Geology And Geophysics

Igneous Rocks in Earths Geology



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The three classes of rocks are formed in different environments and are linked by the rock cycle




There are three major types of rocks : igneous, sedimentary and metamorphic rocks. They are linked by the rock cycle where the natural and continuous repetitions of melting and solidification are one important feature, and moving rocks around is another part of the rock cycle.

The formation of igneous rocks ( e.g. basalt ) occur beneath the surface under high temperatures. There are two main processes : the melting processes, where magma is produced mostly by partial melting of rocks. They need high temperatures to melt and to produce basaltic magma. Partial melting is the reason why the composition of igneous rocks differ from the composition of the original rocks. Melting happens mainly in the mantle.
The other process is fractional crystallization, where on cooling, the magma separates into different fractions. One of them is rich in crystals of minerals, the other fraction is liquid and crystal poor.
Igneous rocks are made of randomly arranged crystals that are interlocking and grown into each other.
There are two types of igneous rocks : extrusive and intrusive.
Magma erupted at the surface form extrusive igneous rocks. They are formed on the surface by rapid cooling and crystallization. Magma will cool rapidly due to low temperatures and produces fine-grained lava, that later hardens and becomes extrusive igneous rock.
Intrusive igneous rocks are course-grained and formed underground by slow cooling because of the surrounding high temperatures, and also by crystallization. Magma chambers crystallize deep beneath the surface to form rocks called plutons. The cooling magma crystallized very slowly to form the grainy rocks.
If cooling is rapid, crystals will be fine-grained, if magma cools and solidifies slowly crystals can grow larger.

Existing rocks, such as extrusive igneous rocks once exposed on the surface will be attacked by weathering and erosion and so they go through another stage of the rock cycle to form a different type of rock - sedimentary rocks.
Sedimentary rocks have fragmental texture with variable grain shapes, and grains are not interlocking.
The formation of sedimentary rocks ( e.g. limestone; sandstone ) happens at the surface through processes of weathering, erosion, transportation and deposition, and under low temperatures.

Weathering can occur in two ways and rocks almost always break down by a combination of these two processes: physical weathering breaks the rocks up by fragmentation, and chemical weathering breaks down rocks and minerals by the
chemical reactions of acidic waters ( e.g. rain ).
The process of erosion follows, removing rock materials with the help of a transporting medium for example wind or river currents or even glaciers.
Sedimentary material is deposited when transporting slows down.
A mixture of fine and course-grained material settles gradually, the course -grained material settling first and the fine grains build up on top pressing the sediment grains together and so eventually a bedding is produced. This is compaction.
When water washes material away from between the grains in the buried sediments, minerals can crystallize in the spaces. This is cementation.
Sediments usually need burial to become sedimentary rocks, and so sediments become lithified through compaction and cementation.
Once buried to great depths they will meet high temperatures and pressures and thus changing their forms to become the third major type of rock, metamorphic rocks.

Metamorphic rocks ( e.g. slate; schist ) have interlocking textures and the crystals align in the same direction.
In metamorphism new minerals grow or old ones recrystallize. These changes can occur when the atoms regroup themselves when reacting to changes in the physical environment, the high pressures and temperatures. Overall the rock's chemical composition does not change.
Metamorphic rocks form deep underground. Some of these metamorphic rocks were sediments at the surface once, got buried under rocks and they were raised to the surface again by tectonic activities ( rock cycle ).
Contact metamorphism involves high temperatures but low pressures, it occurs on the surface when rocks come in contact with hot magma.
Metamorphic rocks produced deep beneath the surface have different textures and minerals due to recrystallization, than the starting material.

At very high temperatures ( e.g. 700 - 1000 C ) rocks (such as schist), may undergo partial melting to produce magmas and so igneous rocks.
The melting of metamorphic rocks that were igneous or sedimentary rocks before completes' a circle of the rock cycle.
The starting material is different for each type of rock that is formed, resulting in differences between them (e.g. grain size ).

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