A volcano is an opening in the Earth where melted rock and gas can come out. Under the ground, the melted rock is called magma. It rises because it is lighter than the rock around it. Gas trapped in it can make a volcano burst out. This is a bit like a shaken bottle of soda when it is opened.

Tephra is the word for all the pieces of rock that are thrown into the air by an eruption, whatever their size.

After the 1991 eruption of Mount Pinatubo, global temperatures dropped for a year or two. The drop was about 1 degree F, which is about half a degree C.

Earthquakes under a volcano usually increase before it erupts. So scientists listen for them. Magma moving underground can make a volcano swell or tilt, and GPS and tiltmeters can measure the change.

Volcano observatories use several kinds of monitoring, such as watching, listening for earthquakes, measuring gas and measuring ground movement. They compare the results, and they do not trust only one warning sign.

Words to know

• observatories: watching stations
• tiltmeters: tilt gauges
• tephra: thrown rock