Which statement about sediment particle settling is correct?

Which statement about sediment particle settling is correct?

• A. Larger particles settle out first
• B. Smaller particles settle out first
• C. Particles settle at the same rate
• D. No particles settle out

Answer: (A)

Larger particles settle out first because settling speed increases with particle size (and density difference between the particle and the fluid). In still water, gravity pulls each particle downward while drag from the water resists that motion. For similar densities and shapes, bigger grains experience a greater downward force relative to the resistance and thus move downward faster. Short-lived, coarser grains reach the bottom sooner, creating a bottom layer before finer grains do. That's why a mix of sediments tends to separate into layers: coarse material settles quickly, followed by medium, then fine particles. Smaller particles don’t settle as fast because the drag they experience quickly balances their weight, so they stay suspended longer. The idea that all particles settle at the same rate or that no particles settle out doesn’t match what happens in still water with different grain sizes.

Larger particles settle out first because settling speed increases with particle size (and density difference between the particle and the fluid). In still water, gravity pulls each particle downward while drag from the water resists that motion. For similar densities and shapes, bigger grains experience a greater downward force relative to the resistance and thus move downward faster. Short-lived, coarser grains reach the bottom sooner, creating a bottom layer before finer grains do. That's why a mix of sediments tends to separate into layers: coarse material settles quickly, followed by medium, then fine particles.

Smaller particles don’t settle as fast because the drag they experience quickly balances their weight, so they stay suspended longer. The idea that all particles settle at the same rate or that no particles settle out doesn’t match what happens in still water with different grain sizes.