Mountains are the result of numerous natural processes that took place over millions of years. Scientists have been researching these processes, out of which weathering seems to be the main reason behind the reformation and deformation of rocks and mountains. You must have heard about exfoliating your skin, but have you heard about exfoliation dome? What is an exfoliation dome? How is exfoliation dome formed? Do the weathering process produces granite exfoliation domes? Let’s begin and find the answers.
1. What is Physical Weathering?
Rocks and minerals are broken down by various mechanical processes without bringing any change in their chemical composition. The process of transforming large rock masses into small pieces is termed physical weathering. These smaller pieces are easier to be transported and get easily eroded. However, physical weathering involves wear and tear of rocks due to various reasons like stress, weakness because of biological and chemical activities, etc. (Read What is Carbonation Weathering?)
2. What are the Processes of Physical Weathering?
Physical weathering is carried out by a number of processes taking place in the rocks depending on their composition, location, and age. Here is a brief description of each of these processes:
- Biological Activities: Microorganisms, trees, and other small living organisms that take shelter in the cracks of these rocks are major reasons for influencing chemical and physical weathering. Acid or other chemical released by them into these cracks promotes chemical weathering and further growing micro cracks. Additionally, the expanding roots of the trees also assist in the opening and expansion of fractures and cracks.
- Exfoliation: Large flat or curved rock sheets are fractured and detached because of the pressure released on them. This process of physical weathering is termed exfoliation. As the exfoliation removes the overburden of the rocks, they become free to expand. Excessive expansion results in the development of fractures and cracks along their sheet joints that are parallel to the erosion surface. Is it better to learn about exfoliation before moving toward how is exfoliation dome formed?
- Hydraulic Action: The process where the air inside the fracture is compressed by the wave that closes the entrance of the fracture which further widens the fracture from within is termed hydraulic action. This is mostly seen on the rocky shores.
- Mineral Crystallization: When the salt inside the cracks of the rocks crystallizes, it expands. The force applied due to this is strong enough to split the rock.
- Natural Zones of Weakness: Huge rocks also have some points where they are naturally weak. In sedimentary rocks, bedding planes are the natural point of weakness. The rocks expand and erode due to these weak points which further enhance the physical and chemical weathering.
- Spherical Weathering: This is the process of splitting off and cracking the curved layers on a smaller scale. This takes place mostly on the spherical boulder. It takes place when a rectangular block of rock gets weathered from 3 sides at its corners and from 2 sides along its edges. Due to its layer-by-layer weathering, it is also known as onion skin weathering.
- Thermal Expansion and Contraction: In a desert, temperature plays an essential role in weathering. Cycles of hot days and cold nights contribute to weathering as different minerals have distinct coefficients of thermal expansion.
3. What are the Characteristics of Exfoliation Joints?
You are close to finding out how is exfoliation dome formed. The cracks and fractures appearing in the rocks are not in random places, and they follow a particular pattern. Therefore, exfoliation is not just any wear and tear process of rocks but possesses some considerable characteristics. Here I have mentioned them in the order they are observed:
- Cracks always follow the topography.
- Rocks are divided into sub-planar slabs.
- There is an increase of about a few centimeters to a few meters in the joint spacing.
- This spacing develops a depth of about 100 meters and not more than that.
- The corners of the landscapes are rounded because the deeper the joint the larger its curvature.
- The point of fractures is malleable (flexible to form shapes).
- The host rock is not completely joined and possesses high compressive strength along with being isotropic.
- The upward curvatures can be convex or concave.
- Exfoliation is related and often leads to secondary compressive forms like buckling, buckled slabs (A-Tents), and arching.
4. What is an Exfoliation Dome?
With continuous changes in temperature and pressure exerted on the rocks, they keep on expanding. The end product of this expansion is large and smooth, these domes are known as exfoliation domes. To know how is exfoliation dome formed, check out the next segment. (See What causes Breaking Down of Rocks into Fragments?)
5. How is Exfoliation Dome formed?
This expansion forces the rocks to move upward along a pre-existing bedding place or fracture. The reason behind the exerted force or pressure can be the activity of tectonic plates or erosion. When the pressure exceeds, the rock breaks leaving behind a dome-shaped feature known as an exfoliation dome. (See How Long do Rocks live?)
6. Which Weathering Process produces Granite Exfoliation Domes?
Process related to physical weathering is responsible for producing granite exfoliation domes. Shells or concentric slabs of the large rocks begin to lose their onion-like layers that are sub-parallel to the exterior through the process of sheet joining, exfoliation, or fractures. With further expansion, the granite layers are exposed to water pressure, freeze and thaw cycles, and functioning vegetation. These large granite sheets shave off the sharp edges, resulting in fine smooth granite domes. (See 3 Agents of Metamorphism)
7. What are Some Examples of Granite Domes?
You must have heard the name of the Ryan Mountains, and they are known for their exfoliation domes because they are the remnants of the molten rocks of the Mesozoic age. Erosion and uplift of the mountains exposed these rocks in the Little San Bernardino Mountains. Other examples of granite domes are as follows:
- Enchanted Rock in Texas
- Cannon Mountain in the White Mountains of New Hampshire
- Royal Arch in the Yosemite National Park
- Half Dome in Yosemite
8. What are Exfoliated Tors?
Even though the process of how is exfoliation dome formed and tor formation is the same, still they are different from each other. The term tor was derived from an old Welsh word twr or twrr which means heap or cluster. Geomorphologists refer to these rocks as kopje or castle koppie. This rock outcrop is the result of the unloading and expansion of rocks. Exfoliated tors are of varied sizes ranging from small to huge. Some examples of the exfoliated tor are as follows:
- Great Tor, Bamford
- Chee Tor, Buxton.
- Dove stone Tor, Derwent Edge (505m)
- Back Tor, Derwent Edge (538m)
- Carl Wark, Hathersage Moor.
9. What is a Rock Monument?
A diabase formation of rock appearing in the sea bed is a rare natural monument termed a rock monument. Stein Erne Rose is one such rock monument that was formed on the sea bed from the out-flowing of lava during the Devonian age. The pillow-like bodies on the internal of the monument are formed as a result of the cooling of lava. This rare monument is in the village of Saalburg-Ebersdorf, Saale-Oria-Kreis district, of the central German state of Thuringia.
So, today you got to know what is an exfoliation dome. You also learned about the process of how is exfoliation dome formed. Did you notice the weathering process produces granite exfoliation domes? Must see What are the Different Layers of Rocks called?