PURGATORY CHASM
by Caitie Sams and Ali Quirk
Weathering
Mechanical Weathering
Mechanical weathering is defined as any type of weathering which alters the size and shape of the rock or will lead to the formation of cracks. This is also known as rock disintegration.
1. Root Wedging: This is a type of mechanical weathering which occurs when tree roots which grow into a rock begin to pry the rock apart.
2. Sheeting, Joints, and Exfoliation: This is a type of weathering which occurs when pressure is removed from a rock by removing overlying material. This results in the rock expanding and creating cracks known as joints. We saw many examples of joints at Purgatory Chasm. When the cracks are parallel to the surface, layers of rock will form, resulting in sheeting. Lastly, when sheets of the rock fall off, it is called exfoliation, and it usually occurs over a large area.
3. Frost/Ice Wedging: This type of mechanical weathering occurs when water gets into a crack in a rock. When water freezes, it expands by 9%, so as the water in the crack freezes, it forces the crack apart which eventually shatters the rock.
Here is an example of root wedging in action. As you can see, the root is beginning to force the rock into two.
Frost wedging caused the rock on the ground to crack and separate from the rock above. As a result, it fell to the ground as a boulder.
Here you can clearly see the joints that are present in this rock. This indicates that at one point, there was a lot of pressure on it that has since been taken off.
Looking closely at this image, you can see the texture of this rock. This has been caused by wind abrasion because of its pitted surface.
Fat Man's Misery is another excellent example of ice wedging. This large break in the rock was caused by a crack which broke apart due to ice action.
4. Abrasion: This is the last type of mechanical weathering that we witnessed at the chasm. This occurs when water or wind pick up grains of sand which then are pelted at the rock, resulting in the rock becoming polished. Because water usually leaves it round and smooth, while wind causes the rock to be pitted.
Chemical Weathering
Chemical weathering is weathering which results in the alteration of the composition of a rock due to chemical reactions. This is also known as rock decomposition.
1. Oxidation: This process occurs when oxygen dissolved in water reacts with a metal. This usually results in color changes as seen in the rust on these rocks.
2. Organic Acids: This is the most common type of weathering seen at purgatory chasm. This occurs when lichen or moss grow on a rock and in the process produce acids which eat away at the surface of the rocks.
3. Spheroidal weathering: This type of weathering occurs in a rock that already has cracks. When water and salt infiltrate these cracks, they eat away at the corners of the rock resulting in the corners of the cracks being rounded, rather than sharp.
In these two images, the moss that is growing on the rocks is dissolving the surface.
The rock to the right is a fantastic example of spheroidal weathering. The cracks that were in this rock were eaten away by water and salt which left rounded cracks.
