A volcano is a feature in Earth’s crust where molten rock is squeezed out onto the Earth’s surface. This molten rock is called magma when it is beneath the surface and lava when it erupts, or flows out, from a volcano. Along with lava, volcanoes also release gases, ash, and solid rock.
Volcanoes come in many different shapes and sizes but are most commonly cone-shaped hills or mountains. They are found throughout the world, forming ridges deep below the sea surface and mountains that are thousands of meters high. About 1,900 volcanoes on Earth are considered active, meaning they show some level of occasional activity and are likely to erupt again. Many others are dormant volcanoes, showing no current signs of exploding but likely to become active at some point in the future. Others are considered extinct.
Volcanoes are incredibly powerful agents of change. Eruptions can create new landforms, but can also destroy everything in their path. About 350 million people (or about one out of every 20 people in the world) live within the “danger range” of an active volcano. Volcanologists closely monitor volcanoes so they can better predict impending eruptions and prepare nearby populations for potential volcanic hazards that could endanger their safety.