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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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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.
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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
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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.
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fractional crystallization: The process by which a magma
becomes progressively more silicic as it cools, because
early-formed crystals settle out.
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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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Sills can raise up or up lift
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country rock when they are
Dikes intrude by extension
of the country rock
emplaced
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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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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.
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This figure shows the example of how magma
chamber emplacement can result the stoping of
country rock. This is an example of
assimilation.
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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.
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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.
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Igneous rocks can have an interlocking crystalline texture, meaning the rock is made of
interlocking minerals. This rock cooled very slowly.
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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.
pegmatit 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.
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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.