Sedimentary RocksSedimentary Rocks•Rocks that form by the compaction and cementation of sediment that transform sediment into solid rock are sedimentary.

Sedimentary RocksSedimentary Rocks• Geologists estimate that sedimentary

rocks account for only 5% (by volume) of the Earth’s outer 16 km but they make up 75% of the rocks exposed as outcrops at the surface of the Earth.

LithificationLithification – the process of turning – the process of turning loose sediment into stone.loose sediment into stone.

1. Compaction – as sediments accumulate through time, the weight of the overlying material compresses the deeper sediments until they harden.

2. Cementation – cementing material through time precipitate on to the sediment, filling the spaces and joining the particles, Calcite, silica and iron oxide are the most common cements.

Sedimentary rocksSedimentary rocks• There are two classes of sedimentary

rocks.1. Clastic (detrital) – formed by the lithification

of sediment formed by weathering, erosion and mass wasting. Example - sandstone

2. Chemical sedimentary rocks – rocks that formed when soluble

material dissolved in lakes or seas precipitates out and hardens. Example - gypsum

Classification of Sedimentary rocksClassification of Sedimentary rocksClastic rocksClastic rocks

• Clastic sedimentary rocks are classified by their grains size.

• Siltstone – gritty between the teeth• Sandstone – feels like sand.• Conglomerate –rounded thumbnail size

and larger • Breccia – angular thumbnail size and

larger.

Chemical sedimentaryChemical sedimentary((sometimes called precipitates or sometimes called precipitates or

evaporites.)evaporites.)• These rocks have a crystalline

structure. Same shape, same color, same size crystals. (Note – These are not interlocking crystals like igneous rocks.)

• They can be called biochemical if they have organic material in it.

Identification CluesIdentification Clues• Sedimentary

rocks can be found in layers.

• Example - sandstones.

• They are sometimes soft.

• example - gypsum.

Identification clues ContinuedIdentification clues Continued• They may contain

fossils.• Example –

limestone

• They look like a broken rock stuck together.

• Example - conglomerate