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VOLCANOES AND PLATE TECTONICS
Since the upper parts of the earth are solid,
special conditions are necessary to form magma.
Volcanism occurs in four principal settings:
1. Along divergent plate boundaries, such as
Oceanic Ridges or spreading centers.
2. In areas of continental extension(they may
become divergent plate boundaries later)
3. Along convergent plate boundaries where
subduction is occurring.
4. In areas called Hot Spots, usually located in
the interior of the plates, away from the plate
margins
Diverging Plate Margins
• Active volcanism is taking place along oceanic
ridges, but most are submarine volcanoes.
• One of the places where oceanic ridge reaches
above sea level is at Iceland, along the MidAtlantic Ridge
• Most eruptions are basaltic in nature,
explosive strombolian types or explosive
phreatic or phreaticmagmatic types
Converging Plate Margin
• Around Pacific Ocean is a zone referred to as
“Ring of Fire”, where dangerous and active
volcanoes occur.
• The “Ring of Fire” coincides with converging
margins along which subduction is occurring.
• Volcanic arcs and Island arcs are formed in
continental areas and oceanic lithosphere
along this plate margin
• Magma generated are andesitic magmas and
volcanism is predominantly andesitic
volcanism
• Magmatic differentiation may change from
andesitic magma to rhyolitic magma; rhyolitic
volcanism is common in the area.
• Magmas are gas-rich and relatively high
viscosity; eruption tends to be violent with
common Strombolian, Vucanian, Plinian, and
Pelean.
• Volcanic landforms tend to be cinder cones,
stratovolcanoes, volcanic domes and calderas
• Repose periods between eruption tend to be
100 – 1000s of years, giving people false sense
of security
Hot Spots
• Volcanism occurs in the interior of plates, not
associated with plate margins.
• Hot Spots results from plume of hot mantle
material upwelling toward the surface,
independent of the convection cells that cause
plate motion.
• Along Hot Spots chains of volcanoes or
seamounts occur, more evident on the sea
floor.
• Hawaiian Islands occur over hot spot
Strombolian-type eruption
Irazú Volcano, Costa Rica, 1965.
Parícutin Volcano, Mexico, 1947.
"Vulcanian"-type eruption
Mount Vesuvius Volcano, Italy, 1944.
"Vesuvian" eruption
Mauna Loa Volcano, Hawaii, 1950.
"Peléan" or "Nuée Ardente (glowing cloud) eruption
Phreatic" (or steamblast) eruptions
Taal Volcano,
Philippines, 1965.
Mount St. Helens about noon, May 18, 1980.
"plinian“ eruptions, most powerful
Mt. Pinatubo, Philippines
Hawaiian Ridge-Emperor Seamount Chain.
Mt. Mayon – stratovolcano
Mt. Pinatubo before and after eruption
Mount Pinatubo's caldera contains a
lake of the same name, pictured here in
2008