Essential Questions
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How do igneous rocks form?
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How can the composition of magma be described?
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What are the factors that affect how rocks melt and crystallize?
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What are igneous rocks?
Vocabulary
Review
New
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silicate
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lava
igneous rock
partial melting
Bowen’s reaction series
fractional crystallization
What are igneous rocks?
Igneous Rock Formation
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Lava is magma that flows out onto Earth’s surface.
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Igneous rocks form when lava or magma cools and minerals crystallize.
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What are igneous rocks?
Igneous Rock Formation
Composition of magma
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The type of igneous rock that forms depends on the composition of the
magma, which is often a slushy mix of molten rock, dissolved gases, and
mineral crystals.
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Magma is classified as basaltic, andesitic, or rhyolitic, based on the
amount of silica it contains. Silica affects melting temperature and impacts
how quickly magma flows.
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What are igneous rocks?
Igneous Rock Formation
Magma formation
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Magma can be formed either by melting of Earth’s crust or by melting within
the mantle.
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The four main factors involved in the formation of magma are temperature,
pressure, water content, and mineral content.
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What are igneous rocks?
Igneous Rock Formation
Magma formation
•
Temperature generally increases
with depth in Earth’s crust. This
temperature increase is known as
the geothermal gradient.
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Temperature in Earth’s crust
increases 30°C every 1 km in depth.
At just over 3 km it is how enough to
make water boil (100°C).
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What are igneous rocks?
Igneous Rock Formation
Magma formation
•
Pressure also increases with depth as a result of the weight of overlying
rock. The increased pressure on a rock also increases the rock’s
melting point.
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Rocks and minerals often contain small percentages of water, which
changes the melting point of the rocks. As water content increases, the
melting point decreases.
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What are igneous rocks?
Igneous Rock Formation
Mineral content
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Rocks melt at lower or higher temperatures because their component
minerals have lower or higher melting points. In general, rocks that are rich
in iron and magnesium melt at higher temperatures than rocks that
contain higher levels of silicon.
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What are igneous rocks?
Igneous Rock Formation
Partial melting
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The process whereby some minerals
melt at relatively low temperatures
while other minerals remain solid is
called partial melting.
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If temperatures are not high enough to
melt the entire rock, the resulting
magma will have a different
composition than that of the original
rock.
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What are igneous rocks?
Bowen’s Reaction Series
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Canadian geologist N. L. Bowen demonstrated that as magma cools and
crystallizes, minerals form in predictable patterns in a process now known as
the Bowen’s reaction series.
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What are igneous rocks?
Bowen’s Reaction Series
Iron-rich minerals
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The left branch of Bowen’s reaction series represents the iron-rich minerals,
which undergo abrupt changes as magma cools and crystallizes.
Feldspars
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The right branch of Bowen’s reaction series represents the plagioclase
feldspars, which undergo a continuous change of composition as magma
cools.
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What are igneous rocks?
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What are igneous rocks?
Fractional Crystallization
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When magma cools, the first minerals that crystallize are the last minerals
that melted during partial melting. This process is called fractional
crystallization.
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Under certain conditions, newly formed crystals can be removed from
magma, ending the chemical reactions between the magma and the minerals.
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What are igneous rocks?
Fractional Crystallization
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What are igneous rocks?
Review
Essential Questions
•
How do igneous rocks form?
•
How can the composition of magma be described?
•
What are the factors that affect how rocks melt and crystallize?
Vocabulary
• lava
• igneous rock
• partial melting
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• Bowen’s reaction series
• fractional crystallization
What are igneous rocks?