Melted rock under the ground is called magma. Out on the surface, it is called lava. Magma rises because it is lighter than the rock around it.

Gas trapped in magma can make a volcano burst out. It is a bit like a shaken bottle of soda when it is opened. Some volcanoes burst with a bang. They send ash and gas high into the air. Others let hot lava ooze out slowly.

Composite volcanoes are also called stratovolcanoes. They get their name from the layers of lava and ash that build them. Shield volcanoes are wide with gentle slopes. Their runny lava flows far before it hardens.

A lava dome is a mound of thick, sticky lava. It piles up over a vent instead of flowing away.

Basalt is the dark rock that forms from runny lava. It makes up most of the ocean floor.

Earthquakes under a volcano usually increase before it erupts, so scientists listen for them.

A lahar is a mudflow of water and volcanic debris. It rushes down the slopes and valleys of a volcano.

Many of Earth's volcanoes and earthquakes are found along the Ring of Fire. It is a path that partly circles the Pacific Ocean.

A volcano is called active if it has erupted in about the last 10,000 years. It is called active even if it is quiet now.

The Hawaiian Islands formed over a hot spot. The Pacific Plate slowly carries volcanoes away from it. So the chain gets older in one direction.

Words to know

• magma: melted rock underground
• active: erupted lately
• lahar: volcanic mudflow