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Volcanic Eruptions: What Causes Volcanoes to Erupt

Volcanic Eruptions: What Causes Volcanoes to Erupt
Pompeii and Herculaneum, the Roman towns, were totally destroyed when Mount Vesuvius erupted, and even the dinosaurs are thought to have become extinct due to an enormous volcanic eruption. But, have you ever wondered what causes a volcano to erupt? If you have, this article has the answer for you.
Maya Pillai
Last Updated: May 31, 2018
What is the first picture that comes to your mind when you hear about a volcanic eruption? I guess, it would be an image of orange-red lava spewing from the top of a giant, cone-shaped mountain, in the middle of an ocean. The stunning display of color and light, though captivating, is nature's most savage tool. Yet, attracted by the fertile, mineral-rich soil, humans have settled close to volcanoes and have made them a part of their lives, culture and religion. There are many mythical stories which explain their formation and causes of eruption. But, to understand the scientific reasoning behind this phenomenon, let's start with the basics.

A volcano is basically a vent from which, magma, gases, and volcanic ash escape to the Earth's surface, through faults in the crust. They may erupt violently like Krakatoa in 1883, or just seep out lava like the Kilauea volcano. Formed as a result of plate tectonics, volcanic eruptions can result in the formation of mountains, craters, crater lakes, plateaus, and islands.
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How are Volcanoes a Result of Plate Tectonics?
Earth is made of four concentric layers - inner core, outer core, mantle, and crust. Outermost layer is the crust, on which we live. This layer is actually made up of seven major tectonic plates. These plates float on the mantle - a semi-solid layer made of molten rock, which we call magma. Convection currents are formed in the mantle as a result of radioactive decay in the Earth's crust. These currents cause the plates in the crust to either move towards each other, away from each other or slide past each other. This movement of plates is called plate tectonics.
Formation of Volcanoes through Convergence
A convergent or destructive boundary is where two plates (usually a continental and oceanic) move toward each other, forcing the denser (oceanic) plate to go under the lighter (continental) plate and sink into the mantle. This is called subduction, and the place where this happens is the subduction zone. When water and rocks from the denser, subjugating plate, come in contact with the hot mantle, they transform into magma. This magma, due to the various gases present, is lighter than the surrounding material and might break through faults in the crust in an violent explosion. These volcanic eruptions are violent and can cause immense damage to all living things in their proximity. Mt. Fuji, in Japan is a volcano found at convergent boundary.
convergent volcano
Formation of Volcanoes through Divergence
Divergence is another cause of volcano formation. At a divergent or constructive boundary, two plates diverge i.e. they move away from each other, forming a gap between them. Hot magma rises from the mantle to fill this gap, cools on the surface and forms a new crust of igneous rock. This type of lava flow, though accompanied by earthquakes, is mostly placid, and usually occurs on the ocean bed. These type of eruptions can continue for years resulting in the formation of oceanic ridges like the mid-Atlantic ridge. The ridges can break the surface of water to form volcanic islands. Iceland, world's second largest volcanic island, is a part of mid-Atlantic oceanic ridge.
divergent volcano
Volcanoes on Hotspot
Due to high pressure, lower mantle is mostly composed of solid rock. These solid rocks rise upwards towards the crust, forming mantle plumes, due to heat from Earth's outer core. As the pressure is lower towards the crust, the rocks begin to melt, transforming into magma. The mantle plumes rise slowly until they break through the plates on Earth's surface. These points of volcanic activity are called hotspots, and can form near or far from the plate boundaries. While the hotspot is stationary, tectonic plates move, resulting in a chain of islands when in the ocean; or a chain of extinct volcanoes, if occurring on continental plates. The islands of Hawaii are a result of hotspot volcanic eruptions.
hotspot volcano
What Causes a Volcano to Erupt?
Volcanic eruptions and earthquakes are a way for Earth to release pressure and heat, much like a safety valve. There are three dominating theories to explain what causes a volcano to erupt -
Volcanic eruption
Due to Density-difference in Magma
As per the first theory, due to heat and pressure in the Earth's mantle, solid rocks melt, to form magma. Magma has the same mass as the solid rock, but more volume, making it lighter and more buoyant. So, it will attempt to rise, if this magma continues to encounter high-density material till it reaches the Earth's crust, volcanic eruption occurs. It can either be in the form of a lava-flow or may be explosive.
Pressure of Released Gases
According to the second theory, magma contains dissolved substances such as water, sulfur dioxide and carbon dioxide. The solubility of magma decreases with the decrease in pressure as it rises up towards the crust, and the gases get released in the form of bubbles. When the volume of the gas bubbles in magma reaches around 75%, magma disintegrates into pyroclasts, a mixture of partially molten and solid fragments. The bursting of pyroclasts is very explosive and the cause of some of the most violent eruptions on the surface of Earth.
Injection of New Magma
The third theory says that when new magma enters a chamber already brimming with magma, the volcano erupts due to the additional pressure exerted by the injection of new magma. This type of eruption can be placid or violent.
The intensity of eruption usually depends on the viscosity of magma and its gas content. High-viscosity magma usually results in bigger, more intense eruptions, whereas magma that flows easily will have lower pressure build-up, so a less violent eruption. Highly viscous magma is characterized by the presence of more silicates and contains less dissolved water. Another important factor is the amount of gases present in the magma. Magma containing large amount of trapped gases will lead to a violent eruption, and less gases in magma will result in an effusive flow.
What are the Types of Volcanic Eruption?
Depending on their intensity, volcanic eruptions can be divided into the following major types;
Common to the islands of Hawaii, these eruptions are usually associated with hotspots, and have non-explosive lava flowing out of fissures or vents, often on the slopes. When the central vent erupts, the volcano throws up a fire fountain- bright orange lava spraying into the air for several hours or few minutes. The lava has very little gas content and crawls slowly to form a lava lake.
Kilauea Hawaii
Kīlauea, Hawaii
Strombolian Eruptions
Packing more power than the Hawaiian, Strombolian eruptions are characterized by short explosive bursts accompanied by booming sounds. Caused by bursting gas bubbles, the eruptions can form columns up to 100 meters tall and can last several centuries.
Vulcanian Eruptions
Vulcanian eruptions operate in a similar manner to the strombolian eruptions, except that these are much stronger and consequently more destructive, but with time, their dome gets damaged and leads to a more continuous lava flow instead of short bursts of lava.
Batu Tara Volcano
Batu Tara Volcano, Indonesia
Plinian Eruptions
The most potent of all eruptions, they have caused massive damage, spreading thousands of miles, obliterating cities and changing the climate. These eruptions are caused by very viscous magma with a high gas content, forming tall columns of gas, and ash, resembling mushroom clouds, from a nuclear explosion, over 35 miles high and can last for days. The tephra, especially when combined with melted snow, flows astonishingly fast and scorches everything in its way. These eruptions often occur abruptly, remaining inactive for centuries, catching life forms unawares as they scramble to escape its pyroclastic flow, poisonous gases and suffocating ash clouds.
Pelean Eruptions
These are very similar to the Plinian eruptions and are equally destructive. The Pelean eruption's major damage comes from avalanches and landslides of glowing ash flowing down the steep slopes at massive speeds whereas Plinian Eruption has tall columns of ash and smoke.
Though geologists have explained many of the mysteries, newer discoveries continuously challenge their theories. Vulcanologist continuously toil hard in an attempt to better understand what causes a volcano to erupt. By understanding this phenomenon, we might be able to reduce its impact on human life, and even harness its power to generate electricity.
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