Volcanoes

• Volcanoes are weak areas of Earth’s crust through which magma and volcanic gases come to the surface.

• Magma that flows onto the Earth’s surface is called lava.

• Magma is a molten mixture of rock-forming substances, gases, and water vapor from the mantle.

• During an eruption, magma is forced to the Earth’s surface.

• When lava cools it forms solid rock.

• Volcanic activity is a constructive force that adds new rock to existing land and forms new islands.

Ring of Fire• There are about 600 active volcanoes on land and

many more beneath the sea.

• Volcanoes occur in belts that extend across continents and oceans.

• One major volcanic belt is the Ring of Fire, formed by the many volcanoes that rim the Pacific Ocean.

• Volcanic belts form along the boundaries of Earth’s plates.

• At plate boundaries, huge pieces of the lithosphere diverge or converge, so the lithosphere is weak and fractured, allowing magma to reach the surface.

Hot Spots• Most volcanoes occur along diverging plate

boundaries, such as the mid-ocean ridge, or in subduction zones around the edges of oceans.

• But some volcanoes form at “hot spots” far from the boundaries of continental or oceanic plates.

Nonexplosive (quiet eruptions)

• Nonexplosive eruptions are the most common type of eruption.

• They produce relatively calm flows of lava, many of which are enormous in size.

Explosive Eruptions• Explosive eruptions are rarer, however they can be incredibly

destructive.

• In an explosive eruption, clouds of hot debris, ash and gas rapidly shoot out from the volcano.

• Instead of lava flows, the molten rock is blown into tiny particles that harden in the air.

• The dust-sized particle, called ash, can reach the upper atmosphere and remain there for years.

• Large pieces of debris fall closer to the volcano. An explosive eruption can demolish an entire mountainside in an instant.

Parts of a Volcano• A magma chamber is a

holding tank of molten rock deep underground that feeds a volcano.

• Magma rises from the magma chamber through cracks in the Earth’s crust to openings called vents.

• The crater is a steep-sided hole at the top of the volcano that sometimes contains a pool of lava.

Magma• The composition of magma affects how explosive a

volcanic eruption is.

 SiO2 MAGMA

TYPETEMPERATURE

(centigrade)  VISCOSITY GAS

CONTENT ERUPTION

STYLE

 ~50% mafic ~1100  low low nonexplosive

 ~60% intermediate  ~1000  intermediate  intermediate  intermediate

 ~70% felsic  ~800 high  high  explosive

• Explosiveness is determined by how much silica, water and gas are in the magma.

• If the magma contains a lot of water, an explosive eruption is more likely.

• Underground, magma is under intense pressure, so the water stays dissolved in the magma.

• As it moves to the surface, the pressure decreases and the water and other compounds become gases.

• As the gases expand rapidly, an explosion can result.

• This is very much like what happens when you shake a bottle of soda and then open the top.

• Magma high in silica also tends to cause explosive eruptions.

• Silica-rich magma has a stiff consistency.

• It flows slowly and tends to harden in a volcano’s vents.

• Pressure builds up until an explosive eruption takes place.

Types of Eruptions• Magma erupts as either lava, or

pyroclastic material.

• Lava is liquid magma that flows from a volcanic vent.

• Pyroclastic material forms when magma is blasted into the air and hardens.

• Nonexplosive eruptions produce mostly lava.

• Explosive eruptions produce mostly pyroclastic material.

Types of Pyroclastic Material• There are 4 basic types of

pyroclastic material:

• Volcanic bombs

• Volcanic blocks

• Lapilli• Volcanic ash

Viscosity• The viscosity of lava affects

lava flow.

• Viscosity is the measure a liquids ability to flow.

• Blocky lava and pahoehoe have a high viscosity and flow slowly.

• Aa and pillow lava have lower viscosities and flow more quickly.

Stages of a Volcano

• An active volcano is one that is erupting or has shown signs that it may erupt in the near future.

• A dormant, or sleeping volcano is one that is expected to awaken in the future and become active.

• An extinct, or dead volcano is unlikely to erupt again.

Types of Volcanoes

• There are three basic types of volcanoes:

• Shield volcanoes

• Cinder Cone volcanoes

• Composite volcanoes

Shield Volcanoes• Shield volcanoes are built

of layers of lava released from repeated nonexplosive eruptions.

• The lava is very runny, so it spreads out over a wide area.

• Over time this creates a volcano that has gently sloping sides.

• Although the sides aren’t steep, these volcanoes can be enormous.

Mauna Loa

Cinder Cone Volcanoes• Cinder cone volcanoes are made

of pyroclastic material usually produced from moderately explosive eruptions.

• The pyroclastic material forms steep slopes.

• Cinder cones are small and usually erupt for only a short time.

• They often occur in clusters and erode quickly since the pyroclastic material is not cemented together.http://encarta.msn.com/media_461532559_761570122_-1_1/Volcanic_Eruption.html

Composite Volcano• Composite volcanoes, or

stratovolcanoes, are one of the most common types of volcanoes.

• They form from explosive eruptions of pyroclastic material followed by quieter flows of lava.

• This creates alternating layers of pyroclastic material and lava.

• They usually have broad bases and sides that get steeper toward the top.

St. Augustine Volcano, Alaska

Other Volcanic Landforms• A caldera is a large, semicircular

depression that forms when the chamber that supplies magma to a volcano partially empties and the chamber’s roof collapses. As a result, the ground above the magma chamber sinks.

Mount St. Helens

Before

After

Kamchatka

Eruption of Klyuchevskaya Volcano in Kamchatka