Volcanic Eruptions Chapter 5.2

Magma becomes a volcano: Lava usually begins as magma, which usually forms in the asthenosphere. Liquid magma is less dense than the solid around it. Therefore the magma flows upward into any cracks in the rocks above.

As magma rises, it sometimes gets caught beneath layers of rock. If an opening allows the magma to reach the surface, a volcano is formed.

A Volcano: A volcano is not simply a cone-shaped mountain. Inside is a system of passageways through which magma moves. Beneath the volcano, magma collects in a pocket called the magma chamber. The magma chamber moves upward through a pipe (a long tube in the round that connects the magma chamber to Earth’s surface.

Molten rock and gas leaves the volcano through an opening called a vent. A lava flow is the area covered by lava as it pours out of a vent. A crater is a bowl-shaped area that may form at the top of a volcano around the central vent.

A Volcanic Eruption: Like carbon dioxide in a bottle of soda, dissolved gases are trapped in magma. These dissolved gases are under tremendous pressure. As magma rises toward the surface, the pressure on surrounding rock decreases.

The dissolved gases begin to expand, forming bubbles. As pressure falls within the magma, the size of the bubbles increase greatly. These expanding gases exert a great force.

When a volcano erupts, the force of the expanding gases push magma from the magma chamber through the pipe until it flows or explodes out of the vent. Once magma escapes from the volcano and becomes lava, the remaining gases bubble out.

Kinds of Volcanic Eruptions: Geologists classify volanic eruptions as quiet or explosive. This depends on the magma’s viscosity and silica content.

Viscosity: The resistance of liquid to flow The greater of viscosity, the slower it flows.

Silica content: Silica is made up of silicone and oxygen. The amount of silica content in magma is between 50-70%

Quiet Eruptions: A volcano erupts quietly if its silica content is low. Low silica has low viscosity and flows easily. Lava can ooze quietly from a vent.

Explosive Eruptions: A volcano erupts explosively if its magma is high in silica. High silica magma has a high viscosity making it thick and sticky. Instead of exploding from the crater, it may build up in the pipes. It plugs like a cork on a bottle.

The dissolved gases cannot escape from the thick magma. The gases build up pressure until they explode. It is all pushed out with incredible force.

Pyroclastic Flow: This happens when an explosive volcano sputs out a mixture of hot gases, ash, cinders, and bombs.

Volcanic Hazards: In a quiet eruption, lava flow can set fire and destroy anything in its path. In an explosive eruption, volcanoes can belch out clouds of deadly ashes, cinders, and bombs. Ash can cover entire towns.

Eruptions can cause landslides, mudslides, an avalanches of mud, melted snow and rock.

Stage of Activity: Geologist often use the terms of active, dormant, or extinct to describe a volcano’s stage of activity.