Main Features of Earthquakes & Volcanoes (Cambridge (CIE) O Level Geography)
Revision Note
Written by: Bridgette Barrett
Reviewed by: Jenna Quinn
Main Features of Volcanoes
A volcano is formed when magma erupts onto the Earth's surface as lava through a vent in the Earth's crust
The magnitude of a volcanic eruption is measured on the Volcanic Explosivity Index (VEI)
The scale is open ended but the highest in recorded human history was an 8 (Tambora 1815)
All volcanoes have features in common, as shown in the diagram below:
Due to the type of lava erupted the formation of a volcano varies
Composite (strato-volcanoes)
Steep-sided
Sticky (viscous) lava
More explosive eruptions
Formed from alternating layers of ash and lava
Tend to form on convergent (destructive) plate boundaries
Shield volcanoes
Gently sloping sides
Runny/thin lava
Less explosive - gentle eruptions
Tend to form on divergent (constructive) plate boundaries or hot spots
Frequent eruptions
Volcanoes may be active, dormant or extinct
Active
The volcano has recently erupted and is likely to erupt again
Dormant
Has erupted in the last 2000 years and may possibly erupt again
Extinct
Shows no evidence of eruption in historic times and there is no evidence of a magma reservoir
Volcanic eruption features
Eruptions may include a range of features as shown in the table below:
Feature | Characteristics |
|---|---|
Lava | When magma erupts to the surface it is known as lava. The lava can be thin and runny or thick and slow moving. This depends on the composition of the magma |
Ash | Pulverised solid lava which measures less than 2 mm in diameter. Ash is ejected into the atmosphere and can travel thousands of kilometres |
Pyroclastic flow | Fast moving, very hot clouds of poisonous gases mixed with ash. Average speeds of about 100km/h but can move at up to 700 km/h |
Lahars | When volcanoes erupt snow and ice on the peak melts and combines with the ash. This creates fast moving mudflows or lahars |
Earthquakes | Magma rising to the surface through the vents in the volcano increases pressure on the Earth's crust leading to earth tremors |
Volcanic bombs | Fragments of molten rock which are ejected from the volcano. These are between 60mm and 5m in diameter |
Worked Example
Shield volcanoes are often formed on constructive (divergent) plate boundaries.
Describe two characteristics of a shield volcano.
[2 marks]
Answer:
Any two of the following
Fluid/thin/runny lava [1]
Low (height to width ratio) [1]
Gentle slopes [1]
Gentle eruption [1]
Frequent eruptions [1]
Main Features of Earthquakes
An earthquake is the sudden, violent shaking of the ground
Earthquakes are the result of pressure building when tectonic plates move
The epicentre is the point on the Earth's surface directly above the focus
The focus is the point at which the earthquake starts below the Earth's surface
The magnitude (amount of energy released) by earthquakes is measured on the Moment Magnitude Scale which replaced the Richter scale
The damage caused by earthquakes is measured on the Mercalli Scale
Earthquakes can occur anywhere but mostly occur at or near plate boundaries
Earthquakes happen at all plate boundaries - constructive (divergent), destructive (convergent), collision and conservative (transform)
At a constructive (divergent) plate boundary, earthquakes tend to be weaker as the plates are moving apart
At destructive (convergent), collision and conservative (transform) plate boundaries earthquakes tend to be stronger
Earthquake sequence
The sequence of an earthquake is the same regardless of the boundary at which it happens:
As the tectonic plates move, they can get stuck
Pressure builds as the plates continue to try to move
Eventually the plates jolt free, and the pressure is released as energy
The point at which the earthquake starts is the focus
The epicentre is the point directly above the focus on the earth's surface
The energy passes through the Earth's crust as waves which is the earthquake
Features of an Earthquake
Earthquakes can also happen as a result of human activity such as drilling into the crust or mining
Worked Example
Tick the one statement in the table below which is the correct definition of an earthquake.
[1 mark]
A crack in the earth's crust caused by a volcano |
|
A vent between two plates of the earth's crust through which molten magma escapes |
|
Movement of large slabs of rock in the mantle |
|
The shaking of the ground caused by plate movement |
|
Answer:
The shaking of the ground caused by plate movement [1]
Examiner Tips and Tricks
When describing the processes which lead to an earthquake or volcanic eruption, it is helpful to write the formation down as a sequence of steps. This will make the process easier to remember.
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