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24-517: Along the trail, the Merced River is a tumultuous mountain stream, lying in a U-shaped valley . Enormous boulders, some the size of a house, are dwarfed by the sheer faces of exfoliating granite , which rise 3000 feet (914 m) from the river. Through it all, the Merced River rushes down from its source in the High Sierra , and broadens on the floor of Yosemite Valley. A trail which links to

48-571: A tectonic process, and are composed of different rock types. He observed that the mountain ranges in this area contained similarities in strata , resembling bubbles and blisters on the earth's surface. He concluded that the mountains were once intrusive magma that had and became visible with erosion. Gilbert presented his findings in a report and contributed significantly in what future generations would know as Geomorphology. Exfoliation joint Exfoliation joints or sheet joints are surface-parallel fracture systems in rock, often leading to

72-480: A study over a three-year period monitoring granite cracks within the park's Valley. Data collected by Park Geologist Greg Stock, and USGS civil engineer Brian Collins using deformation and temperature gauges. They concluded that there is an outward expansion with the change in thermal temperature up to an inch. With prolonged movement, the cracks expand over time and create exfoliation. Twain Harte Lake Dam

96-401: Is appealing, there are many inconsistencies with field and laboratory observations suggesting that it may be incomplete, such as: One possible extension of this theory to match with the compressive stress theory (outlined below) is as follows (Goodman, 1989): The exhumation of deeply buried rocks relieves vertical stress , but horizontal stresses can remain in a competent rock mass since

120-588: Is in Tuolumne County located within the Sierra Nevada Mountain Range. Engineers began construction on the dam in 1928 and placed the structure between two granite domes. In August 2014, the granite developed exfoliating joints and began to leak. The process was captured on video and is one of a couple ever to be caught on film. The lake was completely drained and the cost of reconstruction nine-hundred thousand dollars. In June 2016,

144-548: Is named for the color of the river here. Up the trail, Nevada Fall, a waterfall that is almost twice the height of Vernal Fall, awaits the hiker, although there is no cascade of mist over the trail at that waterfall. Behind Nevada Fall is Liberty Cap , an impressive lump of granite. The Mist Trail then rejoins the John Muir Trail: the hiker can travel in Little Yosemite Valley , or take a side trip to

168-581: Is relieved when the granite is exposed at the surface, allowing it to expand towards the atmosphere. On the surface, if the granite is not jointed, or if it has few joints, then the exposed surface usually expands faster than the underlying granite. The surface layer, often a couple hundred feet thick, separates from the underlying granite along an expansion joint to form a shell. As this continues, several concentric shells may form to depths of 100 feet (30 m) feet or more. Concentric slabs/shells of rock begin to break loose, onion-like layers subparallel to

192-404: Is that erosion of overburden and exhumation of deeply buried rock to the ground surface allows previously compressed rock to expand radially, creating tensile stress and fracturing the rock in layers parallel to the ground surface. The description of this mechanism has led to alternate terms for exfoliation joints, including pressure release or offloading joints. Though the logic of this theory

216-578: The John Muir Trail starts after crossing from the Happy Isles. Depending on one's conditioning, a bridge which re-crosses the river - the halfway point of a hike to the foot of Vernal Fall - can be reached in 15 to 30 minutes. To the side of Vernal Fall, the mist from the river blankets the trail, which was improved during the WPA projects of the 1930s. Here is where raingear becomes handy: in

240-699: The cyclic variations of temperature at the surface of the rock in a process also called spalling . Frost and ice expansion in the joints during the winter accelerate the alteration process while the most unstable loose external layers are removed by gravity assisted by runoff water . Homogeneous granitic plutons are created in high-pressure environments and slowly solidify beneath the Earth's crust . Vertical compression of overburden releases through erosion , or removal of overlying rocks resulting in unloading. Other contributors of unloading are tectonic uplift , glacier retreat , and mass wasting . The pressure

264-410: The erosion of concentric slabs. Despite their common occurrence in many different landscapes, geologists have yet to reach an agreement on a general theory of exfoliation joint formation. Many different theories have been suggested, below is a short overview of the most common. This theory was originally proposed by the pioneering geomorphologist Grove Karl Gilbert in 1904. The basis of this theory

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288-670: The exterior called exfoliating , sheet jointing, or fractures . As the granite expands the outer, most shells become susceptible to weathering by water pressure, freeze/thaw cycles, and functioning vegetation is a process called physical weathering . The sheets of granite are large enough to shave off sharp edges on the granite's surface creating a dome shape. The overall activity creates exfoliation domes. Exfoliating rock can trigger rockfall . Rock Falls in Yosemite National Park are common and pose threats to visitors. United States Geological Survey (USGS) conducted

