3.4.1. Volcanoes

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.

Volcanoes are natural structures that form when magma from inside the Earth escapes to the surface. This can happen during an eruption when gases and molten rock are expelled, creating an active volcano. An active volcano is defined as one that has recently erupted or shows signs of future eruptions.

Think of a volcano like a pressure cooker. Inside, steam builds up pressure until it forces its way out. Similarly, magma builds up pressure until it erupts from a volcano.

The layer below the solid crust is the mantle. It has a 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 a layer of hot, semi-solid rock beneath the Earth's crust. It is denser compared to the crust and contains the asthenosphere, a weaker section that allows magma to flow. Magma forms when the mantle melts. When this molten rock rises to the surface, it is known as lava.

Imagine the mantle like a thick, gooey syrup that can flow. When it gets too hot, it becomes like a bubbling pot that eventually spills over, similar to how lava spills out in a volcanic eruption.

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, and Mid-Ocean Ridge Volcanoes.

Volcanoes fall into different categories based on their shape and the way they erupt. Shield volcanoes have broad, gentle slopes and erupt low-viscosity lava. Composite volcanoes are steeper and erupt more viscous materials, leading to explosive eruptions. Mid-Ocean Ridge volcanoes form underwater and are characterized by continuous eruptions along ocean ridges.

Think of shield volcanoes as large, wide pancakes and composite volcanoes as tall, narrow cakes. Just like how pancakes have a flatter shape due to the liquid batter spreading out easily, shield volcanoes have gentle slopes from the fluid lava. Composite volcanoes are more explosive and shaped like a tall cone, much like a fancy wedding cake.

These volcanoes are characterised by eruptions of cooler and more viscous lavas than basalt. These volcanoes often result in explosive eruptions. Along with lava, large quantities of pyroclastic material and ashes find their way to the ground.

Explosive eruptions are typically associated with composite volcanoes. In these eruptions, hotter, thicker lava traps gases that build up immense pressure. When the pressure is finally released, it results in explosive eruptions that can send ash and rocks flying into the air.

Imagine shaking a soda can and then opening it; the pressure built up inside causes a fizzy explosion. Similarly, when gas builds up in a volcano and can't escape, it results in an explosive eruption.

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 and solidifies, it forms igneous rocks. If this cooling happens above ground, we call it volcanic rock. If it cools beneath the Earth’s surface, it becomes plutonic rock. The distinction depends on where the cooling takes place.

Think of melting chocolate. If you pour it into a mold and let it sit until solid, you've made chocolate bars (like plutonic rocks). If you pour it directly onto a tray and let it harden, you get chocolate bark (like volcanic rocks).

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