Associate Level Material

Sedimentary Rock Worksheet

Use the following table to describe and distinguish between the effects of weathering and erosion. Each response must be at least 200 words.

Effects of Weathering Effects of Erosion What are the Differences?
Mechanical or physical weathering includes two main processes: fracturing and abrasion, which may be intense in wet and dry regions. Wind, rain, snow, ice and other geological events cause mechanical weathering. This type of weathering leads to splitting of rocks and minerals into fragments. Big and sudden changes in air temperature, which are very common in deserts or water temperature changes, always cause expansion or contraction of minerals. For example, when water enters a crack and freezes, it widens the crack and eventually leads to breaking of the surface. Plant roots have a similar effect when they grow so big that they reach the crack in the rock. At some point, the rock will fall apart. This type of weathering changes the mineral composition of rocks by chemical processes and it can sometimes lead to dangerous conditions. Water is the basic agent in chemical weathering because it initiates the whole process. Some of its effects are oxidation (rusting), hydration and carbonization. Chemical weathering also causes loss of chemical elements by solution in water. Caves, stalactites and stalagmites are created by different chemical processes of weathering

Sheet erosion is soil movement from raindrop splash resulting in the breakdown of soil surface structure and surface runoff; it occurs rather uniformly over the slope and may go unnoticed until most of the productive topsoil has been lost. Rill erosion results when surface runoff concentrates forming small yet well-defined channels. These channels are called rills when they are small enough to not interfere with field machinery operations. The same eroded channels are known as gullies when they become a nuisance factor in normal tillage. There are farms in Ontario that are losing large quantities of topsoil and subsoil each year due to fully erosion. Surface runoff, causing gull formation or the enlarging of existing gullies, is usually the result of improper outlet design for local surface and subsurface drainage systems. The soil instability of fully banks, usually associated with seepage of ground water, leads to sloughing and slumping (caving-in) of bank slopes. Such failures usually occur during spring months when the soil water conditions are most conducive to the problem. Weathering can be divided into two categories, physical (also called mechanical) weathering and chemical weathering. Physical weathering includes any process that breaks rocks into fragments. These processes may include breaking the rock by collision with other rocks, fracturing caused by release of pressure as material is removed by erosion from above, growth of ice crystals in fractures, and growth of plants, which extend their roots into fractures and pry the rocks apart. Chemical weathering occurs when water from rainfall, streams, lakes, or oceans, or oxygen from the atmosphere chemically react with the minerals in rocks to produce new minerals that are more stable at the low temperatures and pressures near the Earth’s surface, or in some cases, where the minerals completely dissolve as a result of contact with water. Erosion refers to a group of processes that shape the landscape by wearing down the surface by moving fragments of rocks to lower elevations. The main erosional agents are flowing water, as in rivers and ocean waves, blowing wind, and flowing ice, as in glaciers, all of which are influenced by the force of gravity constantly acting to pull things from higher elevations to lower elevations. They both act together to shape the surface of the Earth into mountains, valleys, plains, and other features that make that surface an interesting and beautiful place to live.

In the following space, explain the role of plate tectonics in the origin of sedimentary rock. Your explanation must be between 250 and 450 words.

Plate tectonics are responsible for many of Earth’s natural occurrences. From volcanoes to earthquakes plate tectonics play are large role. Another natural occurrence that is not is discussed as much as volcanoes and earthquakes is the formation of sedimentary rock. Plate tectonics also plays a major role in the formation of this type of rock. Plate tectonics is a term that is used to describe how the different plates that make up the Earth move and the reactions that occur as they are moving. Before explaining how plate tectonics are related the concept of sediments must be understood.

Sediments are pieces of solid material are deposited on Earth’s surface by way of wind, water, ice, gravity or chemical precipitation. This occurs through weather and erosion. As convergent plates move towards each other, one plate is sub ducted under the other plate and as this is occurring, the plates are moving accumulated sediment and rocks with them. Divergent boundaries also have an effect on sedimentary rock. In divergent movement the plates are moving away from each other and the young sedimentary rock is pushed to the open boundary.
Sedimentary rock actually originates from the disposition of the sediments after they are moved by the movement of plates. The climate controls how sediments are moved and how the rock weather. After weathering and erosion has occurred the sediments stop moving and settle. Where they settle is known as their disposition.

The origination of sedimentary work is a combination of natural processes that result in the disposition of sediment. The way the sediment is actually formed is a process known as Lithification. Lithification is physical and chemical processes which loose sediment is transformed into sedimentary rock. Lithification begins with compaction. The weight of sediments on top of other sediments forces the sediment grains closer together, which results in physical changes.

References

Murck, B. W., Skinner, B. J., & Mackenzie, D. (2010). Visualizing geology (2nd ed.). Hoboken, NJ: Wiley.