Volcanoes form when molten rock from beneath Earth's surface erupts and solidifies on the surface. When beneath the ground, the molten rock is called magma. When it erupts, it is referred to as lava. With each eruption, the volcano increases in size.
Whenever an eruption occurs, gases and rock shoot up through a volcano's opening, spilling over the surface or filling the air with lava fragments. Sometimes there is volcanic lightning during the eruption. Volcanic lightning occurs as a result of the friction of volcanic ash. Volcanic ash is harmful to the health of elderly people, babies and people with respiratory challenges. An erupting volcano can cause earthquakes, tsunamis, mud floods, flash floods and rockfalls.
A volcano can either be active, dormant or extinct. A volcano is said to be active if has erupted within historical memory. A dormant volcano is one that has not only erupted in historical times but is likely to erupt again. An extinct volcano is one that is highly unlikely to erupt again. In the United States, active volcanoes are mainly located in Hawaii, California, Alaska, Washington and Oregon.
The tallest volcano in the world, from base to summit, is Mauna Kea. However, with regards to the solar system, Mars hosts the tallest volcano.