Although volcanoes can look different, they’re all created because of the same event; magma from beneath the Earth reaches the surface and erupts as lava, ash and rock. But what’s going on, how do volcanoes erupt?
To understand how volcanoes erupt, you have to consider the structure of the Earth. You’re standing on the Earth’s crust. This is a relatively small part of the total volume of the Earth. It’s around 30 km thick beneath the continents, and can be less than 10 km thick beneath the ocean floor. Underneath the crust is the Earth’s mantle. This is a vast region beneath the Earth that can be more than 1000 degrees Celsius.
Although the Earth’s mantle is solid rock, it’s under high pressures and temperatures, and this causes rock to melt and squeeze out of cracks in the rock. The magma collects into vast chambers beneath the Earth’s crust. Since this magma is less dense than the surrounding rock, it “floats” up to the surface, seeking out cracks and weaknesses in the rock. When it finally reaches the surface, we see this as an eruption.
When it’s underneath the surface, the molten rock is called magma. When it reaches the surface, it erupts as lava, ash and volcanic rocks. With each eruption, rocks, lava and ash build up around the volcanic vent where the material blasts out of. The nature of the eruption depends on viscosity of the magma. When the lava flows easily, it can travel far and creates wide shield volcanoes. When the lava is very thick, it creates a more familiar cone volcano shape. And when the lava is extremely thick, it can build up in the volcano and explode. This is what happened with Mount St. Helens in 1980.
We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.