Download Chp 7.2 Volcanic Eruptions and Features

What controls volcanic eruptions?
• Composition of Magma
– Based on percentage of silica and oxygen present in
the magma.
– High Silica:
• Thicker and slower moving.
• Usually thick and sticky.
• Viscosity:
– Is a physical property that describes the material’s resistance to
flow.
– Low Silica:
• Low percentage of silica and more Fe and Mg.
• Thin and runny.
• Dissolved Gases and Temperature
– Higher temperature of magma, the more easily it
flows.
– Gases: H2O(v), CO2, SO2, H2S
– The more gases the greater the chance of an
explosive eruption.
Types of Magma and Lava
• Basaltic Magma and Lava
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–
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Low percentage of silica.
Low viscosity.
Much thinner, more fluid magma
Tend to pour from the vent and run down the sides of
the volcano in a non-explosive eruption.
– Pahoehoe lava cools develops ropelike patterns.
– Aa lava is stiff, and slow moving.
– Underwater eruption forms bubble-like pillow lava.
• Granitic Magma and Lava
– High percentage of silica.
– High viscosity, flows slowly.
– Sticky and lumpy.
– Trap gases, builds up pressure and produces
explosive eruptions.
Types of Volcanoes
• Shield volcano:
– Is a huge, gently sloping volcanic landform that is
mainly composed of basaltic lava.
– Develop as layer upon layer of gently flowing
basaltic lava piles up.
• Cinder Cone Volcanoes
– Is mainly composed of solid fragments known as
Tephra.
– Includes fragments of volcanic rock or lava.
– Explosive volcanic eruptions.
– When lava erupts from the vent, it cools quickly in
the air.
• Composite Volcanoes
– Is mainly composed of alternating layers of lava
and tephra.
– Quiet and explosive volcanic eruptions due to the
composition of magma
– Tall mountains.
Volcanoes in California
• Form at a convergent plate boundary
– Part of a volcanic arc.
– Form where the Juan de Fuca Plate subducts
beneath the N. American Plate
Intrusive Igneous Features
• Batholiths
– Largest intrusive igneous features.
– Magma slowly cools and solidifies before reaching
the surface.
– Many kilometers in width and length and several
kilometers thick.
• Dikes and Sills
– Magma squeezes into cracks in rocks below the
surface.
– Magma that cuts across rock layers and hardens is
called dike.
– Magma that is parallel to rock layers and hardens
is called sill.
• Volcanic Neck
– Hardened magma inside the vent is eroded by
water and wind.
– The cone is much softer than the igneous rock in
the vent, so it erodes first.
• Lava Domes
– Rounded volcanic feature that forms when highly
viscous lava erupts from the vent.
– Lava piles up.
– When gases accumulate, pressure increases
within the lava dome, gas, lava and solid materials
are ejected in the air.
• Lava Tubes
– A hollow tube that forms when lava flows through
a channel, cools and hardens on the surface.
– Often underground.
– Magma flows to the sea.
• Caldera
– Is a large, circular depression.
– Forms when the top of the volcano collapses and
becomes wedged into the nearly empty magma
chamber.
– The chamber becomes filled with water, forming
lakes and landscapes.
– Usually the largest eruptions on Earth.