A Volcano is usually a cone-shaped mountain or large hill that has a hole or crater in the top of it.  Hot molten lava, rock fragments, and gas will come up out of the top opening if and when the volcano builds up enough internal pressure and erupts from deep inside the earth's crust.    Volcanoes are very unpredictable as to when they will actually erupt.  The study of volcanoes, lava, and magma is called Volcanology.  Geologists that specialize in observing active volcanoes are called Volcanologists.