Rain

     Rain drops are formed when water vapor in the air condenses around tiny dust particles or ice crystals. Clouds form as the drops of water grow. As rain falls, it returns evaporated water to the surface of the planet, scattering it over the land masses so that plants can grow nearly everywhere on earth. The location of biomes (communities of living organisms) are partly determined by the rain fall at particular locations.

Rainfall on earth varies between almost none (a trace) to over 200 inches a year. Humans live in places with 20 inches or more of rainfall a year, though they can survive with less if they use irrigation or very special dry land farming practices that capture water and deliver it to the roots of their crop plants.

Rivers and Streams

   When rain falls on the ground some of it soaks into the soil. It moistens the soil particles and makes them easier to move. A heavy rain will stir up the surface of the soil and make mud of the small particles. The water that is not absorbed runs off the soil, carrying a little of the soil with it.

       Water flows downhill, and is always seeking a lower path. Little trickles of water flow together, and soon there is a small stream. A small stream has more force than individual rain drops or melting snow. It begins to stir up larger particles and carry them away.

       Here you see a stream bed in the desert.  Notice the steep sides of the stream bed. The stream is dry now, but the water will come back in a big rush after a rain. The fast moving water will carry earth away from the sides of this little valley.

       The sides of many valleys are covered by plants. The roots of the plants hold the soil in place when the rain comes, and their leaves break the force of the falling raindrops.   These valleys do not erode as quickly as valleys of bare earth, but they also change over time. Water is an irresistible force.

Ice and Glaciers
                                                     

    Rocks are constantly being moved around by currents in the mantle, and they crack in response to the strain. Rain seeps into the cracks, and when winter comes the water freezes. Freezing water expands, and it pushes the walls of the cracks apart, making the cracks wider. Over centuries huge rocks can be split. Sometimes rocks are split off from the sides of canyons.

    Glaciers are huge "ice rivers" sliding slowly towards the sea. The underneath part of the glacier may have captured rocks in the ice, and as the glacier moves, it grinds the rocks beneath it. Glaciers move all sizes of rocks, and grind some of the stones to a very fine dust called glacial flour.

     This picture shows an example of glacial polish.  A glacier has smoothed this rock so that it feels as smooth as glass.  The rocks on top of the polished rock may have been left behind when the ice of the glacier melted.  The cracks in the polished rocks create opportunities for ice to break up this surface.  Erosion is a continuous process.

Wave Action

     Tides show erosion in action. The constant movement of the waves shows us how sand grains are tumbled, and the returning water traces miniature drainage channels in the sand. The rocks and stones on the beach are rounded from thousands of years of being rolled against each other or washed by sand-carrying breakers.

     Waves also erode cliffs along the beaches. They undercut the cliffs, and eventually the cliffs fall. The ocean along the coast of California is slowly pushing back the cliffs.

Wind

     Wind picks up dust and sand. Sand storms can act like sand blasters if the wind is moving fast enough. Sometimes when people are driving in the desert they drive through a sand storm which erodes all the paint off their cars! In the desert the wind moves the sand into dunes, piling it up and covering the surfaces with ripples.

Chemical Action

     There are chemicals in the air.   Some of these chemicals can corrode rocks, and fairly quickly, too.  The names on marble gravestones can be dissolved away in only a couple of centuries. Carvings on buildings are being digested by chemicals that cause air pollution.

     Another way that chemical action can break down rock is through crystallization.  If salt water, for instance, gets into cracks in rocks, the salt crystallizes when the water evaporates.  The tiny crystals push against the sides of the cracks and slowly widen them.