Q1.
Have you noticed fine sand collecting on riverbeds or in lakes?
Answer
Yes. That fine sand is formed when rock pebbles, soil and sediments are broken down and moved from one place to another by natural forces like wind and flowing water. This process is called erosion.
Big rock → broken into pebbles and soil (weathering)
Pebbles and soil carried away by wind and flowing water (erosion)
Speed of water or wind falls in a lake or an ocean → material settles down
Sediments harden over thousands of years → new rocks
Pebbles and soil carried away by wind and flowing water (erosion)
Speed of water or wind falls in a lake or an ocean → material settles down
Sediments harden over thousands of years → new rocks
Why it happens: Fast-moving water and wind can carry loose material along with them. When they slow down — as they do where a river enters a lake or the sea — they can no longer hold the load, so the material settles at the bottom. On the way, the pebbles knock against one another and against the riverbed, which is why river pebbles look smooth and rounded.
Tip: Erosion, such as during a landslide, is a physical change — the rock is only broken and moved, not turned into a new substance. Most of these changes take thousands of years and cannot be reversed.