Thirteen million years ago, the crust around the Lake Mead area was stretched and thinned so iron and magnesium-rich magma and basalt, poured out of the ground and covered the landscape. Some magma cooled under the surface of the Earth forming a light colored granite core. The darker lava produced many of the black volcanic flow-capped mesas (flat- topped hills with steep sides) including Fortification Hill. Why can the light colored granite now be seen at the top of the mesa if black volcanic rock covered it in a series of eruptions that occurred thirteen million years ago? Use the picture below as a reference. The outline shows the black volcanic rock that covers Fortification Hill. The yellow arrows are pointing to where the light colored granite can be seen.