DENSITY CURRENTS

David C. Kopaska-Merkel
Geological Survey of Alabama
PO Box O
Tuscaloosa AL 35486-9780


Level: K-6 as a demonstration; 3-8 as a hands-on exercise

 

Estimated Time Required: 15 minutes for a single run, but the students may wish to perform the experiment several different times, varying the conditions and observing the results.

 

Anticipated Learning Outcomes

 

Background

Density is a property of matter. It is the amount of material that fits in a given space. Denser objects are heavier than less dense ones of the same size in the same gravitational field. Thus, a quart of cotton is light but a quart of lead is very heavy; lead is denser than cotton. Liquids have density too. Water is denser than oil, and honey is denser than water. If two liquids of different densities come into contact, then the denser liquid sinks down below the less dense liquid and flows along the bottom. This is a density current.

A density current flows because of the pull of gravity and the density difference, and it stops moving when the two fluids mix, or when the current uses up its energy. This is like what happens when you let a marble roll into a bowl. The marble rolls quickly to the bottom of the bowl, and then it rolls back and forth, gradually slowing, until it stops. The marble uses up its energy of position by moving from the lip to the bottom of the bowl. It uses up its energy of motion by rubbing against the air and against the bowl, turning its energy of movement into heat by friction.

Water is the most important liquid on earth; density currents are common where waters of two different densities meet. How does water change its density? If solid material is mixed into the water, then the density increases. Sea water (which is salty) is denser than fresh water because the salt adds material without making the water take up more space. However, if the fresh water has a lot of mud mixed into it, then it may be denser than the sea water. Cold water is denser than hot water, because when water gets hot it expands. The same amount of material takes up more space, so it is less dense.

A density current in which the denser fluid consists of sediment mixed with water is called a turbidity current, because the water is turbid, or murky. Turbidity currents are among the most common kinds of density currents. Turbidity currents form in lakes in the springtime when heavy rains or melting snow wash mud into rivers. They are also common on the ocean floor. Turbidity currents on the ocean floor can form when storms or earthquakes stir up the sediment from the ocean bottom. The sediment mixes with the water near it, making a denser mass of turbid water. If the place where this happens is on a slope, then the turbid water flows down along the bottom into deeper water. Turbidity currents commonly flow until they reach the flat bottom of a basin, where they slow down, which causes the sediment particles to fall to the bottom. If a turbidity current flows all the way across a basin, it is reflected, or turned back, by the far wall of the basin.

In the following experiment the students will make a turbidity current. Optionally, they will also make a different kind of density current.

 

Materials


Procedures

  1. Have the students read all procedures and prepare needed materials before beginning the experiment. They should never perform any experiment without adult supervision.
  2. The students should place the tank on a table. They should put the small piece of wood under one end to tilt the tank. The students should fill the tank at least half way with cold water.
  3. Have the students mix sediment and water in the smaller container. There should be enough sediment in the water so that the students have to shake the container vigorously to suspend all the sediment. They should shake well, and then slowly and evenly pour the mixture into the high end of the tank.
  4. Have the students observe the turbidity current and the deposit that it forms (a turbidite). They may wish to repeat the experiment with different proportions of water and sediment in the smaller container.
  5. An interesting variation is to perform the experiment by pouring hot water into cold water. The water being poured into the tank should be colored with food coloring. The students should pour the hot water slowly and gently. The temperature difference should be as great as possible. The results of this experiment should be very different from those of the first one. The students need to observe carefully! This experiment should not be performed by unsupervised children.
  6. Have the students clean everything up. Do not pour water containing sediment into sinks because it will clog drains.

