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Igneous Rocks
The Nature of Igneous Rocks
 All igneous rocks are made of interlocking crystals of
minerals that cool and crystallize our of magma.
 Minerals are compounds of chemical elements.
 There are 6 types of minerals that are common in igneous
rocks. They are: quartz, micas, feldspars, pyroxenes,
amphiboles, and olivines.
 Magma – It is a natural occurring molten rock material,
generated within the Earth, from which igneous rocks are
formed from.
Magma, Lava, and Igneous Rock
 Every kilometer into the earth you go, the temperature
goes up about 30 degrees Celsius.
 At about 100 km to 350 km into the Earth, the
temperature is great enough to melt rock, and this melted
rock is magma.
 Magma is less dense than the rock around it, so it rises,
like a hot air balloon.
 Remember hot is less dense than cold, that is why hot air
rises.
Intrusive Igneous Rock
 This type of rock forms underneath the surface of the
Earth.
 It is called “Intrusive” because the magma intrudes into
rock that is already there.
Extrusive Igneous Rock
 When magma cools on the surface, it is called extrusive
igneous rock, because the magma (now called lava since it
is on the surface) “extruded” onto the Earth’s surface like
toothpaste from a tube.
How to classify Igneous Rocks
 There are two ways to classify igneous rocks:
 Color
 Texture
Color
The minerals inside an igneous rock determine the color.
Light color minerals are: quartz, feldspar, and mica.
Dark color minerals tend to be pyroxenes, amphiboles, feldspar,
mica, and olivines.
How to classify Igneous Rocks
 Texture
 The crystal size of an igneous rock depends on how fast or
slow the magma cools.
 Slow cooling magma means large crystals
 Fast cooling magma means many small crystals
 If lava cools really fast, it may result in a glassy looking rock.
Lithospheric Plates and Igneous
Rocks
 Lithospheric Plates – a rigid, thin segment of the outermost
layer of the Earth, consisting of the Earth’s crust and part of the
upper mantle. The plate can be assumed to move.
 Convergent Boundary – A boundary line where lithospheric
plates are moving towards each other.
 Divergent Boundary – A boundary line that is getting wider as
lithospheric plates move away from each other.
 Transform Boundary – A boundary line where two lithospheric
plates are sliding past each other. Like two cars on a road.