Aerosol droplets from a big eruption can cool the whole planet for a time. They do it by blocking some sunlight. The 1815 eruption of Mount Tambora in Indonesia was the largest volcanic explosion in recorded history. Its ash and aerosols helped cause 1816 to become known as the Year Without a Summer.

After Mount Pinatubo erupted in 1991, global temperatures dropped for a year or two. The drop was about 1 degree F (about half a degree C).

Before Mount Pinatubo's big eruption in June 1991, scientists made forecasts and recommendations. They led to more than 60,000 people being evacuated from around the volcano in the Philippines.

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

Observatories use several kinds of monitoring. They watch, listen for earthquakes, measure gas and measure ground movement. They compare the results rather than trusting one warning sign.

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

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

Volcanoes can build whole chains of islands, like the Hawaiian Islands.

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

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

Volcanoes also erupt under the sea. Lava that erupts under water often piles into rounded mounds called pillow lava.

Words to know

• aerosol: tiny airborne drop
• observatories: watching stations
• evacuated: moved away