Igneous Rocks are formed by crystallization indigenous a liquid, or magma. They encompass two species Volcanic or extrusiveigneous rocks kind when the magma cools and crystallizes on the surface ar of the planet Intrusive or plutonic igneous rocks wherein the magma crystallizes in ~ depth in the Earth. Magma is a mixture of liquid rock, crystals, and gas. Defined by a wide variety of chemistry compositions, with high temperature, and properties that a liquid.

Magmas room less dense than surrounding rocks, and will because of this move upward. If magma provides it to the surface it will certainly erupt and later crystallize to type an extrusive or volcanic rock. If that crystallizes before it get the surface it will type an igneous absent at depth dubbed a plutonic or intrusive igneous rock.
Types that Magma

Chemical ingredient of magma is managed by the diversity of facets in the Earth. Si, Al, Fe, Ca, Mg, K, Na, H, and O consist of 99.9%. Because oxygen is for this reason abundant, chemical analyses space usually offered in terms of oxides. SiO2 is the most abundant oxide. Mafic or Basaltic-- SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na intermediary or Andesitic-- SiO2 55-65 wt%, intermediate. In Fe, Mg, Ca, Na, K Felsic or Rhyolitic-- SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na.

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Gases - At depth in the Earth virtually all magmas save gas. Gas offers magmas your explosive character, since the gas increases as pressure is reduced.

largely H2O through some CO2 Minor amounts of Sulfur, Cl , and F Felsic magmas typically have greater gas components than mafic magmas.

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

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

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

Summary Table

Magma Type Solidified volcano 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 %, intermediary 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 of Magma

As we have actually seen the only part of the earth that is liquid is the outer core. But the main point is not most likely to it is in the resource of magmas because it walk not have actually the ideal chemical composition. The outer core is greatly Iron, but magmas are silicate liquids. Thus magmas DO not COME native THE MOLTEN external CORE of THE EARTH. Thus, due to the fact that the remainder of the earth is solid, in order because that magmas to form, some part of the planet must acquire hot sufficient to melt the rocks present. We recognize that temperature increases with depth in the earth along the geothermal gradient. The earth is hot inside due to heat left end from the initial accretion process, due to heat exit by sinking of materials to form the core, and due to heat released by the degeneration of radioactive elements in the earth. Under typical conditions, the geothermal gradient is no high enough to melt rocks, and thus v the exception of the outer core, most of the planet is solid. Thus, magmas type only under unique circumstances. To know this we must first look at exactly how rocks and also mineral melt.

As pressure rises in the Earth, the melt temperature alters as well. Because that pure minerals, there room two general cases.

for a pure dry (no H2O or CO2 present) mineral, the melt temperate increases with increasing pressure.

Three means to generate Magmas

From the above we have the right to conclude that in order to create a magma in the solid part of the earth either the geothermal gradient need to be increased in some way or the melt temperature that the rocks have to be lowered in some way. The geothermal gradient can be increased by upwelling of hot material from listed below either by uprise solid material (decompression melting) or by intrusion the magma (heat transfer). Lowering the melt temperature have the right to be accomplished by adding water or Carbon Dioxide (flux melting).

Decompression Melting - Under normal problems the temperature in the Earth, shown by the geothermal gradient, is reduced than the start of melting of the mantle. Therefore in order for the mantle to melt there needs to be a device to advanced the geothermal gradient. Once such mechanism is convection, wherein hot mantle product rises to reduced pressure or depth, transferring its warm with it.

If the elevated geothermal gradient becomes greater than the initial melt temperature at any type of pressure, climate a partial melt will certainly form. Fluid from this partial melt have the right to be separated native the continuing to be crystals because, in general, liquids have actually a lower thickness than solids. Basaltic magmas show up to originate in this way.

Upwelling mantle shows up to happen beneath oceanic ridges, at warm spots, and also beneath continental rift valleys. Thus, generation of magma in this three atmospheres is likely brought about by decompression melting.

