Earth & climate
Volcanoes
Fire from the deep: how magma rises, why some volcanoes flow calmly and others explode.
What you need first
Maybe you have seen glowing lava flows on the news, or spent a holiday on a volcanic island like Tenerife. Volcanoes are the places where the Earth turns its hot interior inside out. There are about 1500 active volcanoes on land worldwide, and a few dozen of them are erupting at this very moment.
From magma to lava
Deep beneath a volcano, rock melts into magma. Because it is lighter than the solid rock around it, it rises and collects in a magma chamber. There, gas dissolves in the magma like fizz in a bottle of lemonade. If the pressure gets too high, the magma forces its way upward and the volcano erupts: lava, ash and gases burst out. As soon as magma reaches the surface it is called lava, and it can be as hot as 1200 °C.
Pressure is still too low, it only bubbles. Erupts from pressure 4.
Two kinds of volcanoes
How an eruption unfolds depends mainly on how runny the lava is. That is decided by its temperature and how much silica it contains: very hot lava with little silica flows easily, cooler lava with a lot of silica is as stiff as dough. Runny lava lets gases escape easily: it flows calmly and far and builds flat shield volcanoes like those on Hawaii. Viscous lava, on the other hand, traps the gases until they break free explosively: this builds steep stratovolcanoes out of alternating layers of lava and ash. The most famous one is Vesuvius, which buried the Roman town of Pompeii under ash in the year 79 AD.
Where the volcanoes are
Volcanoes are not scattered randomly across the Earth. Most of them sit along plate boundaries, where magma can rise especially easily. Around the Pacific runs the Ring of Fire, home to about 75 percent of all active volcanoes. There are a few exceptions: Hawaii sits in the middle of a plate above a so-called hotspot, a hot patch in the mantle.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
What is molten rock called while it is still underground?
Which volcano shape does runny lava build?
Up to how many degrees Celsius can lava get?
Put the steps of a volcanic eruption in the correct order, from deep down to the surface.
- 1At the surface the molten rock flows out as lava
- 2The magma forces its way upward and the volcano erupts
- 3The lighter magma rises and collects in a magma chamber
- 4Deep beneath the volcano rock melts into magma
- 5Dissolved gas makes the pressure rise higher and higher
About how many active volcanoes are there on land worldwide?
Match each term to its description.
Where this leads