Settling rate-- the speed at which a particle will settle to the bottom of a body of water (deposition) There are 3 factors which affect the settling rate of a particle
1) Size– The smaller the particle (clay, silt) the slower it will settle out. Larger sediments (cobbles, boulders) will settle quickly.
2) Shape– Rounder, more spherical particles settle out faster than flat, angular or irregularly shaped particles.Why is this? Friction is decreased when a particle is rounded and therefore it settles faster. It’s like it’s more aerodynamic - like a sports car. But in this case, it’s hydro-dynamic.
Hydrodynamic particles (round) settle faster than flat or irregular shaped particles
Density– If two particles have the same basic size and shape, the denser particle will settle faster.Which will settle faster – a glass marble or a lead marble? (PS – they are the SAME SIZE marble…)
Settling Rate (cm/sec) Time to settle (sec) Settling Rate vs. Settling Time There is an inverse (indirect) relationship between settling rate and settling time. Think about it, sediments that settle at a faster rate (rate increases) require less time to settle (time decreases) Remember what the graph of an inverse relationship looks like…
Horizontal Sorting Final deposition of particles (sediments) usually occurs at the mouth of a stream. This is due to the faster flowing stream emptying into a slower larger body of water. Horizontal sorting takes place. The sediments that were once carried down the stream are arranged from largest to smallest.
Vertical Sorting • Happens in QUIET WATER (still – lake, pond) • Particles sort out from bottom to top in layers • On bottom – largest, roundest, densest particles • On top – smallest, flattest, least dense particles • Can occur when a depositional event, like a landslide, dumps a large volume of unsorted sediments into a still body of water like a lake • It creates “graded beds” of sediments
Vertical Sorting: Graded Bedding One depositional event One depositional event