Spheroidal forms commonly develop through natural processes, primarily spheroidal weathering, a type of chemical weathering that sculpts rocks into rounded, onion-like shapes.
Understanding Spheroidal Weathering
One of the most common and significant natural mechanisms for the development of spheroidal forms, especially in geological landscapes, is spheroidal weathering. This fascinating process is a direct outcome of chemical weathering acting upon specific types of rock formations.
The Mechanism of Chemical Alteration
Spheroidal weathering begins with the chemical alteration of systematically jointed, massive rocks. These rocks possess a network of intersecting cracks or fractures known as joints.
- Initial State: Rocks are typically fractured into cuboidal or angular blocks by an existing system of joints.
- Water Infiltration: Water, often carrying dissolved acids (like carbonic acid from atmospheric CO2), penetrates these joints.
- Chemical Reactions: The water reacts with the minerals within the rock, particularly along the surfaces of these joints. This chemical alteration, which might involve processes like hydrolysis, oxidation, or dissolution, preferentially attacks the corners and edges of the rock blocks. This is because corners and edges expose a greater surface area to the chemical agents compared to flat faces.
- Progressive Rounding: As weathering proceeds, the corners and edges are rounded off more rapidly than the flatter surfaces. This leads to the gradual transformation of angular blocks into more spherical or ellipsoidal forms.
- Concentric Layers: Over time, the chemical weathering penetrates deeper into the rock. The altered material, often softer and more susceptible to erosion, tends to spall off in concentric layers, resembling the layers of an onion. This process is often referred to as exfoliation or onion-skin weathering, specifically in the context of spheroidal weathering.
Rocks Susceptible to Spheroidal Weathering
This process is commonly observed in various rock types due to their mineral composition and jointing patterns. Spheroidal weathering is the result of chemical alteration along intersecting joints in systematically jointed, massive rocks, including:
- Igneous Rocks:
- Granite: Rich in feldspars and quartz, susceptible to hydrolysis.
- Dolerite: A fine-grained intrusive igneous rock, often jointed.
- Basalt: A common extrusive igneous rock, frequently exhibiting columnar jointing, which can evolve into spheroidal shapes.
- Sedimentary Rocks:
- Silicified Sandstone: Sandstone where silica has filled pore spaces, making it harder but still prone to jointing.
The common thread among these rocks is their massive structure and the presence of systematic jointing, which provides the pathways for chemical attack.
Factors Influencing Development
Several factors contribute to the effectiveness and speed of spheroidal form development:
Factor | Description | Impact on Spheroidal Weathering |
---|---|---|
Rock Type & Composition | The mineralogy of the rock and its susceptibility to chemical alteration. | Rocks with easily weatherable minerals (e.g., feldspars in granite) are more prone. |
Presence of Joints | The density and pattern of pre-existing fractures in the rock. | Provides entry points for water; denser jointing can lead to smaller, more numerous spheres. |
Climate (Water) | Availability of moisture, precipitation levels, and temperature. | Humid climates with abundant rainfall accelerate chemical reactions. |
Vegetation | Plant roots can widen joints, and decaying organic matter can produce humic acids, enhancing weathering. | Indirectly contributes by creating more entry points and slightly acidic environments. |
Time | The duration over which weathering processes can act on the rock. | Longer exposure leads to more pronounced spheroidal shapes and deeper penetration of weathering. |
Examples in Nature
Spheroidal forms are a common sight in many landscapes. You can observe them in:
- Granite outcrops: Often seen as rounded boulders dotting granite batholiths.
- Basalt flows: Sometimes exhibit "pillow lava" formations, though these are typically formed underwater, subsequent weathering can enhance spheroidal shapes on land.
- Road cuts and quarries: Where fresh rock surfaces are exposed, revealing the concentric layers of weathering.
These geological features offer valuable insights into long-term geological processes shaping our planet. For further reading on geological processes, resources like the U.S. Geological Survey (USGS) provide extensive information.
The formation of spheroidal forms is a testament to the persistent and transformative power of Earth's natural weathering processes.