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12.10.07 / 12.11.07
Volcanic eruptions
Factors that determine the violence of
an eruption
• Composition of the magma
• Temperature of the magma
• Dissolved gases in the magma
Viscosity of magma
• Viscosity is a measure of a material's
resistance to flow
Volcanic eruptions
Viscosity of magma
• Factors affecting viscosity
• Temperature (hotter magmas are less viscous)
• Composition (silica content)
• High silica – high viscosity
• Low silica – more fluid
• Dissolved gases (volatiles)
• mostly H2O and CO2
• Gases expand near the surface
Volcanic eruptions
Viscosity of magma
• Factors affecting viscosity
• Dissolved gases (volatiles)
• Provide the force to extrude lava
• Violence of an eruption is related to how
easily gases escape from magma
• Easy escape from fluid magma
• Viscous magma produces a more violent
• Think of boiling water vs spaghetti sauce
A low silica lava flow
A high silica flow
• Pyroclastic material: fragments of rock
formed during eruptions
– Ash and dust: fine, glassy fragments
– Lapilli: walnut sized rocks
– Bombs: magma ejected into the air cools and
forms rounded rocks
– Blocks: ejected as large rocks
General features
• Conduit, or pipe carries gas-rich magma to
the surface
• Vent, the surface opening (connected to
the magma chamber via a conduit)
• Crater
• Steep-walled depression at the summit
• Caldera (a summit depression greater than 1
km diameter)
Types of volcanoes
• Shield volcano
• Broad, slightly domed (like a shield)
• Primarily made of fluid lava
• Generally large size
A shield volcano
Types of volcanoes
• Cinder cone
• Built from ejected lava fragments
• Steep slope angle
• Relatively small size
Sunset Crater – a cinder
cone near Flagstaff, Arizona
Types of volcanoes
• Composite cone (or stratovolcano)
• Most are adjacent to the Pacific Ocean (e.g.,
Mt. Rainier)
• Large size
• Interbedded lavas and pyroclastics
• Most violent type of activity
A composite volcano
Mt. St. Helens – a typical
composite volcano
Mt. St. Helens after eruption
A size comparison volcanoes
Other volcanic landforms
Steep walled depression at the summit
Formed by collapse
Nearly circular
Size exceeds one kilometer in diameter
Crater Lake, OR caldera
Crater Lake in Oregon
Plate tectonics
Plate motions provide the mechanism
by which mantle rocks melt to form
• Convergent plate boundaries
Deep-ocean trenches are generated
Descending plate partially melts
Magma slowly rises upward
Rising magma can form
• Volcanic island arcs in an ocean (Aleutian
• Continental volcanic arcs (Andes
Convergent boundaries
Plate tectonics
Plate motions provide the mechanism
by which mantle rocks melt to form
• Divergent plate boundaries
• The greatest volume of volcanic rock is
produced along the oceanic ridge system
Divergent boundaries
Plate tectonics
Plate motions provide the mechanism
by which mantle rocks melt to form
• Intraplate igneous activity
• Activity within a rigid plate
• Plumes of hot mantle material rise
• Form localized volcanic regions called hot
• One example is the Hawaiian Islands
Hot Spots
Locations of some of
Earth’s major volcanoes
Continental and island volcanic arcs
• Name and describe the three types of
• Describe the different types of
• Explain the factors that affect the severity
(violence) of an eruption.
• Describe how continental and island arcs
are formed.