Rock Cycle

The earth recycles. Igneous, sedimentary and metamorphic rocks are constantly changing to form new rocks. Old rock is eroded and deposited somewhere else to become sedimentary rock. Tectonic plates subduct into the mantle to melt and form igneous rock. Intrusive igneous rocks and sedimentary rocks are buried deep in the earth and are subjected to high temperature and pressure to become metamorphic rock. This never-ending cycle has been going on for over 4 billion years. The earth continually break down and rebuilding itself.

The Rock Cycle is a group of changes. Igneous rock can change into sedimentary rock or into metamorphic rock. Sedimentary rock can change into metamorphic rock or into igneous rock. Metamorphic rock can change into igneous or sedimentary rock.

Igneous rock forms when magma cools and makes crystals. Magma is a hot liquid made of melted minerals. The minerals can form crystals when they cool. Igneous rock can form underground, where the magma cools slowly. Or, igneous rock can form above ground, where the magma cools quickly. When it pours out on Earth's surface, magma is called lava.On Earth's surface, wind and water can break rock into pieces. They can also carry rock pieces to another place. Usually, the rock pieces, called sediments, drop from the wind or water to make a layer. The layer can be buried under other layers of sediments. After a long time the sediments can be cemented together to make sedimentary rock. In this way, igneous rock can become sedimentary rock. All rock can be heated. But where does the heat come from? Inside Earth there is heat from pressure (push your hands together very hard and feel the heat). There is heat from friction (rub your hands together and feel the heat). There is also heat from radioactive decay (the process that gives us nuclear power plants that make electricity).So, what does the heat do to the rock? It bakes the rock. Baked rock does not melt, but it does change. It forms crystals. If it has crystals already, it forms larger crystals. Because this rock changes, it is called metamorphic. Remember that a caterpillar changes to become a butterfly. That change is called metamorphosis. Metamorphosis can occur in rock when they are heated to 300 to 700 degrees Celsius.When Earth's tectonic plates move around, they produce heat. When they collide, they build mountains and metamorphose (met-ah-MORE-foes) the rock. The rock cycle continues. Mountains made of metamorphic rocks can be broken up and washed away by streams. New sediments from these mountains can make new sedimentary rock.

The rock cycle never stops.

Rock Cycle

Metamorphic Rocks

Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means "change in form ".The protolith is subjected to heat and pressure (temperatures greater than 150 to 200 °C) 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. 

Some examples of metamorphic rocks are gneiss, slate, marble and schist.

Formation

They can be formed by 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.Metamorphic rocks are rocks that have "morphed" into another kind of rock. These rocks were once igneous or sedimentary rocks. How do sedimentary and igneous rocks change? The rocks are under tons and tons of pressure, which fosters heat build up, and this causes them to change. If you exam metamorphic rock samples closely, you'll discover how flattened some of the grains in the rock are.

Metamorphic minerals

Metamorphic minerals are those that form only at the high temperatures and pressures associated with the process of metamorphism. These minerals, known as index minerals, include sillimanite, kyanite, staurolite, andalusite, and some garnet.

Other minerals, such as olivines, pyroxenes, amphiboles, micas, feldspars, and quartz, may be found in metamorphic rocks, but are not necessarily the result of the process of metamorphism. These minerals formed during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process. However, all minerals are stable only within certain limits, and the presence of some minerals in metamorphic rocks indicates the approximate temperatures and pressures at which they were formed.

Classification

The two basic types of metamorphic rocks are Foliated metamorphic rock and Non-Foliated metamorphic rock.

Foliated metamorphic rock:

A foliated metamorphic rock will have banded minerals. The mineral flakes will appear to be parallel to the rock and will look layered. When a foliated rock breaks, a thin rock fragment will result.

Foliated rocks are most often formed from mudstones and contain "fine-grained" or "platy" minerals that are usually too small to see with the naked eye; although some can be seen without aid. Examples of foliated rocks are slate, phyllite and schist.

Slate

Non-foliated metamorphic rock:

A nonfoliated rock will have almost the opposite texture. The minerals will appear to be randomly oriented without obvious banding and have a granular appearance. Unlike a foliated rock, there will be no layers and they will not flake apart into thin layers when broken.
Nonfoliated rocks contain more coarse grained minerals and generally have a random shape. Because of this, these rocks are very granular in appearance. Examples of nonfoliated rocks are quartzite, marble and anthracite coal.

Marble

Uses:

1. Marble is mostly used for furnishing surfaces due to its lustrous color and glossy appearance.
2. Slate is commonly used for roofing and flooring or it can also be crushed and used in paints.
3. Quartzite is a decorative stone and may be used to cover walls, as roofing tiles, as flooring, and stair steps. Crushed quartzite is sometimes used in road construction
4. Schist rock is used for decorative rock walls, pillars, paving, jewelry and other pieces.
5. Gneiss has many uses as a building material such as flooring, ornamental stones, gravestones, facing stones on buildings and work surfaces.

Sedimentary Rocks

Sedimentary rock is one of the three main rock groups (the others being igneous and metamorphic rock). Rock formed from sediments covers 75-80% of the Earth's land area, and includes common types such as chalk, limestone, dolomite, sandstone, conglomerate and shale.

