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Why do volcanoes erupt?

And what do tectonic plates have to do with it?

A popular volcanic eruption that people still talk about to this day is the one that occurred in Pompeii.

 Mount Vesuvius would give off minor shakes that the people of Pompeii got used to. That is why, in August 79 AD, they were unprepared for Mount Vesuvius’s eruption. The day after the first eruption (in which 2,000 residents survived), smoke, ash and volcanic mud swept over the city, killing many people. The city was covered in black soot. About 30,000 people died from Pompeii’s destruction.

The earth has four inner layers: the inner core (solid, about 5,400 degrees Celsius which is almost as hot as the Sun), the outer core (liquid, gets heated by radioactive decay (atoms with unstable nuclei release energy, the currents in this layer generate a magnetic field that reverses every 20,000-30,000 years), the mantle (semi-solid, pieces of the mantle are metal), and the crust. Another name for the upper layer of the mantle is the lithosphere. The crust is made up of “jigsaw puzzles” called tectonic plates. Tectonic plates slide on the lithosphere and they move 3-5 centimeters per year, which is actually slower than the speed that our fingernails grow.

The tectonic plates are what cause things such as earthquakes to occur, mountains to form and volcanoes to erupt. When two tectonic plates are sliding towards each other and there is a head-on collision in which both plates rise up, a mountain forms. When a continental crust (a tectonic plate under land) and an oceanic crust (a tectonic plate under a body of water) slide towards each other and collide, the oceanic crust goes underneath the continental crust. This is because the oceanic crust is denser than the continental crust.

Density is mass divided by volume. Mass is the amount of matter in a substance. It is usually measured in grams. Mass is easily confused with weight. Weight is the amount of gravitational force pushing you down. Your weight on the moon is about one-sixth of your weight on Earth. But, your mass on the moon and your mass on Earth is the same.

Density is measured as the mass of an object divided by the volume of the same object. If you put an object in water and that object’s density is less than water’s, that object will float. If the density is more than water’s, it will sink. So, since the density of an oceanic crust is more than the continental crust’s density, it will sink below the continental crust. This creates an area between the sunken oceanic crust and the continental crust called the subduction zone.

When the oceanic crust sinks, it makes the melting point (temperature at which a substance melts) of the mantle lower. That means it does not have to be as hot as it used to be for the substances in the mantle to melt. There are also three geological factors at work here.

The first factor is lithostatic pressure. It is the pressure pushing down on the Earth’s crust which is right above the magma. The second factor is magmastatic pressure which pushes up towards the crust, in the opposite direction of the lithostatic pressure. And the third factor is the rock strength which, because of the lithostatic pressure and the magmastatic pressure pushing both ways on the rock, starts to strain and weaken. Most of the time, the rock is strong enough to make sure the magma stays underneath but sometimes, its equilibrium (equal balance of opposing forces) is thrown off and boom, the rock breaks and the magma travels upwards, causing an eruption.

Usually, eruptions occur because the magmastatic pressure becomes larger, which can happen because water and sulfur do not dissolve in magma (when there is a high amount of magma in a small place) and hence, bubbles are created. When there are a lot of these bubbles, they can shoot upwards like a bullet is shot from a gun.

 Sometimes, eruptions occur because the rock strength is low (this can be due to landslides) and the lithostatic pressure decreases and boom, an eruption occurs. This is called “unloading” and is also why Mount St. Helens in Washington state erupted in 1980. Scientists believe that “unloading” can also occur because of melted glaciers that occur due to climate change and hence glacial melting can lead to increased volcanic activity. Rock strength can also be weakened by tectonic activity such as earthquakes which can create small openings that can allow magma to escape.

New technologies can also help us understand more things about volcanoes. And although understanding why volcanoes erupt does not help us predict when eruptions will occur, it still expands our understanding of the world.

Vaageesha Das  is a junior at Morgantown High School.

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