Ch.4: Igneous Rocks Flashcards
Magma
Molten rock material deep within earth's crust three components: – Liquid – Solid –mineral crystals carried along by the melt – Gas – volatiles like water
Intrusive igneous rocks
Cool slowly underground
Cool at depth
• Lose heat slowly
• Crystals grow large
Phaneritic = see grains
• Most felsic magmas don’t extrude onto surface.
Ex: Gabbro is a coarse-grained, dark-colored, intrusive igneous rock, Granite is a coarse-grained, light-colored, intrusive igneous rock that contains mainly quartz, feldspar
Extrusive igneous rocks
– Cool quickly at the surface → Lava → Pyroclastic debris Aphanitic = don't see grains • Cool at or near the surface. • Cool rapidly • Chill too fast to grow big crystals Most mafic magmas extrude onto surface. Ex: Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase
Lava
hot mixture of gases and molten rocks that come out of the volcano, when magma coems to the surface
Magma formation
Earth's crust does not float on magma Magma forms in special settings that melt existing rocks. Melting is caused by… – Geothermal gradient – Decompression melting – Volatiles – Heat transfer Friction (local effect within subduction zone)
Geothermal gradient
The Earth is hot inside.
– Crustal temperature (T) increases ~25°C per each
kilometer of depth
The geothermal gradient varies from place to place
Decompression melting
release of pressure under a mid-ocean ridge causes solid peridotite to partially melt to form basaltic magma; how ocean crust forms)
– Base of the crust is hot enough to melt mantle rock
– But due to high pressure, the rock does not melt
– A drop in pressure initiates “decompression melting”
• Pressure drops when hot rock rises to shallower depths
Rock melts as T increased
Rock melts as P decreased
Volatiles
addition of volatiles (like water) make rock melt at lower T (evaporate rapidy)
Volatiles lower the melting T of a hot rock.
Common volatiles include water and carbon dioxide.
• Subduction introduces water to the mantle, melting
rock.
Heat Transfer
Rising magma carries mantle heat with it.
This raises the T in nearby crustal rock, which then
melts.
PHANERITIC
grains inside rock are large enough to see
with the unaided eye; magma cooled slowly; minerals had time to grow large
APHANITIC
grains in rock are too small to see without
magnification; magma cooled quickly; minerals didn’t
have time to grow large
PORPHYRITIC texture
two distinct grain sizes;
two rates of cooling, SLOW followed by RAPID
PHENOCRYSTS eurhedral nehedral grew first
GROUNDMASS
anhedral grew after phenocrysts, filled in empty space
Magma Compositions
Four major magma types based on silica (SiO2) percentage:
– FELSIC (feldspar and silica) Granite, rhyolite
– INTERMEDIATE Diorite, andesite
– MAFIC (Mg and Fe-rich) Gabbro, basalt
– ULTRAMAFIC Peridotite
Most mafic magmas extrude onto surface.
• Most felsic magmas don’t.
Upward movement of magma related to:
•Buoyancy (density)
•Viscosity (function of T, silica content, volatiles)
ex) hotter magma flows more easily, has low viscosity
•Size of magma
•Composition of
overlying rock
Magma Variation
Magmas vary chemically due to…
Initial source rock compositions: the source of the
melt dictates the initial composition:
oMantle source → Ultramafic and mafic magmas
oCrustal source → Mafic, intermediate, and felsic
magmas
Assimilation
Magma Mixing
Partial Melting
Fractional Crystallization
Assimilation
Magma moves by…
– Injection into cracks
– Melting overlying rocks
Magma melts the country rock it passes through.
• Blocks of country rock (xenoliths) fall into magma.
• Assimilation of these rocks alters the magma’s
composition.
Assimilation
Magma Mixing
• Different magmas may blend in a magma chamber.
• The result combines the characteristics of the two
magmas.
Fractional Crystallization of Magma
Minerals crystallize in a sequence and settle in the magma chamber. The remaining magma changes in
composition.
Bowen’s Reaction Series
He found with cooling, early-forming crystals settled
out. Bowen discovered that minerals solidify in a specific series. crystallization sequence of a magma is function of silicate structure
Discontinus series
Fe-Mg minerals crystallize in a specific order depending on their silicate structures
• ISOLATED silicates crystallize first
• Followed by SINGLE CHAIN silicates
• Followed by DOUBLE CHAIN silicates
• Followed by SHEET silicates
olivine → pyroxene → amphibole → biotite
Continuous series
Plagioclase changes composition gradually from Ca-rich at higher T to Na-rich at lower T
Ca-rich plagioclase → Na-plagioclase
last minerals to crystallize: K-feldspar + muscovite + quartz
Effects of Fractional Crystallization
Slow crystallization → large grains, phaneritic texture
Rapid crystallization → small grains, aphanitic texture
Slow followed by rapid crystallization → porphyritic texture (phenocrysts and
groundmass)
Magma composition changes due to:
assimilation of country rock as magma rises
magma mixing: combination of magmas as they rise over same source of melting
partial melting: silica-rich minerals melt first; residual rock more mafic
fractional crystallization: mafic minerals crystallize first; remaining melt more sialic
Magmatic intrusive contacts
chilled margins: fine-grained margins produces as magma crystallizes quickly against colder country rock
baked zones: local metamorphism of country rock as it’s “cooked” by magma
xenoliths: blocks of country rock surrounded by magma that cooled into igneous rock
Plutons
batholith: large pluton that forms above subduction zone; may weather into large exfoliation dome
dike: tabular intrusions formed by magma intruding into fracture that cuts across the layering in the country rock
sill: tabular intrusions formed by magma intruding parallel to the layering in the country rock
chilled margins
fine-grained margins produces as magma crystallizes quickly against colder country rock
baked zones
local metamorphism of country rock as it’s “cooked” by magma
xenoliths
blocks of country rock surrounded by magma that cooled into igneous rock
batholith
large pluton that forms above subduction zone; may weather into large exfoliation dome
dike
tabular intrusions formed by magma intruding into fracture that cuts across the layering in the country rock
sill
tabular intrusions formed by magma intruding parallel to the layering in the country rock
mid-ocean ridges
basalt
ocean-ocean convergence
andesite basalt
ocean-continent convergence
andesite, granite
Partial melting
silicon rich materials melt first, residual more mafic
Fractional crystallization
Mafic minerals crystallize first, remaining are more sailic
