils even may have their shapes preserved, though entirely recrystallized, and in many contact-altered lavas the vesicles are still visible, though their contents have usually entered into new combinations to form minerals that were not originally present. The minute structures, however, disappear, often completely, if the thermal alteration is very profound. Thus small grains of quartz in shale are lost or blend with the surrounding particles of clay, and the fine ground-mass of lavas is entirely reconstructed.
By recrystallization in this manner peculiar rocks of very distinct types are often produced. Thus shales may pass into cordierite rocks, or may show large crystals of andalusite (and chiastolite), staurolite, garnet, kyanite and sillimanite, all derived from the aluminous content of the original shale.
A considerable amount of mica (both muscovite and biotite) is often simultaneously formed, and the resulting product has a close resemblance to many kinds of schist. Limestone, if pure, are often turned into coarsely crystalline marbles; but if there was an admixture of clay or sand in the original rock such minerals as garnet, epidote, idocrase, wollastonite, will be present. Sandstones when greatly heated may change into coarse quartzites composed of large clear grains of quartz. These more intense stages of alteration are not so commonly seen in igneous rocks, because their minerals, being formed at high temperatures, are not so easily transformed or recrystallized.
In a few cases rocks are fused and in the dark glassy product minute crystals of spinel, sillimanite and cordierite may separate out. Shales are occasionally thus altered by basaltdikes, and feldspathic sandstones may be completely vitrified. Similar changes may be induced in shales by the burning of coal seams or even by an ordinary furnace.
There is also a tendency for metasomatism between the igneous magma and sedimentary country rock, whereby the chemicals in each are exchanged or introduced into the other. Granites may absorb fragments of shale or pieces of basalt. In that case, hybrid rocks called skarn arise, which don't have the characteristics of normal igneous or sedimentary rocks.
Sometimes invading granite magma permeates the rocks around, filling their joints and planes of bedding, etc., with threads of quartz and feldspar. This is very exceptional but instances of it are known and it may take place on a large scale.
Mississippian marble in Big Cottonwood Canyon, Wasatch Mountains, Utah.
Regional metamorphism, also known as dynamic metamorphism, is the name given to changes in great masses of rock over a wide area. Rocks can be metamorphosed simply by being at great depths below the Earth's surface, subjected to high temperatures and the great pressure caused by the immense weight of the rock layers above. Much of the lower continental crust is metamorphic, except for recent igneous intrusions. Horizontal tectonic movements such as the collision of continents create orogenic belts, and cause high temperatures, pressures and deformation in the rocks along these belts. If the metamorphosed rocks are later uplifted and exposed by erosion, they may occur in long belts or other large areas at the surface. The process of metamorphism may have destroyed the original features that could have revealed the rock's previous history. Recrystallization of the rock will destroy the textures and fossils present in sedimentary rocks. Metasomatism will change the original composition.
Regional metamorphism tends to make the rock more indurate and at the same time to give it a foliated, shistose or gneissic texture, consisting of a planar arrangement of the minerals, so that platy or prismatic minerals like mica and hornblende have their longest axes arranged parallel to one another. For that reason many of these rocks split readily in one direction along mica-bearing zones (schists).
Ingneisses, minerals also tend to be segregated into bands; thus there are seams of quartz and of mica in a mica schist, very thin, but consisting essentially of one mineral. Along the mineral layers composed of soft or fissile minerals the rocks will split most readily, and the freshly split specimens will appear to be faced or coated with this mineral; for example, a piece of mica schist looked at face wise might be supposed to consist entirely of shining scales of mica. On the edge of the specimens, however, the white folia of granular quartz will be visible. In gneisses these alternating folia are sometimes thicker and less regular than in schists, but most importantly less micaceous; they may be lenticular, dying out rapidly. Gneisses also, as a rule, contain more feldspar than schist’s do, and are tougher and less fissile. Contortion or crumbling of the foliation is by no means uncommon; splitting faces are undulose or puckered. Schistosity and gneissic banding (the two main types of foliation) are formed by directed pressure at elevated temperature, and to interstitial movement, or internal flow arranging the mineral particles while they are crystallizing in that directed pressure field.
Rocks that were originally sedimentary and rocks that were undoubtedly igneous may be metamorphosed into schist’s and gneisses. If originally of similar composition they may be very difficult to distinguish from one another if the metamorphism has been great. A quartz-porphyry, for example, and fine feldspathic sandstone, may both be metamorphosed into a grey or pink mica-schist.
Metamorphic Rock Facts
Enjoy our metamorphic rock facts for kids. Find interesting information and a range of examples that help explain what metamorphic rocks are and what makes them different from other kinds of rocks.
* Metamorphic rocks have been changed over time by extreme pressure and heat.
* Metamorphic rocks can be formed by pressure deep under the Earth's surface, from the extreme heat caused by magma or by the intense collisions and friction of tectonic plates.
* Uplift and erosion help bring metamorphic rock to the Earth's surface.
* Examples of metamorphic rocks include anthracite, quartzite, marble, slate, granulite, gneiss and schist.
* Anthracite is a type of coal with a high carbon count, few impurities and with a high luster (meaning it looks shiny).
* Marble is a metamorphic rock that is formed from the sedimentary rock limestone.
* Quartzite is a metamorphic rock that is formed from the sedimentary rock sandstone.
* Slate is a metamorphic rock that is formed from the sedimentary rock mudstone.
* Granulite is a metamorphic rock that is formed from the igneous rock basalt.
* Learn about sedimentary rocks, igneous rocks, other rocks and minerals or fossils.
* Make a fossil cast or check out our rock sorting lesson plan.