Objectives

1. Formation of Sedimentary Rocks Objectives • Sequence the formation of sedimentary rocks. • Explain the formation and classification of clastic sediments. • Describe features of sedimentary rocks. Vocabulary • sediment • clastic • deposition • lithification • cementation • bedding • graded bedding • cross-bedding

2. Formation of Sedimentary Rocks Formation of Sedimentary Rocks • Much of Earth’s surface is covered with sediments. • Sediments are pieces of solid material that have been deposited on Earth’s surface by wind, water, ice, gravity, or chemical precipitation. • When sediments become cemented together, they form sedimentary rocks. • The formation of sedimentary rocks begins when weathering and erosion produce and move sediments.

3. Formation of Sedimentary Rocks Weathering • Wherever Earth’s crust is exposed at the surface it is subject to weathering. • Weathering is a set of physical and chemical processes that break rock into smaller pieces. • Chemical weathering occurs when the minerals in a rock are dissolved or otherwise chemically changed. • Minerals remain chemically unchanged during physical weathering.

4. Types of Weathering Physical Weathering(Four Types) – Processes in which rock is broken into smaller fragments by physical means. The chemical composition of the weathered rock remains the same as the parent material. • Abrasion • Frost Action (Frost/Ice Wedging) • Root Pry • Exfoliation

5. Physical Weathering: Abrasion Abrasion – • Occurs when objects rub against, or collide, with one another • Rounding and a reduction in sediment size are the results of abrasion

6. Physical Weathering: Frost Action Frost Action – • Occurs when water repeatedly freezes, expands, and thaws in the fractures of rock • This is one of the most common forms of physical weathering.

7. Physical Weathering: Frost Action

8. Physical Weathering: Root Pry Root Pry – • Occurs when the roots of plants begin to grow into the fractures of rock • The pressure created as the root grows wedges the rock apart

9. Physical Weathering: Root Pry

10. Physical Weathering: Exfoliation Exfoliation – Occurs in two manners: • occurs when rocks that have been formed under conditions of great pressure are brought to the surface. The “sudden” change in pressure causes the surface of the rock to expand very quickly fracturing the rock. Gravity and erosion then can cause the rock to be removed. • Occurs when there are large shifts in the daily temperature. The surface of a rock will heat and cool much more quickly than the interior the repeated, rapid expansion and contraction cycle cause fractures to form in the surface layer of the rock.

11. Physical Weathering: Exfoliation

12. Types of Weathering Chemical Weathering(Three Types) – Processes in which the chemical composition of the rock material is changed by chemical means. All of these processes involve, either directly or indirectly, the presence of water. • Oxidation • Hydrolysis • Acidification of Water/Acid Precipitation

13. Chemical Weathering: Oxidation Oxidation – Reaction of minerals with atmospheric oxygen

14. Chemical Weathering: Hydrolysis Hydrolysis – Reaction of minerals with water to for new minerals • Water typically acts only as a solvent is chemical reactions, not as a reactant. However there are conditions under which water will act as a reactant and hydrolysis takes place.

15. Chemical Weathering: Hydrolysis

16. Chemical Weathering: Acidification of Water/ Acid Precipitation • Acidification of Water – Reaction of atmospheric gases with atmospheric water; primarily by carbon dioxide • Acid Precipitation – Same as above except it is considered to be a form of pollution

17. Chemical Weathering: Hydrolysis and Acidic Solutions

18. Factors Affecting the Rate of Weathering • Climatic Factors • Characteristics of the Parent Rock • Surface Area • Slope • Vegetation

19. Rate of Weathering: Climatic Factors • Temperature – The higher the average temperature the faster the rate of chemical weathering • Precipitation – The greater the amount of precipitation the faster the rate of chemical weathering

20. Rate of Weathering: Rock Characteristics • Hardness/Resistance to Abrasion – Some rock types are more susceptible to physical weathering due to the method of rock formation. In general sedimentary rocks are less resistant to physical weathering than are metamorphic or igneous rocks • Chemical Composition – The chemical composition of rocks also can affect the type and rate of chemical weathering. For example carbonates are susceptible to water that has been acidified, whereas silicate based rocks are not generally susceptible to this form of weathering.

21. Rate of Weathering: Rock Characteristics

22. Rate of Weathering: Surface Area Since all chemical reactions take place at the surface of materials, any increase in surface area increases the rate of chemical weathering.

23. Rate of Weathering: Slope Slope: Since soils and therefore vegetation has a more difficult time remaining on a steep slope; the steeper the slope the greater the rate of chemical and physical weathering. This is due to the fact that as sediment is produced on a steep slope it is continuously being removed by erosion to a lower elevation. This process continuously exposes new parent material to the elements.

