Lesson 2: Sedimentary Rocks PDF

## Lesson 2: Sedimentary Rocks ### Focus Question: How do rocks become sediment? ### Weathering and Erosion - Weathering breaks down rocks into smaller pieces due to physical and chemical processes - Sediments are these smaller pieces and are transported by water, wind, glaciers, and gravity. - Th...

## Lesson 2: Sedimentary Rocks ### Focus Question: How do rocks become sediment? ### Weathering and Erosion - Weathering breaks down rocks into smaller pieces due to physical and chemical processes - Sediments are these smaller pieces and are transported by water, wind, glaciers, and gravity. - The process of sediments becoming glued together forms sedimentary rocks. ### Weathering - Produces rock and mineral fragments (sediments) ranging from boulders to microscopic particles. - Chemical weathering changes the minerals within the rock by dissolving them or changing their chemical composition. - During weathering, resistant minerals remain chemically unchanged. - Physical weathering breaks the minerals off of the solid rock along fractures or grain boundaries, without changing the minerals. ### Erosion - The removal and transport of sediment is called erosion. - The four agents of erosion are wind, moving water, gravity, and glaciers. - Erosion can be observed when wind carries soil away, water in streams becomes muddy after a storm, or glaciers drag sediments with them. - Eroded material is almost always carried downhill. ### Deposition - Occurs when transported sediments are deposited on the ground or sink to the bottom of a body of water. - Deposition occurs when transport stops, and the sediments settle out. - The largest grains settle out first, forming layers of sediment, with the smallest grains at the top. - Fast-moving water can transport larger particles than slow-moving water. - Wind can only move small grains. - Glaciers can move all material with equal ease. ### Lithification - Most sediments are deposited on Earth in low areas, which are subjected to increased pressure and temperature. - These conditions transform sediments into sedimentary rocks. - The physical and chemical processes of sediment transformation are called lithification. - Lithification begins with *compaction*, in which the weight of overlying sediments forces the sediment grains closer together, squeezing out excess water. - *Cementation* glues the sediment grains together into solid rock when mineral growth occurs between the grains. ### Sedimentary Features - Sedimentary rocks contain information about the history of their formation. - They help geologists interpret how they formed. - Key features include bedding, graded bedding, cross-bedding, and ripple marks. ### Bedding - The most common feature of sedimentary rocks. - Consists of horizontal layers of sediment. - Can range in thickness from a few millimeters to several meters. ### Graded Bedding - The particle sizes become progressively finer and lighter toward the top layers. - Often observed in marine sedimentary rocks. - The largest and heaviest material settles out first, followed by progressively finer material. ### Cross Bedding - Form when layers of sediment are deposited at an incline across a horizontal surface. - Occurs in sandy beaches, sandbars in streams and rivers, and migrating sand dunes. ### Ripple Marks - Form when sediment is moved into small ridges by wind or wave action. - Symmetrical ripple marks are formed by waves pushing sand into symmetrical ridges, where grain size is evenly distributed. - Asymmetrical ripple marks are formed when a current flows in one direction. ### Angular vs Rounded - Individual pieces, when a rock breaks apart, are initially **angular**. - As sediment is transported, they are rounded when they knock into each other. - Rounding is influenced by the time the sediment is transported and how far it travels. ### Clastic Sedimentary Rocks - The most common sedimentary rock. - Formed from loose sediments that accumulate on Earth's surface. - Classified according to the sizes of their particles. #### Coarse-grained Rocks - Consist of gravel-sized rock and mineral fragments. - Include conglomerates, which have rounded, gravel-sized particles. - Are transported by high-energy flows of water, such as mountain streams, flooding rivers, ocean waves and glacial meltwater. #### Fine-grained Rocks - Consist of silt and clay-sized particles. - Form in swamps, ponds, and deep oceans where waters are still or slow-moving. - Tend to be more layered than coarse-grained rocks. ### Chemical and Biochemical Sedimentary Rocks - Formed by the processes of evaporation and precipitation of minerals. - During weathering, dissolved minerals are carried into lakes and oceans. - When water evaporates, the dissolved minerals are left behind. #### Chemical Sedimentary Rocks - Form when the concentration of dissolved minerals in a body of water reaches saturation. - Crystals precipitate out of solution and settle to the bottom. - Mainly found in arid regions, drainage basins on continents that have low water flow, and coastal settings. #### Biochemical Sedimentary Rocks - Formed from the remains of once-living organisms. - The most abundant are limestone, which is composed primarily of calcite. - Shells of marine organisms settle to the bottom of the ocean and form layers of carbonate sediment. - During burial and lithification, calcium carbonate precipitates out of the water, forming limestone. ### Table 2: Classification of Sedimentary Rocks | Classification | Texture/Grain Size | Composition | Rock Name | |---|---|---|---| | Clastic | Coarse (> 2 mm) | Fragments of any rock type -quartz, rounded chert and quartzite common | Conglomerate | | | Medium (1/16 mm to 2 mm) | Quartz and rock fragments | Breccia | | | Fine (1/256 mm-1/16 mm) | Quartz, potassium feldspar and rock fragments | Arkose | | | Very fine (< 1/256 mm) | Quartz and clay | Siltstone | | | | Quartz and clay | Shale | | | | Calcite (CaCO3) | Sandstone | | | Microcrystalline with conchoidal fracture | Quarts (SiO2) | Micrite | | | Abundant fossils in micrite matrix | Calcite (CaCO3) | Chert | | | | | Fossiliferous Limestone | | | Shells and shell fragments, loosely cemented | Calcite (CaCO3) | Coquina | | Biochemical | Microscopic shells and clay | | Chalk | | | Variously sized fragments, ooids | Highly altered plant remains, some plant fossils | Coal | | | | Calcite (CaCO3) | Oolitic Limestone | | | Fine to coarsely crystalline | Calcite (CaCO3) | Crystalline Limestone | | Chemical | Fine to coarsely crystalline | Dolomite (Ca,Mg)CO3 | Dolostone | | | Very finely crystalline | Quartz (SiO2) -light colored; dark colored | Chert; Flint | | | | Calcite (CaCO3) | Micrite | | | Fine to coarsely crystalline | Gypsum (CaSO4 2H2O) | Rock Gypsum | | | | Halite (NaCl) | Rock Salt | ### Medium-grained Rocks - Consist of sand-sized sediments, such as sandstone. - Can contain several features of interest, such as ripple marks and cross-bedding, which indicate the direction of current flow. - Contain relatively high porosity, which makes sandstone layers valuable as underground reservoirs of oil, natural gas, and groundwater.