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.