How does magma make mountains?

Earth’s volcanoes connect the planet’s surface to its fiery core

Volcanoes are mountains and gateways to Earth’s crust. These openings allow magma, volcanic ash and gas to escape during volcanic eruptions. When a volcano erupts it can result in a huge, fiery explosion, throwing scalding lava into the air, or a gentle stream of lava running down the volcano’s surface.

Often volcanoes are found where tectonic plates meet. These rocks make up the Earth’s crust and are continuously moving, causing plates to rub against each other. The friction created by the movement of these plates creates a high temperature that turns the crust into molten hot rock, called magma. Regions with volcanoes formed during the movement of tectonic plates are called hotspots.

High pressures in the Earth’s crust pushes magma up cracks in the tectonic plates until it emerges above ground level. When magma reaches the surface, it is called lava. As lava cools and hardens into volcanic rock, it forms a solid mountain of lava. Every time a volcano erupts, pouring lava over the surface, it adds to the body of the volcano.

Not all volcanoes form in the same way. The viscosity of the lava that’s released determines how steep or gentle the volcano’s slope will become. This is dependent on how quickly lava cools to form the rock.

Volcanoes can also be found underwater in the form of submarine volcanoes. Because the lava instantly comes into contact with cool water, underwater eruptions often go unnoticed. If the top of these volcanoes come close to the water’s surface, it’s possible to see steam and debris being thrown above the sea. There are estimated to be over 1 million submarine volcanoes. Larger ones have the potential to grow above the water and become islands.

How different lava flows

Lava is the product of volcanic eruptions. The two main types of basalt lava are called pahoehoe and a’a. Named in Hawaii, these terms are now widely used.

A’a lava has a rough top surface and sometimes bottom, and an incredibly dense interior. The coarse pieces on top are formed by lava pulling apart and separating as it flows.

Pahoehoe is strikingly different, with its smooth, flowing appearance. It usually flows at least ten times slower than a’a lava. Pahoehoe lava can turn into a’a lava later, but the reverse process is impossible.

They may differ immensely in structure, but their chemical compositions are identical. This means these different lava types differ entirely due to the conditions they face. Pahoehoe is smooth and ropy as a result of lava cooling slowly. When lava cools quickly, it breaks into stoney pieces, which is a’a lava.

This article was originally published in How It Works issue 130, written by Ailsa Harvey 

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