VolcanoesWHAT IS A VOLCANO?

• A volcano is an opening in the earth’s crust which allows molten rock from beneath the crust to reach the earth surface.

• The molten rock is called magma when it is beneath the earth’s crust.

• When magma reaches the earth’s surface it is called lava.

• Other substances that are also released together with magma are gases, ash and rocks.

Places which experience volcanoes• Volcanoes form at the edges of the plate tectonics.• Plate tectonics are huge slabs of the earth’s crust which flows atop

the partly molten mantle.• The mantle is the partially molten layer beneath the earth’s crust.• Plate tectonics have well fitting edges which resemble a jig saw

puzzle.• At the boundaries where the plates meet, there is slight

movements which take place.• During these movements the plates may collide with one another

or pull apart or move parallel to each other in opposite directions.• These places where there is movement of plate tectonic are known

as active zones. • It is on these active zones that most volcanoes erupt.• Volcanoes may also erupt in places where the earth’s crust is very

thin. These areas are known as hotspots.

Landforms that are created by Volcanoes.

(a) Stratovolcano (composite)• Volcanoes erupt in different ways producing different

landforms.• Steep cone shaped, volcanoes form when plates collide• This is because all the pressure and heat of the collision

make for a violent eruption.• The stratovolcano is formed when the lava and other

material is ejected and build up around the opening.• A good example of this type of volcano is Mt. Rainier.• Also known as the composite volcano.

(b) Shield volcanoes• When plates pull apart lava escapes through

the rift. This more gentle flow creates new floor on the sea floor and wide rounded volcanoes on the surface called shield volcanoes. An example of a shield volcano is Hawaii’s Kilauea mountain.

• It is also an example of a volcano that formed in a hotspot.

(c) Cinder cone volcano • This is the result of a moderately explosive eruption of bits

of magma called scoria. • This magma will pile around the crater or vent. • This type of volcanoes are common in Western North

America and other parts of the world.

(d) Caldera• Sometimes an eruption is so violent that the top of the

volcano collapses, leaving a huge pit or caldera. • An example is the Yellowstone National Park and Crater

Lake.

The effects of volcanoes.

• Volcanic eruptions can be catastrophic resulting in loss of human, animal life and damages to infrastructure.

• Dangerous as they are volcanoes also build and shape the land, creating mountains and new sea floor.

• Volcanoes also deposit nutrients and minerals that enrich the soil.

• They also help the authorities to be proactive so as to put in place early warning systems.

• Alerts government to have contingent measures in case of the occurrence of a volcanoes such as evacuation plans and temporary shelter.

The Structure of the Earth

• To draw diagram on page 31 of the textbook.