Volcanism on earth is a process of eruption of Lava onto the surface of earth. Lava in molten phase deep inside the earth, is being called Magma. All the
Magma may not reach the earth's surface. When it solidifies it becomes Igneous rocks of different shapes and sizes.
Types of Volcanic Eruptions
- Volcanic eruptions are broadly classified into Central Eruption, where the eruption will be through a central vent and Fissure Eruption,
where the eruption will be through numerous fissures.
- Within these broadly defined eruptive types, there are various subtypes of eruptions
- Volcanic type of Eruption - These are highly explosive eruptions. Along with lava, solidified lava (pumice), country rocks (volcanic bombs)
and huge amount of dust are thrown out, making it extremely dark, e.g. Sakurajima, Japan.
- Strombolian Variety - It is seen in Mount Stromboli of Sicily. Here, the explosive force is moderate to high. Along with Lava and dust, huge
quantity of water vapour and inflammable gases and incandescent dust are released.
Vapour and gases form orange flame at the top of the mountain and Appears
like a Bunsen Burner. In ancient and medieval era, it acted like a lighthouse and was regarded as lighthouse of Mediterranean sea.
- Plinian Type - It is seen in Mount Pliny in Caribbean islands. Here, there will be moderate to high explosion and eruption of large amount of
Incandescent dust or ash known as Cinder, which turns into hair like (ponytail) feature, is seen on top of the mountain.
- Icelandic Variety or Hawaiian type of Eruption - Here, Lava erupts slowly and quietly devoid of any exclusive force.
Types of Volcanoes
Depending on the volcanic activity, volcanoes are divided into 3 types, Active volcanoes, Dormant volcanoes and Extinct or Dead volcanoes.
- Active Volcanoes - These volcanoes erupt on regular basis. These are concentrated on the margins of Pacific Ocean called Circum-Pacific belt or
Ring of Fire.
Examples of Active Volcanoes are Mt Stromboli and Mt Etna of Sicily, Mt Pinatubo of Philippines, Mauna Loa of Hawaii, etc. In Barren
Island of Andaman & Nicobar Islands, there is active volcanism.
- Dormant Volcanoes - There is no history of recent volcanism here and there is plenty of chances of eruption in future. Here, Central pipe is
intact and it is connected to the Magma chamber. Crater is not occupied by any lake.
The hot gases evaporate the rain water. On a regular basis the water
vapour and gases come out here. Examples are Mt Kilimanjaro in Africa, Mt Vesuvius, etc.
Mt. Vesuvius was regarded as dead volcano. Assuming this, the
city of Pompeii was built as the Lava was very fertile at that time.
Series of explosions occurred from Mt. Vesuvius and flooded the whole city. We have
ruins of city of Pompeii, which is now a centre for international tourism.
- Extinct or Dead Volcanoes - There is no history of recent volcanism here and no chances of eruption in future. If the crater is not connected to the Magma
chamber and the pipe is shattered, it will be dead as Magma cannot come out.
If crater is occupied by a lake, there is 100% of not coming back. This is because
if the water vapours or hot gases are coming out, lake cannot be formed. Examples are Mt Aconcagua in South America, Mt Kenya in Africa, etc.
Igneous Rock Landforms
Igneous rock landforms are formed due to the action of either Magma or Lava. When Magma solidifies inside the crust of earth, the landforms thus formed are
called Intrusive Landforms and when the Lava erupts onto the surface and solidify outside, the landforms thus formed are called Extrusive or Volcanic
Intrusive Landforms of Igneous Rocks
- Batholith - These are the rocks which solidify deep inside the earth's crust.
- Dike or Dyke - These are the rocks which solidify in thin vertically spreading bodies like cracks. Dyke will be having 50 - 100 m in width and
spreading hundreds of kilometers in vertical direction. These are the discarded bodies and they cut across the rocks.
- Sill - These are the horizontal concordant Igneous bodies.
- Lopolith - These are Saucer shaped Igneous rock forms.
- Phacolith - These are Wave shaped Igneous bodies.
- Laccolith - These are Dome or Lens shaped Igneous rock forms.
Extrusive or Volcanic Landforms of Igneous Rocks
- Canonical Shaped Landforms - In this, the Lava flows through a single pipe or vent. A volcanic cone is formed at the top and looks like Conical
We can see cone-crater arrangement here. These are very common type of landforms, e.g. Mount Vesuvius.
- Shield Shaped Landforms - If lava is more mobile, it spreads slightly faster and turns into volcanic dome. It spreads over a wider area, e.g. Mauna
Loa in Hawaii Islands.
- Lava Plateaus - Here Lava erupts through numerous cracks and fissures and the crust is highly fractured. Lava is spread over much larger area, which
forms high and extensive plateaus or tablelands known as Lava Plateaus.
They turn into plateaus when lava is highly mobile like Like Basaltic lava, e.g. Deccan
Plateau is formed due to Basaltic lava.
- Caldera - Sometimes there will be high explosive eruption of Lava. Here crustal rocks are completely shattered and turn into huge depression known
Here, there will be series of explosions, e.g. Krakatoa volcano in Indonesia. Sometimes, they are filled with water forming lakes like in Java.
- Composite Cone - It is formed when there is alternate quiet and explosive eruptions take place. It forms different layers of lava and cinder over
one another. These are also called Stratovolcano, e.g. Mount Fuji.