Transcript Igneous-Rock

Earth Science 3.2
Igneous Rocks
Igneous Rocks
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Remember from the Rock Cycle
that igneous rocks form when
magma or lava cools and hardens
Different types of igneous
rocks form when this hardening
occurs depending on a variety of
factors.
Igneous Rocks
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The word igneous comes from
the Latin ignis, which means fire
Fiery volcanic eruptions are
responsible for some types of
igneous rocks.
Still others are formed deep
inside the Earth’s surface.
Igneous Rocks
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Intrusive Igneous Rocks
 Rocks that form when magma hardens
beneath Earth’s surface are called
intrusive igneous rocks.
Igneous Rocks
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Magma consists mainly of the
elements
 silicon and oxygen,
 plus aluminum,
 iron,
 calcium,
 sodium,
 potassium,
 and magnesium.
Igneous Rocks
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Magma also contains some gases
including water vapor.
These gases are kept within the
magma by the pressure of the
surrounding rocks.
Because magma is less dense it
slowly works it’s way toward the
surface.
Igneous Rocks
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As magma rises, it cools slowly,
allowing elements to combine and
form minerals.
Gradually, the minerals grow in
size forming a solid mass of
interlocking crystals.
Granite is a common
intrusive igneous rock.
Extrusive Igneous Rocks
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Extrusive Igneous Rocks:
 Remember that when magma
reaches the surface, we call it
lava.
 Lava is similar to magma except
that most of the gases have
escaped.
 When lava hardens, the rocks that
are formed are called
extrusive igneous rocks
“This is because they are
extruded onto Earth’s surface”
Classifications of Igneous Rocks
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Many different types of igneous
rocks come from these intrusive
and extrusive processes.
Texture and composition are the
two most important
characteristics to look at when we
classify igneous rocks.
Classifications of Igneous Rocks
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Texture describes the appearance
of an igneous rock based on it’s
size, shape and arrangement of
it’s interlocking crystals.
Composition classes of igneous
rocks are based on the
proportions of light and dark
materials in the rock.
Classifications Igneous Rocks
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Course Grained Texture
 The rate of cooling strongly
affects the textures of igneous
rocks.
 Slow cooling results in the
formation of large crystals.
 Igneous rocks with large crystals
exhibit a course-grained texture.
Classifications Igneous Rocks
Fine Grained Texture
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If cooling occurs rapidly instead,
the ions in the melted material
loose their motion and quickly
combine.
This results in large numbers of
tiny crystals that all compete for
the available ions.
Rapid cooling of magma or lava
results in rocks with small
interconnected grains.
These rocks are said to be fine
grained texture.
Classifications Igneous Rocks
Glassy texture
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When lava spews onto Earth’s
surface rapidly, there may not be
enough time for the ions to
arrange themselves into a network
of crystals.
Solids produced this way are made
of randomly distributed ions.
Such rocks have a glassy texture.
Classifications Igneous Rocks
Porphyritic Texture
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A large body of magma located deep within
the Earth may take tens of thousands of
years to harden.
Minerals that crystallize from the magma do
not form at the same rate or at the same
time.
It is possible for some crystals to get quite
large while others are just starting to form.
Classifications Igneous Rocks
Porphyritic Texture
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The resulting rock can have large
crystals, called phenocrysts, while
the surrounding rock has a fine
grain.
These rocks have a porphyritic
texture.
They are a mixture of large and fine
grains intermingled.
Classification of Igneous Rocks
Granite Composition:
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Igneous rocks in which light colored
silicate mineral quartz and feldspar
are the main ingredient are said to
have a granite composition.
In addition to quartz and feldspar,
most granite rock contains about
10% dark silicate minerals.
Classification of Igneous Rocks
Basaltic Compositions
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Rocks that contain many dark
silicate minerals and plagioclase
feldspar have a basaltic composition.
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Basaltic rocks are rich in iron
and magnesium.
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Because of the iron, they are
usually darker and denser than
granite rocks.
The most common basaltic rock is
basalt.
Classification of Igneous Rocks
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Rocks with a composition between
granitic and basaltic rock have an
andesitic composition.
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This group of rocks is named
after the common volcanic rock
andesite.
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Andesitic rocks contain at least
25 % dark silicate minerals.