Volcanoes And Volcanic Hazards

IB SL

A volcano is an opening or vent in the earth’s surface through which molten material erupts and solidifies as lava.

What is a volcano?

Volcanic vent

Definition

• A volcano is an opening through the Earth’s crust through which hot molten magma (lava), molten rock and ash are erupted onto the land.

• Most volcanoes are found at plate boundaries, although there are some exceptions, such as the volcanoes of Hawaii.

• Some eruptions let out so much material that the world’s climate is affected for a number of years.

Characteristics

• Magma: refers to molten materials inside the Earth’s interior. When the molten material is ejected at the Earth’s surface through a volcano or crack at the surface, it is called lava.

• Chamber: refers to the reservoir of magma located deep inside the volcano.

• Crater: is the depression at the top of a volcano following a volcanic eruption. It may contain a lake.

• Vent: is the channel which allows magma within the volcano to reach the surface in a volcanic eruption.

Where are volcanoes found?

Facts

• The greatest volcanic eruption was Tambora in Indonesia in 1815. Some 50-80 km3 of material was blasted into the atmosphere.

• In 1883 the explosion of Krakatoa was heard as far as 4776 km away.

• The largest active volcano is Mauna Loa in Hawaii, 120 km long and over 100 km wide.

Cross section of a volcano

CraterVolcanic bombs, ash and gases

Main vent

Parasitic cone

Magma chamber

A destructive plate boundary is found where a continental plate meets an oceanic plate.

The oceanic plate descends under the continental plate because it is denser. As the plate descends it starts to melt due to the friction caused by the movement between the plates. This melted plate is now hot, liquid rock (magma). The magma rises through the gaps in the continental plate. If it reaches the surface, the liquid rock forms a volcano.

Destructive Plate Boundary

Why do they happen?

Destructive Plate Boundary

Constructive Plate Boundary

At a constructive plate boundary, two plates move apart.

As the two plates move apart, magma rises up to fill the gap. This causes volcanoes at this type of boundary. However, since the magma can escape easily at the surface, the volcano does not erupt with much force.

Why do they happen?

Constructive Plate Boundary

At destructive plate boundaries the lava is viscous (thick like treacle) and it cannot flow very far from the volcano’s vent. This makes the volcano steep sided.

At constructive plate boundaries the lava is runny and it can flow far away from the volcano’s vent. This makes the volcano gentle sided.

Types Of Volcano

• Two contrasting types exist;

• Those dominated by lava, usually found at diverging boundaries, where basalt can rise freely to the surface (volcanoes of Iceland)

• Those dominated by Ash, usually found along subduction zones where large amounts of pyroclastic material is ejected (Japan - Fuji Yama)

Cone ShapesCone Shape: Characteristics:

FissureVery gentle slope, found at diverging ocean plates, basaltic lava, can flow over large distances.

Basic / Shield

Have gentle slopes, steeper than fissure due to repeated explosions and subsequent build up of basalt based lava (Mauna Loa Hawaii).

Cone Symmetrical in shape, A) acid where thick viscous lava, rapidly cools, B) Ash / Cinder.

Composite Very large old volcanoes. Both ash and lava are deposited (Mt. St. Helens).

Crater / Caldera

Form when a very violent eruption occurs after a build up of gas beneath the volcano. Can destroy the magma chamber leaving a large crater.

Cones

•Active volcano - liable to erupt e.g. Mt. Etna

•Dormant (sleeping) volcano - a volcano which has not erupted for many years. For example, Mt Pinatubo erupted in 1991 after 500 years of dormancy.

•Extinct volcano - a volcano which has not erupted for many thousands or millions of years. e.g. Edinburgh

However, it is often very difficult to tell whether a volcano will erupt again…El Chichon, Mexico erupted in 1982 after being dormant for approximately 1200 years!

Do all volcanoes erupt?

Hot spot

Active volcanoes

Extinct volcanoes

plate movement

In the diagram above, why are the volcanoes to the right of the ‘hot spot’ extinct?

Types Of Lava Flow

• This ranges from being extremely thick and viscous, to highly fluid. The amount of pyroclastic material also varies. The two types are:

• Aa flow: This is a few metres thick, a mix of uneven shaped, sharp edged ash and cinder blocks. It is unstable.

• Pahoehoe flow: Is fluid rather than viscous, but does not move quickly. Often the surface layer is static whilst flow continues beneath.

Types Of Eruption

Types Of Eruption

Hawaiian Type Lava slowly and easily escapes from vent. It is runny, basaltic. Gases escape easily & gentle sided volcanoes result.

Strombolian type Gas explosions occur more frequently, small but frequent eruptions. Cone shaped volcano.

Vulcanian type Gas emissions involved, more violent but less frequent than above. Plugs of cooled lava may be ejected in blast.

Vesuvian typeExtremely strong explosions, often after volcano has been dormant for a while. Gas and ash clouds, which fall over a large area.

Krakatoan type Exceptionally violent.

Plinian typeUsually most violent. Massive amounts of lava, gas and pyroclastic material emitted. Part of the volcano may be removed.

Primary Volcanic Hazards• Lava flows: A greater threat to property than human life due to

opportunity for evacuation. Lava flows are more dangerous when released very quickly. They usually cause large but localised economic losses.

• Pyroclastic flows: These are hot rock fragments, lava particles ash and hot gases. They are linked with subduction zone volcanoes, and the flow moves very quickly from its source.

• Ash and Tephra fall: Ash is material below two millimetres in diameter whilst tehpra is anything above this. It is usually formed when magma is fragmented by explosions, and can stay in the atmosphere causing global variations in weather patterns. Ash fall do not cause many deaths but can lead to breathing difficulties.

• Volcanic gases: Gases emitted from volcanoes include, water vapour, carbon dioxide, sulphur dioxide, hydrogen sulphide, helium and carbon monoxide. They rarely cause death but can be problematic as many are heavier than air.

Secondary Volcanic Hazards

• Lahars: Mud flows of volcanic material due to ash and debris mixing with water. On steep slopes speeds can reach 22 metres per second.

• Volcanic landslides: These are slides of rock and loose volcanic material, which are driven by gravity.

• Tsunamis: A rare event, but the 1883 eruption of Krakatoa, did create a 30m high tidal wave.

Activity

Using Essential AS Geography P38-39, answer the following…

1.What are the differences between “fissure”, and “vent” volcanoes?2.How do silica, and volatiles affect volcanic eruptions?3.Compare composite and shield volcanoes by sketching, labelling, and annotating diagrams of both.