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Sedimentary Rocks •Rocks that form by the compaction and cementation of sediment that transform sediment into solid rock are sedimentary. Sedimentary 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. Lithification – the process of turning 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 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 rocks Clastic 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 sedimentary (sometimes called precipitates or 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 Clues • Sedimentary rocks can be found in layers. • Example sandstones. • They are sometimes soft. • example gypsum. Identification clues Continued • They may contain fossils. • Example – limestone • They look like a broken rock stuck together. • Example conglomerate