Igneous Rocks are created by crystallization native a liquid, or magma. They incorporate two species Volcanic or extrusiveigneous rocks type when the magma cools and crystallizes ~ above the surface of the planet Intrusive or plutonic igneous rocks wherein the magma crystallizes at depth in the Earth. Magma is a mixture of liquid rock, crystals, and also gas. Defined by a wide range of chemistry compositions, v high temperature, and properties the a liquid.

Magmas are less dense than bordering rocks, and will as such move upward. If magma makes it come the surface it will certainly erupt and later crystallize to type an extrusive or volcanic rock. If it crystallizes before it will the surface ar it will kind an igneous absent at depth dubbed a plutonic or intrusive igneous rock.
Types that Magma

Chemical composition of magma is managed by the variety of facets in the Earth. Si, Al, Fe, Ca, Mg, K, Na, H, and O make up 99.9%. Since oxygen is therefore abundant, chemistry analyses room usually provided in regards to oxides. SiO2 is the many abundant oxide. Mafic or Basaltic-- SiO2 45-55 wt%, high in Fe, Mg, Ca, short in K, Na intermediary or Andesitic-- SiO2 55-65 wt%, intermediate. In Fe, Mg, Ca, Na, K Felsic or Rhyolitic-- SiO2 65-75%, short in Fe, Mg, Ca, high in K, Na.

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Gases - At depth in the Earth nearly all magmas save on computer gas. Gas offers magmas their explosive character, because the gas increases as push is reduced.

greatly H2O v some CO2 Minor amounts of Sulfur, Cl , and F Felsic magmas usually have higher gas components than mafic magmas.

Temperature of Magmas Mafic/Basaltic - 1000-1200oC Intermediate/Andesitic - 800-1000oC Felsic/Rhyolitic - 650-800oC. Viscosity the Magmas

Viscosityis the resistance to circulation (opposite of fluidity). Depends on composition, temperature, & gas content.

higher SiO2 contents magmas have greater viscosity than lower SiO2 contents magmas lower Temperature magmas have higher viscosity than higher temperature magmas.

Summary Table

Magma Type Solidified volcanic Rock Solidified Plutonic Rock Chemical Composition Temperature Viscosity Gas Content
Mafic or Basaltic Basalt Gabbro 45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na 1000 - 1200 oC Low Low

or Andesitic

Andesite Diorite 55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K 800 - 1000 oC Intermediate Intermediate
Felsic or Rhyolitic Rhyolite Granite 65-75 SiO2 %, short in Fe, Mg, Ca, high in K, Na 650 - 800 oC High High

Origin the Magma

As we have seen the only component of the earth that is fluid is the external core. But the core is not likely to be the resource of magmas due to the fact that it walk not have the ideal chemical composition. The external core is greatly Iron, however magmas room silicate liquids. Hence magmas DO not COME from THE MOLTEN external CORE of THE EARTH. Thus, due to the fact that the rest of the earth is solid, in order for magmas to form, some component of the planet must gain hot enough to melt the rocks present. We know that temperature rises with depth in the planet along the geothermal gradient. The planet is warm inside due to heat left over from the initial accretion process, because of heat released by sinking of materials to type the core, and due to warm released by the degeneration of radioactive facets in the earth. Under normal conditions, the geothermal gradient is no high sufficient to melt rocks, and also thus through the exemption of the outer core, many of the planet is solid. Thus, magmas type only under special circumstances. To know this we must an initial look at exactly how rocks and mineral melt.

As pressure increases in the Earth, the melt temperature alters as well. For pure minerals, there are two basic cases.

because that a pure dried (no H2O or CO2 present) mineral, the melting temperate rises with increasing pressure.

Three ways to create Magmas

From the above we have the right to conclude the in order to create a magma in the solid component of the earth either the geothermal gradient must be elevated in some way or the melting temperature that the rocks must be lower in part way. The geothermal gradient can be increased by upwelling of warm material from listed below either through uprise solid material (decompression melting) or by intrusion the magma (heat transfer). Lowering the melting temperature deserve to be accomplished by including water or Carbon Dioxide (flux melting).

Decompression Melting - Under normal problems the temperature in the Earth, displayed by the geothermal gradient, is lower than the start of melting of the mantle. Hence in order for the mantle to melt there has to be a mechanism to progressive the geothermal gradient. When such mechanism is convection, wherein warm mantle material rises to lower pressure or depth, transferring its warmth with it.

If the elevated geothermal gradient becomes higher than the initial melt temperature at any pressure, climate a partial melt will form. Fluid from this partial melt have the right to be separated native the remaining crystals because, in general, liquids have a lower density than solids. Basaltic magmas appear to originate in this way.

