The gases in the magma, and how slowly or quickly they are released contributes to how violently a volcano erupts.

Characteristics of Magma

•         high silica content

–        produces light-colored lava that is too sticky to flow very far.

–        when cooled produces

•         rhyolite (same composition as granite)

•         Obsidian (forms by cooling very quickly)

•         pumice (forms when gas bubbles are trapped in cooling lava, leaving spaces in the rock.)

•         Low silica produces free-flowing, dark-colored lava, forming basalt.

•         The type of magma influences how the volcano erupts (quiet or explosive).

–         Quiet Eruptions

•         Magma flows easily (low silica content)

•         Gas dissolved in the magma bubbles out gently.

•         Magma simply oozes from the vents

•         Produces …

–        Pahoehoe lava = fast moving, hot lava. Looks like a solid mass of wrinkles, billows, and ropelike coils.

–        Aa lava = cooler and slower moving. It forms a rough surface consisting of jagged lava chunks.

•         The type of magma influences how the volcano erupts (quiet or explosive).

–        Explosive eruptions

•         Thick and sticky magma plugs the vents.

•         Dissolved gases cannot escape from the magma, so pressure builds.

•         When the pressure of the gases become too great, the vents explode.

•         The expanding gases push the magma out of the volcano with incredible force.

•         The type of magma influences how the volcano erupts (quiet or explosive).

–        Explosive eruptions

•         The explosion breaks the lava into fragments that quickly cool and harden into pieces of different sizes.

–        Ash: fine rocky particles as small as a grain of sand.

–        Cinders: pebble-sized particles.

–        Bombs: baseball to car size

–        Pyroclastic flow occurs when an explosive eruption hurls out ash, cinders, and bombs as well as gases.

Identify some hazards of volcanoes.

•         Hazards

–        Different types of eruptions involve different volcano hazards, but both types can cause damage far from the crater’s rim.

•         Lava flow

•         Falling cinders and bombs

•         Volcanic ash

•         Landslides and avalanches

•         Mudflows

Identify types of volcanic activity other than eruptions.

•         Other Types of Volcanic Activity

–        hot springs

•         forms when groundwater is heated by a nearby body of magma.

•         rises to the surface and collects in a natural pool.

–        geyser

•         a fountain of water and steam that erupts from the ground.

–        Occurs when rising hot water and steam become trapped underground in a narrow crack.

–        Pressure builds until the mixture suddenly sprays above the surface.