MECHANICAL WEATHERING PROCESSES

The simplest and perhaps most effective type of mechanical weathering, fracturing of rock or of sediment particles by IMPACTS OR ABRASION, is often overlooked in discussions of the topic, as it is in your text book.  A rock falling from a cliff or bouncing down a steep slope not only breaks itself as it lands, it damages any other rock it impacts en route.  Sand grains blown by wind or carried by water abrade both themselves and anything they strike as they move.  Boulders carried by glacial ice scour, abrade, and fragment themselves and any rock they contact in the floor or walls of the glacial valley.  This quite likely accounts for most of the mechanical weathering on Earth, but there are other processes as well.

One of these is called FROST ACTION (or sometimes "freeze/thaw effects") and relies upon one result of the strange molecular structure of water.  Molecules of H2O are asymmetrical as shown in the first diagram.

This asymmetry has many important geological and environmental effects, but the one of immediate interest is that changing liquid water to ice requires a significant increase in volume in order to arrange the molecules properly into the crystalline form.  In effect, the molecules have to move farther apart to "swing" the hydrogen atoms around the adjacent molecule and into the proper position.

The volume increase this requires is about 9% of the original liquid volume.  The outward force exerted by this expansion is almost 30,000 lb/in2, much higher than the force required to fracture even very durable rocks.  Granite, for instance, will fracture under a tensile force of only about 3500 lb/in2.  This is in addition to the simple force of crystallization of the water, a separate and similarly powerful force we will return to below.  The photograph shows ice crystals that have expanded greatly in volume as they grew, forcing soil aggregates and small stones as much as 8 cm (3 in) off the ground.

The next diagram outlines how alternate freezing and thawing of water in small cracks can expand those cracks and cause significant mechanical weathering thereby.

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