Upwelling mantle appears to take place beneath oceanic ridges, at warm spots, and beneath continent rift valleys. Thus, generation of magma in this three environments is likely led to by decompression melting.

Transfer the Heat- as soon as magmas the were produced by part other device intrude into cold crust, they carry with lock heat. Top top solidification they lose this heat and transfer it come the neighboring crust. Repetitive intrusions deserve to transfer enough heat to increase the local geothermal gradient and also cause melt of the neighboring rock to generate new magmas.


Transfer of warm by this mechanism might be responsible for generating part magmas in continental rift valleys, warm spots, and also subduction connected environments.

Flux melt - As we observed above, if water or carbon dioxide are included to rock, the melt temperature is lowered. If the addition of water or carbon dioxide takes ar deep in the planet where the temperature is currently high, the lowering of melt temperature could reason the absent to partially melt to create magma. One location where water could be introduced is at subduction zones.Here, water present in the sharp spaces the the subducting sea floor or water current in minerals prefer hornblende, biotite, or clay minerals would be exit by the increasing temperature and also then relocate in come the overlying mantle. Arrival of this water in the mantle would then reduced the melting temperature that the mantle to generate partial melts, which can then different from the solid mantle and rise towards the surface.


Chemical Variability the Magmas

The chemical composition the magma can vary depending on the rock that initially melts (the source rock), and process that occur throughout partial melting and also transport.

Initial composition of Magma

The initial composition of the magma is dictated by the composition of the resource rock and the level of partial melting. In general, melt of a mantle resource (garnet peridotite) results in mafic/basaltic magmas. Melting of crustal resources yields much more siliceous magmas.

In general much more siliceous magmas form by low levels of partial melting. As the degree of partial melt increases, much less siliceous compositions have the right to be generated. So, melting a mafic source thus yields a felsic or intermediary magma.Melting that ultramafic (peridotite source) yields a basaltic magma.

Magmatic Differentiation

But, procedures that operate throughout transportation toward the surface ar or during storage in the tardy can alter the chemical composition that the magma. These processes are referred to as magmatic differentiation and also include assimilation, mixing, and fractional crystallization.

Assimilation - as magma passes v cooler rock on its way to the surface ar it may partially melt the neighboring rock and also incorporate this melt right into the magma.Because little amounts that partial melting result in siliceous fluid compositions, enhancement of this melt to the magma will make it much more siliceous. Mixing - If two magmas with various compositions occur to come in call with one another, they could mix together. The mixed magma will have a composition somewhere between that the the initial two magma compositions. Proof for mix is regularly preserved in the result rocks.

Fractional Crystallization - once magma crystallizes the does so end a variety of temperature. Every mineral begins to crystallize in ~ a various temperature, and if this minerals space somehow gotten rid of from the liquid, the fluid composition will certainly change. The processes is dubbed magmatic differentiation by fractional Crystallization.

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Because mafic minerals prefer olivine and pyroxene crystallize first, the process results in removed Mg, Fe, and Ca, and enriching the liquid in silica.Thus crystal fractionation can change a mafic magma right into a felsic magma.

Crystals can be removed by a variety of processes. If the crystals are much more dense than the liquid, they may sink. If they room less dense than the liquid they will float. If liquid is squeezed out by pressure, then crystals will certainly be left behind. Removed of crystals deserve to thus change the ingredient of the liquid portion of the magma. Let me illustrate this making use of a very an easy case.

Imagine a fluid containing 5 molecule of MgO and also 5 molecules of SiO2. Initially the composition of this magma is expressed as 50% SiO2 and 50% MgO. I.e.

Now let"s imagine I remove 1 MgO molecule by placing it into a crystal and also removing the crystal from the magma. Currently what are the percentages of every molecule in the liquid?

If we proceed the procedure one much more time by removed one an ext MgO molecule

Thus, composition of liquid have the right to be changed.

Bowen"s Reaction Series Bowen found by experiment that the bespeak in which mineral crystallize indigenous a basaltic magma counts on temperature. Together a basaltic magma is cooled Olivine and also Ca-rich plagioclase crystallize first. Upon more cooling, Olivine reacts v the fluid to create pyroxene and also Ca-rich plagioclase react with the liquid to create less Ca-rich plagioclase. But, if the olivine and Ca-rich plagioclase are gotten rid of from the fluid by crystal fractionation, then the staying liquid will certainly be an ext SiO2 rich. If the process continues, an initial basaltic magma can adjust to first an andesite magma climate a rhyolite magma with falling temperature