Results and Discussion

  1. The dense mixture of sediment and water flowed almost vertically down to the bottom of the tank. It then flowed along the bottom, mixing somewhat with the clear water, until it reached the low end of the tank. Then, if it still had enough energy of motion, it turned and flowed at least part way back up the slope towards the high end of the tank.
  2. The students have made a turbidity current. Their turbidity current behaved very much like turbidity currents that form in the ocean. These currents flow from the relatively shallow continental shelves and slopes into the deep central parts of the ocean basins (the abyssal plains).
  3. Turbidity currents are important to people because they damage equipment on the ocean floor. Also, turbidity currents are one of the most powerful mechanisms that move sediment to the deep ocean. In places, much of the ocean-floor sediment was deposited by turbidity currents (for example, Leg 123 Shipboard Scientific Party, 1988).
  4. If the students used sand instead of mud in their experiment, the result was similar, but they did not create a turbidity current; they created a cohesionless particle flow. The differences are discussed in Friedman and others (1992).
  5. If they performed the optional second part of the experiment in which they poured hot water into cold water, then they made a density current created by temperature differences. These are not common in nature, but they are similar to density currents formed by differences in the amount of salt dissolved in water. (The major occurrence on Earth of density currents created by temperature differences in liquids is very slow movement of cold water from the north and south poles, along the bottom of the ocean, towards the equator.) Density currents caused by differences in salt content of water are common where clear fresh water (not mixed with sediment) flows into the ocean, such as near coastal springs or some rivers. Salt makes water denser, so the less-dense fresh water flows on top of the denser sea water. I suggest that the students use a temperature difference to create a density difference in order to show that density currents can be created by completely different mechanisms.
  6. In the students' experiment, the hot water, being lighter than cold water, flowed across the top of the tank, though it did mix with the upper part of the cold-water mass. Eventually, the hot water mixed entirely with the cold water, and this was the end of the density current. This is what happens when clear river water flows into the sea.
  7. Turbidity currents and other density currents are very common in the world around us, and the students should be able to think of some examples. Here are some suggestions: pouring milk into coffee (the milk is denser and sinks to the bottom, then comes back up to the surface because of its energy of motion); ice cubes melting in warm drinks (the cold water coming off the ice cubes is denser and also looks clearer; in a strong light it can be seen swirling into and sinking down through the other liquid in the glass); and muddy water running into a pond or stream after a rainstorm. A density difference can also be seen in the separation of the two liquid components of oil-and-vinegar salad dressing. Vinegar is mostly water and it is denser than oil. If you make dressing by pouring the vinegar into the oil, then a density current is created. If you suspend a tea bag in hot water, the tea mixes with the water closest to the tea bag. This mixture is denser than the pure water, and a density current results.
  8. Older students might want to consider how density currents could form on other planets, or in stars. For example, on the cold moons of Jupiter, compounds that are gases on Earth can be liquid or solid. In the Sun, density currents caused by temperature differences might form in hot plasma. Even here on Earth there are other kinds of density currents. Density currents can form in the hot liquid rock (lava) in active volcanoes. Density currents can form in gases too. At night, cold air flows down mountainsides into the warmer valleys; these are density currents in air.


Additional Activities

The students might like to try using differently colored sediment and creating multiple turbidites in the same tank. (They should wait for each turbidity current to completely settle before creating another one. This might take as long as a day if the sediment contains very fine mud.) The different colors of sediment will allow them to see differences in the patterns of deposition of the turbidites. They could also mix sediments of different sizes (for example, sand and mud) in a single current. The coarser sediment should settle more quickly, resulting in a deposit that fines upward (smaller particles at the top). Most natural turbidites fine upward.

 

Acknowledgments

This paper is based on a science project by the author and Morgan Kopaska-Merkel, and presented by the latter. A similar experiment using salt water was previously published by VanCleave (1991). This paper was reviewed by Leslie Black, Margaret Brown, David J. Davies, Morgan Kopaska-Merkel, Sheila Kopaska-Merkel, R. M. Mink, and Karen Rheams.

 

Selected References

Friedman, G. M., Sanders, J. E., and Kopaska-Merkel, D. C., 1992, Principles of Sedimentary Deposits: Macmillan Publ. Co., New York, p. 335-340.

Kopaska-Merkel, D. C., and Kopaska-Merkel, M. L., 1992, A turbidity current in the classroom: Alabama Geological Society Newsletter, v. 5, no. 3, p. 4-5.
Leg 123 Shipboard Scientific Party, 1988, Sedimentology of the Argo and Gascoyne abyssal plains, NW Australia: report on Ocean Drilling Program Leg 123 (Sept. 1 - Nov. 1, 1988): Carbonates and Evaporites, v. 3, p. 201-212.

VanCleave, Janice, 1991, Earth science for every kid: John Wiley and Sons, New York, p. 196-197.


DENSITY CURRENTS
Student Worksheet

 

Instructions:

  1. Carefully read all instructions before beginning the experiment. Never perform any experiment without adult supervision.
  2. Put the tank on a table. Put the small piece of wood under one end to tilt the tank. Fill the tank at least half way with cold water.
  3. Mix sediment (I hope you are using mud and not sand or gravel!) and water in the smaller container. There should be enough sediment in the water so that you have to shake the container vigorously to suspend all the sediment. Shake well, and then slowly and evenly pour the mixture into the high end of the tank.
  4. Observe the turbidity current and the deposit that it forms (a turbidite).
  5. Optionally, let the first turbidity current settle, then try the experiment again by pouring hot water dyed with food coloring into the cold water in the tank. Be careful pouring hot water! This experiment should be performed with teacher supervision.
  6. Pour slowly and gently. The results of this experiment should be very different from those of the first one. Observe carefully!
  7. Clean everything up. Do not pour water containing sediment into sinks because it will clog drains.



NAME _______________________________

 

Results and Discussion:

  1. Describe what the turbidity current did when you poured the water-sediment mixture into the tank. Where did the mixture go? Why? What happened to the sediment?



  2. Why did the current stop?

  3. Draw a picture of your turbidity current in the box provided. If you think it takes more than one picture to show what happened, draw as many as you need to.


  4. If you performed the second experiment, pouring hot water into cold water, describe how the results of this experiment differed from those of the first one. Did the current go to a different place? Why or why not?
  5. Do you think there is an important difference between the way this current was made and the way the first one was made? Is there an important difference between the results? Explain your answer.
  6. How and why did the current stop?
  7. Have you ever seen a turbidity or density current or something like it? (I'm sure you have!) Describe one example that you can remember seeing.

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