Over long periods of time, many changes have occurred in Earth’s surface features. Forces related to plate tectonics have elevated mountains. Atmospheric constituents (mostly water, oxygen, and carbon dioxide) have interacted with minerals and rocks at Earth’s surface, weakening them and breaking them down through a process called chemical weathering. Physical processes involving, for example, the growth of plants, the release of pressure as overlying material is eroded, and the repeated freezing and thawing of water in cracks, have also helped to break down rocks. Fragments are transported downslope by wind, water, and ice. Gravity by itself moves material by way of landslides and slumps (called mass wasting). The ultimate destination of most of the products of weathering is the ocean. These products arrive in the form of marine sediment deposits. In time the mountains are laid low, the rivers change their courses and disappear, and lakes and seas expand or dry up. Eventually sediments, which have found their way to the oceans along continental margins, are compacted and changed to rock, then uplifted by continental collision or subducted and melted under the crust. New mountains are formed, and the cycle (called the geologic cycle) begins anew. Each cycle takes tens of millions of years. Waterfall.Waterfall.Waterfall.Waterfall.Waterfall.Waterfall.Waterfall.Waterfall. Chapter 4 The Science Content Standards for Grades Six Through Eight Grade Six Focus on Earth Sciences