3.5. VOLCANIC LANDFORMS

You may have seen photographs or pictures of volcanoes on a number of occasions. A volcano is a place where gases, ashes and/or molten rock material – lava – escape to the ground. A volcano is called an active volcano if the materials mentioned are being released or have been released out in the recent past.

A volcano is an opening in the earth's crust where materials like gases and molten rock, known as lava, can escape. These materials can erupt violently or flow gently, leading to various types of volcanic eruptions. An active volcano is one that has had recent eruptions or is currently erupting.

Think of a volcano like a pressure cooker. When the pressure builds up too much, steam (or in the case of the volcano, lava and gases) has to escape. Just as a pressure cooker can release steam through its valve, volcanoes allow molten rock to escape, sometimes violently.

The layer below the solid crust is mantle. It has higher density than that of the crust. The mantle contains a weaker zone called asthenosphere. It is from this that the molten rock materials find their way to the surface. The material in the upper mantle portion is called magma. Once it starts moving towards the crust or it reaches the surface, it is referred to as lava.

The earth's mantle is the layer beneath the crust, which is denser and includes a section called the asthenosphere that allows molten rock (magma) to rise. When magma breaches the crust and flows out, it is termed lava. Understanding this transition from magma to lava is crucial in studying volcanic activity.

Imagine a lava lamp. When heated, the wax inside melts and starts to rise, similar to how magma moves toward the surface. When it escapes the bottle, it’s like lava flowing out of a volcano.

Volcanoes are classified on the basis of nature of eruption and the form developed at the surface. Major types of volcanoes are as follows: Shield Volcanoes, Composite Volcanoes, Cinder Cone Volcanoes.

Volcanoes can be categorized into several types based on how they erupt and their shape. Shield volcanoes have broad, gentle slopes and are built by fluid lava flows. Composite volcanoes are steeper and created from more viscous lava that leads to explosive eruptions. Cinder cone volcanoes are formed from deposits of small lava fragments, creating steep conical hills.

A shield volcano resembles a warrior's shield—broad and flat. In contrast, a composite volcano is like a pyramid, steep and tall, while a cinder cone is akin to a small, steep hill made up of a pile of pebbles.

The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion. Depending on the location of the cooling of the lava, igneous rocks are classified as volcanic rocks (cooling at the surface) and plutonic rocks (cooling in the crust).

When lava cools down, it solidifies into igneous rock. If this cooling happens on the surface, the rocks formed are known as volcanic rocks. If cooling happens below the surface, the rocks are called plutonic rocks. This process creates different kinds of geological formations, based on where and how the cooling occurs.

Think of ice cream melting and then refreezing. If you leave it out in a bowl, it'll freeze at a different rate than if you put it back into the ice cream maker. Similarly, the location where lava cools determines the type of rock it forms.

These are the most explosive of the earth’s volcanoes. They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas.

Calderas form after a massive volcanic eruption when the volcanic structure collapses inward. Instead of building a mountain, this process creates a large depression that can be filled with water over time, often becoming a lake.

Imagine a balloon that you inflate and then suddenly pop. The balloon collapses and creates a large deflated area. Similarly, a caldera is the result of a volcano collapsing after a large eruption, forming a bowl-like depression.

These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are covered by thousands of sq. km of thick basalt lava flows.

Flood basalt provinces occur when large quantities of low-viscosity basaltic lava erupt over extensive areas, creating thick layers of basalt rock. These flows can extend for hundreds of kilometers, often leading to extensive geological formations.

Think of pouring a thick syrup over pancakes. If the syrup is runny and you pour it from a great height, it spreads out over a large area, similar to how fluid lava can spread out and cover vast regions when it erupts.

The lava that cools within the crustal portions assumes different forms. These forms are called intrusive forms. Some of the forms are shown in Figure 3.4.

When lava cools inside the earth before reaching the surface, it creates various intrusive forms such as batholiths, laccoliths, and dykes. These formations result from different cooling conditions and pressure levels in the crust, leading to unique shapes and structures.

Consider how frosting can take different shapes when spread on a cake—smooth and dome-like versus uneven and crusty depending on how it's applied. Similarly, lava creates different forms based on its cooling process underground.

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