Earth’s surface is always changing. Landforms such as mountains, valleys, plains, and plateaus are made by natural forces. These natural forces, whether earthquakes, volcanoes, wind, or rain, wear down and reshape Earth’s landforms over a period of time. Some landforms are only a few hours old, while others are a million years old. Scientists who specialize in landforms are called geomorphologists. For more information, read Pioneers of Earth Scientists: From Mercator to Evans, another book in the Mission: Science series.