What is the passage mainly about? A. The destructive effects of salt on rocks.

In Death Valley, California, one of the hottest, most arid places in North America, there ismuch salt, and salt can damage rocks impressively. Inhabitants of areas elsewhere, wherestreets and highways are salted to control ice, are familiar with the resulting rust anddeterioration on cars. That attests to the chemically corrosive nature of salt, but it is not the way salt destroys rocks. Salt breaks rocks apart principally by a process called crystalprying and wedging. This happens not by soaking the rocks in salt water, but by moisteningtheir bottoms with salt water. Such conditions exist in many areas along the eastern edgeof central Death Valley. There, salty water rises from the groundwater table by capillaryaction through tiny spaces in sediment until it reaches the surface.

Most stones have capillary passages that suck salt water from the wet ground. DeathValley provides an ultra-dry atmosphere and high daily temperatures, which promoteevaporation and the formation of salt crystals along the cracks or other openings withinstones. These crystals grow as long as salt water is available. Like tree roots breaking up asidewalk, the growing crystals exert pressure on the rock and eventually pry the rock apartalong planes of weakness, such as banding in metamorphic rocks, bedding in sedimentaryrocks, or preexisting or incipient fractions, and along boundaries between individualmineral crystals or grains. Besides crystal growth, the expansion of halite crystals (the sameas everyday table salt) by heating and of sulfates and similar salts by hydration cancontribute additional stresses. A rock durable enough to have withstood natural conditionfor a very long time in other areas could probably be shattered into small pieces by saltweathering within a few generations.

The dominant salt in Death Valley is halite, or sodium chloride, but other salts, mostlycarbonates and sulfates, also cause prying and wedging, as does ordinary ice. Weatheringby a variety of salts, though often subtle, is a worldwide phenomenon. Not restricted to arid regions, intense salt weathering occurs mostly in salt-rich places like the seashore,near the large saline lakes in the Dry Valleys of Antarctica, and in desert sections ofAustralia, New Zealand, and central Asia.

What is the passage mainly about?