Magma rises because it is lighter than the rock around it. As magma rises toward the surface, dissolved gas bubbles out of it as the pressure drops, like soda fizzing when a can is opened.

An eruption can be explosive or effusive, and which one happens depends largely on how much gas is in the magma. Magma with a lot of silica is very thick and sticky, and eruptions of it are likely to be explosive.

Scientists watch changes in volcanic gases such as sulfur dioxide, because rising magma releases gas before an eruption.

Many volcanoes have a crater, a round dip at the top or around a vent where the eruption happens. A caldera forms when a very large eruption empties a magma chamber and the ground above collapses into a huge bowl. Calderas are far bigger than craters; the biggest are tens of miles (tens of kilometers) across.

Cinder cones, composite volcanoes and shield volcanoes are the three best-known types of volcano. Scientists can group volcanoes in more than one way, so there is no single agreed count of volcano types.

Yellowstone has had three huge caldera-forming eruptions in the past 2.1 million years, separated by gaps of roughly 600,000 to 800,000 years.

Wind can carry ash hundreds of miles from the volcano, and ash can melt inside a jet engine and make the engine stall or fail.

After the 1883 eruption of Krakatau, sunsets and the sky took on unusual colors around the world.

When lava flows out and cools, it hardens into solid rock.

Lightning in an ash cloud can start with static electricity: ash particles rub together and pick up electric charge in dense ash clouds near the ground.

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

Words to know

• silica: rock ingredient
• dissolved: mixed in
• effusive: gently flowing
• caldera: collapsed bowl