Eventually the pressure from hot gases forces the magma through a weak spot in the surface causing and eruption. The gases emitted from volcanoes include carbon dioxide and nitrogen, as well as carbon monoxide and sulfur dioxide, which can be toxic. Lava is magma that has reached the surface and tephra refers to rock fragments including dust, ash and large particles. When lava and tephra build up a volcanic mountain forms. This buildup can form three types of volcanoes.
Shield volcanoes form from lava and are dome shaped.
Cinder cones form mostly from tephra and are conical in shape. Composite volcanoes, or stratovolcanoes, are formed from alternating layers of lava and tephra. The most active volcano in Costa Rica, Arenal Volcano, has a very interesting history. The area surrounding Arenal had been dormant for nearly five hundred years until it became active in 1968. Then in early July people began to feel tremors in the area.