Magma & Volcanoes, pt. 2, intermediate & felsic rocks

I. What are intermediate and felsic rocks?
A. Intermediate igneous rocks tend to be gray and lighter in color than mafic rocks (basalts & gabbros).
1. Andesites and diorites contain CaNa-feldspar (plagioclase), amphibole (pyroxene + H2O), or black mica (biotite).
2. They form relatively warm magmas (1,650°F, ~900°C)
3. Medium SiO2 content (50-60% by weight)
4. Wet (relatively high volatile content: H2O, CO2, H2S)
5. Does not flow easily
B. Felsic rocks are light gray to pink
1. Rhyolites and granites contain K-feldspar, Na-feldspar (plagioclase), and quartz, with lesser black mica (biotite).
2. They form from the coolest of magmas (1,300°F, ~700°C)
3. Highest SiO2 content (60-70% by weight)
4. Wettest (highest volatile content: H2O, CO2, H2S)
5. Does not flow much at all (highest viscosity)
II. Tectonic Setting/ Origin
A. Intermediate and felsic magmas form around convergent margins with a subduction zone.
1. island arc chain: Aleutian Islands, Japan (Mt. Fuji), and Philippine Islands (Mt. Pinatubo & Mt. Mayon)
2. continental arc mountains: Andes and Cascades (Mt. St. Helens)
3. these volcanic mountain chains often form arc shapes, because that is the shape produced when a flat plate (tectonic plate) intersects a sphere (Earth).
B. Rhyolites may also form around continental hot spots
III. The more silicic magmas (higher SiO2 content) produce more explosive eruptions.
A. Sticky magma blocks vent, magma builds up, makes bulge; bulge breaks, allowing magma to lose volatiles; explosion!
B. Explosive eruptions form various pyroclastic deposits (tephra).
1. ash (sand-sized grains) -- makes flows & cones
2. cinder (pea-sized grains) -- makes flows & cones
3. bomb (orange sized and larger) -- ballistic
C. Composite is a conical mountain of viscous lava flows and pyroclastic deposits (Mt. Fuji)
D. Cinder cone - smaller volcanoes built almost exclusively from pyroclastic deposits.
E. A caldera is a stratovolcano that blew its top; roof fell into magma chamber, making circular crater (Crater Lake).
IV. Types of plutonic bodies depend on size and shape. (a pluton is any body of intrusive rock)
A. Batholith is the largest pluton; it cuts layering in rocks it intrudes
B. Laccolith is a lens-shaped pluton; parallel to layering in rocks
C. A sill is a small tabular sheeted pluton, parallel to layering
D. Dike typically the smallest pluton; it is a tabular sheet that cuts across layering
E. Volcanic neck forms when the central feeder pipe of a volcano is exposed after the volcano erodes away. An example of a volcanic neck is Shiprock New Mexico.

Last update 2/23/2005
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