Onion skins and rock tombstones

This photo shows "onion skin" weathering along joints in lava.Courtesy Jad D'Allura

Rock seems quite solid, doesn't it? Many features, however, such as joints, faults and random fractures act as conduits for root growth, slow percolation of water and weathering far below the earth's surface. Such weathering not only produces soil for plants but some fantastic shapes, such as spheroidal "onion skin" or tombstone boulders.

Drop a sugar cube in water. You'll notice its corners dissolve faster than the edges and the edges dissolve faster than the faces. That's because there's a greater surface area exposed at the corners than the faces. The same phenomenon occurs where water percolates along fractures in rock.

Because this percolation is relatively slow, water remains in contact with the rock for a long time, allowing it to dissolve or alter portions of the rock. As water and other active chemicals within it react with the rock, they weaken it from the outside, causing the rock to crumble until it creates a spherical or oblong shape with rounded corners.

This phenomenon, spheroidal weathering, often produces shapes that appear to poke up from the ground like emergent tombstones or "pod-people" (think: "Invasion of the Body Snatchers"), but that's only because the softer material erodes faster, leaving the solid core intact.

Spheroidal shapes are quite typical of granitic rock, which weathers along joint sets produced as the granite cooled and contracted. The same phenomenon can occur along joints or fractures in sandstone or other hard rock. Spheroidal shapes can be seen in granite along the Mount Ashland access road and in coarse sandstone along the same road near a dirt road that dives toward Colestin Valley.

A closely related phenomenon is known as "onion-skin" weathering. It's commonly associated with spheroidally weathered boulders, but instead of creating smooth boulders it produces thin "shells" that break off parallel to the rounded shapes. These shells appear similar to successive layers of an onion. "Onion skin" rocks may look like huge ball bearings lying one atop another before they fall like disembodied eyeballs off road cuts or cliffs.

Water can percolate a small distance into the solid rock, causing some minerals to chemically alter to clay. Clay absorbs water and expands, causing concentric layers of rock to break loose and fall off, kind of like exfoliating rock skin. Close inspection of the layers will reveal some soft discolored clay and orange-brown iron hydroxide stains in crude concentric layers deposited by the water, the "smoking guns" of chemical weathering.

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net.


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