Volcanoes

Volcanoes A volcano is a weak spot in the crust

where molten rock comes to the surface.

Molten Rock There are two types of molten rock:

› Magma – molten rock found below the surface of the earth

› Lava – molten rock that has reached the surface of the earth Once lava or magma cools, it becomes solid

rock.

Volcanoes Volcanoes form at:

› Divergent plate boundaries› Convergent plate boundaries› Hot spots

Divergent Plate Boundaries Volcanoes form along mid-ocean ridges

at divergent boundaries.› Lava pours out of the ridges, creating new

oceanic crust. Volcanoes can also form at rift valleys

on land.

Convergent Plate Boundaries

Volcanoes will form at convergent plate boundaries where subduction is occurring.› This means at two oceanic plates or at an

oceanic and a continental plate.

Convergent Plate Boundaries

As the old, dense oceanic plate is moving back into the mantle, it is going to melt some of the rock above it.› This happens because of friction.

Convergent Plate Boundaries

Because liquids are less dense than solids, the molten rock is less dense than the rock above it.

This is going to cause the molten rock to start to move toward the surface of the earth.

Convergent Plate Boundaries

When the molten rock reaches the crust it will push the rock up forming a mountain and eventually break through the crust, forming a volcano.

Hot Spots A hot spot is an area where material

deep within the mantle rises and melts, forming magma.

Hot Spots Just like at convergent plate

boundaries, this molten rock will move towards the surface until it breaks through the crust.

This will form a string of volcanic mountains as the plate above it continues to move.

Viscosity Viscosity is the resistance of a liquid to

flow. Some liquids can flow more easily than

others because they have different viscosities.

Viscosity Liquids with high viscosity flow slowly.

› Honey is a high viscosity liquid. What else might have a high viscosity?

Viscosity Liquids with a low viscosity flow

quickly.› Water is a low viscosity liquid.

What other liquids might have a low viscosity?

Lava Not all types of magma and lava are

the same.

Lava One way that lava is different is its

viscosity. The viscosity of lava depends on its

silica content and its temperature.

Silica Content A major ingredient in lava and magma

is silica. Silica is made of oxygen and silicon.

Silica Content The silica content in molten rock

ranges from 50% to 70%.

Silica Content The more silica molten rock has, the

higher its viscosity. The less silica molten rock has, the

lower its viscosity.

Silica Content High silica lava is light in color, sticky,

and does not flow very far. Low silica lava is dark in color and can

flow much further than high silica lava.

Temperature Viscosity increases as temperature

decreases.› The hotter it is, the lower the viscosity.› The colder it is, the higher the viscosity.

Pahoehoe Pahoehoe is a fast moving, hot lava

with low viscosity. It is dark in color and has a small

amount of silica.

Aa Aa is cooler and slower-moving than

pahoehoe. It has a higher viscosity, is lighter in

color, and has more silica.