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
Webpage designed by Hiram Jackson.
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