Desert Pavement

This black rocky area is known as desert pavement.  Notice basically no plants grow in the desert pavement, instead growing in the gray/tan surface where the pavement is absent.  The interlocked rocks and dense soils of desert pavement prevent water from penetrating the soil, therefore preventing plant growth.

Traveling from east to west in the Sonoran Desert rainfall decreases from 12 inches annually near Tucson to the hyper-arid two to four inches of rain received along the Colorado river.  This results in extremely sparse vegetation across vast areas of the landscape within this hyper-arid region.  Often the desert ground appears as painted black over broad areas, forbidding nearly all life.  Taking a closer look at the black covered soil you would notice a network of black rocks that appear to have been puzzle pieced together.  These areas, have very hard ground not unlike pavement, which is in-fact where it derives the name desert pavement.  Walking across this pavement is quite easy being it is a relatively smooth, hard, and level surface.  Though easy to walk across, summertime on this pavement is quite forbidding with scorching sun and heat, being only a little better than walking across a blacktop parking lot.

Close up of some desert pavement.  Notice how the rocks are closely interconnected, often like a puzzle.  This results in rainwater not penetrating the soil and instead running off.  Plant and other live are nearly absent to areas like this.

When scientists initially found these vast areas of pavement more then 100 years ago they thought overgrazing by cattle had trampled and compacted the ground making it infertile and inhospitable to plant life. Later it was determined desert pavement was actually a natural phenomena, not caused by cattle grazing.  Over long periods of time, decades to millenniums, a desert soil surface has smaller soil particles like sand and clay either washed or blown away.  Heavier rocks stay in place becoming interlocked.  Most of these rocks are not normally black in coloration but over time become covered with desert varnish.  This varnish forms when magnesium and iron rich dust falls on the rocks staining them through the action of bacteria.  Both desert pavement and varnish are more common the drier a desert is.  This is why Tucson has far less desert pavement and varnish than areas around the Colorado River.  The more rain an area has, the more unstable the soil surface is.  Significant rainfall amounts result in rocks being washed away through erosion, preventing pavement and varnish from forming.

In our previous entry we discussed desert soil horizons, and if you were wondering, desert pavement is part of the A horizon.  Soil horizons are extremely important in determining where water accumulates and therefore where and what types of plants grow.  Desert pavement, as mentioned before, prevents water from penetrating deep into the soil due to its hard pavement like surface.  Typically, desert pavement also indicates underlying thick layers of caliche, both of which can strongly impede water and plant roots from penetrating.  Water may only penetrate a few centimeters deep, even after a good rain, and many years may pass between times where there is enough rainfall to penetrate to adequate depths to support plant life.  Outside of the pavement though water penetrates more often and a lot deeper as evidenced by obviously thicker vegetation. Every five or so years through there is significant rainfall to penetrate deeper into desert pavement soils and short lived flowers can color the normally black desert ground yellow.

Normally black and barren desert pavement covered with wildflowers after strong winter rainfall.  Picture from:http://www.desertmuseum.org/programs/flw_mohave.php