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Sedimentary Rocks What is a Rock? • A rock is any naturally formed solid on Earth or in any part of the universe. • Rocks are grouped into three categories: 1. Sedimentary 2. Igneous 3. Metamorphic • Rocks are described by their composition and texture. Sedimentary Rocks • Sedimentary rocks are made from pieces of other rocks and/or organic materials. • Most sedimentary rocks form under large bodies of water. – Horizontal layers • Once deposited, methods of formation include cementation, compaction, chemical action, and organic processes. How do Sedimentary Rocks form? 1. Cementation: occurs when clasts are cemented together. – Calcite, quartz, & hematite are common cements. – Clastic Sedimentary Rocks: mainly composed of solid sediments 2. Compaction: occurs when sediments are squeezed together. 3. Chemical Action: occurs when minerals dissolved in water precipitate out from the water, and form a crystalline mass of intergrown crystals. – “evaporites” Salt Mine in Geneseo, NY Limestone towers at Mono Lake, California 4. Organic Processes: any rock made by living organisms or mostly composed of materials from life forms is an organic rock. – “Bioclastic Sedimentary Rock” Characteristics of Sedimentary Rocks • Composed of fragments (sand, pebbles, etc.) • Contain fossils • Often layered The Grand Canyon is made of layers of sedimentary rocks that are 200 million to nearly 2 billion years old, many containing fossils. Identifying Sedimentary Rocks • Clastic rocks are identified based on particle size. Identifying Sedimentary Rocks • Non-clastic rocks are usually made of one mineral, so they can be identified like minerals. Limestone is made of the mineral calcite Rock Salt is made of the mineral halite Rock Gypsum is made of the mineral gypsum