CHAPTER 4 - VOLCANIC ACTIVITY (P. 69-96)
Materials extruded during a volcanic eruption include:
- GASES - H2O, CO2, SO2, CH4, N2, etc.
- LAVA FLOWS - which solidify to form two types of lava rock: PAHOEHOE
(ropey texture), and AA (sharp-edged, blocky texture)
- PYROCLASTICS - bombs, blocks, cinders, lapilli, ash
- PYROCLASTIC FLOWS - giant ash clouds blown out of a great volcano,
covers a great area with ash; sometimes it remains hot enough to solidify as a
massive rock layer. A dense fire cloud from a volcano is called a NUEE ARDENTE.
One such eruption in 1902 killed 28,000 people in the town of St. Pierre on the
Caribbean island of Martinique.
- LAHAR - a mudflow created by a the heat of a volcanic eruption that melts
ice trapped on the top of a volcano. The cities of Tacoma, Olympia, and Seattle
are built on Lahar deposits from Mt. Ranier (Washington).
Types of Volcanos
- SHIELD VOLCANO - forms from hot mafic lava; has very gentle slopes fromed
from lava flows; examples include Hawaiian and Iceland volcanos - many 1000's
feet high (rising from the depths of the oceans).
- CINDER CONE - gassy lava creates cinders which pile up to form steep cones
- typically about 1000 feet max.
- COMPOSITE CONES - typically many thousands of feet high, forms from both
lava flows and cinders: examples include Mt. Fuji, Mt. Vesuvius, Mt. Ranier, Mt.
St. Helens. Composite cones tend to blow themselves apart then slowly rebuild
themselves. Most volcanos of the "Ring of Fire" region are composite cones.
- CALDERA - a huge hole that forms when a massive volcanic chamber under a
volcano collapses during or after a major eruption: examples include Yellostone
(WY), Crater Lake (OR), Long Valley Caldera in California is on the "most
- FISSURE FLOWS form FLOOD BASALTS - these types of eruptions form from
huge volumes of basaltic lava venting of the surface along huge fractures through
the earth's crust. The lava flows cover 1,000s to 100,000s of square miles in a
single eruption: examples include the Columbia River region (OR and WA), huge
regions of Siberia and India.
Distribution of Volcanic Activity
- SPREADING CENTER VOLCANISM - volcanic regions were plates are moving
apart, such as along the Mid Atlantic Ridge, the African Rift Valleys, the Red Sea,
- SUBDUCTION ZONE VOLCANISM - practically everywhere around the Pacific
Rim (where deep ocean trenches are adjacent to island arcs): examples include the
Cascades (USA), the Aleutian Islands, Japan, Philippines, Malaysia, etc.
- INTRAPLATE VOLCANISM "HOT SPOTS" - very hot "plumes" of magma
rising beneath a moving plate: examples include the "Hawaiian chain" and many
volconic islands of the Microneasia region.
- "FISSURE BASALTS" Columbia River Plateau (mentioned above as fissure
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