Sheeting, exfoliation, or unloading is a physical weathering process. It happens when deeply buried rocks break into concentric slabs or onion-like layers parallel to the rock body due to pressure release when exposed to the surface.
This happens because the deeply buried rocks are initially under pressure or compressive stress. When exposed to the surface, they will expand or rebound as the pressure is released. These expansions cause linear to curvilinear fractures or joints.
In geology and Earth Sciences, a joint is an extensional fracture on a rock body that doesn’t involve visible or measurable parallel movement. It happens in a direction perpendicular to the fracture plane or surface, and the jointed rocks remain in situ.
However, erosion, gravity, or mass movement can make some separated rocks move. As they tumble down the slope, they further break into smaller pieces.
Lastly, sheeting is also called offloading or jointing. However, jointing can also occur due to tensional forces as magma or lava cools to form rocks or crustal extensions. An example is columnar jointing in basalt.
How does unloading or sheeting happen?
Sheeting happens due to unloading, i.e., removal of overburden or overlaying material in initially deeply buried rocks. Such deeply buried rocks are under immense confining pressure or compressional forces from overlaying material above.
Examples include plutons like granites, some metamorphic rocks, and thick sandstones. Such rocks formed or have spent some time under pressure from overlaying crustal or lithospheric material.
However, erosion can expose such deeply buried plutonic rocks, or uplift can bring them to the surface. This removes the overburden. Also, unloading can happen by melting or removing glaciers or ice sheets.
The overburden removal releases the confining compressive forces from the rock’s surface. This causes the rock portion on the surface to expand or rebound upwards more than below the surface since the unloading pressure decreases with depth.
Usually, this inelastic expansion or dilation is in the direction of pressure release or perpendicular to the rock body surface.
It will extend existing cracks and create linear to curvilinear sheet joints or joint-like fractures, parallel and subparallel to the ground or rock’s surface. This happens when extensional forces from expansion due to pressure release overcome tensile forces.
Put differently, it will cause slabs or layers of rocks on the surface to separate or peel off from the rest of the rock body, i.e., exfoliation. These slabs or layers can remain in situ or peel, exposing the inner surface. If this happens, the process will repeat.
Also, it will increase spaces between individual mineral grains, or what we call particle disintegration. Particle disintegration happens along individual grain boundaries.
Something worth noting is that expansion causes most fractures. However, it can increase and enlarge preexisting ones that form during emplacement or those from tectonic activities. For instance, differential cooling can cause parallel joints or sets of fractures.
However, fractures from tectonic forces like mountain building and magma cooling will create unique jointing patterns when crystallizing.
Characteristics
In unloading weathering, the joints or fractures occur parallel to the unloading surface, i.e., parallel to the face or side of an outcrop or perpendicular to the pressure release or expansion direction.
It forms large or small linear to curvilinear slabs parallel to the rock body surface. For instance, the slabs will also be curved if the surface is curved.
The linear or arched onion-like slabs or fracture patterns formed are 0.5 to 10 meters thick.
Exfoliation domes
Exfoliation refers to successive spalling or peeling of onion-like layers, sheets, or shells from a rock body due to unloading. It can, over time, create an exfoliation dome as the shells fall off the rock body.
Exfoliation domes are large or massive rounded rock outcrops that resemble a dome. They occur commonly in exposed plutonic rocks, mainly granites. However, it can occur in granodiorites, diorites, and other rocks.
Examples of exfoliation domes include
- Liberty Cap and Half Dome in Yosemite National Park in California
- Moxham Mountain in New York
- Stone Mountain, Georgia, USA (an exfoliated inselberg)
However, exfoliation domes can also form from thermal expansion weathering, or insolation.
Sudden sheeting or catastrophic rock bursting
A demonstration of sheeting weathering occurs when human activities cause sudden unloading.
For instance, during deep mining, large pieces of rocks often explosively burst in a newly cut tunnel wall in granites or other rocks under pressure. It happens due to a sudden decrease in confining pressure.
Similarly, removing large rocks during quarrying can cause rocks to crack parallel to the surface. Again, it happens due to pressure release.
Unloading weathering is common in which rock?
Sheeting is common in homogeneous, crystalline plutonic rocks like granite, granodiorites, diorites, or gabbro. Also, it occurs in metamorphic rocks formed at depths beyond 1 km where pressures are high.
Also, it can happen in massive, layered sandstones formed under tremendous pressure.
Since the form is under tremendous pressure, its compressional force exceeds extensional force. Therefore, they will easily fracture when exhumed.
Lastly, it develops better in these rocks as they are brittle and less common in ductile rocks like shale. However, crystalline plutonic rocks will have a more irregular jointing pattern than sedimentary rocks.
Where is sheeting common
Sheeting can occur anywhere where erosion or uplift has exposed rocks formed under pressure. However, it is more visible on bare rocks. Also, it may occur in outcrop slopes mantles with soil.
Surface expression of sheet joints is common in arid and semi-arid areas since they have little to no regolith. Also, hillslopes have a bedrock morphology.
Unloading weathering landforms
The exfoliation dome is a typical landform from sheeting. However, this physical weathering mechanism is associated with inselbergs, tors, and bornhardts.
The base of these landforms may have debris build-up. This debris or fragments are from jointed rocks that fall under graving, erosion, or other mass wasting ways.
It can speed up other weathering processes
Sheeting can allow other forms of weathering, like salt wedging, frost weathering, or chemical weathering, as it will enable water to infiltrate rocks easily.
Also, it can encourage root wedging or biological weathering and represents a plane of weakness where fractures can occur much more easily.
References
- Huggett, R. J. (2011). Fundamentals of geomorphology (3rd ed.). Routledge.
- Turkington, A. (2004). Mechanical weathering. In Goudie, A. (ed.) Encyclopedia of geomorphology (vol. 1, pp-657-659). Routledge.
- Bierman, P. R., & Montgomery, D. R. (2014). Key concepts in geomorphology. W.H. Freeman and Company Publishers
- Plummer, C. C., Carlson, D. H., & Hammersley, L. (2016). Physical Geology (15th ed.). McGraw-Hill/Education, Inc
- Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.
- Dixon, J. C. (2004). Weathering. In Goudie, A(ed.) Encyclopedia of geomorphology (vol. 1, pp 1108-1109). Routledge.