• Inform students that this presentation is designed to help them develop a firm understanding that there is a gradual and continuous change taking place to the physical Earth. They will also learn that slow, gradual processes over time result in very large-scale changes in the Earth’s surface.

• Explain to students that 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 a mountain range flat to sea level.

Note: While the main thrust of Investigation 1 is to introduce physical and chemical weathering and to model these two processes in the Lab, the next two slides are aimed at giving students a 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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Note: This is the first of two slides that attempt to impress upon students the extremely long periods of time we must consider when talking about changes to the Earth’s surface by weathering and erosion.

• Inform students that 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.

• Explain to students that, according to this time scale, the Earth formed on September 10^th.

• Show students that, even though they 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 26^th!

• Show students that, according to this calendar, dinosaurs did not become extinct until December 30^th, 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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• Explain to students that since the concept of geologic time is so abstract, a second means of visualizing time is shown on this slide.

• Show students that man does not appear until the most recent Period (circled in red in the slide).

• Point to the 65 million years ago mark. Explain that it is at this time that dinosaurs and many other species became extinct, probably due to a large asteroid impact.

• Share with students that, while extinct for 65 million years now, dinosaurs inhabited the Earth for 165 million years. According to this diagram, modern man has been on Earth for less than 2 million years.

• Explain that 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.

• Show students that the diagram coils all the way down to 4.5 billion years ago, the approximate age of the Earth.

• Explain that, according the information on the previous slide, this did not occur (the formation of the Earth) until September 10^th in the cosmological calendar.

• Explain to students that these extreme lengths of time are almost impossible for humans to comprehend.

• Tell students that the main point for discussing geologic time at this juncture is so that they can simply understand that there has been plenty of time for even very 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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Note: This slide begins our discussion of weathering and erosion directly.

• Explain to students that this slide begins our formal 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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• Inform students that this slide addresses the differences and similarities between weathering and erosion.

• Tell students that, in general, one may think in terms of a sequence in which weathering precedes erosion.

• Read the slide to the students.

• Explain that through both physical and chemical weathering, rocks are softened and broken into smaller and smaller pieces.

• Explain that erosion uses wind or torrents of rushing water to move released mineral debris down the slopes of mountains and hills into rivers.

• Explain that 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).

• Explain that 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 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.

• Explain that the interplay between weathering and erosion will ultimately chisel the mountain slope completely away.

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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.

• Inform students that this slide shows four of the forms of physical weathering – frost wedging, plants, animals, and gravity.

• Explain that 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.

• Explain that 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).

• Explain that 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 allowing them to be smashed to smaller pieces by ocean waves and surf.

• Finally, explain that 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 rock) as well as a large-scale (e.g. the force behind falling rocks in a major avalanche).

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• Tell students that 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.

• Explain that chemical weathering 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.

• Explain that 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.

• Tell students that the top of this slide shows the weathered surfaces of a group of tombstones in Ohio.

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

• Tell students that the pair of photographs at the bottom illustrates the acceleration of chemical weathering as the result of air pollution.

• Explain that 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 the twentieth century industry.

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• Tell students that, in the Lab for Investigation 1, they will use hydrochloric acid to demonstrate how acid can break down rock.

• Explain to students that they will use marble chips (also known as Calcium Carbonate) in the experiment.

Note: The concentration of acid students will use in Lab is fortunately 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 more realistic acid concentrations.

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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• Remind students that they must wear lab coats, goggles, and gloves when working with hydrochloric acid.