Weathering Part 1: Physical Weathering

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Jan 21, 2002

What is the most destructive force on our planet? Is it the earthquake, with the power to move the earth meters at a time? Or maybe it's a volcano, with the explosive force to destroy mountains and bury entire landscapes? Typhoons and hurricanes annually flood entire regions and cause considerable damage to our lives and landscapes. Tornadoes strike with quickness and power, able to tear the very earth from the ground. Or is it the raindrop?

A raindrop, you might exclaim, ranked with the likes of hurricanes, earthquakes and volcanoes? Preposterous? Not likely. Raindrops, and more importantly water is probably the most destructive force on our planet. Over the hundreds of millions of years that our planet has existed, water has been the driving force behind shaping the landscape, making the soil, and carrying sediment. Water, along with other agents, has slowly but inexorably altered the landscape of our planet through weathering, erosion and transport. Many of the spectacular features we travel to see, such as this arch in Bryce Canyon National Park, would not be possible without these agents.

So what are these processes and how do they work and what are the agents of weathering and erosion? From our study of the rock cycle, we know that weathering and erosion plays an important role in altering rocks and the formation of new rocks. Without these processes, 2/3 of the terrain we see, the sedimentary rocks would not exist. Weathering is the process that changes the physically and chemical character of rocks at or near the surface. You can see it whenever you visit a graveyard, especially a very old one. Walk among the headstones and look at the dates on them. As you find older and older headstones, you will notice that the older the grave the harder the lettering is to read. The edges are worn, and small pits might have formed as time has allowed wind, rain, and maybe frost to break up the rock. Weathering can occur when a rock has not moved, or while it is being moved.

When someone refers to erosion, many times they are erroneously referring to weathering. Erosion only occurs when an outside agent, such as water, wind, or ice, physically removes the rock. The actual movement of the rock particles by these agents is called transportation. During transport, a rock may continue to be weathered; further altering itself until it reaches a state of equilibrium with the environment it is in. If the environment changes, then the rock will weather again until equilibrium is reached.