Physical Geology - Volcanoes and Volcanic Rocks

Our Hazardous Environment
GEOG 1110
Dr. Thieme
Lecture 8:
Volcanoes and Volcanic
• volcano - a conical or dome-shaped
landform created when lava and/or
tephra accumulates on the earth’s
• lava - molten material on the surface
• magma - molten material below the
• tephra - igneous material, ranging in size
from dust to boulders, that is explosively
ejected from a volcano
Distribution of Volcanoes
Most frequently associated with subduction zones
Also common along mid-ocean ridges
Some at hot spots, but much more rare
magma chamber - an accumulation of molten rock beneath the Earth’s
vent - the opening magma uses to move from the magma chamber to the
Earth’s surface
crater - the opening through which lava and tephra issues
caldera - a very large crater created by explosion or collapse
cone - a build-up of lava and/or tephra around a vent, creating a hill or
Types of Volcanoes
• There are 3 basic types of volcanoes
1) Shield volcanoes - largest
2) Stratovolcanoes (also called composite cones)
3) Cinder cones - smallest
Shield Volcanoes
• Created by mafic magma
• Thin and flow easily in comparison to felsic magmas (less “viscous”)
• Usually hold little gas in comparison to felsic magmas
• Eruptions of volcanoes consisting of mafic magmas are relatively quiet
because of the thin, fluid nature and the absence of gas under
• Mafic lava flows tend to travel long distances across the surface, and
form broad-based volcanoes with gently sloping sides
• Fissure eruptions on the sides of the volcanoes are common, called
flank eruptions
• Calderas often develop
Shield Volcanoes
• Composed of multiple stacked lava flows
• Famous examples include Hawaii, Iceland, and the
• Hawaii is formed over a hot spot, Iceland and the Azores
over the Mid-Atlantic Ridge
• hot spot - an area where a plume of magma is welling up
from deep within the asthenosphere
’a ’a
• Created by intermediate to felsic
thick and gummy magma/lava
magma/lava frequently high in gas content
eruptions can be very explosive
cones are composed of interlayered lava and tephra
tall, steep-sided cones
• also called composite volcanoes or composite cones
• stratovolcanoes are known for their scenic beauty
• famous examples include:
– Mount Rainier and Mount St. Helens in Washington
– Mount Shasta in California
– Mount Fujiyama in Japan
• volcanoes that most concern scientists
– explosive eruptions
– glowing avalanches, scorching hot clouds of ash and debris
– glowing avalanche from Mount Pelée on the Caribbean island of Martinique
killed all but 2 of the 30,000 inhabitants of the city of St. Pierre in 1902
Composite Cone, Mount Shasta, northern California
Mount Lassen
• east of
Redding, CA
• erupted in
June, 1914
Formation of Crater Lake
Figure 4.13
Long Valley
• caldera
• 700,000
years ago
• tephra used
to date
other rocks
“Peléean” eruptions
Silica-rich (“felsic”) magma
form domes
glowing avalanches
ash flows
Cinder Cones
• made up entirely of tephra
• form when frothy mafic magma is ejected from a vent
under high pressure
• rarely get more than a few hundred meters high, and
they are the most easily eroded volcanic cone
Volcanic Hazards
• gases
• CO2
• SO2
• H2S
• F2
• lava flows
• bombs, ash
• pyroclastic flows
• landslides
• mudflows (“lahars”)
Molten Sulfur and
Sulfur Gases in an
Indonesian volcano
Lake Nyos,
West Africa
Carbon Monoxide
(CO) is poisonous
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