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Weathering, Erosion & Deposition

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1 Weathering, Erosion & Deposition
6th Grade Earth Science S6E5

2 Weathering vs. Erosion Weathering is the process by which rocks and minerals are gradually broken down. Erosion is the movement of rocks and soil by ice, water, wind, or gravity. Weathering can be classified in two ways: Mechanical and Chemical

3 Mechanical Weathering
Mechanical weathering breaks rock into smaller pieces. These smaller pieces are just like the bigger rock; they are just smaller! The rock has broken without changing its composition. The smaller pieces have the same minerals in the same proportions. You could use the expression "a chip off the old block" to describe mechanical weathering! The main agents of mechanical weathering are water, ice, and wind.

4 The above diagram illustrates ice wedging
The above diagram illustrates ice wedging. This is how ice wedging works. When liquid water changes into solid ice, it increases in volume. You see this when you fill an ice cube tray with water and put it in the freezer. The ice cubes go to a higher level in the tray than the water. You also may have seen this if you put a can of soda into the freezer so that it cools down quickly. If you leave the can in the freezer too long, the liquid expands so much that it bends or pops the can.

5 Mechanical Weathering (continued)
Abrasion is another type of mechanical weathering. With abrasion, one rock bumps against another rock. Gravity causes abrasion as a rock tumbles down a slope. 

6 Plants and Animals in Mechanical Weathering
A plant's roots grow into a crack in rock. As the roots grow larger, they wedge open the crack. Burrowing animals can also cause weathering. By digging for food or creating a hole to live in the animal may break apart rock. Today, human beings do a lot of mechanical weathering whenever we dig or blast into rock. This is common when we build homes, roads, and subways, or quarry stone for construction or other uses.

7 Mechanical Weathering vs.Chemical Weathering
Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases. With more surfaces exposed, there are more places for chemical weathering to occur.

8 Make the connection… Let's say you wanted to make some hot chocolate on a cold day. It would be hard to get a big chunk of chocolate to dissolve in your milk or hot water. Maybe you could make hot chocolate from some smaller pieces like chocolate chips, but it is much easier to add a powder to your milk. This is because the smaller the pieces are, the more surface area they have. Smaller pieces dissolve more easily.

9 Chemical Weathering Chemical weathering is different than mechanical weathering. The minerals in the rock change. The rock changes composition and becomes a different type of rock. Chemical weathering is important. It starts the process of changing solid rock into soil. We need soil to grow food and create other materials we need. Chemical weathering works through chemical reactions that change the rock.

10 Chemical Weathering agents
There are many agents of chemical weathering. Remember that water was a main agent of mechanical weathering. Well, water is also an agent of chemical weathering. That makes it a double agent! Carbon dioxide and oxygen are also agents of chemical weathering. 

11 Weathering Happens at Different Rates
Each type of rock weathers in its own way. Certain types of rock are very resistant to weathering. Igneous rocks tend to weather slowly because they are hard. Water cannot easily penetrate them. Granite is a very stable igneous rock. Other types of rock are easily weathered because they dissolve easily in weak acids. Limestone is a sedimentary rock that dissolves easily. When softer rocks wear away, the more resistant rocks form ridges or hills.

12 Let’s review what we know about weathering

13 Remember… Weathering is the process by which rocks and minerals are gradually broken down. Erosion is the movement of rocks and soil by ice, water, wind, or gravity. Weathering can be classified in two ways: Mechanical and Chemical Let’s talk about erosion.

14 Erosion Erosion happens when a bit of earth material is separated from the surface. Once detached, a force like moving water or wind transports the material to a new location where it is deposited.

15 Erosion is responsible for some of the earth's most remarkable natural features, like the Grand Canyon. The term "erosion" comes from "erodere," a Latin verb meaning "to gnaw." Erosion is what usually happens to the material loosened by weathering. Plant roots help to hold soil togethr. When plants and vegetation are not present, erosion can be more dramatic. Erosion can create sandbars, floodplains, and river deltas. Four main processes have an eroding effect: gravity wind water ice

16 Gravity Gravity is a force that will move material, once it is broken down by weathering, downhill. Gravity is always working on all things on Earth. Gravity pulls materials toward the center of the Earth making rocks fall from mountain tops and sand to settle to the bottom of oceans. It causes mass movement, any one of several processes that move sediment downhill. Mass movement can be rapid or it can be slow. There are different types of mass movement. landslides mudslides slump creep

17 Wind Wind is an effective agent in causing erosion in dry, arid climates like deserts. The main way that wind causes erosion is by deflation. Deflation is the process by which wind removes surface materials. Wind blows over the land and picks up the smallest particles of sediment. Clay and silt make up this sediment. Stronger winds can pick up larger particles. Some materials that are somewhat heavy might gain lift from strong winds but will appear to almost bounce or skip along. Deflation can move the topsoil in an area leaving only stones and rocky materials.

