Restless earth

CHARACTERISTICS

• invisible from the ground
• forms a depression, called a CALDERA
• has a ridge of higher land around it
• erupts less frequently - eruptions are hundreds of thousands years apart.

CASE STUDY - YELLOWSTONE

Effects - 

• 75% of America would be directly affected; total destruction would occur, leading to 87,000 deaths 
• within 1,000Km of the eruption; ash layers would be atleast 15cm deep leading to roof collapses
• all aircraft would need to be grounded as ash stops the engines and dust blocks a clear view
• in the tropics, temperatures would fall by 15 degrees

1) rising magma cannot escape, and a large bulge appears on the surface

2) cracks appear in the surface and gas and ash erupt from the magma chamber

3) the magma chamber collapses, forming a depression called a caldera. 

There are two types of volcaones; they are: COMPOSITE and SHIELD

• Composite volcanoes are a tall cone shape with a narrow base and steep sides. The volcano is made of alternate layers of ash and lava.  Composite volcanoes give off violent eruptions with the lava being vicious (sticky).
• Shield volcanoes are cone shaped but have a wide base with gentle sloping sides; the lava pours out with little violence and the lava isn't very viscous. Shield volcanoes have frequent eruptions. 

Composite volcanoes are found on DESTRUCTIVE plate boundaries

Shield volcanoes are found on CONSTRUCTIVE plate boundaries. 

Composite volcanoes are formed at destructive plate boundaries because an oceanic plate is pushed towards the continental plate by convection currents. The oceanic plate is subducted ; this creates a deep ocean trench. 

Shield volcanoes are formed at constructive plate boundaries becaus ethe two plates are moving apart from each other in opposite directions. Convection currents moving in opposite directions in the mantle move the two plates apart. As the plates move apart, magma from the mantle escapes and the magma fills the gap and eventually erupts onto the surface and cools as land. A good example of this is the mid atlantic ridge, where the eurasian plate moves away from the North american plate. 

In 1991 Mt Pinatubo erupted

• killed more than 300 people
• 655 people lost their jobs
• 50,000 people/families lost their homes

Mt Pinatubo is in the pacific ring of fire, on the Phillippines. The volcano was on a destructive subductive plate boundary, because it was on an oceanic plate margin. (the Eurasian and Phillipine plate). 

Immediate responses:

• emergency food, medicines and tents were sent from Japan

Long term responses:

• Various countries helped by granting money, materials and equipment for rehabilitation