Transfer of Heat- once magmas the were produced by part other device intrude into cold crust, they carry with castle heat. Upon solidification they lose this heat and also transfer it to the surrounding crust. Repetitive intrusions deserve to transfer enough heat to boost the local geothermal gradient and also cause melt of the neighboring rock to generate brand-new magmas.


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

Flux melt - As we experienced above, if water or carbon dioxide are added to rock, the melt temperature is lowered. If the addition of water or carbon dioxide takes location deep in the earth where the temperature is already high, the lowering of melt temperature could cause the rock to partly melt to create magma. One location where water could be introduced is in ~ subduction zones.Here, water existing in the spicy spaces that the subducting sea floor or water existing in minerals prefer hornblende, biotite, or clay minerals would be released by the increasing temperature and also then relocate in to the overlying mantle. Arrival of this water in the mantle would then lower the melt temperature that the mantle to generate partial melts, which might then different from the solid mantle and also rise toward the surface.


Chemical Variability the Magmas

The chemical composition of magma have the right to vary depending upon the absent that originally melts (the resource rock), and procedure that occur throughout partial melting and transport.

Initial composition of Magma

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

In general an ext siliceous magmas type by low levels of partial melting. Together the level of partial melt increases, less siliceous compositions have the right to be generated. So, melt a mafic source thus yields a felsic or intermediary magma.Melting that ultramafic (peridotite source) yields a basaltic magma.

Magmatic Differentiation

But, processes that operate during transportation towards the surface or throughout storage in the late can transform the chemical composition the the magma. These processes are described as magmatic differentiation and include assimilation, mixing, and fractional crystallization.

Assimilation - together magma passes through cooler absent on its way to the surface it might partially melt the neighboring rock and also incorporate this melt into the magma.Because small amounts that partial melting result in siliceous liquid compositions, addition of this melt come the magma will make it much more siliceous. Mixing - If two magmas with various compositions happen to come in contact with one another, they might mix together. The blended magma will have a composition somewhere in between that that the initial two magma compositions. Evidence for mixing is frequently preserved in the resulting rocks.

Fractional Crystallization - when magma crystallizes that does so end a variety of temperature. Every mineral starts to crystallize at a various temperature, and also if these minerals are somehow gotten rid of from the liquid, the fluid composition will certainly change. The processes is referred to as magmatic differentiation by fractional Crystallization.

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Because mafic minerals choose olivine and also pyroxene crystallize first, the procedure results in remove Mg, Fe, and also Ca, and also enriching the fluid in silica.Thus decision fractionation can adjust a mafic magma into a felsic magma.

Crystals can be removed by a range of processes. If the crystals are an ext dense 보다 the liquid, they may sink. If they room less thick than the liquid they will float. If liquid is squeezed the end by pressure, then crystals will be left behind. Removed of crystals deserve to thus change the composition of the liquid part of the magma. Permit me highlight this making use of a very basic case.

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

Now let"s imagine I eliminate 1 MgO molecule by putting it right into a crystal and removing the decision from the magma. Currently what are the percentages of each molecule in the liquid?

If we continue the procedure one an ext time by removing one an ext MgO molecule

Thus, ingredient of liquid can be changed.

Bowen"s Reaction Series Bowen discovered by experiment the the bespeak in which mineral crystallize indigenous a basaltic magma relies on temperature. As a basaltic magma is cooled Olivine and also Ca-rich plagioclase crystallize first. Upon additional cooling, Olivine reacts v the liquid to develop pyroxene and Ca-rich plagioclase react v the fluid to develop less Ca-rich plagioclase. But, if the olivine and Ca-rich plagioclase are removed from the fluid by crystal fractionation, then the staying liquid will be an ext SiO2 rich. If the procedure continues, an original basaltic magma can change to an initial an andesite magma then a rhyolite magma with falling temperature