Fractures as a tourist destination?
Much of the landscape of Yosemite National Park was influenced by the presence of a fracture system in the Cretaceous granite bedrock. The Fissures, located on the south rim of Yosemite Valley near Taft Point, are named in honor of the fractures that contribute to this beautiful landscape.
The Fissure pictured here is nearly vertical, as is typical of these fractures. The aperture in the upper part of its exposure is wide, reaching more than one meter. This wide aperture is not due solely to the mechanical opening of a propagating, opening-mode fracture. If it was then the granite would have needed to plastically deform locally to accommodate the strain. The need for continuous plastic strain is particularly evident in the convex-lensoid shape in the foreground. However, this planar fracture formed when the rock was in the brittle-elastic domain, not in a magma chamber. The lensoid shape is due to weathering and erosion, not plasticity.
The kinematic, or maximum mechanical aperture, may well be represented by the thin crack emerging near the lower terminus of the Fissure. The wide aperture that defines most of this Fissure is likely due to a combination of ice wedging and to the action of flowing water flushing away the weathering walls of the Fissure and abrading them with its sediment load.
Two boulders are visible that have apparently fallen into the Fissure. As fracture proppants they are very large. Their long-term movement within the fracture – most likely downwards – might be an indication of the rate of changing aperture.
Other Fissures occur in this section of the park. Such fractures have controlled many of the tall, vertical walls characteristic of the Yosemite Valley. Take a look at the Google map image for the area near Taft Point. The well-developed network of fractures in this area form several sets of parallel fractures. The subject Fissure strikes NE, parallel to one of the dominant trends in the satellite image. Zoom in on the Google image to see the serrated appearance of fractures along the canyon edge near Taft Point.
Many people have the mistaken notion that fractures in granitic rock will show a random pattern, but The Fissures, as well as fractures evident on the satellite image, belie that opinion. They are regularly spaced and show both consistency of strike within sets, as well as consistency of near-vertical dip. Fractures in igneous rock can be complex in places, but generally they are similar in character to those in strata.
The Fissures are highly unusual in that they define a popular destination in a national park. The public comes, seeking them, on a quest to know more about fractures! Fractures can enhance your social standing among friends and relatives. Try livening up the next party you attend by turning the conversation to natural fractures. They may change your life.