1. Volcanoes

2. How and why do volcanoes erupt?
Hot, molten rock (magma) is buoyant (has a lower density than the surrounding rocks) and will rise up through the crust to erupt on the surface.
Same principle as hot air rising, e.g. how a hot air balloon works
When magma reaches the surface it depends on how easily it flows (viscosity) and the amount of gas (H2O, CO2, S) it has in it as to how it erupts.
Large amounts of gas and a high viscosity (sticky) magma will form an explosive eruption!
Think about shaking a carbonated drink and then releasing the cap.
Small amounts of gas and (or) low viscosity (runny) magma will form an effusive eruption
Where the magma just trickles out of the volcano (lava flow).

3. Volcanoes Form at any point where the Crust is WEAK?
DIVERGENT CONVERGENT HOT SPOTS
Magma rises through the Earth's crust,
Pressure builds up inside the Earth.
When this pressure is released, eg as a result of plate movement, magma explodes to the surface causing a volcanic eruption.
The lava from the eruption cools to form new crust.
Over time, after several eruptions, the rock builds up and a volcano forms.

4. The type of eruption depends on…..
-Amount of gas (volatile compounds) in the magma.
High Volatility Low Volatility
LOTS OF GAS NOT MUCH GAS
-VISCOSITY (resistance to flow) of the magma
High Viscosity Low Viscosity
Sticky/Thick Runny

5. Explosive Formed from cooler, more viscous magma
Explosive Formed from cooler, more viscous magma. (Gasses become trapped in thick magma, pressure increases)
Can send ash clouds into the stratosphere
Magma has High VISCOSITY & High VOLATILITY
Produces thick/sticky lava flows
Build up steeply sloping volcanoes

6. Effusive Formed from hot, runny magma that leaks out onto the surface passively as lava flows.
Outpourings of lava onto the ground.
Magma has low viscosity and low volatility
Builds up gently-sloping volcanoes

7. Where do volcanoes erupt???
Convergent plate boundaries – where two of Earth’s plates join together RING OF FIRE
Divergent plate boundaries – where two of Earth’s plates move apart MID OCEAN RIDGES
Hot spots – active areas below the earths crust

8. Structure of a Volcano magma central vent side vent lava old layers
of lava
ash cloud
Earth’s crust

9. Structure of a Volcano
Magma – the molten, or liquid-like, rock within the Earth
Lava – magma that reaches Earth’s surface
Vent – an opening through which molten (liquid-like) rock flows onto Earth’s surface
Fissures: side vents, usually accompanying the main vent.
Crater/caldera: large, depression created by the emptying of the magma at great speed. The volcano collapses creating a large crater.
Ash cloud: smoke and debris usually consisting dust, hot rock fragments (tephra), gases, water vapor, and other particles.

11. Global Impact
When a volcano erupts it throws out a lot of ash.  At short notice this ash can be very harmful to the environment, but on the long term the ash layer, which contains many useful minerals, will be converted to a very fertile soil.
Volcanoes provide resources for energy extraction, also called geothermal resources.  Heat from the earth's crust is being converted to energy.  The big advantages to this type of energy are that it is very clean and the resources are nearly inexhaustible.

12. Global Impact
Sea Floor Spreading- new crust is formed at the bottom of the ocean floor.