Debris Flow Caused by Dirt Cracking

April 11, 2013

Debris flow main 2

Photographer: Thomas McGuire
Summary Author: Thomas McGuire

Debris Flow rocks 2The top photo shows a debris flow in a forest of saguaro cacti near my home in Cave Creek, Arizona. Physical weathering is sometimes caused by ice forming in rock cracks. Water expands when it freezes and as it does, the ice pries the rock apart. Deep frost is relatively rare in the Arizona desert, so frost action is less common than dirt cracking by expansive clay. Rocks initially fracture due to internal stress (right). Weathering of basalt produces expansive clay, which can fall or blow into the cracks. When the clay gets wet, it expands exerting a force on the rock. This increases the size of the cracks. Eventually, the rock will pry apart and may cause a steep rocky slope to fail. If a rapid debris flow occurs, it can make a deep channel and a zone of accumulation below the point where the slope begins to flatten out.

The channeled debris flow in the top photo occurred, unseen, during foggy, wet weather between January 18 and 22, 2010. It’s about 0.25 mi (0.5 km) long, approximately 10 ft (3 m) wide and about 6 ft (2 m) deep. Scientists estimate that this dramatic event started and ended in just a few minutes. The scar of the debris flow now clearly shows in the view of the nearby mountains from our patio. Fortunately, it occurred in a conservation area with no human injury or property loss. Nearby homes in the Phoenix valley may one day be in harm’s way.