Volcanic Eruptions 11/5/14 9-1/9-2 pgs  IN: What do you think causes a volcanic eruption to sometimes be explosive and others to just run?

Volcano  A mountain that forms when molten rock, called magma, is forced to the Earth’s surface.

What determines how it will erupt?  Magma composition  High water content can cause increased pressure as the water turns to steam and expands  Silica rich magma is sticky and clogs the vents of volcanoes plugging them and preventing the release of the pressure. basalt granite

What is difference between lava and magma? Magma is inside the Earth. Lava is on the Earth’s Surface.

Nonexplosive Volcano  Volcanoes that have relatively calm outpourings of lava releasing a huge amount of molten rock.

Lava flows from non explosive volcanoes  Blocky lava: jumbled heaps of sharp-edged chuncks  Pahoehoe: flows slowly like candle wax with glassy surface and rounded wrinkles

 Aa: Flows fast forming a brittle crust with jagged pieces  Pillow: Forms under water in lumps that look like pillows

Explosive Volcano  Clouds of hot debris and gases shoot out from the volcano often at supersonic speeds.

Pyroclastic Material  Rock fragments created by explosive volcanic eruptions.

Four types of Pyroclastic Material  Volcanic blocks: the largest pieces of solid rock blasted out of the volcano.  Volcanic bombs: large blobs of magma that harden in the air.

 Lapilli: “little stones” of pebble-like bits of magma that become solid before they hit the ground.  Volcanic ash: form when the gas bubbles of stiff magma explode into tiny glasslike slivers.

9.2 Volcanoes’ Effects on Earth  Flows and Fallout: clouds of hot ash flow at super sonic speeds downhill, choking and searing everything in their path. Ash combines with glaciers creating a cement like mixture that can dam up rivers…called a lahar. It is an effective fertilizer, but when it reaches the upper atmosphere it blocks out the sun cooling average global temperatures.

3 Types of Volcanoes  Shield volcanoes  Cinder cone volcanoes  Composite volcanoes

Shield Volcanoes  Built out of layers of lava from repeated nonexplosive eruptions.

Cinder Cone Volcanoes  Small volcanic cones made entirely of pyroclastic material.

Composite Volcanoes  Form by explosive eruptions of pyroclastic material followed by quieter outpourings of lava.  Most common.

 Forms when a magma chamber that supplies material to a volcano empties and its roof collapses leaving a large circular depression. Calderas

Out  Explain how some volcanoes erupt explosively and some do not.