A volcano is an opening in the Earth. Melted rock and gas can come out. Many volcanoes have a crater. It is a round dip at the top or around a vent. This is where the eruption happens. 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. Lava flows out. It cools. It hardens into solid rock.
Some volcanoes burst with a bang. They send ash and gas high into the air. Others let hot lava ooze out slowly. Gas is trapped in magma. It can make a volcano burst out. It is a bit like a shaken bottle of soda when it is opened. Ash is made of tiny pieces of rock. Wind can carry ash hundreds of miles from the volcano.
Cinder cones, composite volcanoes and shield volcanoes are the three best-known types of volcano. Cinder cones are small volcanoes. They are usually less than 1,000 feet (330 meters) tall. Composite volcanoes are also called stratovolcanoes. They get their name from the layers of lava and ash that build them. Shield volcanoes are wide. They have gentle slopes. Their runny lava flows far before it hardens.
Pahoehoe is lava with a smooth, ropy surface. Lava called 'a'a has a rough, jagged, chunky surface. A lava dome is a mound of thick, sticky lava. It piles up over a vent. It does not flow away. Basalt is the dark rock that forms from runny lava. It makes up most of the ocean floor.
Volcanoes also erupt under the sea. They can build whole chains of islands. The Hawaiian Islands are one. As of November 2026, a Kilauea eruption in 2018 added about 875 acres of new land to the island in Hawaii. As of November 2026, Mauna Loa in Hawaii is among the largest active volcanoes in the world.
Words to know
- magma: melted rock underground
- ash: tiny rock pieces
- crater: round dip
- basalt: dark rock
The short version
A volcano is an opening in the Earth where melted rock and gas can come out. Melted rock under the ground is called magma, and once it comes out onto the surface it is called lava. Magma rises because it is lighter than the rock around it.
Quick facts
- Magma rises because it is lighter than the rock around it.
- Wind can carry volcanic ash hundreds of miles from the volcano.
- When lava flows out and cools, it hardens into solid rock.
What is the difference between magma and lava?
Melted rock under the ground is called magma. When it comes out onto the surface, it is called lava.
What makes a volcano burst out?
Gas trapped in magma can make a volcano burst out, a bit like a shaken bottle of soda when it is opened.
