Igneous rocks and Plate Tectonics

divergent margins: mantle partially melts to form mafic magma.

Subduction zones:
partial melting of mafic and ultramafic rocks (with water) forms intermediate rocks
Partial melting of continental crust forms felsic magma

How magmas move
Move upward due to lower density
Flow slowed by:
 overlying rock
magma viscosity
greatest at low temperature and high Si

Plutons: bodies of intrusive igneous rock

Batholith: >100 km2, often made of granite

Volcanoes occur at plate boundaries.
 

Igneous Rocks and Plate Tectonics
mafic magma: partial melting of mantle.
Found at divergent margins

basalt
 
 

gabbro

Mafic magma also at  “hot spots” (Hawaii)
 
 
 

intermediate magma:
partial melting of mafic and ultramafic rocks (with water)
 found at subduction zones

Andesite:
 

Diorite:

Felsic magma: Partial melting of continental crust.
Found at subduction zones.
Rocks are:
rhyolite
 
 
 
 
 

granite
 
 

Felsic magma also at continental hot spots

– Divergent: mafic
• basaltic volcanoes and fissure eruptions

– Subduction zones: intermediate magma
•  andesitic volcanoes
   rare rhyolite eruptions

Hawaii: “Hot spot” of magma from mantle.
• Forms chain of basaltic volcanoes as plate rides over.
        for web site with pictures of Hawaiian volcanoes and more information, click here.

• Why are some volcanoes more explosive than others?
– More gases  (water, sulfur gases, carbon dioxide)
– Viscosity of the magma

Andesitic volcano:Mt. St. Helens             (For web site with pictures and more  about Mt. St. Helens, click here.)
prior to explosion, pressure builds
Lateral blast
Eruption column
pyroclastics (ash to “bombs”)
forms tuff or
           breccia
pyroclastic flows: ash mixed with hot gases
high speeds (700 km/hr)
so hot that ash fuses together (welded tuff)
Mudflows (Lahars) and  debris flows
ash and volcanic gases can remain in atmosphere for years, affect climate
composite volcano: layers of pyroclastic and lava flows

Rhyolite eruptions: Yellowstone, Long Valley, CA
primarily pyroclastic, deposit tuff
leave large depressions, calderas

Basaltic volcanoes: Hawaii
 less viscosity, less explosive
lava flows through vents, rifts, or fissures

Forms shield volcanoes

Underwater, forms pillow basalts

Volcanic Monitoring
History
Earthquakes
T, gases
ground levels