Chapter 13: Landscapes: Dunes, Glaciers, Karst, and Subsidence

This book contains exercises for a physical geology lab class. It is under development, with a full 1st edition release planned for Fall 2024.

The goals of this chapter are:

• Understand forces that change the landscape in different environments
• Interpret rates of change of landscapes
• Evaluate the impacts of these changes on humans

Geomorphology (from Ancient Greek: geo “earth”; morphe, “form”; and logos, “study”) is the scientific study of the origin and evolution of topographic and bathymetric features generated by physical, chemical or biological processes operating at or near Earth’s surface. Geomorphologists seek to understand why landscapes look the way they do, to understand landform and terrain history and dynamics and to predict changes through a combination of field observations, physical experiments and numerical modeling.

There are many forcing agents that control the shapes of landscapes. In the last chapter, you learned about how rivers and streams on Earth’s surface influence landscapes. Other forces include gravity, wind, glaciers, oceans, wildfire, people and even water beneath the surface. Also important is climate – more about that in Chapter 15. In this chapter, you will explore a few different landscapes.

Deserts

The first you’ll explore is the effect of wind in deserts. Sand dunes commonly form in arid environments with a large sediment supply and wind strong enough to transport sand. Dunes form where a dry climate prevents vegetation can interfere with their development. Sand dunes, however, are not restricted to deserts, and can be found along seashores, along streams in semiarid climates, in areas of glacial outwash, and in places where sandstone bedrock disintegrates to produce an ample supply of loose sand.

Dune shapes are dictated by two main factors: the amount of loose sediment as well as whether there is one or more prevailing wind direction. The strength, direction, and persistence of winds helps determine how dunes develop. In some areas, winds will blow one direction in summer and another in winter. Other areas there are multiple wind directions. This variability in wind changes the shape of the dunes as well as their names (Figure 13.1).

Vegetation acts as a barrier to wind and thus sand may build up behind the vegetation. Vegetation also makes the sand surface rougher which can affect sand movement. Also, deep roots can stabilize sand and prevent it from moving. These affects greatly affect sand dunes along the coast and at the edge of a desert where there is enough moisture for plants to grow.

Exercise 13.1 – Observing Sand Dunes

 

Dunes are made of sand-sized particles, and may consist of quartz, calcium carbonate, snow, gypsum, or other materials. Sand is pushed or bounces (saltation) up the shallow side and then slides down the steep side (slip face). This characteristic shape is easy to see from a side view but harder to see in map view. Figure 13.2 shows different types of sand dunes. Be sure to look at the scales on each image to get a sense of their size.

1. Observe the similarities and differences between the dunes in each photo. Trace the crest of as many dunes as possible.
2. For each image, describe the geometry of the dunes.
   1. White Sand Dunes National Park, NM:
      

      ---

      ---

   2. Great Sand Dunes National Park, CO:
      

      ---

      ---

   3. Sahara Desert, Algeria:
      

      ---

      ---

   4. Dunas de la Soledad, Mexico:
      

      ---

      ---

3. For each image, draw arrows to indicate the wind direction.
4. Which do you think has a higher sediment supply, White Sand Dunes National Park or Dunas de la Soledad? Explain.
   

   ---

   ---

5. What do you think gives the sands of White Sand Dunes National Park their unique color?
   

   ---

   ---

6. Do you notice vegetation in any of these arid settings?
   

   ---

   ---

7. How do you think vegetation affects dune formation?
   

   ---

   ---

8. Figure 13.3 is a close-up from the Sahara Desert. Do these dunes look the same as they did at a larger scale. Do they look similar to another desert in Figure 13.2.
   

   ---

   ---

9. Critical Thinking: Why can dunes vary in shape at different scales?
   

   ---

   ---

   ---

The next exercise is an exercise in measuring rates of landscape change. This movement of sand is emerging as a significant problem as desertification, or the process by which land becomes desert, is threatening about one fifth of the world’s lands. In fact the United Nations has declared this to be our planets greatest environmental hazard and predicts over 50 million people will be displaced by this process in the next ten years.

Exercise 13.2 – Measuring Dune Migration

One of the filming locations for the Star Wars movie series was in the Saharan desert of Tunisia, where sand dunes migrate across the landscape. Since it was built in 1976, it has been become a tourist attraction. Figure 13.4 shows four Google Earth images of the set from different times over a 5.5-year period. A single barchan dune is outlined with a red dashed line. A barchan dune is crescent-shaped and has a shallow side and a steep side. Wind transports sand up the shallow side of the dune, and sand tumbles down the steep side. Based on the images, you can see that the barchan dune is approaching the film set, which is a popular tourist attraction. Let’s figure out the rate at which this dune is moving.