24. Rate of Weathering: Vegetation Vegetation: • The presence of vegetation can both increase and decrease the rate of weathering. • Since the presence of vegetation inhibits the flow of water across the surface of a slope this tends to increase the rate of deposition of sediments (or from another perspective it decreases the rate of erosion). This in turn exposes less of the parent material to the elements thus slowing physical weathering. • However, as vegetation undergoes the decay process it naturally releases acids which can affect an increase in the rate of chemical weathering.

25. Rate of Weathering: Vegetation

26. Formation of Sedimentary Rocks Weathering

27. Formation of Sedimentary Rocks Weathering • Clastic describes rock and mineral fragments produced by weathering and erosion. • Clastic sediments range in size from huge boulders to microscopic particles.

28. Formation of Sedimentary Rocks Erosion and Transport • After rock fragments have been weathered out of outcrops, they are transported to new locations. • Erosion is the removal and movement of surface materials from one location to another. • The four main agents of erosion are wind, moving water, gravity, and glaciers. • All agents of erosion move sediments in the down hill direction due to the pull of gravity. Wind is an exception. It has the ability to move sediments up slope

29. Formation of Sedimentary Rocks Erosion and Transport Deposition • 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. • As water or wind slows down, the largest particles settle out first, then the next-largest, and so on, so that different-sized particles are sorted into layers. • Since wind can move only small grains, sand dunes are commonly made of fine, well-sorted sand. • Sediment deposits from glaciers and landslides are not sorted because both move all materials with equal ease.

30. Formation of Sedimentary Rocks Erosion and Transport Burial • Most sediments are ultimately deposited on Earth in depressions called sedimentary basins. • These basins may contain layers of sediment that together are more than 8 km thick. • As more and more sediment is deposited in an area, the bottom layers are subjected to increasing pressure and temperature which causes lithification. • Lithification includes the physical and chemical processes that transform sediments into sedimentary rocks.

31. Formation of Sedimentary Rocks Lithification • Lithification begins as the weight of overlying sediments forces the sediment grains closer together, causing the physical changes. • Layers of mud shrink as excess water is squeezed out.

32. Formation of Sedimentary Rocks Lithification • Sand resists additional compaction during burial. • Grain-to-grain contacts in sand form a supporting framework that helps maintain open spaces between the grains.

33. Formation of Sedimentary Rocks Lithification • The temperature in Earth’s crust increases with depth by about 30°C per kilometer. • Sediments that are buried 3 to 4 km deep experience temperatures that are high enough to start the chemical and mineral changes that cause cementation. • Cementation occurs when mineral growth cements sediment grains together into solid rock.

34. Formation of Sedimentary Rocks Lithification • There are two common types of cementation. • A new mineral, such as calcite (CaCO3) or iron oxide (Fe2O3) grows between sediment grains as dissolved minerals precipitate out of groundwater. • Existing mineral grains grow larger as more of the same mineral precipitates from groundwater and crystallizes around them.

35. Formation of Sedimentary Rocks Features of Sedimentary Rocks • Bedding, or horizontal layering, is the primary feature of sedimentary rocks. • The type of bedding depends upon the method of transport. • The size of the grains and the material within the bedding depend upon many factors.

38. Formation of Sedimentary Rocks Features of Sedimentary Rocks • Graded bedding is bedding in which the particle sizes become progressively heavier and coarser towards the bottom layers. • Graded bedding is often observed in marine sedimentary rocks that were deposited by underwater landslides.

41. Formation of Sedimentary Rocks Features of Sedimentary Rocks • Cross-bedding is formed as inclined layers of sediment move forward across a horizontal surface.

44. Formation of Sedimentary Rocks Features of Sedimentary Rocks • Small-scale cross-bedding can be observed at sandy beaches and along sandbars in streams and rivers. • Most large-scale cross-bedding is formed by migrating sand dunes. • Small sedimentary features such as ripple marks are also preserved in sedimentary rocks. • If a rippled surface is buried gently by more sediment without being disturbed, it might later be preserved in solid rock.

49. Formation of Sedimentary Rocks Features of Sedimentary Rocks Evidence of Past Life • Fossils are probably the best-known features of sedimentary rocks. • Fossils are the preserved remains, impressions, or any other evidence of once-living organisms. • Fossils are of great interest to Earth scientists because fossils provide evidence of the types of organisms that lived in the distant past, the environments that existed in the past, and how organisms have changed over time.