ZIMSEC O Level Geography Notes: Vulcanicity
- Vulcanicity refers to all the various way by which molten rock and gasses are forced into the earths and onto its surface.
- It includes volcanic eruptions which lead to the formation of volcanoes, lava plateaus and geysers.
- It also includes the formation of other vulcanic features such as batholiths, sills and dykes in the earth’s crust.
- Volcanic materials include rock fragments, smoke, magma and lava.
- Magma is molten rock.
- Magma comes from the mantle.
- When magma reaches the earth’s surface it is known as lava.
- Vulcanicity is common at plate boundaries.
- Convergence and Divergence plate boundaries see most of the world’s volcanic activity.
- Vulcanicity is divided into two main types:
- Extrusive vulcanicity and,
- Intrusive vulcanicity
- Intrusive vulcanicity occurs within the earth’s crust while extrusive volcanoes occur on the surface of the earth’s crust.
- A volcano is an opening in the earth’s surface through which magma is injected into the earth or ejected as lava onto the earth’s surface.
- Rocks below the earth’s crust have a very high temperature but the great pressure exerted on them by the earth’s crust keeps them in a a semi-solid state.
- Friction at plate boundaries raises their temperature and the simultaneous fall in pressure caused by faulting and folding causes the rocks to become molten and semi-fluid.
- It is these rocks that are known as magma which then flows up into cracks in the earth’s surface.
- When the magma stays in the earth’s crust (intrusive vulcanicity) it may cool and solidify forming such features as dykes, batholiths, sills and lapoliths.
- It may reach the earth’s surface as either a violent or quiet eruption.
- If the magma contains gasses especially steam, as the magma approaches the surface there is a sudden reduction in pressure causing the gases to expand rapidly forming a violent eruption.
Types of volcanoes
There are three types of volcanoes:
- Active volcanoes
- Dormant volcanoes
- Extinct volcanoes
1. Active volcano
- The volcano erupts either occasionally or periodically
- A volcano is considered active if it is currently erupting, is likely to erupt or is showing signs of unrest for example if it is making gas emissions.
- Most volcanoes are found along the Pacific Ring of fire.
- Mount Nyiragongo and the neighboring Nyamuragira are considered Africa’s most active volcanoes.
- Active volcanoes are sometimes linked to Tsunamis.
2. Dormant volcano
- The volcano is inactive and has not been known to erupt in recent times.
- Dormant may erupt again and recent studies have shown that some of the dormant volcano volcanoes are only going through what is known as the recharge period which may last for tens of thousands of years after which they will erupt again.
- Dormant volcanoes are currently quiet but may erupt again.
- An examples include Mount Vesuvius which erupted suddenly in AD 79, Mount Pinatubo in 1991 and Soufrière Hills in 199 all of which erupted again after being considered dormant.
- An example of a dormant mountain is Mount Kilimanjaro in Tanzania.
3. Extinct volcano
- Are those which are considered by scientists as being unlikely to erupt again
- Usually because they have exhausted their magma supply.
- Hawaii’s Emperor sea mount chain is an example of an extinct volcano.
To access more topics go to the Geography Notes page.