So far in 2021 we have had over 50 volcanic eruptions around the world and there are over 1,500 potentially active volcanoes world-wide! This includes everything from plinian and vulcanian eruptions (the type in movies) to phreatic eruptions, those driven by the heat of magma interacting with water. Volcanoes can form on land or below the sea, in fact the largest volcanoes in the world are below the ocean’s surface. But how do volcanoes form and why do they erupt?
Why Do Volcanoes Erupt?
Volcanoes mostly form on or near the edge of a tectonic plate boundary. Tectonic plates are large slabs of the Earth’s crust that get jostled and moved around by the circulation of the Earth’s mantle. The mantle is made of scalding, liquid rock and is thousands of kilometers thick that lays beneath the tectonic plates that make up the Earth’s crust.
When the tectonic plates jostle and move, one of the plates may begin to slide beneath the neighbouring plate in a process called subduction. The downward movement of the plate carries rock into the mantle where the heat and pressure is extremely high. This causes the rock to melt, and because the liquid rock is lighter than the surrounding materials it heads back towards the Earth’s surface. When this magma finds a weak spot in the surface it breaks through and develops into a new volcano.
In fact, many oceanic islands started as volcanoes! A great example of this is the Hawaiian islands which formed over a mantle plume from the Pacific plate. The Pacific plate has slowly moved northwest and a series of new volcanoes have punched their way through to the surface. Over time, this led to the formation of the Hawaiian archipelago.
What Are The Parts of a Volcano?
Magma is the hot fluid or semi fluid rock material from below the Earth’s crust. Magma is made of a mix of minerals and often contains small amounts of dissolved gases including water vapor, sulphur, and carbon dioxide. Magma is broken into three types: basaltic, adesitic, and rhyolitic. Basaltic magma ranges in temperature from 1000ºC to 1200ºC is high in iron, calcium, and magnesium with low levels of potassium and sodium. Andesitic magma has a temperature range between 800ºC and 1000ºC with moderate amounts of all minerals. The final kind of magma is rhyolitic magma which has a temperature range between 650ºC and 800ºC, this magma is high in potassium and sodium with low levels of iron, calcium and magnesium.
The temperature of the magma, along with its mineral content affects the viscosity of the magma and the way it erupts from a volcano.
The central vent is the main way through which magma and pyroclastics escape the Earth’s crust. The eruptive products have their highest levels of accumulation which creates a low-angle shield or higher-angle cone that creates the volcanic pile. The vent often grows over time as the magma heats up the walls of the vent which causes the peripheral walls to collapse and expand.
Lava is what magma is called once it reaches the Earth’s surface and interacts with the atmosphere. The term “lava” is also used to describe the solidified rock that forms from lava flows after the eruption. The lava flow from a volcano can be extremely stiff, barely flowing at all or it can be almost syrup-like in its consistency, this is all dependant on the silica content of the lava as the higher the silica content, the more viscous the lava.
Volcanic Craters and Calderas
Volcanic activity can often create craters or calderas, ranging in size and shape. A crater is caused by the outwards explosion of materials from a volcano. Craters most frequently occur as a summit crater, like Mount Fuji in Japan but can also occur in multiple locations on a volcano, for example, Mount Etna in Italy has four craters. The crater can even have lava lakes that form and can be used to predict the future behaviour of the volcano.
Calderas, on the other hand, are caused by the inward collapse of the volcano’s magma chamber. When the chamber empties, the lack of support from the magma can cause the sides and top of the volcano to fall inwards. The size of the caldera can vary in size from one to 100 kilometres in diameter. These calderas can, over time, become lakes like Crater Lake in Oregon that was created when 7000 years ago Mt Mazama erupted.
Volcanic ash is comprised of a mixture of minerals, rocks, and glass particles that burst free from a volcano during an eruption. These particles are extremely small, regularly with a diameter of less than 2 millimeters. Along with hot gas and water vapor, volcanic ash is part of the dark column that shoots from a volcano upon its eruption. Due to the small size of the particles and their low density, as they are mostly pitted and filled with holes; the ash can travel large distances and last in the air for days after the eruption. This can lead to flights being grounded and making it difficult to reach the site of the eruption. When the Eyjafjallajokull volcano in Iceland erupted in 2010, it grounded over 100,000 flights over an 8 day period, almost half of the world’s air traffic!
A pyroclastic flow is often described as lava in news reports but it is different on a fundamental level. While lava flows are relatively viscous fluids that advance slowly over time; a pyroclastic flow is made of solid volcanic fragments and hot gas and it moves like a dense gas pouring down a slope and can even move uphill if it gains enough momentum. The downhill velocity of a pyroclastic flow can exceed 100 km per hour and is the main cause of damage in a volcanic eruption. In fact, a pyroclastic flow was the cause of the destruction of Pompeii after the eruption of Mount Vesuvius.
Try It Yourself!
At Street Science, we have great options for making your own DIY Volcano and to learn more about volcanoes overall with our Make Your Own Volcano Kit that allows you to make a lava lamp and create a mini-Vesuvius in your own home. We have worked with students and teachers around Australia to help with science education and have a wide range of DIY Experiments for you to try at home!