Chapter 9 Earth Science

Volcanic Eruptions  Volcanoes – are areas of Earth’s surface through which magma and volcanic gases pass.  Explosion of a volcanic eruption can turn an entire mountain into a billowing cloud of ash and rock in a matter of seconds.  Help form fertile land.  Create some of the largest mountains on earth.

Volcanic Eruptions  Magma – melted rock below the Earth’s surface.  Lava – Melted rock above the Earth’s surface.

Mt. Cleveland- Aleutian Islands, Alaska

Stromboli Volcano - Sicily

Mount Redoubt - Alaska

Nonexplosive Eruptions  Flows of lava are calm.  Releases huge amounts of lava.  Vast areas of the Earth’s surface, are covered with lava from non-explosive eruptions such as Hawaii.

Types of Lava Flows  Pahoehoe lava flows – sheet like lava flows that result in continuous surfaces.  Flow smoothly  Move forwards in tongues or lobes and are characterized by a glassy, plastic skin.

Pahoehoe Lava Flow

Types of Lava Flows  Aa – lava pours out quickly and forms a brittle crust.  The crust is torn into jagged pieces as molten lava continues to flow underneath.  Got its name because of the painful experience of walking barefoot across the jagged surfaces.  Associated with lava fountaining.

Aa Lava Flow

Lava Fountaining  Lava Fountaining – spraying of lava into the air pulsing with the pressure of escaping gases.

Types of Lava Flows  Pillow lava – forms when lava erupts underwater.  Lava forms rounded lumps that are the shape of pillows.

Types of Lava  Blocky Lava – cool, stiff lava that does not travel far from the erupting vent.  Normally forms jumbled heaps of sharp-edged chunks.  Get it’s shape from oozing from a volcano.

Blocky Lava

What is Inside a Volcano?

Water and Magma Are an Explosive Combination  If the water content of magma is high, an explosive eruption is more likely.  Underground the water and magma are under extreme pressures.  When water and magma rise to the top the gases expand rapidly, an explosion can result.  Sometime so rapid a water floating version of lava called Pumice is formed.

Silica-Rich Magma Traps Explosive Gases  Magma that has a high silica content will tend to cause explosive eruptions.  Silica-rich magma has a stiff consistency.  Flows slowly and hardens in a volcano’s vents plugging the vents.

Gas Trapped in a Volcano  The more that is trapped in a volcano the more explosive it will be due to the rapid rate in which the gas escapes from the volcano.

What erupts from a Volcano?  Pyroclastic material - forms when magma is blasted into the air and hardens.  Volcano’s eruptions may alternate between lava and pyroclastic eruptions.

Volcanic Bombs  Volcanic Bombs – large blobs of magma that harden in the air.  The shape of the bomb was caused by the magma spinning through the air as it cooled.

Lapilli  Lapilli – pebblelike bits of magma that hardened before they hit the ground.  Means “little stones”

Volcanic Ash  Volcanic Ash – forms when the gases in stiff magma expand rapidly and the walls of the gas bubbles explode into tiny, glasslike silvers.  Makes up most of the pyroclastic material in an eruption.

Volcanic Ash

Pyroclastic Flows  Pyroclastic Flow – produced when enormous amounts of hot ash, dust, and gases are ejected from a volcano.  Can move downhill at 200km/h.  Center of the flow can exceed 700 C.