7.1 Weathering

Weathering occurs when a rock is exposed to the “weather”, or to the forces and conditions at Earth’s surface. A rock exposed at the surface can experience changes in temperature, oxygen and other gases, and water. Each interacts with rocks to break them down physically and/or chemically. Physical weathering, also called mechanical weathering, is the breakdown of rocks into smaller fragments, called sediment. The mineral chemistry is not altered during physical weathering. Chemical weathering does alter the chemistry of minerals, either by dissolving minerals in a solution of water, or by replacing elements in a mineral. Chemical weathering typically results in ions (elements) remaining dissolved in a solution.

Physical and chemical weathering go hand in hand because physical weathering produces fresh surfaces for attack by chemical processes, and chemical weathering weakens the rock so that it is susceptible to physical weathering.

Exercise 7.1 – Weathering Rates

The weathering rate of rocks and minerals is affected by the composition of the atmosphere, temperature and rainfall rates, even at a local level. Below are the results of a study of marble gravestones in Australia (Figures 7.1 and 7.2). The data is from two areas: an urban residential area in Sydney and an industrial area in Wollongong (65 km or 45 miles south of Sydney). The year is when the gravestones were inscribed. The surface reduction is a measurement of how much weathering has occurred on the gravestones’ surface since they were emplaced.

1. Describe the relationship between weathering and age of the gravestones in Sydney.
   

   ---

   ---

   ---

2. What is the relationship between weathering and age of the gravestones in Wollongong? How does it compare to Sydney?
   

   ---

   ---

   ---

Another way to look at this data is to analyze the weathering rate per 100 years (Figures 7.3 and 7.4), but we don’t have 100 years worth of data for each gravestone. So, we extrapolate the available weathering data to 100 years, assuming a consistent weathering rate for each gravestone. For example, the gravestone in Wollongong that was emplaced in 1940 would only be a 46-year record (measurements were taken in 1986). In that 46-year period, the gravestone weathered a total of 0.13 mm, or about 0.0028 mm/year. Multiply the yearly weathering rate by 100 and you get a weathering rate of 0.28 mm/100 years.

3. What is the average weathering rate per 100 years for Sydney? Draw a line of best fit on Figure 7.3. ____________________
4. Has the rate of weathering changed for Sydney? If so, can you locate when the change occurred?
   

   ---

   ---

   ---

5. What is the average weathering rate per 100 years for Wollongong? Draw a line of best fit on Figure 7.4. ____________________
6. Has the rate of weathering changed for Wollongong? If so, can you locate when the change occurred?
   

   ---

7. On average, gravestones are about 4 inches thick (102 mm). Using the weathering rate for the 1940 gravestone in Wollongong of 0.28 mm/100 years (0.0028 mm/year), how many years would it take for this gravestone to completely weather away? ____________________
8. Critical thinking: Compare the weathering rate over time for both sites. Which are the principal factors controlling weathering rate between the two sites? Do you think this similarity or difference will continue in the future?
   

   ---

   ---

This exercise is adapted from G.S. Hancock and C.M. Bailey.

7.2 Sediment Erosion, Transport, Deposition, and Lithification

What happens after weathering? The particles need to be moved, or eroded. Sediment particles that were physically weathered are moved by water, wind, and ice. When the water or wind slows down, or when the ice melts, the sediment stops moving and is deposited. These mechanisms can transport sediment hundreds of kilometers. For ions that were chemically weathered, they can precipitate out of the solution under certain conditions, forming solid particles that can be deposited.

In all cases, sediments need to undergo lithification to transform from loose grains to a solid rock. Typically, two steps are involved in turning sediment into a sedimentary rock; compaction and cementation. When grains are deposited, they typically have empty spaces between them called pores (porosity). Compaction reduces porosity by bringing the grains closer together. The best analogy to think about is your garbage can at home. When the garbage can is full, do you change the bag right away or do you push down on it to create more room? By pushing down on trash, you reduce the space between the pieces of garbage and create more room at the top; this is compaction. Sediments work the same way, but what’s causing the compaction is the accumulation of more sediment on top of the previously deposited material. The more sediment piled on top, the more compact the sediment beneath it becomes.

Cementation is a process that happens during either at the surface or during compaction. As sediments interact with water, minerals precipitate in pore space acting as a glue that holds the sediment together. During compaction, water is squeezed out of the pore spaces; this enhances cementation. The common minerals that make up cement are calcite, quartz, and pyrite.

