A volcano is an opening in the Earth's surface where molten rock, ash, and gases can escape from below. This occurs when pressure builds up in the Earth's mantle, causing magma to rise. When the pressure becomes too great, it can lead to an eruption, which can vary in intensity and type. Volcanoes can be found all over the world, often forming mountain ranges or islands. They are classified into different types, such as shield volcanoes, composite volcanoes, and cinder cone volcanoes, based on their shape and eruption style. Some famous volcanoes include Mount St. Helens and Kilauea.