Adrian Hoban, age 12, of San Francisco, California, for his question:
What causes erosion?
Erosion covers the beaches with sand and changes the shapes of the shorelines. It digs deep valleys and floods the plains with soggy mud. It chews steep canyons in the slopes and wears down the loftiest mountains. It shifts vast masses of desert sand and molds the mesas. Natural erosion is a planetary project that continually remodels the wrinkled ups and downs of the earth's surface. Enormous planetary forces are needed to cause it and keep it going.
This non stop planetary operation occurs where the earth's surface is exposed to the weathery atmosphere and the restless oceans. In a general way, it chews up hard crustal rock and shifts countless tons of loose debris from place to place. Plants cannot live in bare, hard bedrock and their roots hold onto their thin layers of surface soil. People cause erosion when they cut down forests and strip the grassy plains, exposing the loose soil to the winds and water. Modern farmers are learning ways to protect their fielas from such disasters.
Naturally, we do our best to stop the erosions of our fertile farmlands. But nobody can stop the vast planetary system of erosion. Nobody realAy wants to, because this is how the changing earth renews itself and stays forever yang. Erosion chomps up tough rocks and mixes them with organic material to build new soils. It shifts mountains of dirt, so that barren regions have a chance to thrive and weary old places have a chance to rest. This patient erosion is a community project on a grand scale. It is caused by gravity and running rivers, winds and rains, the seasonal sun and even the tidal moon.
Gravity strives to slide loose material down the slopes. Rainwater seeps into porous rocks and runs downhill to the seas. Moving water dissolves soft minerals, erodes hard rocks and sweeps along muddy gravel as it dashes to the base of the slopes. There gravity relaxes its pull. The steams slaw down and dump their eroded loot on the plains.
The changing seasons also help to crack the tough peaks into moveable pieces. Hard rocks expand and contract with summer heat and winter cold. Rains and melting snows fill the fractures. The water washes out the rocky pockets and the frost freezes it solid. The jagged ice expands and digs the cracks deeper. The spring streams carry off the loose fragments as they dash down the slopes. Sometimes the mountains are shaken by storms and earthquakes. Weakened pinnacles lose their balance and gravity brings masses of eroded material plummeting down in thundering landslides.
Most, but not all, erosion is caused by the team of gravity, weather and running water.
In the deserts, where water is scarce, the work of erosion is done by blustering breezes. They hurl gritty grains of sand in the faces of bald rocks. This wind erosion carves canyons in the desert cliffs and molds the mesas. Along the coast, the sea hurls pounding waves at the beaches and the rocky cliffs. This wave erosion is caused by wild gales and lunar tides and gradually it reshapes the shorelines.