NC Earth Science Essential Standards

Transcription

1 NC Earth Science Essential Standards EEn. 2.1 Explain how processes and forces affect the Lithosphere. EEn Explain how the rock cycle, plate tectonics, volcanoes, and earthquakes impact the Lithosphere. EEn Predict the locations of volcanoes, earthquakes, and faults based on information contained in a variety of maps. EEn Explain how natural actions such as weathering, erosion (wind, water, and gravity), and soil formation affect Earth s surface. EEn Explain the probability of and preparation for geohazards such as landslides, avalanches, earthquakes, and volcanoes in a particular area based on available data. [National Science Content Standards:] UPC.1, UPC.2, UCP.3, UPC.5, UPC.5; A.1, A.2; B.2, B.3, B.4; C.6; D.2; F.1, F.3, F.5, F.6;

2 Reading Assignment: Read ; pages: Objective: Vocabulary: - Explain how sedimentary rocks are formed through weathering, erosion, deposition, and lithification; - Know the three types of sedimentary rocks classes and be able to describe how each of them are formed. - Recognize the importance of sedimentary rocks. Sediment Clastic Deposition Erosion Lithification Cementation Compaction Bedding Stratification Graded bedding Cross-bedding Reverse grading Fossils Concretions Ripple marks Mud cracks Porosity evaporite Clastic sedimentary rocks Organic sedimentary rocks

3 Sedimentary Rocks

4 Sedimentary Rocks Much of Earth s surface is covered with sediments pieces of solid material deposited on Earth s surface by wind, water, ice, gravity, or chemical precipitation Sediments are formed through weathering (breakdown of rocks) When sediments become cemented together, they form sedimentary rocks (forming layers)

5 Weathering physical and chemical processes that break rock into smaller pieces. Chemical weathering minerals in a rock are dissolved or chemically changed (the rock changes) Physical weathering minerals are not chemically changed, rock fragments simply break off the solid rock (rock does not change) Weathering produces rocks and mineral fragments known as clastic sediments. The word clastic means broken.

6 Erosion and Transport Erosion is the removal and movement of surface material (sediments) from one location to another location. Four Main Agents: Water, Wind, Gravity, and Glaciers Particles transported by water, almost always move downward; wind can sometimes carry fine sand and dust to higher elevations, but will eventually be pulled down by gravity; gravity is also responsible for glacier movement, which erodes materials as it travels downhill Streams and rivers carry the small bits towards the sea (continually wearing them down as the they progress).

7 Deposition Occurs when sediments are laid down on the ground or sink to the bottoms of bodies of water. Sediments are deposited when transport stops Sediments drop by size: largest sediments will drop first and the smallest will drop last. Faster moving water can transport large particles but as the water slows down, the largest particles settle out first, then the next-largest, and so-on; Such sorted deposits are characteristic of sediment transported by wind and water.

8 Unsorted Disposition Glaciers move all materials with equal ease; large boulders, sand, and mud are all carried along by the glacier ice and dumped in an unsorted pile at the end of the glacier. Land slides and mud slides create similar deposits when sediment moves downhill in a jumbled mess.

9 Burial and Lithification Most sediments are ultimately deposited on Earth in depressions called sedimentary basins, which can be more than 8 km thick. As more and more sediment is deposited in a basin, the bottom layer are subjected to increasing pressure and temperature resulting in lithification. Lithification is the physical or chemical process that transforms sediments into sedimentary rocks. The word Lithify comes from the Greek word lithos, which means stone.

10 Lithification Lithification begins with compaction. The weight of overlying sediments forces the sediment grains closer together, squeezing out, up to 40% of the water content. Earth s crust temperature increases with depth (30ºC - Km) Sediments that are buried 3-4 km deep experience temperatures that are high enough to start chemical and mineral changes that cause cementation. Sand, however, has irregular grain shapes which prevents compaction, resulting in more open spaces between the gains.

11 Lithification Sand, however, is usually well compacted during deposition, and resists additional compaction during burial. Grain-to-grain contacts in sand form a supporting framework that helps maintain open spaces between the gains. Groundwater, oil, and natural gas are commonly found in these spaces in sedimentary rocks. Oil shale

12 Lithification Cemention occurs when mineral growth cements sediment grains together into solid rock. 1 st type occurs when a new mineral, such as calcite (CaCO 3 ) or iron oxide (Fe 2 O 3 ) grows between the sediment grains as dissolved minerals precipitate out of ground water. 2 nd type occurs when existing mineral grains grow larger as more of the same mineral precipitates from ground water and crystallizes around them. From pile of sand to solid rock

13 Features of Sedimentary Rock The primary feature of sedimentary rocks is horizontal layering, called bedding. Bedding can range from a mm thick layer of shale to sandstone deposits meters thick. Types of bedding depends upon the method of transport, which the size of the grains and the material within the bedding depend upon many factors. Bedding in which the particle sizes become progressively heavier and coarser towards the bottom layers is called graded bedding.

14 Graded bedding is often observed in marine sedimentary rocks that were deposited by underwater landslides; as the sliding material slowly came to rest underwater, the largest and heaviest material settled out first and was followed by progressively finer material.

15 Cross-bedding Another characteristic feature of sedimentary rocks is cross-bedding, which are formed as inclined layers of sediment mover forward across a horizontal surface.