Sedimentary rocks are classified by the source of their sediments, and are produced by one or more of;

clastic rock formed from fragments broken off from parent rock, by weathering in situ or erosion by water, ice or wind, followed by transportation of sediments, often in suspension, to the place of deposition; biogenic activity; or precipitation from solution.

The sediments are then compacted and converted to rock by the process of lithification.

Formation

Sedimentary rocks are formed because of the overburden pressure as particles of sediment are deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension. As sediment deposition builds up, the overburden (or 'lithostatic') pressure squeezes the sediment into layered solids in a process known as lithification ('rock formation') and the original connate fluids are expelled. The term diagenesis is used to describe all the chemical, physical, and biological changes, including cementation, undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface weathering.

Sedimentary rocks are laid down in layers called beds or strata. That new rock layers are above older rock layers is stated in the principle of superposition.There are usually some gaps in the sequence called unconformities. These represent periods in which no new sediments were being laid down, or when earlier sedimentary layers were raised above sea level and eroded away.

Sedimentary rocks contain important information about the history of Earth. They contain fossils, the preserved remains of ancient plants and animals. Coal is considered a type of sedimentary rock. The composition of sediments provides us with clues as to the original rock. Differences between successive layers indicate changes to the environment which have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remains.

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 5% of the total. As such, the sedimentary sequences we see represent only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks.

Classification

Sedimentary rocks are classified into three groups. These groups are clastic, chemical precipitate and biochemical or biogenic.

Clastic

Clastic sedimentary rocks are composed of discrete fragments or clasts of materials derived from other rocks. They are composed largely of quartz with other common minerals including feldspar, amphiboles, clay minerals, and sometimes more exotic igneous and metamorphic minerals.

Clastic sedimentary rocks, such as breccia or sandstone, were formed from rocks that have been broken down into fragments by weathering, which then have been transported and deposited elsewhere.

Clastic sedimentary rocks may be regarded as falling along a scale of grain size, with shale being the finest with particles less than 0.002 mm, siltstone being a little bigger with particles between 0.002 to 0.063 mm, and sandstone being coarser still with grains 0.063 to 2 mm, and conglomerates and breccias being more coarse with grains 2 to 263 mm. Breccia has sharper particles, while conglomerate is categorized by its rounded particles. Particles bigger than 263 mm are termed blocks (angular) or boulders (rounded). Lutite, Arenite and Rudite are general terms for sedimentary rock with clay/silt-, sand- or conglomerate/breccia-sized particles.

Conglomerate

Chemical

The second type of sedimentary rocks are the result of chemical deposits. Halite is an example of a rock that formed when water evaporated and left behind minerals. Sedimentary rocks that result from chemical deposits are called chemical rocks.

The formations you see in caves are made when water drips from the top of the cave and leaves behind minerals. These formations are another example of chemical rocks. 

Halite

Organic

The third form of sedimentary rocks are known as organic rocks. Organic rocks are formed from the life processes of living organisms or the remains of the organisms. Limestone that is formed from the shells of clams and other organisms is organic. When you write with chalk on the chalkboard, you are actually using the shells of organisms that lived long ago.

Chalk

Uses:

1. Coal is mainly used in power plants to generate electricity.
2. Limestone is used mainly in the manufacture of Portland cement, the production of lime, manufacture of paper, petrochemicals, fiberglass.
3. Shale can be a component of bricks and cement.
4. Conglomerate is mainly used in the construction industry.
5. Halite is used in the food industry and as rock salt to melt snow on roads.

Igneous Rocks

CLASSIFICATION OF ROCKS

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Igneous rocks (etymology from Latin ignis, fire) are rocks formed by solidification of cooled magma (molten rock), 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 the Earth's mantle or crust. 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. These have diverse properties, depending on their composition and how they were formed.

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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 and metamorphic rocks.

Igneous rocks are geologically important because:

1. 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.
2. 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.
3. their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions (see plate tectonics).

Classification

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

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. 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.

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. 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. The extrusive 

rocks often produce lava flows.

Granite

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 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:

1. divergent boundary: 73%
2. convergent boundary (subduction zone): 15%
3. hotspot: 12% 

Basalt

Uses:

1. Granite is used for long lasting monuments and for trim and decoration on buildings. Granite looks attractive when polished and is often used for the facades(fronts) of buildings, expensive kitchen worktops and gravestones.
2. Pumice is actually a kind of glass and not a mixture of minerals. Because this rock is so light, it is used quite often as a decorative landscape stone. Ground to a powder, it is used as an abrasive in polish compounds and in Lava© soap.
3. Basalt can be rich source of iron and is commonly used as an aggregate in the building of roads.
4. Gabbro can be polished to a brilliant black luster and can be used to make cemetery markers, kitchen counter tops, floor tiles, facing stone and other dimension stone products.
5. Pegmatite rock has limited use as an architectural stone. However, pegmatite deposits often contain gemstones, industrial minerals and rare minerals.

Reference