Weathering and Erosion
Investigation 2 – Concept Day
Weathering and Erosion
- During this Investigation, you will be introduced to two major forms of erosion on Earth, water and wind erosion.
Note: Just as in Concepts for Investigation 1, there is not a tremendous amount of theoretical background for this Investigation, so we will take the opportunity to introduce a concept related to weathering and erosion. Just as weathering and erosion cause slow, steady changes of the Earth’s surface, plate tectonics and continental drift also cause major changes to occur over geologic time. The next three slides will be devoted to the forces and mechanism by which continents move and change positions over time.
- This slide depicts the three major regions of the Earth’s surface and interior: the crust, mantle, and core.
Note: Depending on the source, there are more divisions than the three major ones shown here. For example, the core may be divided into an inner core and an outer core. Both are thought to be composed of mainly iron with some nickel. While the outer core is clearly liquid, there are apparently some solid-like properties associated with the inner core, perhaps due to the extreme pressures present at this great depth.
- the mantle is a very hot semi-solid, but as shown in the slide, has enough liquid-like properties that it can flow.
- this ability of the mantle to flow is very important in the establishment of convection currents, which, in turn, is directly related to continental drift and plate tectonics.
Note: Continental drift and plate tectonics will be the topic of the next slide.
- The crust is the cooled, solid layer that floats on the more liquid mantle. In terms of relative thickness, the crust is somewhat comparable to the shell on a hard-boiled egg! That is, it is an extremely minor component of the entire planet, a very thin skin.
- The crust is not only present on the continents, but on the seafloor as well.
- This slide shows a diagram of the convection currents in the mantle that lead to the deposit of new material to the crust on the ocean floor.
- The convection current cycle is caused by the hot mantle semi-solid molting rock rising to the surface due to its decreased density on account of its extreme heat.
- As the hotter mantle material reaches near the crust, it cools somewhat and becomes denser and sinks. Hotter material again rises toward the crust, cools somewhat, and sinks again, thus perpetuating and sustaining the convection current cycle.
- As new crust material, derived from the upward-moving mantle, is added, it forces older crust away from the site of the deposit area. This, in turn, pushes the entire plate of crust away.
- The moving crust will then interact with other plates that are in motion from other sites like the one depicted in this slide. As a result, entire continents slowly and steadily move across the surface of the Earth.
- The ultimate result of such continental drift is shown over geologic time on the next slide.
- This slide shows the result, over millions of years, of the constant motion of the plates of crust.
- the 415 mya (million years ago) date during the Paleozoic Era corresponds roughly to the time that land animals appeared on Earth.
- the 200 mya date at the beginning of the Mesozoic Era is when early dinosaurs appeared on Earth.
- the modern Earth is depicted by the globe in the front.
- The Earth the dinosaurs inhabited was much different than today’s Earth.
- For example, 200 million years ago animals could have migrated by land from North America, South America, Africa, and what is now Europe.
- Convection currents in the mantle and continental drift are still very active processes, so we can be sure that the Earth, as we know it today, will not always look as it does now.
Note: This is a simple introductory slide for the discussion of wind and water erosion. You should now be aware that as the continents move in plates across the planet, the solid rock surface of the crust is constantly being reshaped by the forces of physical and chemical weathering and wind and water erosion.
- When plates of moving crust come into contact with each other, the material at their impact sites can be pushed up to form high mountain ranges.
- This is how the Rocky Mountains and Himalayan Mountains and many other ranges were formed.
- The combined action of weathering and erosion then acts to slowly break down entire mountain ranges, eventually lowering them to sea level.
- The amount of time required for this to happen is enormous.
- Nonetheless, mountain chains have been continually created and worn down as the slow pace of geologic time passes.
- It is truly difficult to conceive of the lengths of time involved in these natural processes.
- Direct your attention to the coastline in the photograph on the right.
- This picture was taken at Cannon Beach on the northern Oregon coast.
- The landmass once extended out beyond the rocks to the right.
- Wave action, which has tremendous kinetic energy, constantly hammers at the coastal rocks and, in combination with both biological and chemical weathering, crumbles once enormous cliffs into sand.
- Wind erosion occurs as the result of solar energy being transformed into the kinetic energy of wind.
- Strong enough winds have enough kinetic energy to carry small bits of debris and sand.
- These moving particles can act like sandpaper to grind away the surface of rocks.
- The material thus removed from rock surfaces may then be carried great distances by the wind or be washed downhill and into streams and rivers by rain.
- The photograph on the right was taken during the Dust Bowl in Oklahoma in the 1930s.
- In this case, the billows of dust are actually composed of valuable top-soil needed for the farming of crops.
- Thousands of farms were entirely stripped of their top-soil layer, creating infertile fields of dry clay.
- The situation was devastating to the farmers of the region.
Note: The American author, John Steinbeck’s Pulitzer Prize-winning novel The Grapes of Wrath (1939), recounts the struggle of the Joad family who emigrated from the Dust Bowl to California because their farm was destroyed.
Water erosion begins with a single raindrop.
Note: Each raindrop disrupts the soil it hits as it transfers its kinetic energy of motion to the Earth’s surface.
- With heavy rains, the soil is unable to soak up a sufficient volume of water; the excess water follows gravity and the contour of the land.
- It can form rills in fields that eventually merge into streams and rivers.
- The kinetic energy of the moving water carries soil and other small particles with it.
- The area of land around a system of streams and rivers is called a watershed.
- The soil from the surrounding watershed is constantly being moved from its source by water erosion.
- Under natural conditions, soil production by the weathering of rock and soil deposition from wind to the watershed act to replace the soil lost by erosion for long periods of time. However, if the land is modified sufficiently, for agricultural or urban uses or even by deforestation by wildfires, the runoff of soil by erosion can easily overcome the recruitment of new soil, thus throwing off the natural balance of the watershed.
- There are three factors that influence the amount of erosion as shown on this slide. Explain them.
- Rainfall intensity and runoff:
- Heavy bursts of hard rainfall have more of an impact on soil erosion than a light rainfall.
- Depending on the surface, light of medium amounts of rain will percolate down into the soil and cause little erosion.
- Heavy rains, however, can saturation soil quickly, causing puddles that will begin to follow gravity and initiate erosion.
- Slope Gradient:
- The steeper a surface or field, the more likely water erosion will occur.
- This is largely due to the fact that water moves very quickly down the side of a steep hill compared to the flat land of a gradual slope.
- This does two things. The rapidly moving water doesn’t have time to seep or percolate into the soil and its higher amount of kinetic energy acts to carry more soil faster and further away.
- Plants in general and grasses in particular act to hold soil in place by entwining it with their root systems.
- Plants also absorb some of the kinetic energy of raindrops with their leaves, branches, and stems.
- They also absorb some of the water into their roots, stems, and leaves as well.
- This final slide shows the experimental setup for Investigation 2 Lab.
- Read the slide carefully.
Note: Using a paint tray and ring stand support, you will be able to manipulate the rate of “rainfall”, the slope of the soil surface, and also model the effect of vegetation on soil erosion by water.