Deep under our feet, the Earth is extremely hot. Tens of kilometers below the surface, the heat can be strong enough to melt rock. This melted rock is called magma. Because magma is lighter than the solid rock around it, it slowly rises, and sometimes it collects in a large underground chamber.
When the pressure in the chamber becomes too great, the magma pushes its way up through cracks in the ground. When it reaches the surface, it is called lava. An eruption can be gentle, with rivers of glowing lava, or it can be violent, sending ash and gas many kilometers into the sky. The ash can cover whole towns and even stop airplanes from flying.
Most volcanoes are found where the giant plates of the Earth's crust meet. Around the Pacific Ocean there are so many volcanoes that the area is called the Ring of Fire. Japan, Indonesia and Chile all lie on this ring.
Israel has no active volcanoes today, but it has many old ones. Large parts of the Golan Heights are covered with dark basalt rock, which formed from lava that cooled down. Hills such as Mount Avital and Mount Bental are actually ancient volcanoes.
Volcanoes are dangerous, but they are also useful. Volcanic soil is very rich, so farmers often grow fruit and vegetables on the slopes of volcanoes. In some countries, such as Iceland, the heat under the ground is even used to produce electricity and to warm homes.