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Volcanism and Its Landforms
Objectives
• Describe the distribution of volcanic activity and
explain its relationship with plate boundaries
• Explain how the composition of magma
influences the processes that occur during
volcanic eruptions
• Discuss the major types of volcanic landforms,
and the hazards associated with them
• Cite some dramatic historical examples of human
interaction with volcanic environments
• Describe the landscapes that result from
volcanism
Volcanoes
• The eruption of molten rock at Earth’s surface
• Process of creating new lithosphere
Kanaga stratavolcano, Alaska. © USGS
Distribution of Volcanic Activity
• Volcanoes occur along
plate boundaries
• Divergent boundaries
– mid-ocean ridges
• Convergent boundaries
– Subduction zones
Volcanoes
• Active Volcanoes
– One that has erupted in
recorded history
• Dormant Volcanoes
– No evidence of eruption,
but shows evidence of
recent activity
– Shows little sign of being
worn down
• Extinct Volcanoes
– Shows no sign of eruption
and long-term weathering
and erosion
Belknap Shield Volcano, Oregon, is an extinct volcano.
© USGS
Properties of Magma
• Viscosity
– A fluid’s resistance to flow
– Some magmas have higher
viscosity than others
because of their
composition
– Higher viscosity magmas
typically have higher silica
content and produce
explosive eruptions
• Pyroclastics – solid fragments
erupted from a volcano
– Basaltic lavas have low
viscosity and have effusive
eruptions
Volcanic Landforms
• Calderas
– After an eruption, the magma chamber empties and no
longer supports the overlying surface
– Surface rocks collapse where the magma chamber
once subsisted leaving a large depression
[Insert Fig. 32.4 - caldera]
Composite Volcanoes
• Also called stratovolcanoes
• Explosive volcanoes
• Composed of alternating layers of lava and
pyroclastics
Deadly Composite Volcanoes
• Lahars
– Viscous mudflow of
pyroclastic debris and
water
– May be triggered by rapid
snow melt or rain
Insert Fig. 32.6 - Nevado del Ruiz
Composite Volcanoes
• Pyroclastic Flows
– Ground-hugging avalanche
– Composed of hot ash,
pumice, rock fragments,
and volcanic gas
– May move as fast as 100
km/hr and be up to 500°C
Right: Mayon pyroclastic flow, Philippines © USGS
Composite Volcanoes
• Predicting Risks
– Understanding
precursors to explosive
eruptions
– Frequent earthquakes
– Growing bulge due to
rising magma chamber
– Increased gas
emissions
Cinder Cones
• Single eruptive event
• Consists primarily of small fragments (cinders)
• Low silica content
[Insert Fig. 32.8 - East Africa cinder cone]
Shield Volcanoes
• Fluid basaltic, lowsilica magma produce
quiet eruptions
• Result in smooth, ropy
lava called pahoehoe
• Broad, gentle-sloping
flanks
Shield Volcanoes
Basalt Plateaus and Plains
• Flood basalts represent the accumulation of
many lava flows that spread over large areas
[Insert Fig. 32.3 - Columbia Plateau and Snake River Plain]
Hot Spots
• Intra-plate volcanism
occurs over a stationary
hot spot
• Lithospheric plate moves
over the hot spot
producing an active
volcano
• As the plate moves off
the hot spot, it produces
a long chain of mostly
extinct volcanoes
Landscapes of Volcanism
• Composite cones are limited to their geographic
extent, but dominate the landscape
Landscapes of Volcanism