Creating volcanoes is a natural way for the Earth and other planets to cool off. The heat inside the earth is hot enough to melt rocks, which is called magma. When the magma breaks through the surface, at this state called lava, it causes an eruption and the buildup of ashes and lava around the hole forms the volcano. Some volcanoes only erupt for a short time, a few days or weeks and will never erupt again, while the bigger ones can erupt thousands of times and for millions of years. There are three places of origin for volcanoes. Some form when oceanic or continental plates are moving apart and magma flows up, filling the space that is created. Volcanologists are not sure if the magma makes the plates move or if it is the other way around that means the plates are moving and giving the magma an opportunity to emerge. Other volcanoes are created when two plates hit each other, one occurring on top of the other. Material from the lower plate will move towards the mantle under the top plate, this lowers the melting point and magma is generated. Part of the magma might also be the lower plate that actually melts when it gets closer to the hot interior. Arenal is an example of a volcano that has been formed by the collision of two plates. Hotspots are formed by magma generated from the upper mantle and at a depth not greater than 100 kilometers. Little is known about the hotspots, which can emerge even where there is no movement of continental or oceanic plates. The volcano grows on the inside due to intrusion, which is magma that arises to the surface but never erupts and on the outside because of extrusion meaning lava that erupts and covers the slopes. Magma has low density and as it rises to the surface, bubbles form from the gas dissolved in the magma and generates a high pressure that provokes the eruption and typical volcanic explosion.