1. Transportation:
- When rocks break down into smaller fragments (sediment), they are picked up by wind, water, or ice.
- These agents transport the sediment, causing them to collide with each other and with other surfaces.
2. Impact and Collision:
- As sediment moves, it constantly bumps and grinds against other particles and the surrounding environment.
- The harder the sediment and the longer the transport distance, the more pronounced the rounding.
3. Wear and Tear:
- These collisions cause the sharp edges and corners of the sediment to wear away, gradually smoothing them out.
- The constant grinding also reduces the overall size of the grains.
Factors Influencing Roundness:
- Hardness of the sediment: Hard minerals like quartz are more resistant to abrasion and take longer to round. Softer minerals like calcite will round more quickly.
- Distance of transport: Longer transport distances result in more collisions and more rounding.
- Transporting agent: Water is generally a more effective rounding agent than wind or ice.
- Presence of obstacles: Obstacles like rocks or bedrock along the transport path can increase abrasion and accelerate rounding.
Example:
Imagine a piece of angular, jagged rock breaking off a cliff. As it tumbles down a mountain stream, it repeatedly collides with other rocks and the streambed. Over time, the sharp edges are worn away, and the rock becomes smoother and more rounded.
Types of Roundness:
- Subangular: Some sharp edges and corners remain.
- Subrounded: Edges are mostly worn away, but the overall shape is still somewhat angular.
- Rounded: Smooth, well-rounded surfaces with no sharp edges.
The degree of rounding in sedimentary grains can tell us about the history of the sediment, including the distance it has traveled and the environment it was transported in.
