Compaction and Cementation

The processes by which sediments are changed into rock are complex, but can be simplified into two processes, called compaction and cementation.

Rounded sediment grains (ooliths) bound together with crystalline calcite.

Compaction:

happens when sediments are deeply buried, placing them under pressure because of the weight of overlying layers. This squashes the grains together more tightly.

Cementation:

is where new minerals stick the grains together – just as cement (from a bag) binds sand grains in a bricklayer’s mortar. If you look carefully at the microscope photo, you can see mineral crystals that have grown around the sediment grains and bonded them together.

Common cementing minerals are calcite (CaCO3), silica (SiO2), iron oxides and clay minerals.