Download Igneous Rock Composition

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Study Guide 4-2
Igneous Rocks
Ms. Grady
How is an igneous rock formed?
 When lava or magma cools and hardens, it forms igneous rocks.
 The two main categories are extrusive and intrusive.
 Because Magma is less dense than the surrounding rock, it is forced upward to the
surface.
 When magma reaches the Earth’s surface, and flows from volcanoes, it is called lava.
Intrusive Rocks
Magma is melted rock material made of
common elements and fluids.
As magma cools, atoms and compounds (in
the liquid) rearrange themselves into new
crystals called mineral grains.
Rocks form as these mineral grains grow
together.
Rocks that form from magma below the
surface are called intrusive igneous
rocks.
Intrusive rocks are found at the surface
only after the layers of rock and soil have
been removed by erosion.
Extrusive Rocks
Extrusive igneous rocks are formed as lava cools
on the surface of Earth.
When lava flows on the surface, it is exposed
(open) to air and water.
Erosion happens when the rocks are
pushed up by forces within Earth.
Mineral grains can’t grow large because the atoms
in the liquid don’t have time to arrange into large
crystals. It has a quick cooling rate.
Extrusive igneous rocks are fine grained.
Intrusive rocks form at depth below the
surface and they are surrounded by other
rocks so it takes a long time for them to
cool.
Lava such as the basaltic lava cools quickly under
these conditions. See picture below.
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Slowly cooled magma produces or makes
mineral grains that are large enough to
be observed by the eye.
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 An igneous rock can be from basaltic, andesitic or granitic magma.
There are 3 types of magma:
Rhyolite Rock (Felsic)
(also known as Granitic
Rocks)
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Light colored
Lots of silica if it
contains quartz and
feldspar.
Low density
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Thick – flows slowly
Ex. rhyolite
Andesitic Rock
Intermediate in color
and silica amounts.
It contains feldspar
and amphiboles.
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Ex. Andesite
Basaltic Rock (Mafic)
Dark colored minerals
Poor in silica
Rich in iron and
magnesium
High density
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Thin - watery
Ex. Basalt
Igneous Rock Classification
 Igneous rocks are classified according to their composition and
texture.
 Composition refers to the minerals of which rocks are formed (how
they are made up).
 Texture means the shape, size, arrangement (order) and distribution
of the minerals inside.
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Igneous Rock Composition
There are 3 basic igneous rock composition which produce 3 colors of
igneous rocks.
 Dark (Mafic)
 Intermediate
 Light (Felsic)
Formed from basaltic magma
Formed from
Formed from granitic or
andesitic magma
rhyolitic magma
Igneous Texture
 There are 4 textures of igneous rock:
 Fine
 Coarse
 Glassy
These grained rocks
have crystals too
small to be seen
without the help of a
microscope.
These extrusive rocks
grains cool quickly on
the surface of the
earth. The fine grained
rock is rhyolite.
Basaltic rocks are found
mainly in the crust
under the ocean and
volcanic islands like
Hawaii.
Consist of
interlocking
mineral crystals
which are all
roughly the
same size and
visible to the
eye.
Ex. Granite
Are shiny and
look like glass.
They cool so
quickly, the
crystals do not
have a chance to
grow.
Ex. obsidian
 Vesicular (full
of holes or
frothy)
Are formed when
lava is full of gas
bubbles cools and
produces a rock
with open holes.
Examples: Pumice
and Scoria
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