Types of Volcanoes Chapter 11 Section 2

Types of Eruptions: Quiet Explosive Determined by: Amount of water vapor/trapped gases in the magma. Amount of silica (silicon and oxygen) in the magma.

Quiet Eruptions Contain basaltic magmas Low in silica Flows very easily Contain gases that are easily released due to the high viscosity (ability to flow) of the magma.

Quiet Eruptions Pahoehoe Aa Pillow lava Lava cooled in a rope-like structure Aa Forms at lower temperatures than pahoehoe Pillow lava Igneous rock structures shaped like baloons, tubes, or pillows.

Quiet Eruptions Examples: Hawaii Rift zones (Iceland)

Explosive Eruptions Contain granitic magma High in silica Thick Low in viscosity (ability to flow) Contains many trapped gases May also contain andesitic magma Intermediate silica content

Explosive Eruptions Examples Convergent boundaries

Forms of Volcanoes Dependent upon type of magma and type of eruption. 3 basic forms: Shield volcanoes Cinder cone volcanoes Composite volcanoes

Shield Volcano Broad volcano with gently sloping sides. Result of basaltic magma and quiet eruptions. Example: Hawaii

Flood Basalts Occur when basaltic magma flows onto the surface of the earth through large cracks called fissures. When the magma cools, it covers large areas with thick igneous rock. Accounts for largest volume of erupted volcainc material on Earth. Examples: Columbia Plateau New ocean floor

Cinder Cone Volcano Form when tephra fall to the ground and form a loosely-packed steep-sided structure. Tephra consist of bits of rock or solidified lava drpped from the air. Includes volcanic ash, cinders, and larger rocks called bombs and blocks Example: Paricutin – Feb 20th, 1943

Types of Tephra Volcanic Ash Volcanic Dust Lapilli Tephra particles less than 2 mm in diameter. Volcanic Dust Particles less than 0.25 mm in diameter. Lapilli Larger tephra particles, less than 64 mm in diameter. Generally fall near the vent.

Types of Tephra Volcanic Bombs Volcanic Blocks Tephra particles with round or spindle shape. Volcanic Blocks Solid rocks blasted from a fissure.

Composite Volcano Formed from alternating quiet and explosive eruptions, depending on the amount of trapped gases and silica content at the time of eruption. Result is alternating layers of tephra and lava. Examples: Convergent boundaries – Mount St. Helen’s, Mount Raineer

Krakatau Island in the Sunda Straits near Indonesia. Erupted in August of 1883, causing the “island” to collapse into the empty magma chamber (caldera-forming). More than 36,000 people were killed (mainly by a tsunami) Global temps dropped as much as 1.8 degrees Celsius for five years.

Effects of Eruptions Lava flows Collapse of buildings from ash fall Air pollution (lung disease/stress) Acid rain Pyroclastic flows Huge, hot, fast rush of ash and debris down the mountain (can result in fires) Permanent/Temporary Evacuation