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Igneous Rocks
Igneous Rocks
 Lava is melt that has erupted from a volcano at the
Earth’s surface.
 When the melt is underground, it is called magma.
 Igneous rocks are classified according to texture and
composition.
Igneous Rocks
 Magma forms when rock in the Earth partially melts.
 This can occur during the following conditions:
a. Pressure decreases
b. Volatiles are added
c.
Heat is transferred into the crust by rising magma
Igneous Rocks
 Magma can have various compositions. These
include felsic (silicic), intermediate, mafic, and
ultramafic.
 Composition of the magma is a reflection of the
composition of the original rock from which it forms
and the way the magma forms.
Igneous rocks
 Magma rises because of its buoyancy and pressure
caused by overlying rock.
 The viscosity of magma depends on composition.
 Felsic magma is more viscous than mafic magma.
Igneous Rock
 Intrusive magma cools at a rate that depends on
depth, size, and shape of magma body, and whether
groundwater is present.
 How quickly the magma cools has an effect on the
texture of the igneous rock.
Igneous Rocks
 Extrusive igneous rocks form from lava.
 Intrusive igneous rocks form from magma.
 Lava may solidify to form flows, or it may explode to
form ash.
Igneous Rocks
 Intrusive igneous rocks form when magma intrudes
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into preexisting rock below the Earth’s surface.
Plutons are blob-shaped intrusions.
Tabular intrusions cut across layering.
These layers can be called dikes.
Huge intrusions are batholiths, and they are often
made of many plutons.