2.Sedimentary Rocks- form by the compaction ofsediments, like gravel, sand, silt or clay
(Figure4.5). Sediments may include fragments of other rocks that have been worn down
into small pieces, materials made by a living organism ororganicmaterials, or chemical
precipitates, which are the solid materials left behind after a liquid evaporates. For exam-
ple, if a glass of salt water is left in the sun, the water will eventually evaporate, but salt
crystals will remain behind as precipitates in the bottom of the glass.
Figure 4.5: This sandstone is an example of a sedimentary rock. It formed when many small
pieces of sand were cemented together to form a rock. ( 12 )
- Metamorphic Rocks- form when an existing rock (of any type) is changed by heat or
pressure within the Earth, so that the minerals undergo some kind of change (Figure4.6).
Figure 4.6: This quartzite is an example of a metamorphic rock. It formed when sandstone
was changed by heat and pressure within the Earth. ( 2 )