1. Three points of the dune have been marked with a yellow dot: the northern point, central, and southern point. Measure the distance that these points move between each time. It may help to outline the dune for each image.
   

   Table 13.1 – Dune movement distance

   Time Period	Northern Point (m)	Central (m)	Southern Point (m)
   1/2008-9/2009
   9/2009-6/2012
   6/2012-7/2013
   Total Movement

2. What is the dune movement rate for each part of the dune? The rate is distance/time.
   

   Table 13.2 – Rate of dune movement

   Time Period	Northern Point (m/yr)	Central (m/yr)	Southern Point (m/yr)
   1/2008-9/2009
   9/2009-6/2012
   6/2012-7/2013

3. Why do you think there are different movement rates for the dune?
   

   ---

   ---

4. What is the overall movement rate for each part of the dune over the 5.5-year period?
   1. Northern Point: ____________________
   2. Central: ____________________
   3. Southern Point: ____________________
5. How long before each part of the dune makes it to the westernmost buildings of the film set?
   

   ---

   ---

6. Critical Thinking: Figure 13.5 is the latest image from the film set from 2023. What happened to the dune?
   

   ---

   ---

Glaciers

Next, let’s explore icy landscapes in which snow turns into dense glacial ice. A glacier is a body of glacial ice that moves under its own weight.  Currently, about 10% of Earth’s land surface is currently covered with glacial ice, mostly in Antarctica and Greenland. At various times during the past, glacial ice has been much more extensive, covering at least 30-90% of the Earth’s land surface at times. Often, glaciers are many meters thick and hundreds of meters in extent. Thus, they are the largest repository of fresh water on Earth (~69% of all fresh water). They are highly sensitive to changes in climate, and in recent decades have been melting rapidly worldwide. For mountainous regions, glaciers are an important sources of drinking water.

There are two types of glaciers: alpine (found at high altitudes) and continental (found at high latitudes). The largest glaciers can cover entire continents and be several kilometers thick. Ice masses of this size leave behind very distinct features. As they advance, they carve through the surrounding landscape and transport sediment. As they retreat, large volumes of melting water carry and deposit sediment. These carved or deposited features can not only tell scientists that a glacier used to lie over an area, but they can also inform them about how the glacier behaved.

Many glacial features are very large, larger than what can be observed in a single outcrop. One way that geologists can interpret large-scale landscape features is through LiDAR imaging (Light Detection and Ranging). LiDAR is an imaging technique that can make a detailed elevation map of an area without tree cover.

Exercise 13.3 – Observing Glacial Landscapes

To observe some glacial features in LIDAR for yourself, first visit the Washington LIDAR Portal. Once you have entered the site, select “Hide All” towards the top of the page. Then scroll down the grey list of maps until you see “Island 2014” and select the box next to DTM Hillshade. After a second, you should see a greyscale digital hillshade model covering Whidbey Island in northern Puget Sound.

1.  Look at the southern half of Whidbey Island. Describe the pattern(s) you see in the landscape. It may help to zoom in to either 2 km or 1 km on the scale bar.
   

   ---

   ---

2. How do you think these patterns could have formed?
   

   ---

   ---

3. What can you infer about the ice flow direction in this area?
   

   ---

   ---

4. Look to the far west side of the island, just west of Penn Cove. Describe the pattern(s) you see in the landscape. Zoom in to 100 m on the scale bar for the best view.
   

   ---

   ---

5. What are these features?
   

   ---

   ---

6. Look at the northern part of the island, northeast of Crescent Harbor. Describe the pattern(s) you see in the landscape. Zoom to 500 m on the scale bar for the best view.
   

   ---

   ---

7. Where else on the island did you see these features? What are the differences in their orientation?
   

   ---

   ---

8. How would you explain the difference in orientation?
   

   ---

   ---

   ---

9. Figure 13.6 shows the extent of the Puget Lobe of the Cordilleran ice sheet 16,000 years ago. Why do you think Mount Baker was not covered by the glacier?
   

   ---

   ---

10. What about the area just south of Mount Baker?
    

    ---

    ---

Another factor in understanding landscapes is the height of a region. In lecture, you may have been introduced to the concept of isostasy or the principle that the Earth’s crust is floating on the mantle, like a raft floating in the water. This also applies to high topography in mountainous regions as erosion reduces the height, the  weight of the crust decreases and it rebounds. This same principle applies to glaciers as thick glacial ice adds weight to the crust and causes the crust to subside. For example, the Greenland Ice Sheet is over 2,500 m thick, and the crust beneath the thickest part has been depressed to the point where it is below sea level over a wide area (Figure 13.7). When the ice sheet melts, the crust and mantle will rebound.

