This map of the ocean floor is actually two maps. In normal light, it shows a topographic map of the terrain submerged beneath the seas. In the dark, a tectonic map glows to reveal the dynamics that shape the earth’s thin crust. The crust of the earth consists of two components. The lighter continental crust, depicted here in gray, "floats" on the denser ocean floor, which is shown in the colors of the sand, silicates and reddish mud that cover the basalt basement rock. The oceanic crust is fragmented into sections, called plates, that migrate around the earth and are recycled over geologic time back to the deep mantle from which they came. This movement is explained by the theory of plate tectonics. The Topographic Map The plates obey the laws of spherical geometry, rotating more like "caps" over the earth's rounded surface. To best portray global crustal dynamics, this map presents the earth as a large sphere. Each half, top and bottom, shows more than 360° of the earth's surface. The top half is centered on the Greenwich Meridian (0°), emphasizing the Atlantic and Indian Oceans. The bottom half, centered on the International Dateline (180°), showcases the Pacific Ocean. The Arctic Ocean is shown in scale on the upper right sphere, Antarctica is shown on the sphere at the lower left. The band at the center of the large sphere divides the two views of the earth and shows the meridian coordinates for each half. Parallel coordinates are shown along the sides. Minimum distortion is present above 60°S for the top view and below 60°N for the bottom view. Listed at the upper left side are major topographic features found in the four oceans. To locate a specific feature on the large map, refer to the corresponding number on the small sphere above the list. Each feature is also color-coded to indicate its tectonic structure. Color codes refer to the schematic cross-section at the bottom of the map, which illustrates the relationships between topographic features and underlying tectonic forces.