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Weathering and Erosion

Investigation 1 – Concept Day

 

 

 

 

 

 

 

SLIDE W&E-1-1

  • This presentation is designed to help you develop a firm understanding that there is a gradual and continuous change taking place to the physical Earth. You will also learn that slow, gradual processes over time result in very large-scale changes in the Earth’s surface.
  • The small amount of soil and mud dislodged by a single raindrop can, in combination with trillions of other raindrops and hundreds of thousands and millions of years, wear an entire mountain range flat to sea level.

Note: While the main thrust of Investigation 1 is to introduce you to physical and chemical weathering and to model these two processes in the Lab, the next two slides are aimed at giving you an important first glimpse of geologic time. Without a concept of the extreme age of the Earth, the slow processes of erosion and weathering cannot be fully appreciated.

 

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SLIDE W&E-1-2

Note: This is the first of two slides that attempt to impress upon you the extremely long periods of time we must consider when talking about changes to the Earth’s surface by weathering and erosion.

  • This slide represents the nearly 14 billion years since the Big Bang origin of the Universe and the beginning of time itself, as a single calendar year.
  • According to this time scale, the Earth formed on September 10th.
  • Even though you may think of dinosaurs as living a very long time ago, on the timescale presented on this slide, dinosaurs did not even appear until December 26th!
  • According to this calendar, dinosaurs did not become extinct until December 30th, just a single day before humans came onto the scene. In fact, according to this time scale, humans do not occur until late in the evening of the very last day of the year.

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SLIDE W&E-1-3

  • Since the concept of geologic time is so hard to imagine, a second means of visualizing time is shown on this slide.
  • Notice that man does not appear until the most recent Period (circled in red in the slide).
  • Look at the 65 million years ago mark. At this time dinosaurs and many other species became extinct, probably due to a large asteroid impact.
  • While extinct for 65 million years now, dinosaurs inhabited the Earth for 165 million years. According to this diagram, modern humans have been on Earth for less than 2 million years.
  • As we go back further in time in this diagram, less and less detail is visible. Stretching back one or two billion years ago, only privative forms of life were present.
  • Notice that the diagram coils all the way down to 4.5 billion years ago, the approximate age of the Earth.
  • According to the information on the previous slide, this did not occur (the formation of the Earth) until September 10th in the cosmological calendar.
  • These extreme lengths of time are almost impossible for humans to comprehend.
  • The main point for discussing geologic time at this point is so that you can simply understand that there has been plenty of time for even very slow and subtle changes in the Earth’s surface to accumulate. And, with the passage of long periods of time, major changes result from the processes of weathering and erosion.

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SLIDE W&E-1-4

Note: This slide begins our discussion of weathering and erosion directly.

  • This slide begins our discussion of weathering and erosion. It depicts the weathering of a stone statue on the left and the larger results of erosion on the right.

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SLIDE W&E-1-5

  • This slide addresses the differences and similarities between weathering and erosion.
  • In general, one may think in terms of a sequence in which weathering precedes erosion.
  • Read the information on the slide carefully.
  • Through both physical and chemical weathering, rocks are softened and broken into smaller and smaller pieces.
  • Erosion uses wind or torrents of rushing water to move released mineral down the slopes of mountains and hills into rivers.
    • Rivers transport millions of metric tons of weathered materials into the oceans each year (the photo on the right is of the Yangtze River in China. Its current carries incredible amounts of weathered and eroded materials through Shanghai to the East China Sea).
  • The double arrow at the bottom of this slide points out that weathering and erosion impact each other.
    • Take, for example, the steep slope of a section of the mountain. Weathering, both physical and chemical, slowly degrades its surface.
    • Water seeps into small cracks in large boulders and then freezes and expands in the winter. This weakens the rock along with chemical etching on the surface of the rock by carbonic acid (more about carbonic acid below).
    • Eventually, the force of gravity, perhaps with the assistance with a slight Earth tremor, causes an avalanche. Large and small rocks crash downhill as far as their kinetic energy will take them. As a consequence, new rock faces are exposed to the surface for fresh weathering action.
    • The interplay between weathering and erosion will ultimately chisel the mountain slope completely away.

