If the accumulation of sediments continues over a long period, as it usually does in the ocean, great thicknesses of material may be placed on top of the original layers. Burial beneath thousands of feet of other sediments is what begins to turn sediments into rock. The weight of overlying layers squeezes particles together into the tightest arrangement possible; under extreme conditions, it may crush some or all of the grains. As the water that originally filled the pore spaces between particles is squeezed out, the volume occupied by the sediments is reduced. This process is called compaction. Different clastic materials pack differently.

A dune sand composed entirely of rounded grains of similar size may be compacted very little. In a poorly sorted sand, however, smaller particles can be rearranged to fill the spaces between the larger grains, thus reducing the pore space. Plate-like clay particles, however, become aligned with one another when compacted, fitting together like bricks with very little space between. Some clays become compacted to less than half their original volume under the pressure of deep burial.

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