NAVIGATION:  BACK TO MODULE TWO INTRODUCTION

Physical Weathering
(some material on this page borrowed from USGS)

Physical Weathering

The two main types of weathering are physical and chemical weathering.  This page describes mechanical (physical) weathering (and more).

Rocks are naturally fractured at several levels. Mineral grains have boundaries, which can be areas of weakness in the rock. Sedimentary rocks (next time) often are layered and the layers sometimes are not bound together well. More massive rocks can have joints (cracks which have no relative motion across them, only spreading) which will open as the rocks are exposed due to erosion. Physical weathering acts to widen these rock fractures.

Some examples of physical weathering mechanisms:

• Frost wedging
  Frost wedging happens when water filling a crack freezes and expands (as it freezes, water expands 8 to 11% in volume over liquid water). The expanding ice imparts a great amount of pressure against the rock (as much as 30,000 pounds/square inch) and wedges open the crack.  We see this type of weathering from a distance in the San Francisco Peaks (near Flagstaff) and up close in the La Sal Mountains in Southeast Utah (near Moab). 

• Heat/Cold Cycles
  As rocks are alternately heated and cooled (that is, as they go through cycles of hot/cold), they expand and contract; minerals expand and contract different amounts, and this differential expansion and contraction may stress rocks and crack them. This is mostly found in deserts; the actual mechanisms by which it happens are still under study (Brandon Vogt did his thesis on insolation weathering of sandstone)!  Forest fires (and even lightening!) can crack rocks too!  These hot/cold cycles can occur in a matter of moments (say, as the sun suddenly passes behind a cloud) or in a matter of hours (diurnal temperature change) or in the matter of days (air mass behind the passage of a warm/cold front). 

• Unloading
  Plutonic igneous rocks form at depth in the crust with an immense volume of rock (overburden) above them.  When these rocks are exposed, and the immense weight of the overburden rock above them is removed, they expand, and zones of weakness open up as joints. Granite tends to open and peel away sheets of rock (think of peeling an onion); the domes in Yosemite are formed by this process, which is called unloading.  There are examples of this type of weathering in the Landslide module as well as the throughout the Weathering module.

Frost wedging.  Notice how these granite rocks have been "pried" apart? Image is from Sierra Nevada Mountains.

Heat/cold cycles.  Has the sun been weathering this rock equally; though the sun is always in the Southern Hemisphere? Image is from Southeast Utah.

Rock is broken into gently dipping plates by unloading joints. Unloading joints probably form as the rock is exposed by erosion. These joints, and others that are more steeply oriented, provide pathways for water to enter the rock.  Image is from Yosemite National Park in California.  From here.

Biological Weathering

Biological weathering is, of course, weathering done by living things. I suppose it could really be called a special case of either physical or chemical weathering, but it is kind of neat that life on the planet can weather rocks. Some examples:

• Tree roots
  Tree roots grow into cracks and widen them, which helps physical weathering.

• Bacteria
  Some bacteria and other organisms secrete acidic solutions, which helps chemical weathering.

Different Weathering Types Help Each Other

Keep in mind that you really can't separate physical from chemical or biological weathering, because all three proceed at the same time (though not at the same rates, necessarily).

• IMPORTANT -- Physical weathering helps chemical weathering by breaking rocks up into smaller chunks, thus exposing more surface area. With more surface area exposed, chemical reactions happen faster. Think of dissolving sugar in water. Sugar lumps will dissolve more slowly than the same amount of sugar which is granulated --- because the surface area is higher for the granulated sugar. Try it if you don't believe me.

  Also, if slopes are flattened by physical weathering, water and soil can build up and thus speed chemical weathering.

• Chemical weathering helps physical weathering by weakening the mineral grains which make up rocks. This makes the rocks easier to break up by mechanical means.

• Biological weathering helps both, as I described above. Trees fracture rock with their roots, which makes them easier to break up physically and exposes more surface area for chemical weathering. Bacteria which secrete acid solutions speed chemical weathering.