It may not seem like an important part of the landscape, but ground water flow controls many aspects of geomorphology underground. Huh? Well, caves are a geomorphic feature formed beneath the surface by dissolution of limestone. Limestone at the Earth’s surface can dissolve to form karst topography. Both caves and karst form as carbon dioxide from the atmosphere dissolves in rainwater making the water slightly acidic. The acidic water widens existing cracks or crevices which leads to even larger cracks and more water flow and dissolution. In addition to chemical weathering, mechanical erosion occurs as loose rock fragments transported by water erode away the sides of the openings.

A critical requirement for the development of karst is water. Without water there would be no karst or caves! On a global scale a significant portion (15-20%) of the Earth’s surface is underlain by limestone that have the potential to form karst. An understanding of karst processes is therefore important, particularly where humans interact with this landscape (Figure 13.7). Karst landscapes have features and resources that are not present in non-karst landscapes. Karst aquifers provide the main source of water; for example, 25% of US groundwater come from karst such as the Edwards aquifer in central Texas.

Exercise 13.5 – Karst and Sinkholes

Extensive carbonate occurs in southern Virginia and northern Tennessee. In fact, there is a state park, near Clinchport VA, with a natural tunnel that is over 255 m (838 ft) long that once had a railroad running through it. Compare the satellite image (Figure 13.8) with LiDAR image of the same area (Figure 13.9) for an area east of Natural Tunnel State Park.

1. Make observations about the landscape in the tree covered area in the LiDAR.
   

   ---

   ---

2. Compare this with the farmland surrounding the wooded area. Use Figure 13.7 and name the landforms in the farmland.
   

   ---

   ---

3. Use the geologic map and rock descriptions in Figures 13.10 and 13.11 and list the different geologic units in each area.
   

   ---

   ---

4. Critical thinking: Almost every unit in this area is a type of limestone. Why is some of the area filled with sinkholes and some does not have any karst features?
   

   ---

   ---

 

As you just discovered, both surface water and groundwater play a significant role in landscape development. Groundwater is stored in the open spaces within rocks and within unconsolidated sediments. Porosity is the percentage of open space within an unconsolidated sediment or a rock. Primary porosity is represented by the spaces between grains in a sediment or sedimentary rock. Secondary porosity is porosity that has developed after the rock has formed. It can include fracture porosity—space within fractures in any kind of rock. Some volcanic rock has a special type of porosity related to vesicles, and some limestone has extra porosity related to cavities within fossils.

Do you think that water can flow through a basalt with lots of vesicles? No, as there is another parameter, permeability, that describes how those pores are shaped and interconnected. This determines how easy it is for water to flow from one pore to the next. Larger pores mean there is less friction between flowing water and the sides of the pores. Smaller pores mean more friction along pore walls, and also more twists and turns for the water to have to flow-through.

Permeability is the most important variable in groundwater. Permeability describes how easily water can flow through the rock or unconsolidated sediment and how easy it will be to extract the water for our purposes. A permeable material has a greater number of larger, well-connected pores spaces, whereas an impermeable material has fewer, smaller pores that are poorly connected. The characteristic of permeability of a geological material is quantified as the hydraulic conductivity (K) measured as centimeters per second (Figure 13.12).

Exercise 13.6 – Groundwater Flow – Porosity and Permeability

Let’s look at some of the factors that control groundwater flow. Your instructor will provide 3 or 4 containers with sediment with a screened bottom.

1. Pour the same amount of water into each container. Record the time it takes for the water to flow through the sediment. Fill in Table 13.3. Your instructor will determine how many trials you need to do to complete this table.
   

   Table 13.3 – Factors affecting groundwater flow

   Observation	Container 1	Container 2	Container 3	Container 4
   Grain size, sorting, and rounding, and sediment type
   Time elapsed
   Flow rate (volume/time)
   Color of water
   Volume of water retained?

2. Which sediment transmitted the most water? Which sediment transmitted the least water?
   

   ---

   ---

3. What properties of these sediments are correlated with good water transmission? Which properties are correlated with flow retardation and water retention?
   

   ---

   ---

   ---

4. Critical thinking: Which of these sediments will provide a clean source of water? Will any of these materials be a good filter for either organic matter or pollutants?
   

   ---

   ---

5. Now take two samples of porous material – vesicular basalt and coquina and put them across a beaker. Which of these do you think is more permeable?
   

   ---

6. Use an eye dropper and pour water over these two samples. Which sample allows water to pour through it? Was your hypothesis correct?
   

