Tectonic Hazards Revision

Extinct = Not erupted for thousands of years

Dormant = (sleeping) not erupted for years, or even centuries, but still releases gas sometimes

Active = Has erupted recently and is likely to do so again

-Can form in the middle of a plate

-Isolated columns of hot magma rising slowly within the mantle

-Melts overlying crust, "burning a hole through"

-Can be under continental or oceanic plates

-A conveyer belt of volcanes are formed

-Example: Hawaii and the island surrounding it

-Usually found at DESTRUCTIVE plate boundaries

-Cone volcanoes are tall and steep-sided

-Cone volcanoes are formed from eruptions of thick, viscous lava. The lava is generally like this, as the magma is mixed with water and other materials from the sea bed, when the oceanic plate is subducted under the continental.

-The thick lava moves relaticely slowly and hardens quickly to form new rock - this explains steeps sides and a cone shape.

-Eruptions tend to be violent

-These volcanoes are composed of alternating layers of lava and ash.

-The eruptions from these volcanoes may be a pyroclastic flow rather than a lava flow. Pyroclastic flow is a mixture of hot steam, ash, rock and dust.

-A pyroclastic flow can roll down the sides of a volcano at very high speeds and with temperatures over 400°C. 

-Usually found at CONTRUCTIVE plate boundaries

-They are low with gently sloping sides.

-Shield volcanoes are formed by eruptions of thin, basaltic, runny lava, which is very hot, therefore travels farther before cooling. 

-Eruptions tend to be frequent but relatively gentle.

Destructive:

-Volcanoes on a destructive/convergent plate boundries, where subduction occurs, erupt magma that is high in silica and therefore more viscous. 

-This silica comes from the slab of oceanic crust sliding underneath the continental crust.

-As the melting oceanic crust mixes with the clean magma from the mantle/centre of the earth, it pollutes the clean magma with water and carbon dioxide.

-The polluted magma becomes sticky and thick, so it often sticks to the inside of the vent, causing a great build up of pressure, particularly when lava has hardened, creating a cap in the vent.

-When this pressure is eventually released, it causes a large and explosive release of lava and other materials from the volcano.

Contructive:

-At a constructive/divergent plate boundary, the two plates move apart, leaving a gap through which clean, unpolluted magma can flow up to the surface. 

-This type of magma has not been 'polluted' by mixing with melting oceanic crust and so it is fluid and runny. 

-On reaching the Earth's surface, it can flow for a long time before cooling and hardening, so it produces low, wide volcanoes.

-The fact that there is no mixing of the magma, and no build up of pressure means that the eruptions are relatively gentle.

-Fertile soils - good for farming

-Tourist Attractions - people want to visit volcanoes

-Cheap electricity - Geothermal energy can be harnessed

-Mineral deposits - for example: gold, silver, diamonds, copper, zinc and building stones