Andy Jones, age 14, of Lancaster, Pa., for his question:

WHAT CAUSES VOLCANOS?

When melted rock from deep within the earth's surface comes up through an opening with lava, hot gases and rock fragments, you have a volcano. Most are at the tops of mountains which were formed by the materials thrown out during past eruptions.

Volcanos start with melted rock called magma. Much gas is produced and gas filled magma is gradually moved toward the earth's surface because it is lighter than the solid rock around it. Under great pressure, it can eventually break through to the surface, erupting with a burst of magma and other volcanic materials.

The world's largest volcano is Mauna Loa in Hawaii. It rises almost 30,000 feet from the ocean floor and is about 60 miles wide at its base.