Results of plate tectonics:
1.Volcanism
2.Diastrophism (folding and faulting)
3.Earthquakes
Volcanism
Definition: movement of molten rock (and gases) from earth’s interior to surface
Why are some volcanoes explosive and others effusive (not violent)?
Determined by Volcano’s location and composition of magma.
Types of Volcanism:
A. Extrusive
1. Lava Flows
flood basalts
2. Volcanoes
a) shield
b) cinder cone
c) strato-volcanoes (composite)
d) caldera
B. Intrusive
A. Extrusive (Magma reaches the surface (lava) and cools above the surface).
1. Lava Flows
Effusive (nonviolent) eruptions
At spreading boundaries and hot spots
Lava spreads out over surface; coming out of fissure (rift)
example: Lake Superior shore
1.2 billion yrs BP
FLOOD BASALT is a type of lava flow: extremely large outpourings of lava : accumulate
layer upon layer. Examples: Mid-continent rift (1.2 billion years B.P.) North Shore;
Columbia plateau; Deccan Plateau
2. Volcanoes:
hills or mountains of volcanic material released under pressure from a vent.
> 500 active (4/5 “ Ring of Fire”)
Where do volcanoes occur? Subduction zones, rift valleys, mid-oceanic ridges, hot spots
Types of Volcanoes:
a) Shield Volcanoes: broad, gently sloping surfaces
examples: Hawaii, Azores, Mt. Hekla in Iceland
(tallest mountains on earth are shield volcanoes from the ocean floor)
effusive eruption (nonviolent eruption) due to type of magma:
low viscosity; 1200 degrees C, little gas in magma
Composed of Iron, magnesium, low in silica: from asthenosphere (Primarily basalt)
long continuous eruptions (vent doesn’t block)
Lava flows travel long way, Result : broad domes, with gentle slopes, as in Kilauea,
Hawaii
b) Cinder Cones: steep, cone-shaped hills of pyroclastics
pyroclastics: rock pieces ejected: Tephra: pulverized rock and Scoria: cindery vesicular
rock
moderately explosive
most abundant and smallest type of volcano (<1500 ft. high)
has a central crater
Cinder cones form rapidly, erode rapidly.
examples: Surtsey, Iceland; Paricutin, Mexico; Idaho Snake River Plain; Sunset Crater,
Arizona
c) Stratovolcano (Composite) Volcanoes
steep-sided; alternating layers of pyroclastics and lava from ash falls and lava flows
Where do they occur? At subduction zones
examples: Mt. Pinatubo, Philippines; Mt. St. Helens, Mt. Rainier, Mt. Fuji, Mt.
Hood
explosive eruption due to type of magma:
higher viscosity, 700 C; contains gases;
from melting of lithosphere: more silica
Eruptions unpredictable and hazardous to a region
Can sometimes cause short time cooling events, e.g. Pinatubo
d) Calderas: large bowl-shaped depression left after a composite volcano blows;
mountain collapses inward
associated with some devastating composite volcanoes
Example: Crater Lake, Yellowstone
B. Intrusive Volcanism :
Magma solidifies beneath surface; intrudes into cracks in crust. Intrusions can later be exposed
at surface; they are more resistant than surrounding rock.
Intrusive landforms:
Neck: narrow steep-sided peak , formed in central vent of an extinct volcano
E.g., Ship Rock, Devil’s Tower
Dike: thin layer of igneous rock ; vertical; cuts across other rock layers
Sill: thin layer of igneous rock between rock layers
Plutons: large igneous bodies at depth
batholiths
laccolith: dome-shaped