Defination
This is the movement of magma from the mantle into the earth's crust and onto the earth's surface. That is, the process by which solid and gaseous material (magma) from them the mantle is injected into the crust or ejected onto the surface. From this definition, two types of Volcanicity can distinguished, namely; intrusive and extrusive Volcanicity.
- Intrusive Volcanicity: occurs when magma rises and solidifies within the earth's crust. This often produces Intrusive structures or plutons of various sizes and shapes such as dykes and sills.
- Extrusive Volcanicity: Occurs when magma reaches the earth's surface as lava before cooling and modification take place. When magma if forced out of the crust to the surface, the gases are given off and it is called lava. This often accumulates to form volcanic cones of various types and other features.Volcanoes: Origin, types and General structure
A volcano is a cone with a hole where lava (hot, liquid rock) comes from a magma chamber under the ground. Most volcanoes have a volcanic crater at the top. Volcanoes sometimes form mountains. Mauna Loa, which is one of five volcanoes on the Big Island of Hawaii, is the world's biggest volcano. It has spewed lava 33 times in the past 170 years.
The main parts of a volcano as given in the fig above are:
1. The crater: A depression at the top of the volcano from which volcanic material has been ejected.
2. The vent: The conduit between crater and the magma chamber.
3. The cone: The area around the crater at the top of the volcano, made up of material ejected during an eruption. Many volcanoes have several craters, cones and vents.
When a volcano is active, materials come out of it. The materials include lava, steam, gaseous sulfur compounds, ash and broken rock pieces.
Formation of Volcanoes
Volcanoes are formed when magma from within the Earth's upper mantle works it's way to the surface.
- The first stage is therefore that of the formation of magma. Magma formation takes place at the base of the crust and in the upper mantle. This occurs when solid rock under pressure exerted (by the overlying layers) is changed into liquid whenever the pressure is released.
- Pressure is released when the crust fractures or develop faults or when overlying layers are uplifted during folding.
- As the pressure is released, the semi-solid rock of the upper mantle melts and becomes liquid or molten rock. This is magma which comprises of mix of liquid, gases and remaining solid fragments.
- The magma formed usually collects in a large reservoir or magma chamber a few kilometers below the surface.
- Since molten material is less dense than the surrounding rocks, the magma then rises from the source regions and through a variety of means (vents or fractures), is forced into the crust above it or onto the surface.
- At the surface, it erupts to form lava flows and ash deposits. Over time as the volcano continues to erupt, it will get bigger and bigger.
Types of Volcano
A popular way of classifying volcanoes is by their frequency of eruption into 3 types: Active, dormant and extinct volcanoes.
- Active volcanoes: Are those that erupt regularly or that have erupted in some way relatively recently: say, within the last few hundred of few thousand years. Kilauea volcano on Hawaii is the world's most active volcano, followed by Etna in Italy and Piton de le Fournaise on La Réunion island.
- Dormant Volcanoes: They are volcanoes that have not erupted for some time, perhaps several hundred or thousand years, but are still considered likely of erupting sometime in the future. They are sleeping volcanoes.
- Extinct volcanoes: They are volcanoes that have not erupted in historical times. They are considered incapable of erupting.
Hope this was helpful to make you move an step further in understanding Volcanicity. There is still more untold and will be given in a nearest feature.
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