Stones arrive on beaches primarily through the process of erosion, transportation, and deposition by natural forces.
Here's a breakdown of how this happens:
Erosion
- Weathering: Over time, larger rocks on land are broken down into smaller pieces through weathering. This can involve:
- Physical weathering: Actions like freeze-thaw cycles (water freezing and expanding in cracks) and the abrasive force of wind and rain.
- Chemical weathering: Chemical reactions that dissolve or weaken the rock.
- Attrition: As rocks break down, they collide with each other, further reducing their size and smoothing their edges. This process is called attrition. This tumbling action is crucial in creating the rounded pebbles often found on beaches.
Transportation
- Rivers and Streams: Rivers play a major role in transporting eroded rock fragments (sediment) from inland areas to the coast. The force of the water carries the stones downstream.
- Glaciers: In some regions, glaciers can transport large amounts of rock and sediment, depositing it along the coast as the glacier melts.
- Ocean Currents and Tides: Once the stones reach the coast, ocean currents and tides distribute them along the shoreline. These currents can also carry smaller rock fragments further along the coast.
Deposition
- Wave Action: Waves deposit the transported stones onto the beach.
- Longshore Drift: The movement of sediment along the coastline by wave action and currents (longshore drift) can concentrate stones in certain areas, forming beaches.
- Gravity: Gravity plays a role, particularly with larger rocks, as they are pulled down slopes toward the sea.
In summary, stones on the beach are the result of a continuous cycle of erosion, transportation, and deposition. Larger rocks break down into smaller pieces, are carried to the coast by rivers or other forces, and are then shaped and deposited on the beach by wave action and currents. The "attrition" process is key to creating the smooth, rounded stones common to beaches.
