Why Volcanoes Exist A volcano is a weak spot in the crust where magma comes to the surface. When magma comes to the surface, it becomes lava. Magma is a molten mixture of rock forming substances, gases, and water from the mantle. After lava has cooled , it forms solid rock. The lava released during volcanic activity builds up Earth's surface.
Volcanic activity is a constructive force that adds new rock to existing land and forms new islands.
In a shield volcano, repeated lava flows during quiet eruptions gradually build up a broad, gently sloping volcanic mountain. Shield volcanoes are more flat than all of the other volcanoes. They can also be the largest of all volcanoes. The Hawaiian Islands are a series of shield volcanoes. When cinder cones erupt explosively from a volcanic vent, they pile up around the vent, forming a cone-shaped hill called a cinder cone.