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– 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.