Understanding the Process of Weathering

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1 NAME PER DATE Understanding the Process of Weathering Part One: Weathering Reading Comprehension Directions: Read and annotate this article, in order to answer the attached follow-up questions. Weathering is the process of breaking down the Earth s surface (rocks) by wind, water, ice, plants, animals, or chemical change. For millions and billions of years, weathering has continued to wear down and change the landscape of Earth s surface. Weathering creates sediments, which are loose materials such as small rock fragments, mineral grains, and bits of plants/animals. In other words, sediments are evidence of the weathering process occurring in the environment. There are two kinds of weathering that occur in nature: mechanical and chemical. Mechanical weathering, or physical weathering, is the process of breaking down rocks into smaller pieces without changing the chemical composition. The chemical makeup of the rock is not altered, only its size. There are five different categories in which mechanical weathering can occur in the environment. First off, temperature can produce mechanical weathering. Hot temperatures cause materials, including rocks, to expand. On the other hand, cold temperatures cause things to contract (tighten/shrink). This is why we see roads cracking in the winter and buckling in the summer. The same can be seen with rocks. During the day, the sun's energy heats a rock's surface. The internal temperature does not change. During the night, the rock's surface will cool. The following day, the heating and cooling cycle begins again. This repeated change from hot to cold may cause the rock to peel or flake layers that are parallel to the rock's surface. This peeling or flaking of the rock surface is known as exfoliation. A second type of mechanical weathering may be caused by frost (ice). When temperatures are above 32 F, water remains a liquid, but below 32 F, water freezes into solid ice. Water may drip down and into a small crack or hole in a rock and remain there. As temperatures drop below freezing, the water will expand and solidify into ice. As the freezing water expands, it will cause the crack or hole in the rock to grow larger. When the water later melts at warmer temperatures, it then moves deeper into the bottom of the crack or hole and refreezes. Eventually the rock will break into pieces. This cycle of repeated freezing, expansion, breaking, and melting is known as ice wedging. We are familiar with the effects of ice wedging, because it creates giant cracks and potholes in the road during the winter season. The third type of mechanical weathering is biotic or organic activity. This may be in the form of tree roots growing in a crack in a rock. As the tree grows and the roots expand, they will pry material loose. This is known as root wedging. Animals can also cause mechanical weathering, as well. For example, animals such as ants, worms, and woodchucks, burrow (dig) into the ground to create their homes, causing mechanical weathering. In addition, humans can also be responsible for this form of mechanical weathering by digging, cutting stone, clearing land for building, or even driving on roads. In fact, humans have been one of the biggest mechanical weathering forces over the past few centuries! As civilizations evolved and populations boomed, we have slowly broken down the natural landscape of our planet in order to build our own habitats. Gravity is the fourth type of mechanical weathering. Landslides move downhill due to the force of gravity. As the rocks cascade down the hillside, they collide with other rocks and break into smaller pieces.

2 The last type of mechanical weathering is called abrasion. Abrasion is the mechanical scraping of a rock surface by friction between rocks and moving particles during their transport by running water, wind, glaciers, waves, or erosion. The moving particles cause friction with can dislodge loose and weak debris from the side of the rock. Typically this is a slower process, but over time, the rocks become smoother as rough and jagged edges break off from the abrasive friction. Have you ever notice the rounded shape and smooth surface of rocks and pebbles at the beach or in a river? The rocks shape and texture was created by the abrasive friction from the running water! In fact, wind-blown sand is so abrasive, that it can weather even some of the hardest rocks. Construction workers mimic this abrasion process when using a sander to smooth down a piece of wood. While mechanical weathering alters only the size of the rocks, chemical weathering alters the mineral composition, or the chemical makeup, of the rocks as well. Chemical weathering takes place in almost all types of rocks. However, smaller rocks are more susceptible to this process, because they have a greater amount of surface area exposed. There are three different ways in which chemical weathering can occur in the environment. Chemical reactions break down the bonds holding the rocks together, causing them to fall apart, forming smaller and smaller pieces. Chemical weathering is much more common in locations where there is a lot of water. This is because water is important to many of the chemical reactions that can take place. Warmer temperatures are also more friendly to chemical weathering. The most common types of chemical weathering are oxidation, hydrolysis and carbonation. Oxidation is a chemical weathering process combining oxygen with another substance. The end result is the creation of an entirely new substance. Typically, metal-like minerals are susceptible to the oxidation process. An example of oxidation is the combination of iron and oxygen to form rust. Reddish-orange drip-like streaks on rocks are evidence that this type of chemical weathering has occurred. The second type of chemical weathering is caused by naturally occurring acids in the environment. Carbonation occurs when carbon dioxide dissolves in water, carbonic acid is formed. This acid reacts with other substances, creating chemical changes. An example is found with acid rain, created when carbon dioxide dissolves in rain water. Acid rain will dissolve some kinds of rocks. More evidence of the effect of carbonic acid can be seen in Mammoth Cave and Carlsbad Cavern, both formed by this process when rain water seeped underground. Plant acids, known as humic acids, can also cause chemical weathering. Mosses and lichens provide a good example. As these plants grow on rocks, they produce acids that, in turn, break down the minerals found within the rocks. The acids create tiny pits or holes in the rock that become bigger over time. The final type of chemical weathering is hydrolysis, which occurs when water combines with certain substances in rocks to form new types of substances, which are softer than the original rock types. This allows other forces, such as mechanical weathering, to more easily break them apart.

3 Understanding the Process of Weathering Part One: Weathering Reading Comprehension Directions: Use the information from the attached article to answer the questions below. 1. What is weathering? 2. What has weathering done to the Earth over time? 3. What are sediments? 4. How is weathering related to sediments? 5. How many different types of weathering are there? 6. What is mechanical weathering? 7. Fill out the table below summarizing the categories/types of mechanical weathering: Categories/Types Describe HOW this Causes Mechanical Weathering Related Processes 8. What is chemical weathering? 9. Why are smaller rocks more likely to experience chemical weathering? 10. Where is chemical weathering most common?

4 11. Fill out the table below summarizing the categories/types of chemical weathering: Categories/Types Describe HOW this Causes Chemical Weathering Part Two: Identifying the Types of Weathering Directions: Decide if the following descriptions are examples of mechanical weathering or chemical weathering. Write the word mechanical or chemical in the blank at the left. 12. Mosses growing on the surface of rocks, producing pits in the rocks 13. The wedging of tree roots along natural joints in granite outcrops 14. Limestone dissolved by carbonic acid 15. The oxidation of minerals that contain iron 16. Animal burrows dug in rock that let in water and air 17. Repeated freezing and thawing of water that cracks rock 18. The action of water, salt, and air on car fenders and panels 19. Acids from plant roots which break up rocks 20. Formation of potholes in streets during severe winters 21. Lifted sections of sidewalk along tree-lined streets Part Three: Identifying the Types of Weathering Directions: Recall that there are five different categories of mechanical (physical) weathering. Analyze each picture below and determine which type of mechanical weathering is illustrated

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