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Part 3 EXFOLIATION IN MASSIVE ROCKS Profuse sheet jointing developed in a glacial cirque on Little Shuteye Pass in the Sierra National Forest, CA. The rock is the Mt. Givens granodiorite. Note instructor’s Jeep for scale (arrow). Exfoliation, or sheet joints, are common in massive plutonic rocks, like this Sierra granite. These are likely produced by a combination of mechanisms, not simply load removal. The stronger the rock, the more brittle it becomes. This shows a closer view of the glacial cirque in Little Shuteye Pass of the Sierra Nevada Mtns. Although it is arguably, the strongest rock in California (qc = 37,000 psi), it also exhibits the closest sheet joint spacings, because brittle rocks have a low strain tolerance. Cut in glaciated plutonic rock in the Rocky Mtns. In glaciated areas the weathered regolith has been stripped away. Note the blocky nature of the ground mass and the presence of valley-side discontinuities, semi-parallel to the cut slope. Secondary joints are those that form locally, in consequence to landforms, unloading, weathering, and/or the works of man, such as excavations. Inclined secondary joints are responsible for numerous rock slides along steeply-inclined discontinuities, as shown here in the Lower Granite Gorge of the Grand Canyon in the Precambrian Zoroaster Granite. Sheet joint exfoliations can form in any massive rock, so long as it retains some portion of its original elasticity and is relatively devoid of fractures. This shows profuse exfoliation of the Esplanade sandstone member of the Pennsylvanian age Supai Group in North Canyon, a tributary of Marble Canyon, Arizona. Ayers Rock in central Australia is a monolith of Precambrian age arkosic sandstone It exhibits exfoliation sheets as thick as 35 feet, as shown here. It has been under subaerial exposure for many millions of years Valley-side sheet joints can even develop in limestone. This shows a wall arch developed along valley-side secondary joints in the Mississippian-age Redwall Limestone, near River Mile 32.6 in Marble Canyon, Arizona The spacings between sheet joints tend to increase with depth and confinement (overburden), as shown in this Yosemite granite quarry Fire-Induced Exfoliation Heat from forest and brush fires is often sufficient to cause thermally-induced exfoliation These examples are granite in the southern California Batholyth, described by K. O. Emery in 1940 Sheet joints form through induced tension whenever the limiting tensile stress is exceeded. In this case a 9o F temperature variance is sufficient to spawn tensile fracture. Thermallyinduced tensile fractures could be expected to exhibit a pattern of increased fracture spacings with depth, as shown here and observed in the field (data from Rogers, 1982). Field measurements suggest that thermal effects on sheet jointing tend to control plate thickness closest to the ground surface (i.e. 20 ft), beyond which, other factors, such as glacial rebound, likely exert controlling influence.