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SLIDE W&E-1-6

Note: We will discuss erosion in greater detail in Investigation 2. We will briefly discuss physical weathering on this slide and then chemical weathering on the next.

  • This slide shows four forms of physical weathering – frost wedging, plants, animals, and gravity.
  • Frost wedging is caused by the freezing of water that has seeped into the crevices of rocks. When water freezes, it expands by about 10%. This force essentially pries the rock apart. When a rock cracks by frost wedging, more rock face is exposed for chemical weathering. Physical weathering can therefore speed the rate of chemical weathering.
  • Plant roots cause weathering of rock by mechanical force. Roots can lift and displace rock just as it can crack and shift sections of the sidewalk. In addition to direct physical weathering by plants, plant roots also secrete weak acids that can contribute to the chemical weathering of rock (more on the next slide).
  • Animals burrow into the soil, exposing new surfaces to weathering and erosion. On the right is a drawing of the mollusk, Penitella penita. Penitella burrows into solid rock on the northwest coast of the United States. Interestingly, as it grows and moves forward, it is prevented from ever backing up as it outgrows the diameter of its escape route. They can only dig further into the rock. Rocks in the coastal tide regions can become infested with these mollusks, their combined borrowing greatly diminishing the strength of the rocks and allows them to be smashed to smaller pieces by ocean waves and surf.
  • Gravity is the ever-present force that pulls down on all loosened pieces and particles of matter on Earth. Gravity acts on both a small-scale (e.g. forcing seepage of water into a crack in a rock) as well as a large-scale (e.g. the force behind falling rocks in a major avalanche).

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SLIDE W&E-1-7

  • All of the mechanisms of physical weathering have one important thing in common, they do not change the chemical composition of the materials they act upon. Rocks are broken into smaller and small pieces, but they are simply smaller pieces of the same compounds.
  • Chemical weathering, however, changes the chemical composition of the materials it acts upon. In nature, a leading agent of chemical weathering is carbonic acid. Carbonic acid is naturally produced by the combination of water and carbon dioxide. Both of these reactants are around in abundant amounts.
  • Chemical weathering by acids, naturally occurring or accelerated by various types of pollutants introduced into the ecosystem by industry, acts by softening and decomposing rock. However, it is often most obvious by looking at its effect on manmade objects made of stone.
  • The top of this slide shows the weathered surfaces of a group of tombstones in Ohio.

Note: You may be familiar with this type of weathering. It is largely caused by the slow action of carbonic acid on limestone, marble, or granite.

  • The pair of photographs at the bottom illustrates the acceleration of chemical weathering as the result of air pollution.
  • Sulfur released into the air as a waste product of various industrial processes can combine with water and form weak sulfuric acid. This, combined with naturally occurring carbonic acid, has caused more decomposition of the statue in 61 years (between 1908 and 1969) than the previous 206 years! This is undoubtedly largely due to pollution associated with the industrial revolution and twentieth-century industry.

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SLIDE W&E-1-8

  • In the Lab for Investigation 1, you will use hydrochloric acid to demonstrate how acid can break down rock.
  • You will use marble chips (also known as Calcium Carbonate) in the experiment.

Note: The concentration of acid you will use in Lab is very much more concentrated than even the worst acid rain. This is because the reaction would be too slow to observe within a lab period using the much lower acid concentrations found in acid rain.

Note: While carbonic acid is produced naturally by the interaction of water and carbon dioxide, hydrochloric acid is not a normal component of the atmosphere or lithosphere (the outer part of the earth, consisting of the crust and upper mantle). Nonetheless, the action of hydrochloric acid is worth considering as the fracking procedure for mining natural gas uses dilute hydrochloric acid and could, therefore, be an issue in the future.

 

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SLIDE W&E-1-9

  • You must wear lab coats, goggles, and gloves when working with hydrochloric acid.