How are Volcanoes formed?
- Magma rises through cracks or weaknesses in the Earth's crust ( a vent) Usually at plate margins
- 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. (this is thrown out from the circular hole at the top called the crater)
- The lava from the eruption cools to form new crust.
- Over time, after several eruptions, the rock builds up and a volcano forms
Inside a Volcanoe
- The magma chamber is a collection of magma inside the Earth, below the volcano.
- The main vent is the main outlet for the magma to escape.
- Secondary vents are smaller outlets through which magma escapes.
- The crater is created after an eruption blows the top off the volcano.
An eruption occurs when pressure in the magma chamber forces magma up the main vent, towards the crater at the top of the volcano. Some magma will also be forced out of the secondary vent at the side of the volcano
Different Types of Volcanoe
Volcanoes can be described in terms of activity and can be: Still active and erupt frequently, Dormant (temporarily inactive but not fully extinct), Extinct (never likely to erupt again).
Volcanoes can also be described by their shape or type:
- Shield volcanoes are usually found at constructive or tensional boundaries.
- They are low, with gently sloping sides.
- They are formed by eruptions of thin, runny lava.
- Eruptions tend to be frequent but relatively gentle. (Hekla in Iceland)
- Composite volcanoes are made up of alternating layers of lava and ash (other volcanoes just consist of lava). (Cone shaped)
- They are usually found at destructive or compressional boundaries.
- The eruptions from these volcanoes may be a pyroclastic flow rather than a lava flow. A 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 of over 400° C. (Etna in Italy)
Erupt a massive volume of material, 1000 times bigger than the large erruption of Mount St Helens in 1980.
Would alter the landscape over thousands of kilometres. There would be a "volcanic winter" as less sunlight would reach earth so global freezing would occur.
The last supervolcanoe eruption was Toba in Indonesia 75 000 yeas ago
Yellowstone in the USA is a supervolcanoe and a famous landmark for tourists due to its hot pools and geysers.
Effects of Volcanoes on people ( Negative)
Many lives can be lost and people injured as a result of a volcanic eruption.
If the ash and mud from a volcanic eruption mix with rain water or melting snow, fast moving mudflows are created. These flows are called lahars.
Lava flows and lahars can destroy settlements and clear areas of woodland or agriculture. Communications and public services disrupted.
Human and natural landscapes can be destroyed and changed forever.
Shortages of food, water and shelter, spread of disease from contaminated water
Economic problems for rebuilding, lose of farming and tourism. Social problems for familly loses and stress
Effects of Volcanoes on People ( Positive)
The dramatic scenery created by volcanic eruptions attracts tourists. This brings income to an area.
The lava and ash deposited during an eruption breaks down to provide valuable nutrients for the soil. This creates very fertile soil which is good for agriculture
The high level of heat and activity inside the Earth, close to a volcano, can provide opportunities for generating geothermal energy.
Mount St Helens Primary Effects
Mount St Helens is on the plate boundary between the Juan de Fuca plate and North American plate. When it erupted it permanently changed the surrounding landscape.
Primary effects :
Nuée ardente (hot ash and gas) destroyed forests and logging camps.
63 people were killed, mainly by poisonous gases.
7000 big game animals killed in lateral blast zone
Lahars (mudflows of ash and water) covered an extensive area surrounding the volcanoe
Short term results and responces:
People were rehoused and communications and stuctures repaired
Mount St Helens Secondary Effects
Ash blocked rivers destroying popular fishing sites and causing flooding. This in turn destroyed crops and livestock.
Flooding destroyed communications such as road and railway bridges.
Sediment carried downstream ruined barge transport on the Columbia River.
Long term responses and results
- Soil fertility improved due to the ash deposits.
- The volcano is now more carefully monitored.
- Tourism has increased, boosting the local economy.
Mount St Helens
Mount St Helens erupted on May 18th 1980 following a period of activity which began in March 1980 with an earthquake measuring 4.0 on the richter scale. What followed was 3 months of seismic activity as magma rose within the mountain. As the magma rose, a large bulge grew on the north flank of the volcano, this was due to a blockage in the main vent .
On May 18th, an earthquake measuring 5.1 on the richter scale caused a landslide on the northern flank of the volcano, this casued a release of pressure and a lateral (sideways) blast
Overall cost estimated $1.1million
300 people left the area after lossing jobs in forestry