Pushing Up Earth's Surface
Earth's surface is not only built from above, but also it is built up from below. Not far below Earth's surface, temperatures are quite high. In some places the conditions are hot enough to melt rock. Melted rock is called magma. Magma originates in a layer of Earth just below the crust called the mantle. Pressure below the surface can cause magma to push up on Earth's crust, creating round, dome-shaped mountains. In some places, magma can work its way up through the crust and flow out onto the surface as lava. As lava flows, it cools and hardens into rock. In some places, enough lava will build up from a huge deposit with gently sloping sides. Such deposits are called shield cones. Shield cones often form on the ocean floor. For example, the Hawaiian Islands are actually the tops of several giant shield cones. The base of Mauna Loa, the largest of these cones, is about 15,000 feet below the surface of the Pacific Ocean. Its peak rises over 14,000 feet above the oceans surface.