Volcanoes form when magma rises through cracks in the Earth’s surface. Pressure builds up inside the Earth. When this pressure is released, magma explodes to the surface causing a volcanic eruption. The lava from the eruption cools to form new crust. Over time, the rock builds up and a volcano forms. Volcanoes occur at destructive and constructive boundaries.
Volcanoes have parts:
Magma Chamber (Reservoir) is a collection of magma inside the Earth, below the volcano.
Main Vent is the main outlet for the magma to escape.
Secondary Vents are smaller outlets where magma escapes.
Crater is created after an eruption blows the top off the volcano.
Volcanoes can be described in terms of activity and can be:
- Active – erupt frequently
- Dormant – temporarily inactive but not fully extinct
Extinct – not likely to erupt again
Types of Volcano:
A Composite Volcano is a cone shaped volcano which is made up of layers/strata of lava, ash and rock debris
They are steep volcanos and are explosive when they erupt
The explosiveness of their eruptions is due to the thick, highly viscous, acidic lava
This viscous lava traps hot gases
The thick lava cannot travel far down the slope of the volcano before it cools. This makes the sides of the composite volcano steep.
As a result, composite volcanos are made of different layers
Example: Mount Vesuvius
Shield Volcanos are found at constructive plate boundaries or hot spots
Shield volcanoes are broad, domed-shaped volcanoes with long, gently sloped sides
These volcanoes can cover large areas but never grow very tall
These volcanoes flatten out due to the composition of the lava that flows from them.
This more fluid lava spreads out in all directions but cannot pile up in steep mounds.
Subduction zone volcanoes happen when two plates collide. One plate is subducted at a subduction zone. (Normally the oceanic plate).
At the plate is subducted, it melts in the mantle. This melted plate is in the form of magma in the mantle. This magma rises and escapes, creating volcanic activity.
Hazards of the volcano:
- Pyroclastic Flows – hot ash, rocks, gases and steam moving at high speed (up to 320km/h). Common with Explosive eruptions
- Ash Cloud – Blocks out the sun, suffocation, health problems (from air/soil etc)
- Lahar – Mudslide as a result of melted snow and volcanic ash
- Gases – volcanoes produce many gases (carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), fluorine gas (F2), hydrogen fluoride (HF)). All of these gases can be hazardous to health
- Lava Flow – Damage to buildings, vegetation (plants), burns
An Island arc is a chain of volcanic islands located off of continental coasts. The Japanese and Philippian archipelagos are examples. Island arc volcanoes result from the subduction of oceanic plates under ocean plates, while near coastal are the result of oceanic plates being subducted under continental plates.
A hot spot is an area of the sea that is pushed up by super-heated magma plumes originating from the core and often result in a volcano or series of volcanoes like those found in Hawaii.
Unlike other volcanoes, hot spots are not the result of the interaction of plate boundaries. They are often found near the middle of plates.
Sea floor spreading and rifts both occur at divergent plate boundaries.
However, rifts are found on land, such as the Great Rift Valley in Africa.
In contrast, sea floor spreading occurs at oceanic boundaries.
Example: Mid-Atlantic ridge
Positive and negative effects of an eruption
|The dramatic scenery created by volcanic eruptions attracts tourists. This brings income to an area.||Many lives can be lost as a result of a volcanic eruption.|
|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||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.|
|The high level of heat and activity inside the Earth, close to a volcano, can provide opportunities for generating geothermal energy.||Lava flows and lahars can destroy settlements and clear areas of woodland or agriculture.|
|Human and natural landscapes can be destroyed and changed forever.|
|Short Term effects||Long Term effects|
|Communications (roads/rail) effected
Housing is destroyed
Abandoning the area
Funding to rebuild
Decrease in tourism
|Soil fertility improves
Monitoring of the volcano
Increase in tourism/economy
How can volcanoes be predicted?
|Warning signs||Monitoring techniques|
|Hundreds of small earthquakes are caused as magma rises up through cracks in the Earth’s crust.||Seismometers are used to detect earthquakes (seismic activity)|
|Temperatures around the volcano rise as activity increases.||Thermal imaging techniques and satellite cameras can be used to detect heat around a volcano.|
|When a volcano is close to erupting it starts to release gases. The higher the sulfur content of these gases, the closer the volcano is to erupting.||Gas samples/emissions may be taken and chemical sensors used to measure sulfur levels.|
|Movement of the Earth’s surface||GPS monitors the movement
Tiltmeters measure changes in the slope
Preparing for a volcano
-creating an exclusion zone around the volcano
- -being ready and able to evacuate residents
- -having an emergency supply of basic provisions, such as food
-good communication system needs to be in place