Dr Pusey

Learning OutcomesDescribe the structure of two main types of volcanos

and the differences between them: Strata (Composite) and Shield types.

Recall the term Viscosity and the relative viscosity of different fluids.

Label the parts of a volcano including the Magma Chamber, Vents, Ash Cloud, Pyroclastic Flow and Lava

Explain how a hot-spot volcano is formed

Volcanoes A volcano is a place where extremely hot material from

inside the Earth erupts at the earth’s surface.

This material includes: Gas such as steam and hydrogen sulphide (H2S)

Lava (molten rock)

Lumps of solid volcanic rock

Volcanoes form when there are weak spots in the Earth’s crust. Magma is forced upwards under great pressure through the crust forming the volcano.

When the magma reaches the surface it is called lava.

Structure of a Volcano

Magma chamber• Large underground

pool of molten rock sitting underneath the Earth’s crust.

• When magma reaches the surface, it results in a volcanic eruption.

Structure of a VolcanoLava• Molten rock expelled

from a volcano during an eruption. When it first comes out, the lava can have a temperature higher than 1000 °C. It then flows downhill from the eruption point until it cools and hardens.

Structure of a Volcano

Main Vent• The point in the

Earth’s crust where hot magma has reached the surface.

Secondary Vent• The point in the

Earth’s crust where hot magma has reached the surface.

Structure of a Volcano

Pyroclastic Flow• Fast moving hot gas,

ash and rock which move rapidly down the side of the volcano – Very dangerous!

Structure of a Volcano

Ash Cloud• Volcanic ash consists

of small pieces of pulverized rock and glass created during volcanic eruptions. can be carried in the air for many kilometres.

Types of Volcanoes There are two types of volcanoes we can compare:

Shield volcanoes

Composite volcanoes (Strato volcanoes)

http://www.bbc.co.uk/schools/gcsebitesize/geography/natural_hazards/volcanoes_rev3.shtml

Shield Volcanoes Forms from non explosive eruptions of flowing lava.

Creates broad volcanoes with gentle slopes (like a shield).

Large, over 10’s of kilometres wide.

Consist almost entirely of frozen lavas (same layer).

Lava is low viscosity. (runny)

The individual islands of Hawaii are large shield volcanoes.

Composite Volcanoes Gentle lower slopes but steep upper slopes.

Build up alternating layer by layer (lava/ash/lava/ash).

High viscosity lava. (not very runny)

Largest type of volcano, 1-10 kilometres in diameter

Lava Lava can be over 1000°C.

The lava changes colour as it cools, from white to yellow/orange/red, until finally it becomes black when it hardens and solidifies into rock.

Lava can be highly viscous (flow fast) or of low viscosity (flow slowly).

Volcanic Eruptions When magma is viscous (slow

flowing) and contains lots of water and gas, the volcanic eruption is explosive.

The gas bubbles grow larger as the magma rises to the surface.

When the magma cools and starts to solidify, the expanding gas bubbles can suddenly break the rock apart explosively.

Where Do Volcanoes Form?

Locations Volcanoes occur at:

Divergent boundaries

When plates move apart, the crust becomes thinner, lowering the pressure on the underlying rocks and they melt

Convergent boundaries

When two plates collide, particularly at subduction zones, the crust is weakened and a lot of heat is generated.

If the two plates collide under the ocean, then a chain of volcanic islands (island arcs) are formed at the edges of the plates.

Hot Spot Volcanoes Some volcanoes form away from plate boundaries.

Magma plumes reach the crust, forming volcanoes.

When the Earth’s crust is moved (plate tectonics), a new part of the plate sits over the hot spot and another volcano is created.

The old volcano is dormant and the new volcano is active.

A chain of volcanic islands are created from this process.

Hot Spot Volcanoes - Hawaii

http://en.wikipedia.org/wiki/Hawaiian_Islands

Dr Pusey on the Big Island

Punalu’u Beach (Black Sand Beach)

Dr Pusey on the Big Island

Steam Vents (Moana Loa) – Volcanoes National Park

How did you go?Describe the structure of two main types of volcanos

and the differences between them: Strata (Composite) and Shield types.

Recall the term Viscosity and the relative viscosity of different fluids.

Label the parts of a volcano including the Magma Chamber, Vents, Ash Cloud, Pyroclastic Flow and Lava

Explain how a hot-spot volcano is formed