Spheroidal Weathering of boulders at the Oriole Walkway
This beautiful walkway is covered with large granite boulders. But why are these large stones all round and why are they lying around here? Come with me on a little geological tour into the world of woolsack weathering
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Tasks:
1. At the listing coordinates (WP1) you are standing in front of a 2 x 3 meter great granite boulder. Explain the typical characteristics of the woolsack weathering you can see.
2. How came this stone on its place?
3. Is all the spheroidal weathering on the boulder uniform all around or not? What do you feel is the reason for what you observed?
4. Optional but definitely funny: What weight might this boulder at WP2 approx. have? (little help: The estimated weight of granite is in the range of 2650 – 3200 kg / m3. As a rough estimate, a typical granite countertop that measures 3 feet by 6 feet and is about 1.25 inches thick can weigh approximately 250 to 300 pounds.
4. Add a photo of you or your GPS close to the given coordinates.
Geological lesson about spheroidal weathering
You may have wondered where did all these boulders came from? Well, they were actually always there! These boulders formed in place by spheroidal weathering, which is a form of chemical weathering in which concentric shells of decayed rock (ranging from a few millimeters to a couple meters, WP1 & WP2 ) are successively loosened and separated from a block of rock, transforming angular blocks into round boulders as you see today.
These boulders are remnants of rock that cooled below the earth's surface. As cooling progressed the rock contracted and cracked. Millions of years later, after uplift brought the rock closer to the surface, warmer temperatures, elevated atmospheric carbon dioxide, and decaying vegetation combined with rainfall to chemically weather the surfaces of the rocks. The degree of weathering was controlled by the ability of these solutions to penetrate rock. As the decomposed material was stripped away, large round boulders were left behind to form boulder strewn hills like here at Oriole Walkway.
On many rocks in this mountainous area we can impressively observe spheroidal weathering.
This is a form of chemical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock that is known as saprolite. When saprolite is exposed by physical erosion, these concentric layers peel (spall) off as concentric shells much like the layers of a peeled onion. Within saprolite, spheroidal weathering often creates rounded boulders, known as corstones or woolsack, of relatively unweathered rock. Spheroidal weathering is also called onion skin weathering, concentric weathering, spherical weathering, or (specially in german speaking countries) woolsack weathering.
Spheroidal weathering is the result of chemical weathering of systematically jointed, massive rocks, including granite, dolerite, basalt and sedimentary rocks such as silicified sandstone. It occurs as the result of the chemical alteration of such rocks along intersecting joints. The chemical alteration of the rocks results in the formation of abundant secondary minerals such as kaolinite, sericite, serpentine, montmorillonite, and chlorite and a corresponding increase in the volume of the altered rock. When the joints within bedrock form a 3-dimensional network, they subdivided it into separated blocks, often in the form of cubes or rectangles, that are bounded by these joints. Because water can penetrate the bedrock along these joints, the near-surface bedrock will be altered by weathering progressively inward along the faces of these blocks. The alteration by weathering of the bedrock will be greatest along the corners of each block, followed by the edges, and finally the faces of the cube.
The differences in weathering rates between the corners, edges, and faces of a bedrock block will result in the formation of spheroidal layers of altered rock that surround an unaltered rounded boulder-size core of relatively unaltered rock known as a corstone or wollsack. Spheroidal weathering has often been incorrectly attributed solely to various types of physical weathering.
Frequently, erosion has removed the layers of altered rock and other saprolite surrounding corestones that were produced by spheroidal weathering. This leaves many corstones as freestanding boulders on the ground's surface. Often the spheroidal weathering, which created this corestones and the enclosing saprolite occured in the prehistoric past during periods of humid, even tropical climates. Frequently, the removal of the saprolite by erosion and exposure of corestones as freestanding residual boulders, tors, or other landforms occurs many thousands of years later and during vastly different climatic conditions.
References:
Ollier, C.D. 1971. Causes of spheroidal weathering. Earth-Science Reviews. Vol. 7, Issue 3
http://en.wikipedia.org/wiki/Spheroidal_weathering