Weathering Module Introduction

Rock does not weathering uniformly due to variations in local and regional factors. Weathering becomes interesting, in terms of its manifestation in form across the landscape, when rocks weather differentially - differential weathering can produce spectacular landforms and landscapes.

The canyon walls in the Grand Canyon are a series of uniquely shaped "steps". The steps are actually different strata or different rock types. Some of the layers of rocks weather very easily (are not very resistant) while others weather very slowly (are very resistant). The rocks that weather very easily (like mudstones and shales) generally tend to form gentle ramps into the canyon below. The rocks that weather very slowly, i.e., the rocks that are highly resistant to weathering, generally tend to have much steeper slopes - some approach a 90 degree slope (vertical).

"Steps" in the Grand Canyon.

Differential weathering in the Painted Desert (not too far north of Flagstaff, AZ). See the resistant rock (the layer that is making the shadows). Notice how the weaker layers (least resistant) have the most gentle profiles along the ridge.

Here's a profile of JFK in basalt. If it were not for differential weathering, we would see this profile as merely a pile of rocks. This image was borrowed from here.

So far we have talked about differential weathering in terms of variations in the resistances in strata as seen on a scarp or cliff face (such as the walls of the grand canyon). Other conditions that yield distinctive landforms are resistant cap rocks. A more resistant cap rock (such as sandstone) at the top of a mesa does not weather or erode until the softer underlying rock weathers and erodes away - when the underlying rock is no longer supporting the cap rock, it will break away and fall down the slope of less resistant underlying rock.

The plateau in the above image is covered with a flat protective rock (probably basalt). The protective rock is called a "cap rock". Cap rocks protect weaker layers below from erosion.

Other examples of differential weathering are Devil's Tower, Wyoming and weathering forms controlled by jointing.

Devils Tower, Wyoming. Devil's Tower is a very resistant "volcanic plug" that was surrounded by weaker shales that have since eroded away. All that is left is the resistant tower.

The profile through this rock is loaded with fractures or joints. The joints or breaks in the rock give weathering a head start... the increased surface area in the joints lead to accelerated weathering along the joints. This diagram borrowed from here.

The Colorado Plateau is full of cliffs (cliffs and scarps are synonymous, but scarp is a cooler word than cliff). Cliffs weather and erode, just like anything... but, because they are vertical faces of rock, we say that they, "retreat". How does this work, you ask?

The image here shows a typical scarp. As it weathers and material falls down the steep face and piles up at the base, the whole scarp tends to "retreat" (scarp retreat) from left to right. The material that weathers is eroded away; exposing the next surface to be weathered. The cycle continues and the scarp continues to retreat. If you had a time lapse video of this area (say, a picture taken every 100 years), the scarp would eventually march out of the image and you would only see a flattish plane (that transports away the weathered material) and the blue sky.

John and I think the reason the scarp retains a nearly "vertical" or very steep surface as it retreats is as follows: Weaknesses in the layers of rock serve as a "failure" zone... In other words, in this image, there is a very weak layer of strata near the base of the cliff that erodes very easily. Once the weak layer has eroded, there is no longer any "support" for the main cliff face -- the cliff becomes undermined. After it is undermined, the next segment of rock fails and breaks away and falls down the side of the slope. The process continues and the scarp retreats.