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Transcript
Volcanoes
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
eruption
• Think of boiling water vs spaghetti sauce
A low silica lava flow
A high silica flow
Pyroclastics
• 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
Volcanoes
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)
Volcanoes
Types of volcanoes
• Shield volcano
• Broad, slightly domed (like a shield)
• Primarily made of fluid lava
• Generally large size
A shield volcano
Volcanoes
Types of volcanoes
• Cinder cone
• Built from ejected lava fragments
• Steep slope angle
• Relatively small size
Sunset Crater – a cinder
cone near Flagstaff, Arizona
Volcanoes
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
(stratovolcano)
Mt. St. Helens – a typical
composite volcano
Mt. St. Helens after eruption
A size comparison volcanoes
Other volcanic landforms
Calderas
•
•
•
•
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
magma
• 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
Islands)
• Continental volcanic arcs (Andes
Mountains)
Convergent boundaries
Plate tectonics
Plate motions provide the mechanism
by which mantle rocks melt to form
magma
• 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
magma
• Intraplate igneous activity
• Activity within a rigid plate
• Plumes of hot mantle material rise
• Form localized volcanic regions called hot
spots
• One example is the Hawaiian Islands
Hot Spots
Locations of some of
Earth’s major volcanoes
r
Continental and island volcanic arcs
Review
• Name and describe the three types of
volcanoes.
• Describe the different types of
pyroclastics.
• Explain the factors that affect the severity
(violence) of an eruption.
• Describe how continental and island arcs
are formed.