#18: The Expanding Rim Width of the Grand Canyon
A cracking sound from below the rim may announce a geological event that lasts only seconds after thousands of years of preparation. Water enters fractures, winter freezing widens them, summer heat expands exposed rock, and gravity eventually pulls loose blocks into the canyon. Flash floods then sweep smaller debris through side channels toward the Colorado River. Softer shale layers erode into slopes, undermining resistant limestone and sandstone that break into cliffs, giving the Grand Canyon its familiar staircase profile.
