Igneous rocks: : Rock that forms when hot molten rock (magma or lava) cools and freezes solid. Can be intrusive (formed deep in the earth) or extrusive (formed at the surface of the earth). Magma: : Molten rock beneath the Earth s surface Magma chamber: : A space below ground filled with magma.
In this figure, the rocks formed at the surface through the volcanic eruption are extrusive igneous rocks, while the rocks formed deep in the magma chamber are intrusive igneous rocks. Magma chamber Close up of a magma chamber
Melting occurs in the earth s mantle and lithosphere through three main types: 1) Melting as the result of decrease in pressure (decompression melting). This happens at mid-ocean ridges, mantle plumes and continental rifts. 2) Melting as the result of the addition of volatiles. This happens at subduction zones. 3) Melting as the result of heat transfer from rising magma. This happens in all of the above settings, but at mid-ocean ridges to a lesser degree. Melting in the mantle and lithosphere is what causes magma to form.
This figure is an example of decompression melting for a rock called peridotite, which makes up the mantle underneath oceanic crust. As a rock is brought up from depth to the solidus it begins to melt, and as it reaches the liquidus it will become a complete liquid. Between the two curves, both liquid and solid peridotite exists. The geotherm is the variation of temperature with depth within the earth. The average geothermal gradient in the earth is ~25 C/km.
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06_04b.jpg This figure shows how a solidus changes with the introduction of the volatile water. Melting becomes easier and occurs at lower temperatures when volatiles are introduced into a rock.
06_04c.jpg This is a diagrammatic example of a volatile being introduced into a rock and inducing melting.
There are 4 major types of magmas: Felsic: : igneous rocks or magma that are rich in elements forming feldspar and quartz. Has 66%-76% silica in the melt. Intermediate: : igneous rocks or magma that have between 52%-66% silica in the melt. Transitional between Felsic and Mafic compositions. Mafic: : igneous rocks or magma that are relatively poor in silica and rich in iron and magnesium. Contains 45%-52% silica. Ultramafic: : igneous rocks or magmas that are rich in iron and magnesium and very poor in silica. Contains 38%-45% silica. Felsic intermediate mafic ultramafic
06_05b.jpg Wall rock Magma chamber Magma assimilation: A magma chamber may incorporate the rocks that make up the wall of the chamber by rocks falling into the magma chamber and melting, or when heat from the magma chamber melts the wall of the chamber.
06_05ac.jpg fractional crystallization: : The process by which a magma becomes progressively more silicic as it cools, because early-formed crystals settle out.
06_06.jpg Bowen s reaction series: : The sequence in which different silicate minerals crystallize during the progressive cooling of a melt.
Magma mixing: : Multiple magmas of different compositions combining together to form a magma of a new, integrated composition.
Intrusive igneous settings: Dike: : A tabular (wall-shaped) intrusion of rock that cuts across the layering of country rock. Sill: : A nearly horizontal table-top-shaped tabular intrusion that occurs between the layers of country rock. Laccolith: : A sill that domes upward (convex up). Lopolith: : A sill that domes downward (concave down). Pluton: : An irregular or blob-shaped intrusion; can range in size from tens of m across to tens of km across. Batholith: : A vast composite, intrusive, igneous rock body up to several hundred km long and 100 km wide, formed by the intrusion of numerous plutons in the same region. Xenolith: : A relict of wall rock surrounded by intrusive rock when the intrusive rock freezes. Stoping: : A process by which magma intrudes; blocks of wall rock break off and then sink into the magma. Country rock: : preexisting rock that magma intrudes.
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Dikes intrude by extension of the country rock 06_15ab.jpg Sills can raise up or up lift country rock when they are emplaced
06_12.jpg As a magma chamber is active, it creates igneous forms such as dikes and sills above; as well as extrusive igneous rocks at the surface by volcanic eruptions. After time, the magma chamber can become exposed to form a batholith.
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06_14c.jpg The Sierra Nevada Mts represents the remains of a large volcanic arc that existed ~100Ma along the west cost of north America.
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Behind me is a batholith in the cascade mountains. It also is one of these ~100Ma plutonic rocks that occur in western North America.
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Baked zone: : When the country rock is heated and altered by the intrusion of very hot magma. Chill zone or chill margin: : At the contact with country rock, magma cools quickly due to the heat loss from the magma chamber into the country rock. This results in finer grained rocks at this margin than in the main body of the intrusion.
06_15cd.jpg This figure shows the example of how magma chamber emplacement can result the stoping of country rock. This is an example of assimilation.
06_16.jpg The cooling time is the time needed to turn a liquid magma into a rock. This depends on the depth of a magma; the shape and size of the magma body; and if water is present.
06_17h.jpg Igneous rocks can have a glassy texture, meaning the rock is made of glass or very small minerals surrounded by glass. This rock cooled very quickly.
This is a microscopic photo of very small minerals in a glassy rock.
06_17d.jpg Igneous rocks can have an interlocking crystalline texture, meaning the rock is made of interlocking minerals. This rock cooled very slowly.
06_17b.jpg This microscopic picture shows how igneous rocks are made up of interlocking minerals. These minerals are feldspar, quartz, and mica in this example.
One type of crystalline igneous rock is a very coarse-grained type called a pegmatite. Minerals become very large in pegmatites, and these types of rocks cooled very slowly.
Igneous rocks can also be fragmental (broken-up) when they are formed from a magma. These types are called pyroclastic rocks. To the right is a basalt that was broken-up when it formed; so, now we call it a broke pillow basalt and it is a pyroclastic rock.
Rock classification scheme. 06_18.jpg
Geologists also use a more advanced form of igneous rock classification scheme. This one is based on the % of quartz (Q) feldspar (A and P) and a mineral called feldspathoids (F). This diagram to the right is for intrusive igneous rocks.
This diagram to the left is for extrusive igneous rocks.
Igneous rocks can also be classified by their chemical composition. Below is an example of a classification scheme for extrusive igneous rocks based on the amount of Si, Na, and K in the rock.