Exfoliation of Sloop Skin

In the vicinity of GZ you will see a massive chunk of granite. Please walk around it and have a good look. Observe the layers of rock that are in the process of peeling off the large outcrop and the layers that have already parted ways with the rock and litter the ground at the base of it.

Logging Requirements:

1.) Briefly describe your understanding of exfoliation? Is it a mechanical, chemical or biological form of weathering?

2.) What type of rock is most commonly associated with exfoliation? (Hint, it’s the kind of rock you are looking at)

3.) What is the primary driving force behind exfoliation in this type of rock?

Then post a photo of you at the location in your log, preferably with a view of the water in the background. Of course, if you do not want to appear in the photo, a personal item in the photo is enough proof of your presence. You may log the cache as soon as you submit your answers to us via messenger.

Logs without accompanying answers sent or without a photo uploaded may be deleted without notice. Sending the answers is a requirement not a request. Please note, answering the logging tasks is based on your observations at the site and using your understanding of  content of the cache page, there’s no need to do any further research. 

The Lesson:

Granite batholiths (large bodied intrusions that form when a magma chamber cools and solidifies) form deep within the Earth under tremendously high pressure.

When the overlying material (rock, soil, glacier, etc) is removed due to erosion, glacial retreat, tectonic uplift or mass wasting, the underlying rock becomes exposed and the pressure is released. No longer under this confining weight, the rock responds to the decrease in pressure by expanding upward.

The expansion causes the granite to fracture as it releases stress along planes parallel to the surface of the batholith. If the granite is minimally jointed, then the exposed surface of the rock usually expands at a faster rate than the underlying granite. This process is known as unloading and is the primary driving force behind exfoliation.

Exfoliation is a type of mechanical weathering, it is the physical force that causes rocks to break down and disintegrate.

As the granite continues to expand and fault, the process of exfoliation accelerates. The expanding gaps between the outermost shell layer and the main body of the rock exposes it to all 3 types of weathering processes. This includes other physical types, like freeze—thaw cycles, as well as chemical weathering like oxidation and hydrolysis and biological weathering like the effects plants have on the rocks. Chemical weathering can change the mineral composition in the rock. Studies of thin spalls (fragments of rock that have broken off a larger solid rock body) that separate from rock exposed to the weathering reveal a common cause of the separation is the slow development of clay minerals within the decomposing rock. These clays can cause an increase in volume from their ability to hold water and a breaking of the bonds that hold the rock together.

The layered eroding slabs of rock eventually break loose and slough off along the  subparallel fractures called exfoliation joints. As the surface rock is removed more pressure is removed from the underlaying batholith thus exposing fresh rock below and the process begins anew.

Exfoliation gives rise to a variety of terms but in essence, they all describe the same geological phenomenon: the breaking away of rock layers due to stress release, often resulting in the characteristic onion-like appearance of the affected rock formations…..some of these terms include Sheeting, Onion-skin weathering, Spalling, Spheroidal weathering, Concentric weathering.

The sheets of granite are often large enough to shave off sharp edges on the granite's surface creating a dome shape. The overall activity creates exfoliation domes. This exfoliation or separation of successive thin shells, (or spalls), from the parent rock; it is common in regions that have moderate rainfall. The thickness of individual sheet may be from a few millimetres to a few metres.

Resources: https://www.nps.gov/flfo/learn/nature/virtual-roots-of-pikes-peak.htm