Download Magma Formation and Behavior

11/2/2009
Magma Formation and Behavior
Questions
• What causes mantle rock to melt, resulting in
magma formation?
• Why is magma formation restricted to specific
plate tectonic settings?
• Why are mafic (basaltic) magmas erupted along
mid-ocean ridges if they form by the melting of
ultramafic rock (peridotite)
• Why do granitic (felsic) magmas usually
crystallize before reaching Earth’s surface?
(Most intrusive rocks are granite)
• Why are mafic (basaltic) magmas usually
erupted as lavas at Earth’s surface? (Most
volcanic rocks are basalt)
Subduction Zones
• According to plate tectonic theory, most
magmas form in the following settings:
– Subduction zones, along convergent plate
boundaries
– Rift zones, along divergent plate boundaries
– Mantle plumes (hot spots)
Rift Zone
Mantle Plume
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How can magma form by melting of
mantle rock?
How can magma form by melting of
mantle rock?
• Increase Temperature:
• Decrease Pressure
– Least important cause of melting within the
Earth
• There is no source of heat to increase
temperature of mantle/lower crustal rocks
to their melting temperatures.
At high pressures deep within the mantle, atoms in
minerals are so tightly compacted that chemical
bonds cannot be broken to transform the solid
minerals to a liquid (a magma).
Melting by Decrease in Pressure Occurs
Along Mid-Ocean Ridges
– High pressures – atoms in minerals are so
tightly compacted that chemical bonds cannot
be broken to transform the solid minerals to a
liquid (a magma)
– Low pressure – atoms in solid minerals are
spaced apart so that atoms have room to
vibrate and chemical bonds between them
can be broken to form a liquid (a magma)
At low pressures, atoms in solid minerals are
spaced apart so that atoms have room to vibrate
and chemical bonds between them can be broken
to form a liquid (a magma)
Melting by Decrease in Pressure Occurs
Within Mantle Plumes
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Peridotite – ultramafic rock which makes up
Earth’s mantle
Phase Diagram for Melting of Mantle Rock
(peridotite) by Decrease in Pressure
solidus
Phase Diagram – A graph which shows the various
physical conditions under which a phase (such as a solid,
liquid, gas) can exist
Why do magmas form along
subduction zones?
• Rocks within the ‘mantle wedge’ are
‘squeezed’, resulting in an increase in
pressure.
– Therefore, ‘decompression melting’
cannot explain the formation of magmas
within a subduction zone
What unique feature of a subduction zone might
cause melting of mantle rock in the mantle wedge?
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How can magma form by melting of mantle
rock?
Melting by Addition of Water Occurs Within
Subduction Zones
• Addition of Water
– Small amount of water acts as a catalyst to
lower the melting temperature of most
minerals.
• Unequal distribution of electrons around the
water molecule (electrical polarization)
results in decreased anion-cation attraction
within minerals, so that chemical bonds in
the mineral structure are more easily
broken
Phase Diagram for Melting of Mantle Rock
by Addition of Water
Why does cold beer sometimes partially freeze
when the can is opened?
solidus
Partial Melting
Why are mafic (basalt) lavas
erupted along mid-ocean ridges if
the rock being melted to form the
magma is ultramafic (peridotite)?
• Minerals with highest Si content melt at
the lower temperatures than minerals with
low Si content (reverse of Bowen’s
Reaction Series)
– Silicate rocks melt over a range of
temperatures.
– Magmas produced by partial melting of a rock
will have a higher Si content than the ‘parent
rock’ which underwent melting.
• Example: partial melting of an ultramafic rock
forms a mafic magma
• Explains that mafic lavas (basalts) erupted along
MOR’s form by partial melting of ultramafic mantle
rock (peridotite)
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Low temperature minerals at the ‘bottom’ of BRS melt
first, producing a ‘partial melt’ with a higher Si content
than that of the rock being melted.
Partial Melting
– Silicate rocks melt over a range of
temperatures.
– Magmas produced by partial melting of
a rock will have a higher Si content than
the ‘parent rock’ which underwent
melting.
• Partial melting of an ultramafic rock forms a
mafic magma
• Explains that mafic lavas (basalts) erupted
along MOR’s form by partial melting of
ultramafic mantle rock (peridotite)
Partial melting of a mafic rock produces a magma
of what composition?
Why are most mafic (basaltic) magmas
erupted at volcanoes, while most felsic
(granitic) magmas crystallize beneath the
surface?
Felsic Magma vs. Mafic Magma
• Felsic Magmas – most crystallize as intrusive
rocks (granite) beneath Earth’s surface
– High Si content – results in greater viscosity
(resistance to flow); flows to surface very
slowly
– High dissolved water content – water escapes
as steam as magma ascends, increasing
temperatures at which minerals will start to
crystallize
• Higher crystallization temperatures
increase the likelihood that complete
solidification will occur before reaching the
surface
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Why does cold beer sometimes partially freeze
when the can is opened?
‘CO2 – Rich’ Beer
(liquid)
‘CO2 – Dry’ Beer
(solid)
Felsic Magma vs. Mafic Magma
• Mafic Magmas – most are erupted as
basaltic lavas at Earth’s surface
– Low Si Content – results in lower viscosity
(flows to surface quickly)
– Low dissolved water content – loss of water
as magma ascends has little effect on mineral
crystallization temperatures
Summary
• To Crystallize a Magma:
– Decrease temperature
– Increase pressure
– Remove water
• To Melt a Rock:
– Increase temperature
– Decrease pressure
– Add Water
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Fractional Crystallization
• Involves removal of crystals from the
surrounding melt
– Chemical reactions between crystals and
surrounding melt cannot occur
– Results in solid rock of entirely different
chemical composition than that of the melt
from which it formed
Equilibrium Crystallization
• Occurs when crystal remain in contact with
the surrounding melt
– Chemical reactions between solid crystals
and surrounding melt can occur
– Results in solid rock of the same chemical
composition as that of the melt from which it
crystallized
Fractional Crystallization
Equilibrium Crystallization
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