Erosion

Studies of erosion and the landscapes it leaves behind have been going on for over a century. This area of geologic inquiry, known as geomorphology, has long recognized that a balance exists between the erosion of land and its rejuvenation. If this were not the case, after a few tens to hundreds of millions of years, the earth's mountains would wear down to flat, relatively featureless plains and the basins would fill up with the sediment shed by the mountains. Instead, after billions of years of erosion, we still have mountains such as Mt. Everest in the Himalayas, which stands over 5.5 mi (8.8 km) above sea level, and ocean trenches such as the Marianas Trench, which reaches depths of more than 6.5 mi (10.4 km) below sea level.

The continued existence of rugged landscapes on the face of the earth is a result of a process of rejuvenation known as plate tectonics. Forces within the interior of the earth periodically re-elevate, or uplift, the earth's surface in various regions, while causing the lowering, or subsidence, of other regions. Plate tectonics therefore serves to maintain existing landscapes or build new ones. Currently, the Himalayas are an area of active uplift, but someday uplift will cease and erosion will slowly, but completely, wear them down. Someday the Marianas Trench may be filled in with sediment deposits. At the same time, new dramatic landscapes will be forming elsewhere on the Earth.