Volcanic Eruptions: What Causes Volcanoes to Erupt

What is the first image that comes to your mind when you hear about volcano eruption? I guess, it would be an image of orange-red lava spewing from the top of a giant, cone-shaped mountain. The entire human race is in awe of the destructive power of volcanoes. Though geologists have solved many of the mysteries regarding the eruptions of volcanoes, they believe there is a wider geological phenomenon that needs to be solved. The study of volcanoes and their related phenomena is known as volcanology or vulcanology. The scientist who study volcanoes are known as volcanologists.

How are Volcanoes Formed?

Volcanoes are formed in various sizes and shapes. Cone-shaped volcanoes are the most common ones. Some volcanoes are only a long crack in the earth's crust. Volcanoes that resemble high mountains are the ones that are formed due to many volcanic eruptions.

According to the theory of plate tectonics, the crust of the earth has many rigid plates that are floating. It is the pressure created by the currents caused by the heat energy from the earth's core that causes the movement of these plates. When the pressure becomes intense, the plates either move towards each other or away from each other horizontally.

When two plates collide, one of the plates goes beneath the other, causing a friction in the earth's crust. The pressure caused by the friction will cause the underlying rocks to melt and make the magma rise. The hot, orange-red lava is thrown out of the mountaintop. Volcanoes of this type are considered threats to living beings. Their eruptions are violent and cause destruction to all living things in their proximity.

A gap is formed when two layers of plates move away from each other. The hot lava rises through this gap. Volcanoes of this type occur on the ocean bed and is not visible. However, if a volcano erupts violently from an ocean, it rises above the ocean to create an island.

Some volcanoes are formed in hotspot areas. The hotspots are the center-points of the earth plates. These plates are connected to the hot mantle of the earth. In brief, volcanoes are formed when the hot lava (magma) shoots out of the earth's crust. The lava becomes hard on cooling down to form a volcanic mountain.

Causes of Volcanic Eruption

The buoyancy and pressure of the gas within the earth's crust cause a volcano to erupt. Magma is formed when the upper mantle of the earth melts. A volcano erupts when the magma (the hot liquid) rises upwards by the pressure of gas that is dissolved in it. This is one of the three predominant theories.

According to the second theory, magma contains dissolved substances such as water, sulfur dioxide and carbon dioxide. The solubility of the gases is high as the pressure increases. The solubility of water decreases as the magma moves closer to the earth's surface and eventually separates from the magma.When the ratio of the gases becomes more in magma it causes the magma to disintegrate into pyroclasts, a combination of partially molten and solid fragments, and the volcano erupts explosively.

The third theory says that a volcano erupts when new magma is injected into a chamber that is already brimming with magma of similar or different compositions. The eruption occurs when the magma moves upwards due to the additional push exerted by the injection of new magma.

Types of Volcanic Eruption

Volcanic eruptions are classified according to the different phases of eruption. There are three major types of volcanic eruptions.

• Hawaiian Eruptions

Hawaiian eruptions are neither explosive nor destructive. The lava thrown out is low in gas content and it flows down slowly. Sometimes the volcanoes throw up a fire fountain where bright lava is sprayed into the air for several hours or few minutes. However, the most common form is the lava lake where the lava forms craters or depressions.

• Strombolian Eruptions

Though these eruptions are not dangerous, they are more impressive than Hawaiian eruptions. There are regular eruptions where the small amounts of lava are exuded into the air followed by booming sounds. The Strombolian eruptions produce small quantities of ashy tephra.

• Plinian Eruptions

This was the eruption that not only destroyed but also buried Pompeii and Herculaneum. These eruptions are caused by magma, which has high gas content and has high viscosity. The lava is spewed as high as 50 km in the air. This eruption lasts for days. Plinian eruptions throw out large quantities of tephra which is heaped on one side, depending upon the direction of the wind. Here, the lava flows quickly and will destroy anything that it comes across.

To sum up, volcanic eruptions can cause destruction to all living things. They can also cause substantial changes in the climate. The particles that are thrown into the atmosphere hinder the sunlight from reaching the surface of the earth. This can lead to low global temperatures. Large quantities of gases such as sulfur dioxide, hydrogen chloride and hydrogen fluoride are sometimes thrown out. These react with atmospheric water to produce acid rain. Such acid rain will not only destroy the crops but also harm all living things.