The sight of massive granite peeling away in curved, concentric sheets, often described as an “onion-skin” appearance, is known as exfoliation or sheeting. Granite is an intrusive igneous rock, forming from magma that solidifies deep beneath the Earth’s surface. The dramatic layering seen in granite domes is the result of forces acting on the rock long after its initial formation.
Granite’s Deep Origins
Granite begins its existence miles below the surface, crystallizing slowly from a silica-rich magma within the continental crust. This slow cooling process allows the constituent minerals—primarily quartz, feldspar, and mica—to grow into the large, interlocking crystals that define granite’s coarse-grained texture. The formation environment is characterized by intense lithostatic pressure, the uniform confining stress exerted by the weight of the overlying rock.
At typical formation depths (2 to 8 kilometers), the confining pressure can reach between 50 and 200 megapascals (MPa). This immense pressure compresses the granite body, stabilizing the rock in a highly stressed state. The massiveness and lack of internal fractures in granite make it particularly susceptible to the effects of this deep-seated stress.
The Primary Mechanism: Pressure Release
The principal cause of the granite’s layering is pressure release, also termed unloading. Over millions of years, the rock mass situated above the granite body is gradually removed by natural forces like erosion, glaciation, and tectonic uplift. The removal of this enormous overburden eliminates the confining pressure that kept the granite tightly compressed.
Once this massive weight is lifted, the granite, formed under compression, begins to expand. This expansion is a volumetric rebound, similar to a compressed spring attempting to return to its original state. The release of stress happens most dramatically near the newly exposed surface, causing the granite to undergo a slight but measurable increase in volume.
The expansion creates internal tensile stress within the rock body, which is greatest perpendicular to the relieved pressure. This stress is relieved by fracturing the rock. The process is particularly effective on massive, homogeneous rocks like granite because they lack the pre-existing weaknesses that would allow for other types of fracture.
How Pressure Release Creates Layers
The tensile stress caused by volumetric expansion results in the formation of large-scale fractures known as sheeting joints or exfoliation joints. These fractures develop subparallel to the exposed rock surface, curving to mirror the surface topography. Because the pressure release is most significant near the surface, the outer layers are the first to fracture and separate.
These sheeting joints propagate inward, creating a series of concentric, curved slabs that detach from the main rock mass. The thickness of these layers can vary considerably, ranging from mere centimeters to several meters, depending on the rock’s characteristics and the magnitude of the pressure reduction. As the outermost sheet peels away, the next layer beneath it experiences a reduction in confining stress, allowing the process to repeat.
The continuous removal of these sheets, aided by gravity on sloped surfaces, results in characteristic landforms known as exfoliation domes. Examples like Half Dome in Yosemite National Park illustrate how this large-scale physical weathering sculptures the landscape into smooth, rounded forms. The formation of these curved, onion-like shells is a direct physical response to the removal of the overlying geologic load.
Secondary Factors That Enhance Layering
While pressure release is the primary force that creates the large-scale sheeting joints, other weathering processes play a secondary role in enhancing the separation and breakdown of the layers. These factors typically work to widen or loosen the fractures that have already been established by unloading. The cyclic changes in temperature, for instance, can contribute to the process.
Diurnal heating and cooling cause the rock’s surface minerals to expand and contract slightly, generating minor tensile stresses. This thermal expansion and contraction can accelerate the propagation of existing joints and help pry the sheets apart. Water is also a factor, particularly through chemical weathering processes like hydrolysis.
Water and dissolved carbonic acid react with feldspar minerals within the granite, altering them into softer clay minerals like kaolinite. This chemical decomposition weakens the rock material along the sheeting joints, making the granite more susceptible to physical disintegration. The presence of water and ice within the fractures can also exert immense pressure, further aiding the detachment of the concentric slabs.