The single most important force shaping Earth's surface is moving water. Falling and running water weathers and erodes rocks, creating features such as canyons. Great rivers and small streams carry sediments and deposit them to create new land. And water invisible to us, locked up in rocks up to 200 kilometers (125 miles) below our feet, enables the vast, slow movements of Earth's continents. Water wears away mountains; it also makes possible the powerful planetary forces that build them.
When water flows over relatively soft rocks, the effects can be dramatic. Over its nearly four-million-year history, the Colorado River, in what is now the western United States, has eroded and dissolved two billion years' worth of rock.
Water has shaped our familiar landscapes. For instance, ice can gouge scars on the surface of the planet. The great ice sheets that spread repeatedly across the Northern Hemisphere within the past two million years carved valleys like Yosemite, in western North America. Rocks dragged by moving glaciers cut the deep grooves you can see--and touch--in an outcrop from Central Park in the heart of New York City.
Great rivers such as India's Ganges carry sediment that creates fertile deltas--land gradually built into the sea. Geologists estimate the total sediment delivered to the sea by Earth's rivers to be about 11 billion metric tons (12 billion U.S. tons) each year.
The oceans are never still. Periodic storms and the regular pulse of the tides claw endlessly at the shore. Off the coast of Australia, the power of the stormy Southern Ocean carved these sea stacks, known as the Twelve Apostles, from the limestone cliffs of Victoria, Australia. In the 1950s there were nine stacks; today there are only eight.
Geologically speaking, waterfalls have short lives. The immense power of falling water erodes the base of the falls and undercuts the lip, causing it to collapse and move upstream. Horseshoe Falls is one of the three waterfalls that make up Niagara Falls, where Lake Erie empties into Lake Ontario. It has migrated upstream 150 meters (500 feet) since European explorers encountered it in the 1700s.
On the Earth we see, the cycling of water from the atmosphere to the surface and back again happens over days, weeks, and years. But Earth's rocks have a water cycle, too--one played out over tens of millions of years. In the rock cycle, great slabs of Earth's crust called tectonic plates, made more pliable by the water they include, are forced under other huge plates. As slabs descend, the water is squeezed out, promoting melting in the rocks above. Eventually, mountains rise and volcanoes erupt, returning water vapor to the atmosphere.