16 Ripple Marks Small sedimentary features, such as ripple marks form when sediment is moved into small ridges by wind or wave action, or by a river current. Back-and-forth movement of waves create ripples that are symmetrical, while a current flowing in one direction, such as a river or stream, produces asymmetrical ripples. If a rippled surface is buried gently by more sediment, it may be preserved in solid rock.

17 Evidence of Past Life (Fossils) The best-known features of sedimentary rock are fossils, which are preserved remains, impressions, or other evidence of once-living organisms. As sediments piled up, plant and animal remains were buried. Hard parts of these remains may be preserved in the rock. More often, even the hard parts dissolve and leave only impressions in the rock. Brachiopods (lampshells) Trilobites (3-lobed) Crinoids (feather stars or sea lilies)

18 Types of Sedimentary Rocks Classification of sedimentary rocks is based on how they are formed. There are three main groups of sedimentary rocks: clastic, chemical, and organic. Clastic sedimentary rocks are further classified according to the sizes of their particles: coarse, medium and fine grained (see Table 6-2, pg 128).

19 Coarse-Grained Clastics Gravel-sized and mineral fragments are classified as coarse-grained clastics. There are two types of coarse grained clastics: conglomerates and breccias. Conglomerates are composed of rounded fragments that range from 2mm to boulders, where as, breccia s fragments are angular and have sharp corners. Conglomerates provide evidence of water transport. Conglomerate Breccia

20 Medium-Grained Clastics Sand-sized grains and mineral fragments are classified as medium-size clastic. When these sediments are buried and lithified, sandstone is formed, which may show ripple marks or cross bedding patterns, providing evidence of an old stream and river channels.

21 Medium-Grained Clastics Another important features of sandstone is its relative high porosity, which is the percentage of open spaces between its grains in a rock. The incomplete cementation of mineral grains can result in porosities as high as 30%; when pore spaces are connected to one another, fluids can move through sandstone. This feature makes sandstone layers valuable as underground reservoirs of oil, natural gas, and groundwater.

22 Fine-Grained Clastics Sedimentary rocks consisting of silt and mud are called siltstone and mudstone. Siltstone is mostly composed of silt-sized grains, while, shale is composed mostly of silt and clay-sized particles, and breaks along thin layers. Unlike sandstone, these fine-grained sedimentary rocks have very low porosity, and hinders the movement of ground-water and oil. Siltstone Mudstone Shale

23 Chemical Sedimentary Rocks During chemical weathering minerals can be dissolved and carried into lakes and oceans, as the water evaporates, the dissolved minerals are left behind. In arid regions, high evaporation rates can increase the amount of dissolved minerals in bodies of water and as the water evaporates chemical sedimentary rocks are formed.

24 Rocks Formed from Evaporation When the concentration of dissolved minerals in a body of water reaches saturation, crystal grains precipitate out of solution and settle to the bottom, which are called evaporites. Evaporites most commonly form in arid regions, in oceans and in drainage basins on continents that have low water flow. As more dissolved minerals are carried into the basin, evaporation continues to remove freshwater and maintain high mineral concentrations. Overtime, thick layers of evaporite minerals can accumulate on the basin floor.

25 Rocks Formed from Evaporation The three most common evaporite minerals are: calcite (CaCO 3 ), halite (NaCl), and gypsum (CaSO 4 ). Layers of these minerals are often mined for their chemical content. Calcite Halite

26 Organic Sedimentary Rocks Organic sedimentary rocks are formed from the remains of once-living things. The most abundant organic sedimentary rock is limestone, which is composed primarily of calcite. Some organisms that live in the ocean use the dissolved calcium carbonate to make their shells. When these organisms die, their shells settle to the bottom of the ocean and can form thick layers of carbonate sediment. During burial and lithification, calcium carbonate precipitates out of water, crystallizes between the grains of carbonate sediment and forms limestone.

27 Organic Sedimentary Rocks Limestone is common in shallow water environments such as those in the Bahamas, where coral reefs thrive in m of water just offshore. Many types of limestone contain evidence of their biologic origin in the form of abundant fossils, ranging form large corals to microscopic unicellular organisms.

28 Organic Sedimentary Rocks Another type of organic sedimentary rock, coal, forms from the remains of plant material. Over long periods of time, thick layers of vegetation slowly accumulate in swamps and coastal areas. When these layers are buried and compressed, (before they decay), are slowly lithified into coal, which is mostly of carbon, which can be burned for fuel to heat homes and or heated to generate electricity.

29 Importance of Sedimentary Rocks All the characteristic features of sedimentary rocks, such as cross-bedding, ripple marks, layering, and fossils provide a geologic snap-shot of surface conditions in Earth s past. Other features indicate the location and direction of the flow of ancient rivers, wave and wind action over lakes and deserts, and possible ancient shore lines. Rock fragments found in conglomerates and breccias are large enough to identify what types of bedrock they eroded from. By considering all this information, geologists can reconstruct the nature of Earth s surface at various times in the past, and better understand how geologic changes occur over time.

30 Energy Resources Many of the natural resources used by humans come from sedimentary rocks. For example oil, natural gas, and coal are found in porous sedimentary rocks like sandstone. Uranium, used for nuclear power, is often mined from sandstone. Large deposits of phosphate, used for fertilizer, and to make steel are found in sedimentary rocks. Limestone is used to make cement, and both sandstone and limestone are used for walls and buildings in the construction industry.

31 Great Job!