Download 06 Intrusions

Magma & Intrusions
Learning objectives:
1. Learn to interpret igneous textures.
2. Understand igneous intrusive structures.
Where we are
• Today: Plate Tectonics & Intrusive Rocks
- Magma creation
- Magma changes
- Intrusive Features
• Next: Volcanoes & Hazards
[show pillow lava]
Convergent Boundaries
“Subduction Zones”
• It’s all about the water!
• What’s getting subducted?
Convergent Plate
Boundaries
Convergent Plate
Boundaries
Heating of ocean
crust releases fluids
(mostly water) into
mantle above.
Convergent Plate
Boundaries
Heating of ocean
crust releases fluids
(mostly water) into
mantle above.
Added fluids produce
basalt magma from
melting mantle.
Convergent Plate
Boundaries
Buoyant magma rises
to base of crust.
Heating of ocean
crust releases fluids
(mostly water) into
mantle above.
Added fluids produce
basalt magma from
melting mantle.
Convergent Plate
Boundaries
Buoyant magma rises
to base of crust.
Heating of ocean
crust releases fluids
(mostly water) into
mantle above.
As basalt moves through
continental crust, it melts
and absorbs some crust,
becoming more silica-rich.
Added fluids produce
basalt magma from
melting mantle.
Convergent Plate
Boundaries
Continental arc volcanoes
violently erupt silica-rich
magma as ash from
composite volcano.
Buoyant magma rises
to base of crust.
Heating of ocean
crust releases fluids
(mostly water) into
mantle above.
As basalt moves through
continental crust, it melts
and absorbs some crust,
becoming more silica-rich.
Added fluids produce
basalt magma from
melting mantle.
Convergent Plate
Boundaries
• Continental
Volcanic Arc
- e.g., WA
• Volcanic Island
Arc
- e.g., Japan
A Cooperative Quiz
• Where mantle convection causes rising
asthenosphere:
A. The mantle rock will erupt on the seafloor
B. The mantle rock will melt (at least partially)
producing basaltic magma
C. The molten mantle will crystallize
D. The subducting slab will rise
Magma evolution
• Magmas can change their composition. How?
- assimilation (melt the rocks around them)
- crystallization (crystallize Si-poor minerals)
basalt
andesite
granite
Show Fractional
Crystallization
animation
A Cooperative Quiz
• By removing early-formed crystals from a
______ magma, we can obtain a ______ magma.
A. granite ... rhyolite
B. basalt ... granite
C. granite ... basalt
D. Si-rich ... Si-poor
A Cooperative Quiz
A Cooperative Quiz
• A magma body will rise from its point of origin
until:
A. Its density equals that of the surrounding rock
B. Its density exceeds that of the surrounding
rock
C. It solidifies to the point it cannot flow
D. It gets to the surface
Magma density
• Magmas are (usually) less dense than the rocks
around them.
• If they reach the surface: Volcanism!
• If not...
Intrusive features
• Represent magma that solidified (froze) after it
stopped moving
- dike
- sill
- pluton
- batholith
a large body of
igneous rock
Show Igneous
Features animation
A Cooperative Quiz
A Cooperative Quiz
• True (A) or false (B): Igneous intrusions range in
scale from a few cm3 to thousands of km3
d’Alessio, UC Berkeley
• Wrong: Sketch with caption
• Right: explanatory labels on sketch
Isostasy Rubric
• Thickness effect
• Density effect
• Elevation differences
• Mountain roots
• Crust / lithosphere “floating” on mantle /
asthenosphere
MOR Rubric
• Lithosphere-20
• Crust-20
• Asthenosphere-20
• Asthenosphere
convection-30
• Rising asthenosphere/
mantle-30
• Melting due to
decompression-30
• Crustal age
progression-20
• Topographic ridge-30
• Plates moving
apart-30
• Thinner lithosphere
near ridge-20
• Reason for ridge-20
• Shallow magma
chamber-20
• Transform faults-30
• Magnetic stripes-30
• Earthquakes-10
• Rock types-10 each
• Total possible: 370+
Sketch Problems
• Isostasy
•
•
Must relate water
sketch to earth!
Mid-ocean ridges
Lithosphere is not
below crust
What causes the
spreading?
• Not rising magma
• Mostly slab-pull
-
Magma Quiz
1.
2.
3.
4.
What causes melting at mid-ocean ridges?
What causes melting in Hawaii?
What causes melting below Mt. Baker?
What is the composition of the melt produced
in each of the above locations?
5. What is the composition of the melt erupted in
each of the above locations?
Locale
Plate Tectonic
Environment
Reason for
Magma
Production
Typical
Rock
mid-ocean
ridge
divergent
boundary
decompression
basalt
wetting
andesite
convergent
subduction
boundary (w/at
zone
least 1 ocean plate)
“hot spot”
within plate
basalt/
decompression
rhyolite
Questions to think about
in video
A. What volcanic hazard affects many, but kills
few?
B. What volcanic hazard is the most hazardous to
people far from the volcano?
C. What is the main volcanic hazard for
Bellingham?
D. What is the least important hazard for
Bellingham?
Turn on the
International Volcanic
Hazards
Training Video now.