18 Water

19 Summary of Water Erosion
Splash Erosion The force of falling irrigation or rainwater displaces soil particles. Sheet Erosion Impermeable surfaces, compacted soil, or bare soil lets water run across it, washing away disturbed surface particles. Rill Erosion Sheet erosion wears down soil to establish a definite path, forming rivulets in the soil referred to as rills. Rill erosion is much more visible to humans than splash or sheet erosion. Gully Erosion Over time, rills widen and deepen into a gully, accelerating the effects of erosion by creating more and more surface area susceptible to disturbance. Bank Erosion Fast water flows (often caused by influx of stormwater from impermeable surfaces) wear away stream sides at an accelerated pace, often causing bank failure.

20 Ice Ice erosion is the process of large chunks of ice, known as glaciers, eroding an area over a long period of time with the help of gravity. Ice erosion is caused by movement of ice, typically as glaciers. Glaciers can scrape and break up rock and then transport it, leaving moraines, drumlins, and glacial erratics in its wake typically at the terminus or during glacial retreat. Ice wedging is the weathering process where water trapped in tiny rock cracks freezes and expands, causing the breakup of the rock. This can lead to gravity erosion on steep slopes. The scree which form at the bottom of a steep mountainside is mostly formed from pieces of rock broken away by this means. It is a common engineering problem wherever rock cliffs are alongside roads and morning thaws can drop hazardous rock pieces onto the road.

21 Deposition The erosional transport of material through the landscape is rarely continuous. Instead, we find that particles may undergo repeated cycles of entrainment, transport, and deposition. The process of erosion stops when the transported particles fall out of the transporting medium and settle on a surface. This is called deposition.

22 Deposition can also be caused by particle precipitation and flocculation. Both of these processes are active only in water. Precipitation is a process where dissolved ions become solid because of changes in the temperature or chemistry of the water. Flocculation is a chemical process where salt causes the aggregation of minute clay particles into larger masses that are too heavy to remain suspended.

23 Soil Four Major Components of Soil
Once you step out onto a piece of ground, you step out onto something that is alive. Soil is not just a piece of dirt. Soil is made up of living and nonliving material spread as a very thin layer over the entire surface of the planet we call earth. Soil must provide nutrients, water, and air and helps to support the plant.

24 Soil Composition Another material is called organic matter. It is made up of decaying plant and animal matter.

25 Soil Types Sand is the largest particle in the soil. When you rub it, it feels rough. This is because it has sharp edges. Sand doesn't hold many nutrients. Silt is a soil particle whose size is between sand and clay. Silt feels smooth and powdery. When wet it feels smooth but not sticky. Clay is the smallest of particles. Clay is smooth when dry and sticky when wet. Soils high in clay content are called heavy soils. Clay also can hold a lot of nutrients, but doesn't let air and water through it well.

26 Soil Erosion and Humans
Soil erosion is a natural process. It becomes a problem when human activity causes it to occur much faster than under natural conditions

27 CAUSES OF SOIL EROSION Wind and water are the main agents of soil erosion. The amount of soil they can carry away is influenced by two related factors: speed - the faster either moves, the more soil it can erode; plant cover - plants protect the soil and in their absence wind and water can do much more damage.

28 POLITICS, ECONOMICS AND SOIL EROSION
To understand soil erosion we must be aware of the political and economic factors affecting land users. In South Africa apartheid policies ensured that 42% of the people lived on 13 % of the land (the "homelands"). This overcrowding has resulted in severe erosion. As the land became increasingly degraded and thus less productive, subsistence farmers were forced to further overuse the land. The intensive agriculture and overgrazing that followed caused greater degradation. Soil erosion can be seen as both a symptom of underdevelopment (i.e. poverty, inequality and exploitation), and as a cause of underdevelopment. A reduced ability to produce, invest one's profit and increase productivity, contributes to increasing poverty, and can lead to desertification, drought, floods, and famine. On commercial farm lands, overstocking, mono-cropping, and the plowing of marginal lands unsuitable for cultivation has led to soil erosion and desertification. Frequently these practices have been unwittingly encouraged by the state offering subsidies which made it profitable to exploit the land in the short-term.

29 PREVENTING SOIL EROSION
Political and economic changes need to address the distribution of land as well as the possibility of incentives to encourage farmers to manage their land sustainably. Aspects of technical changes include: the use of contour plowing and wind breaks; leaving unplowed grass strips between ploughed land; making sure that there are always plants growing on the soil, and that the soil is rich in humus (decaying plant and animal remains). This organic matter is the "glue" that binds the soil particles together and plays an important part in preventing erosion; avoiding overgrazing and the over-use of crop lands; allowing indigenous plants to grow along the river banks instead of plowing and planting crops right up to the water's edge; encouraging biological diversity by planting several different types of plants together; conservation of wetlands


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