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).
Magma flows easily (low silica content)
• Gas dissolved in the magma bubbles out gently.
• Magma simply oozes
from the vents
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).
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.
– Different types of eruptions
involve different volcano hazards, but both types can cause damage far from the crater’s rim.
• Lava flow
• Falling cinders
Landslides and avalanches
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.
• 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.