VOLCANOES AND PLATE TECTONICS

VOLCANOES AND PLATE TECTONICSSince 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 Mid-Atlantic 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

"Vesuvian" eruption

 Mount Vesuvius Volcano, Italy, 1944.

Mauna Loa Volcano, Hawaii, 1950.

"Peléan" or "Nuée Ardente (glowing cloud) eruption

Phreatic" (or steam-blast) eruptions

 Taal Volcano, Philippines, 1965.

"plinian“ eruptions, most powerful

 Mount St. Helens about noon, May 18, 1980.

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