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
Anticipated Learning Outcomes
- The students will learn how fluids of differing densities
interact with one another.
- The students will learn some ways the densities of fluids
can be changed.
- The students will learn how density currents transport
and deposit tremendous amounts of sediment in lakes and in the ocean.
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
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.
- A large elongate clear-walled container (a 10-gallon
fish tank works well)
- A smaller container (pint or quart size) with a tight-fitting
- Sediment (mud is best; do not use sand or gravel; if
the sediment is clayey, be sure to mix it up especially well before pouring)
- A block of wood a few inches on a side
- A place to work where, if you spill water, no one will
- Food coloring (if you are doing the temperature experiment)
- Have the students read all procedures and prepare needed
materials before beginning the experiment. They should never perform any
experiment without adult supervision.
- 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.
- 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.
- 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.
- 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.
- Have the students clean everything up. Do not pour water
containing sediment into sinks because it will clog drains.
Results and Discussion
- 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.
- 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).
- 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).
- 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).
- 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.
- 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.
- 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
- 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.
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.
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.
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.
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.
- Carefully read all instructions before beginning the
experiment. Never perform any experiment without adult supervision.
- 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
- 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.
- Observe the turbidity current and the deposit that it
forms (a turbidite).
- 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.
- Pour slowly and gently. The results of this experiment
should be very different from those of the first one. Observe carefully!
- Clean everything up. Do not pour water containing sediment
into sinks because it will clog drains.
Results and Discussion:
- 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?
- Why did the current stop?
- 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.
- 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
- 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.
- How and why did the current stop?
- Have you ever seen a turbidity or density current or
something like it? (I'm sure you have!) Describe one example that you can