Download IGNEOUS ROCKS

IGNEOUS ROCKS
• EXTRUSIVE (Volcanic)
– Fine-grained
• INTRUSIVE (Plutonic)
– CoarseCoarse-grained
Rock Cycle
• Equilibrium
• Interrelationships
between:
–
–
–
–
–
igneous rocks
sediment
sedimentary rocks
metamorphic rocks
weathering and erosion
Intrusive structures
• PLUTON
• BATHOLITH
– Large intrusive body
– Exposed in an area greater than 100 square km
– Coalesced smaller plutons
• Smaller bodies are called STOCKS
• Magma moves upward from depth as diapirs
Building a batholith out of plutons
Classification of Igneous Rocks
• Mineralogy
• Chemistry
• Mafic to Silicic
• Ultramafic rock
– Mantle
peridotite
• Varieties of
granite
– Proportions of
qtz/plag/K-spar
Sierra Nevada Batholith
California
Granodiorite
(plagioclase-rich granite)
Torres del Paine, Patagonian Batholith, southern Chile
Granite pluton
Intrusive contact
Chalten or Cerro Fitzroy
Patagonian Andes,
Argentina
Granite
quartz + plagioclase
+ K-spar + biotite)
Gabbro
Adamello Complex, Italian Alps
Diorite pluton, Plag + hbl
+ xenoliths of country rock
Distribution of Plutonic Rocks
• Granite most abundant in continents
– Common in mountain ranges
– Ancient mountain ranges that are now plains
• e.g, Wolf River Batholith in Wisconsin
• Ultramafic rock comprises the upper mantle
Cordilleran
Batholiths
North and
South
America
How does magma form?
• Partial melting of rock
at depth
• Source of heat
– Geothermal gradient
• Rate of Temperature
increase with depth
• Average 30 oC/km depth
• Not same everywhere
• Tells us that heat is
being conducted to the
surface of the earth from
the interior (Core > 5000
deg C)
How does magma form?
• Factors that control melting temperatures
– Pressure
• melting points of minerals increase with pressure
– This is why increasing temperature along the geotherm
alone fails to melt crustal rocks
• reduction in pressure can therefore induce melting
– Water added under pressure
• lowers melting point of minerals
– Mixing of minerals
• lowers melting point of minerals
Partial melting at top of mantle plume
• Mantle plumes
– Narrow upwelling of
hot mantle rock from
great depth
– Brings deep rocks
rapidly to pressures
at which they can
melt
500 km
Partial melting beneath
the Mid-Ocean Ridge
• Diverging plates
– Lithospheric plates spread apart
– Creates narrow upwelling zone
where hot mantle rock of the
asthenosphere rises to fill in the
gap
– Decompression results in partial
melting
– Spreading animation
– Whole mantle upwelling zones
Partial melting in subduction zones
• Converging
plates
•
Addition of water from
subducted crust into
mantle lowers melting
point of mantle rock
•
Mantle rock partially melts
to form basalt
•
Basalt rises into crust
– Evolves to andesite
and rhyolite
– Partially melts crust to
create granite
– Forms Granite
batholiths
Sea level
Oceanic crust
Sedimentary rocks
Basalt
Mantle
(“rigid”)
Oceanic
lithosphere
Kilometers
0
100
Asthenosphere
(mantle)
Continental crust
Continental
lithosphere
Asthenosphere
Sea level
Oceanic crust
Sedimentary rocks
Oceanic
lithosphere
Kilometers
0
100
Basalt
Mantle
(“rigid”)
Asthenosphere
(mantle)
Continental crust
Continental
lithosphere
“Dewatering” of oceanic
crust; water triggers
melting in
asthenosphere
Asthenosphere
Water from subducting crust
Sea level
Oceanic crust
Sedimentary rocks
Continental crust
Basalt
Mantle
(“rigid”)
Oceanic
lithosphere
Kilometers
0
100
Asthenosphere
(mantle)
Continental
lithosphere
“Dewatering” of oceanic
crust; water triggers
melting in
asthenosphere
Asthenosphere
800°C
1000°C
1200°C
1
1 2
80 00 00º
0º 0ºC C
C
Water from subducting crust
Sea level
Oceanic crust
Sedimentary rocks
Oceanic
lithosphere
Kilometers
0
100
Continental crust
Basalt
Mantle
(“rigid”)
Asthenosphere
(mantle)
Continental
lithosphere
“Dewatering” of oceanic
crust; water triggers
melting in
asthenosphere
Asthenosphere
800°C
1000°C
1200°C
1
1 2
80 00 00º
0º 0ºC C
C
Zone where wet mantle
partially melts
Water from subducting crust
Sea level
Oceanic crust
Sedimentary rocks
Oceanic
lithosphere
Kilometers
0
100
Continental crust
Basalt
Mantle
(“rigid”)
Continental
lithosphere
Asthenosphere
(mantle)
Mafic magma
“Dewatering” of oceanic
crust; water triggers
melting in
asthenosphere
Asthenosphere
800°C
1000°C
1200°C
1
1 2
80 00 00º
0º 0ºC C
C
Zone where wet mantle
partially melts
Water from subducting crust
Sea level
Mixing of magma
Intermediate
magma
Granitic plutons emplaced
Silicic magma
Partial melting
of silicic crust
Mafic magma
Kilometers
0
100
How do magmas evolve?
• Partial melting of mantle or crust is
followed by:
• Differentiation
– Crystal Settling
• Assimilation of country rock
• Mixing of magmas
How do magmas evolve?
• Differentiation
– Crystal Settling
How do magmas evolve?
• Assimilation of country rock
How do magmas evolve?
• Mixing of initially separate magmas
Review: Plate Tectonics & Igneous activity
• DIVERGENT BOUNDARY
– Notably at mid-oceanic ridges
– Sea floor Spreading
– Magma from asthenosphere
• Partial melting
– Due to reduced pressure
• Produces mafic magma
• Solidifies into basalt and gabbro
• Becomes oceanic crust
• Unmelted residue remains as ultramafic rock
Review: Plate Tectonics & Igneous activity
• INTRAPLATE IGNEOUS ACTIVITY
– Attributed to mantle plumes
• Partial melting due to reduced pressure on
upwelling mantle rock
• Creates mafic magma (basalt)
– Ocean Island chains
– “Hot spot” magmatism
Review: Plate Tectonics & Igneous activity
• CONVERGENT BOUNDARY
– Origin of Andesite
• Partial melting of asthenosphere above subducted
crust
• Water lowers melting temperature producing
mafic magma
• Ascending magma evolves into intermediate
magma
– Origin of Granite
• Partial melting of lower continental crust
• Heat from mafic magma underplating the crust