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Volcanoes




Structure of the Earth

Crust - this is the thinnest layer of the earth and various from 5-75km in thickness.

Mantle - thickest layer of the earth - upper part is solid and forms the plates with the crust above. The lower part of the mantle is viscous and able to move.

Core - this is divided into the inner and outer core. The inner core, at the centre of the earth has temperatures up to 5,500oC and is solid, made up of iron and nickel.


There are two types of crust:

1. Continental crust - this is permanent and cannot be destroyed, it usually consists of granite rock and it is buoyant and less dense than oceanic crust - thickness may be up to 75-90km thick at its highest point.

2. Oceanic crust - this is constantly being renewed and destroyed at constructive plate boundaries, it consists of basaltic rock and it denser than continental crust - thickness varies between 5-10km.

There is a useful link here


Tectonic Plates

There are a number of large plates and several smaller plates - see map below.

Types of Plate Boundaries

1. Destructive Boundary - two plates moving together - one oceanic and one continental - Nazca and South American plate. - The Andes

2. Collision Boundary - two plates moving together - two continental plates - Indo-Australian and Eurasian plate. - The Himalayas

3. Constructive Boundary - two plates moving away from each other - oceanic crust - Eurasian and North American plates - Mid-Atlantic Ridge

4. Conservative Boundary - two plates which move side by side - San Andreas Fault - California.

There is a useful link here which looks in more detail at types of plate boundaries.

How do volcanoes occur at destructive boundaries? (learn the sequence):

- two plates (one oceanic and one continental) move together due to forces such as convection and slab pull / ridge push

- the oceanic plate is denser and is subducted (goes down) underneath the continental plate

- as it descends it is heated and melts to form magma

- this magma rises and erupts at the surface to form a volcano

- the pressures during subduction can also result in earthquakes.



How do volcanoes occur at constructive boundaries?

- two oceanic crustal plates move away from each other - due to forces of convection, ridge push and slab pull.

- magma wells up between the two plates to fill the gap

- new crust is created and where the magma erupts at the surface it forms volcanoes




How do earthquakes occur at conservative boundaries?

- here two plates move side by side

- the plates ‘stick’ and pressure and friction builds up between them

- eventually the pressure is released as the plates spring apart releasing seismic energy in the form of earthquakes.

- the point at which the earthquakes starts underground is the focus and the point directly above on the ground surface is known as the epicentre.




What happens at a collision boundary?

- two continental crustal plates collide

- as neither plate can be subducted, folding and faulting of the crustal rocks occurs

- this results in uplift and the formation of fold mountains such as the Himalayas

- there are no volcanoes here due to the lack of magma source however earthquakes can occur here.




Structure of a Volcano

Main Vent - this is the passage through which magma passes from the magma chamber to the surface.

Crater - this is the opening at the top of the volcano formed during an eruption.

Secondary Cone - this is a small volcano formed on the side of the main volcano due to a blockage of the main vent.

Secondary Vent - if the main vent becomes blocked, magma finds a weakness and forms another passage to the surface, this is known as a secondary vent.

Magma Chamber - the area beneath the volcano where magma is stored.

Volcanic Hazards

Ash Cloud - huge plumes of ash are released into the atmosphere

Pyroclastic Flow - these are clouds of hot ash, lava and gas which flow at great speeds down the side of a volcano - they may move at up to 450mph and have temperatures up to 1000oC

Poisonous Gases - many gases are released in huge quantities during an eruption, including sulphur dioxide and carbon dioxide - at high levels these can be very poisonous.

Lava Flows - the speed of these will depend on different types of lava, but where there is basaltic lava, flows can move quite fast.

Lahars - these are volcanic mudflows, they are formed when there are high levels of rainfall which mix with ash to form mud.

Volcanic Bombs - these are blocks of solidified lava which are thrown from the volcano during an eruption - they can vary greatly in size.

Montserrat Case Study

Causes:


Effects:


Source: http://volcanoes.usgs.gov/Hazards/Effects/SoufriereHills_PFeffects.html

How do we monitor volcanoes?

Earthquakes / Seismicity - as magma rises it can sometimes cause mini earthquakes as it forces its way through the crust. An increase in these small earthquakes can suggest that a volcano is increasingly active.

Poisonous Gases - an increase in gases such as sulphur can suggest that magma is rising - a spectrometer can be used to monitor changes in gas levels.

Tiltmeters / GPS - as magma rises it can cause changes in the shape of the volcano and may cause the sides to bulge. These changes in shape can be detected using GPS measurements and tiltmeters located on the sides of the volcano.

Thermal Cameras - thermal photography can be used to pick up signs of increasing heat from the ground as magma rises.

There is an interesting link supporting this here

Why do people continue to live near volcanoes?

GEOTHERMAL ENERGY - many volcanic areas areas are able to use the natural heat of the earth to provide cheap electricity and energy. Water is pumped down to the hot rocks, creating steam and turning turbines to create electricity. 87% of homes in Iceland use electricity and hot water from geothermal sources. Geothermal energy is also a sustainable energy and doesn't release greenhouse gases.

MINING - volcanic areas have many precious minerals from which people can make money from jobs in mining - for example sulphur mining in Indonesia (Kawah Ijen) and copper, gold and silver at Mount St Helens.

TOURISM - people can earn money in jobs associated with tourism as volcanic areas such as the Blue Lagoon in Iceland and Mount Fuji in Japan are very popular with tourists. Jobs include in hotels, tour guides, souvenir selling etc.


FARMING - ash and lava weathers down to produce fertile soils, these enable lots of crops to be grown so people can make a living in farming - e.g. Olive groves and citrus farms around the slopes of Mount Etna.

APATHY and UNWILLINGNESS TO LEAVE - some people have lived near volcanoes all of their lives and been unaffected by them and don't think it is going to happen to them so become unwilling to leave (e.g. Harry Truman - an 83 year old who refused to leave before the 1980 Mount St Helens eruption).

Make sure that you learn both causes and effects!

The following should help you when revising your volcanoes work for the end of unit Volcanoes test.  It will also provide good consolidation of the work you have done in class.

    REMEMBER:

   Active Volcano – a volcano which is or has    very recently been erupting e.g. Mt Etna    (Sicily, Italy).


   Dormant Volcano - A volcano which is    presently inactive but which may erupt again


   Extinct Volcano – a volcano that will not    erupt again