   ---

   ---

   ---

Subsidence may seem like a minor change in the landscape as in most areas sinking is negligible. To those living near the coast, it is a major threat combined with sea level rise and climate change. Not only will coastal communities be affected, but their water supplies are also susceptible to influx of saltwater. This problem makes many more susceptible to sea level changes such as the Mississippi delta and America Samoa. This process results in coastal retreat and land loss. In the Mississippi delta, about 60 km²/year is vanishing. Not only is this important for those who live there, the Atchafalaya River basin (Figure 13.14) provides flood relief for the Mississippi River, slowing its flow as well as trapping nutrients and pollution improving water quality as it flows into the Gulf of Mexico.

Exercise 13.7 – Groundwater Flow – Subsidence

Now that you’ve learned about porosity and permeability, let’s use your knowledge to interpret groundwater flow. Houstonians started pumping ground water about 1836 and almost immediately this started to cause subsidence with most of the Houston-Galveston area subsiding (sinking) more than 0.3 m (~1 foot). Some areas near Baytown subsided as much as 4 m (~13 feet); an entire subdivision sunk below sea level, and it is now all marsh. In addition, prior to 1977 Houston aquifers (Chicot, Jasper and Evangeline) lost ~100 m (300-400 feet). In reaction, the Texas legislature created the Houston-Galveston subsidence district in 1975. One of their first initiatives was to install a network of wells to measure ground water and subsidence; data collection started in 1977 and continues today. Use this link for recent water well data around Houston. This will show you how water well data is made into a contour map.

1. Figure 13.15 shows the water level in wells around Houston TX in 1977. The USGS has shown contour lines relative to sea level (0 feet).  You have worked with topographic maps showing relief with contour lines that connect equal points of elevation. You can also use contour lines for other types of data. However, the USGS only contoured part of the data. Finish the contour map.
   

   

2. When this data was collected in 1977, Houston was withdrawing ~138 million gallons of water per day, and it is now about pumping only 8 million gallons per day. How has this affected ground water levels? Compare your contour map with the map for the Houston region in 2020 (Figure 13.16).
   

   ---

   ---

   ---

   

3. Is the total range of water depths similar? Has the water level increased or decreased from 1977 to 2020?
   

   ---

   ---

4. How does the sea level contour look in both maps? Has it moved relative to downtown Houston?
   

   ---

   ---

5. Critical Thinking: Do you think that the water level will keep getting shallower through time? Or will it reach a steady-state value?  Explain.
   

   ---

   ---

Groundwater exists everywhere there is porosity. However, whether that groundwater is able to flow depends on the permeability. An aquifer is defined as a body of rock or unconsolidated sediment that is permeable. An aquitard or confining layer does not allow significant water flow. Figure 13.17 shows a cross-section through the Greater Houston area with various aquifers and confining layers as well as the direction of water flow from where it enters the system in the recharge area to the discharge area.

 

• 1. Next, let’s compare the water level with subsidence rates in these water wells. Contour one of the subsidence maps in Figure 13.18 and compare your results to the rest of the class.
  2. Where is the most subsidence located?
     

     ---

     ---

  3. Are the patterns from Figure 13.18 similar to the water well contours of Figures 13.16 or 13.17? How are they different?
     

     ---

     ---

  4. Critical thinking: Even without making a contour map, you can see that the amount of subsidence changes with time. Since groundwater withdrawal has been stopped. What other factors could explain the shifts in subsidence rates in this area?
     

     ---

     ---

 

Additional Information

Exercise contributors

Virgina Sisson, Daniel Hauptvogel and Michael Comas

References

Brent, W.B., 1963, Geology of the Clinchport quadrangle, Virginia, Virginia Division of Mineral Resources Report of Investigations 5, 47 pp., Map Scale: 1:24,000 

Chowdhury, A.H., and Turco, M.J., 2006, Chapter 1, geology of the Gulf coast aquifer, Texas. In R. E. Mace, S. C. Davidson, E. S. Angle, & W. F. Mullican (Eds.), Aquifers of the Gulf Coast of Texas (Vol. 365, pp. 23–50). Austin, Texas: Texas Water Development Board. Report. 

Freeze, R.A. and J.A., Cherry, 1979, Groundwater, 604 pp. CC-NC available as pdf

Taylor, C.J. and E.A. Greene, 2008, Hydrogeologic Characterization and Methods Used in the Investigation of Karst Hydrology, in Field Techniques for Estimating Water Fluxes Between Surface Water and Ground Water Edited by D.O. Rosenberry and J.W. LaBaugh ,U.S. Geological Survey Techniques and Methods 4–D2, 128 pp.

Varugu, B., and Jones, C., 2023. Delta-X: Land Subsidence Rate, Mississippi River Delta (MRD), Louisiana, USA. Oak Ridge National Laboratory Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC), Oak Ridge, Tennessee, USA. DOI: 10.3334/ORNLDAAC/2307