How do sediments become sedimentary rock? What processes are involved?

1 Answer
Apr 2, 2016

Sand is eroded from rock, deposited on the ocean floor, and, after millions of years of heat and pressure, forms rock again.


Sediment in the ocean is mostly sand, which floats around as small particles. Eventually the sand is deposited on the ocean floor, and builds up over long periods of time. The process of sedimentation takes millions of years at the least.

When a lot of sediment is deposited, it will put pressure on the sediment below, and this pressure combined with the pressure of the ocean, heat of deep Earth and the amount of time taken will compact it into a harder, denser form, more like rock.

The chemical formulae of sand and many rocks is actually largely the same: #SiO_2#, silicon dioxide or silica. This is because sand comes from erosion of rock into tiny granules, and rock is built up again out of sand in sedimentation. It's a circular process.

A key signature of sedimentary rock is that it looks like it is made up of layers, like slate, because sand is deposited more or less evenly on the ocean floor like a blanket, and so when it is condensed into rock from heat and pressure it retains the layered structure.