Part 3Part 3

EXFOLIATION IN EXFOLIATION IN MASSIVE ROCKSMASSIVE ROCKS

Profuse sheet jointing developed in a glacial cirque on Profuse sheet jointing developed in a glacial cirque on Little Shuteye Pass in the Sierra National Forest, CA. Little Shuteye Pass in the Sierra National Forest, CA. The rock is the Mt. Givens granodiorite. Note The rock is the Mt. Givens granodiorite. Note instructor’s Jeep for scale (arrow). instructor’s Jeep for scale (arrow).

Exfoliation, or sheet joints, are common in massive Exfoliation, or sheet joints, are common in massive plutonic rocks, like this Sierra granite. These are likely plutonic rocks, like this Sierra granite. These are likely produced by a combination of mechanisms, not simply produced by a combination of mechanisms, not simply load removal. load removal.

The stronger the rock, the more brittle it becomesThe stronger the rock, the more brittle it becomes. This shows a . This shows a closer view of the glacial cirque in Little Shuteye Pass of the closer view of the glacial cirque in Little Shuteye Pass of the Sierra Nevada Mtns. Although it is arguably, the strongest rock Sierra Nevada Mtns. Although it is arguably, the strongest rock in California (qin California (qcc = 37,000 psi), it also exhibits the closest sheet = 37,000 psi), it also exhibits the closest sheet joint spacings, because brittle rocks have a low strain tolerance. joint spacings, because brittle rocks have a low strain tolerance.

Cut in glaciated plutonic rock in the Rocky Mtns. In glaciated Cut in glaciated plutonic rock in the Rocky Mtns. In glaciated areas the weathered regolith has been stripped away. Note the areas the weathered regolith has been stripped away. Note the blocky nature of the ground mass and the presence of valley-side blocky nature of the ground mass and the presence of valley-side discontinuities, semi-parallel to the cut slope. discontinuities, semi-parallel to the cut slope.

Secondary jointsSecondary joints are those that form locally, are those that form locally, in consequence to landforms, unloading, in consequence to landforms, unloading, weathering, and/or the works of man, such as weathering, and/or the works of man, such as excavations.excavations.

Inclined secondary jointsInclined secondary joints are responsible for numerous are responsible for numerous rock slidesrock slides along along steeply-inclined discontinuitiessteeply-inclined discontinuities, as , as shown here in the Lower Granite Gorge of the Grand shown here in the Lower Granite Gorge of the Grand Canyon in the Precambrian Zoroaster Granite.Canyon in the Precambrian Zoroaster Granite.

Sheet joint exfoliationsSheet joint exfoliations can form in any massive can form in any massive rock, so long as it rock, so long as it retains some portion of retains some portion of its original elasticity and its original elasticity and is relatively devoid of is relatively devoid of fractures. fractures.

This shows profuse This shows profuse exfoliationexfoliation of the of the Esplanade sandstoneEsplanade sandstone member of the member of the Pennsylvanian age Pennsylvanian age Supai Group in North Supai Group in North Canyon, a tributary of Canyon, a tributary of Marble Canyon, Arizona. Marble Canyon, Arizona.

Ayers RockAyers Rock in in central Australia central Australia is a monolith of is a monolith of Precambrian age Precambrian age arkosic sandstonearkosic sandstone

It exhibits It exhibits exfoliation sheetsexfoliation sheets as thick as 35 as thick as 35 feet, as shown feet, as shown here.here.

It has been under It has been under subaerial subaerial exposure for exposure for many millions of many millions of yearsyears

Valley-side sheet jointsValley-side sheet joints can even develop in can even develop in limestonelimestone. This . This shows a shows a wall archwall arch developed along valley-side secondary joints developed along valley-side secondary joints in the Mississippian-age Redwall Limestone, near River Mile 32.6 in the Mississippian-age Redwall Limestone, near River Mile 32.6 in Marble Canyon, Arizonain Marble Canyon, Arizona

The spacings The spacings between sheet between sheet joints tend to joints tend to increase with increase with depth and depth and confinement confinement (overburden), as (overburden), as shown in this shown in this Yosemite granite Yosemite granite quarryquarry

Fire-Induced Fire-Induced ExfoliationExfoliation

HeatHeat from forest and from forest and brush fires is often brush fires is often sufficient to cause sufficient to cause thermally-induced thermally-induced exfoliationexfoliation

These examples are These examples are granite in the granite in the southern California southern California Batholyth, described Batholyth, described by K. O. Emery in by K. O. Emery in 19401940

Sheet joints form through induced tension whenever the limiting Sheet joints form through induced tension whenever the limiting tensile stress is exceeded. In this case a tensile stress is exceeded. In this case a 99oo F F temperature temperature variance is sufficient to spawn tensile fracture. variance is sufficient to spawn tensile fracture. Thermally-Thermally-induced tensile fracturesinduced tensile fractures could be expected to exhibit a pattern of could be expected to exhibit a pattern of increased fracture spacings with depth, as shown here and increased fracture spacings with depth, as shown here and observed in the field (data from Rogers, 1982). observed in the field (data from Rogers, 1982).

Field measurements suggest that Field measurements suggest that thermal effects on sheet jointingthermal effects on sheet jointing tend tend to control plate thickness closest to the ground surface (i.e. 20 ft), to control plate thickness closest to the ground surface (i.e. 20 ft), beyond which, other factors, such as glacial rebound, likely exert beyond which, other factors, such as glacial rebound, likely exert controlling influence. controlling influence.