Science 7: Unit E: Planet Earth Topic 3 - Erosion

How are Sedimentary Rocks Formed? Sedimentary rocks are formed from the compaction and cementation of sediments into layers called strata. Sedimentary rocks are formed from the compaction and cementation of sediments into layers called strata. So the question remains: what are sediments and how are they formed? So the question remains: what are sediments and how are they formed?

How are sediments formed? Sediments are tiny particles of rocks. They are formed from other rocks and recycled into sedimentary rocks. Sediments are tiny particles of rocks. They are formed from other rocks and recycled into sedimentary rocks. It takes three processes to go from one type of rock (eg. obsidian) and into a sediment strata being compacted. These processes are: I. weathering, II. Erosion, and III. Deposition. It takes three processes to go from one type of rock (eg. obsidian) and into a sediment strata being compacted. These processes are: I. weathering, II. Erosion, and III. Deposition.

I. Weathering The wearing away of bits of rock from the larger rock. There are three types of weathering: The wearing away of bits of rock from the larger rock. There are three types of weathering: I. Mechanical: the physical breaking up of rock particles through wind, water, ice, or other objects pushing pulling, and splitting rocks apart. I. Mechanical: the physical breaking up of rock particles through wind, water, ice, or other objects pushing pulling, and splitting rocks apart. An example of mechanical weathering is frost wedging where water freezes and expands in a crack in the rock and splits the rock further. An example of mechanical weathering is frost wedging where water freezes and expands in a crack in the rock and splits the rock further.

Weathering Cont’d II. Chemical: Chemical reactions breaking down minerals and small particles of rocks. II. Chemical: Chemical reactions breaking down minerals and small particles of rocks. The chemicals could be water dissolving minerals, acid rain and pollution reacting with the rock. The chemicals could be water dissolving minerals, acid rain and pollution reacting with the rock. Note that chemical weathering doesn’t just affect rocks/mountains; old structures such as the Collosseum are wearing away because of air pollution. Note that chemical weathering doesn’t just affect rocks/mountains; old structures such as the Collosseum are wearing away because of air pollution.

Weathering Cont’d III. Biological: The physical or chemical breakdown of rocks and rock particles caused by living organisms. III. Biological: The physical or chemical breakdown of rocks and rock particles caused by living organisms. Physical biological weathering includes tree roots splitting apart a rock or breaking up a sidewalk. Physical biological weathering includes tree roots splitting apart a rock or breaking up a sidewalk. Chemical biological weathering includes acids from living organisms wearing away the rock. Chemical biological weathering includes acids from living organisms wearing away the rock.

II. Erosion Erosion: the movement of the weathered rock particles from the source rock to another area. Erosion: the movement of the weathered rock particles from the source rock to another area. The two most common sources of erosion are wind and moving water. The two most common sources of erosion are wind and moving water. Wind blowing rock particles against another rock is a form of erosion and mechanical weathering. This process is called abrasion. Wind blowing rock particles against another rock is a form of erosion and mechanical weathering. This process is called abrasion.

III. Deposition Deposition: The piling up of eroded rock particles. These piles over time will form a stratum that when pressurized over time with other strata will form a sedimentary rock. Deposition: The piling up of eroded rock particles. These piles over time will form a stratum that when pressurized over time with other strata will form a sedimentary rock.

Glaciers are Agents of Erosion Glaciers advance and retreat very slowly and pick up large rocks and carry them hundreds of kilometers before depositing them. These rocks are called erratics. Glaciers advance and retreat very slowly and pick up large rocks and carry them hundreds of kilometers before depositing them. These rocks are called erratics. As they move, glaciers push and pile soil sediments on either side forming ridges. These ridges will eventually form into large sedimentary rocks called moraines. As they move, glaciers push and pile soil sediments on either side forming ridges. These ridges will eventually form into large sedimentary rocks called moraines. Gravity (falling rocks), floods and human activities are other causes of erosion. Gravity (falling rocks), floods and human activities are other causes of erosion.