7.3 Sedimentary Rock Classification

Sedimentary rocks (Figure 7.5) are classified as clastic, chemical, and organic based on how they form. The most common way sedimentary rocks form is when other rocks weather into small particles and are transported by wind, water, or ice to an area where they are deposited. These are called clastic sedimentary rocks and are classified based on their grain size. Sedimentary rocks can also form through chemical or organic processes. If you have a body of water containing dissolved salt and then that water evaporates, it will leave behind the salt as a solid mineral form called a precipitate, which is a chemical sedimentary rock. Organic sedimentary rocks form from the accumulation of organic debris. This organic material would need to be buried very quickly, so it doesn’t decay. Organic material in sedimentary rocks is where coal, oil, and natural gas come from. Then, there are sedimentary rocks that we can classify as biochemical. For example, foraminifera are tiny, single-celled organisms that build shells out of calcium carbonate. After they die and decay, their shells may sink to the bottom of the ocean and accumulate to form a sedimentary rock. In this case, biological processes caused the chemical precipitation of the mineral for the shell, but it doesn’t contain organic compounds, so we call it biochemical.

The size and shape of grains within sedimentary rocks can also help you interpret that sediment’s history (Figure 7.6). When all the grains in the rock are about the same size, it is called well-sorted. Usually, sediment needs to travel a far distance from its source to be well-sorted. In contrast, poorly sorted sediment is deposited very close to its source. The rounding of sediment grains is also an indicator of how far the sediment traveled. Grains that have smooth edges are considered well-rounded and have traveled a far distance. These grains start out with rough edges and become smoother as they travel further and further. In contrast, grains with sharp edges are considered angular and have not traveled a far distance (Figure 7.6).

7.4 Depositional Environments

Sedimentary rocks contain clues that tell you about the environment was like at the time of their deposition. Figure 7.7 contains imagery for several common sedimentary environments. Interpreting depositional environments based on the sedimentary rock characteristics is a very important part of a sedimentary geologist’s job. Many environments share characteristics in their deposits with other environments; for example, deserts and beaches commonly leave behind deposits of quartz-rich, well-sorted sands. There may also be a wide variety of deposits within a single type of environment; for example, a slow-moving river may deposit silty mud in its channel, while a rapidly flowing river may deposit coarse sands and pebbles. Due to the variety in deposits and overlapping characteristics, it takes a well-trained eye to accurately determine the correct depositional environment for many sedimentary rocks.

Exercise 7.4 – Sedimentary Environments

Below, you have been provided with branching diagrams that group depositional environments based on shared characteristics (Figures 7.8 – 7.12). Note that environments grouped in one diagram are often not grouped in others. The characteristics of sediments deposited in a system are broadly controlled by the proximity of the deposit to the sediment source and how much energy is moving through the system.

1. Use Figures 7.8 – 7.12, along with your knowledge of sediments and their structures, to help discern a possible depositional environment for each of your sedimentary rocks provided by your instructor. Fill in this information in Table 7.1.
2. Critical thinking: Which sedimentary features are useful for determining whether the depositional environment is continental versus marine? Are there any sedimentary features that will determine the sedimentary environment with just one feature?
   

   ---

   ---

   ---

Recall from Figure 4.2 that the most common rock type covering North America are sedimentary rocks! Now that you can recognize different rock types and interpret where they were formed, you can start to explore geologic maps and better-understand the physical world around you. This information can be use to determine where to find economic resources such as limestone for concrete, shale to make bricks, sandstone as a place for fossil fuels and drinking water, and halite deposits for salt. Or use them to help you find fossils such as the Texas state fossil wood or a dinosaur. Some use sedimentary rocks for jewelry and ornamental stones. The next exercise will explore some of the sedimentary rocks in Texas.

Additional Information

Exercise Contributions

Daniel Hauptvogel, Michael Comas, Virginia Sisson

References

D. Dragovitch, 1986, Weathering rates of marble in urban environments, eastern Australia; Zeitschrift für Geomorphologie, v. 30, p. 203-214

Image credits for Figures 7.8-7.12

• Glacier – Jacob Frank, Public Domain
• Desert – Pixabay user kuloser, Public Domain
• Swamp – Wikimedia user Pierre5018, CC BY-SA
• River – Simon Ledingham, CC BY-SA
• Deep Ocean – Johann Piber, Public Domain
• Alluvial Fan – NASA, Public Domain
• Beach – Michigan Department of Natural Resources, Public Domain
• Delta – NOAA, Pubic Domain
• Lake – Wikimedia user Zeete, CC BY-SA
• Salt Flat – Wallpaper Flare, Public Domain
• Shallow Ocean – Wallpaper Flare, Public Domain