312-545: The granite rock had to be closed off for a second time because of the continued exfoliation. Grove Karl Gilbert is considered one of the most influential geologists of his generation. He was the only president of the Geographic Society to receive the highest honor for geographic accomplishment; Hubbard Medal . Working his way down the Great Basin , Gilbert knew the general formation of mountains through

336-419: The laboratory during uniaxial compression tests. High horizontal or surface-parallel compressive stress can result from regional tectonic or topographic stresses, or by erosion or excavation of overburden. With consideration of the field evidence and observations of occurrence, fracture mode, and secondary forms, high surface-parallel compressive stresses and extensional fracturing (axial cleavage) seems to be

360-569: The laboratory since at least 1900 (in both uniaxial and biaxial unconfined compressive loading; see Gramberg, 1989). Tensile cracks can form in a compressive stress field due to the influence of pervasive microcracks in the rock lattice and extension of so-called wing cracks from near the tips of preferentially oriented microcracks, which then curve and align with the direction of the principle compressive stress. Fractures formed in this way are sometimes called axial cleavage, longitudinal splitting, or extensional fractures, and are commonly observed in

384-580: The medium is laterally confined. Horizontal stresses become aligned with the current ground surface as the vertical stress drops to zero at this boundary. Thus large surface-parallel compressive stresses can be generated through exhumation that may lead to tensile rock fracture as described below. Rock expands upon heating and contracts upon cooling and different rock-forming minerals have variable rates of thermal expansion / contraction. Daily rock surface temperature variations can be quite large, and many have suggested that stresses created during heating cause

408-423: The most plausible theory explaining the formation of exfoliation joints. Recognizing the presence of exfoliation joints can have important implications in geological engineering . Most notable may be their influence on slope stability. Exfoliation joints following the topography of inclined valley walls, bedrock hill slopes, and cliffs can create rock blocks that are particularly prone to sliding. Especially when

432-426: The near-surface zone of rock to expand and detach in thin slabs (e.g. Wolters, 1969). Large diurnal or fire-induced temperature fluctuations have been observed to create thin lamination and flaking at the surface of rocks, sometimes labeled exfoliation. However, since diurnal temperature fluctuations only reach a few centimeters depth in rock (due to rock's low thermal conductivity ), this theory cannot account for

456-469: The observed depth of exfoliation jointing that may reach 100 meters. Mineral weathering by penetrating water can cause flaking of thin shells of rock since the volume of some minerals increases upon hydration . However, not all mineral hydration results in increased volume, while field observations of exfoliation joints show that the joint surfaces have not experienced significant chemical alteration, so this theory can be rejected as an explanation for

480-411: The origin of large-scale, deeper exfoliation joints. Large compressive tectonic stresses parallel to the land (or a free) surface can create tensile mode fractures in rock, where the direction of fracture propagation is parallel to the greatest principle compressive stress and the direction of fracture opening is perpendicular to the free surface. This type of fracturing has been observed in

504-405: The spring, hikers often become completely drenched from water sprayed off Vernal Fall. This is one of the few places where a complete circular rainbow can be viewed. The trail now crosses a sheer face which is made somewhat easier by a single guard rail. At the top of Vernal Fall, the Merced River traverses a shelf of granite, and the hikers frequently sun themselves to dry off. The Emerald Pool

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528-453: The toe of the slope is undercut (naturally or by human activity), sliding along exfoliation joint planes is likely if the joint dip exceeds the joint's frictional angle. Foundation work may also be affected by the presence of exfoliation joints, for example in the case of dams . Exfoliation joints underlying a dam foundation can create a significant leakage hazard , while increased water pressure in joints may result in lifting or sliding of

552-574: The top of Half Dome , using cables to climb the rock. Permits must be obtained in advance to hike Half Dome, and back country permits are required to stay overnight along the John Muir Trail. The distance from Happy Isles trailhead to the Vernal Fall foot bridge is 0.8 miles (1.3 km) with an elevation gain of 400 ft (120m); to the top of Vernal Fall is 1.2 miles (1.95 km ) with an elevation gain of 1000 ft (300m); and to

576-463: The top of Nevada Fall is 2.7 miles (4.35 km) one way with an elevation gain of 1900 ft (610m)( data per NPS information). 37°43′37″N 119°32′40″W  /  37.72694°N 119.54444°W  / 37.72694; -119.54444 Exfoliating granite Exfoliating granite is a granite undergoing exfoliation , or onion skin weathering ( desquamation ). The external delaminated layers of granite are gradually produced by

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