Download convection current demonstration

Day 2 Explore
ƒ Have the convection current demonstration that is described below set up
when students enter the classroom. Begin class by reviewing yesterday’s take
home points.
o The Earth has several distinct layers. Ask the students what these are.
o The lithosphere is made up of the crust and rigid upper mantle.
o Lithospheric plates “float” on top of the partially molten asthenophere.
o Earthquakes occur as a result of the movement of these plates.
ƒ Tell the students that today they are going to explore the motion of tectonic
plates. First they are going to learn what causes the plates to move. Hand out
Convection Current and Plate Tectonics Worksheet.
ƒ Convection current demonstration (modified from
http://web.ics.purdue.edu/~braile/edumod/convect/convect.htm, last accessed
2/28/10): This can either be done as a demonstration by the teacher or, if
enough materials are available and the safety concerns with students working
with candles or sterno cans are acceptable, this can be a hands on lab done by
groups of 2 or 3 students. The instructions below and the materials list
assume that it is being done as a demonstration by the teacher.
o Ask the students if they have any ideas about what makes the Earth’s
tectonic plates move. After a few minutes of discussion tell them that
you have a demonstration to help explain why the Earth’s plates move.
o Mix the vegetable oil and thyme in the loaf pan. Stir to thoroughly
distribute the thyme leaves. (Note – if left unstirred for a long time, the
thyme will separate and not be evenly distributed in the oil. If this
happens, stir to mix thoroughly, let the mixture stand w/out heat until
the thyme stops moving, and begin the experiment again by adding
heat).
o Arrange loaf dish and other materials as shown in Figure 1. Do not
light the sterno or candles yet)
o Tell the students to consider question 1 on their Convection Current
and Plate Tectonics Worksheet. “Without any heat being applied to
the oil-thyme mixture, how does the thyme behave? Is it moving at
all? If so, what do you think is causing it to move? Is it moving
slowly or quickly?” If it is difficult for students to see the demo, have
them come up for closer observation in small groups.
o Light the sterno and let the oil-thyme mixture heat up for a few
minutes. Once the oil heats up and begins to flow have the students
think about question 2 on the Convection Current and Plate Tectonics
Worksheet. “Describe and draw the pattern of flow in the hot oil by
noting the location of individual flakes of thyme over time. Is the flow
approximately symmetric on the two sides of the heated area? Where
do you observe upward flow? Where do you see downward flow?
Where do you observe horizontal flow?” Again, if necessary have the
students come up in small groups for closer observation.
Thermal Convection Experiment
Thyme
Glass Bread Loaf Dish
Oil
Flow
Cup
Sterno
Tabletop
Figure 1. – configuration of convection current demonstration. From,
http://web.ics.purdue.edu/~braile/edumod/convect/convect.doc
o Explain the following: The flow you are observing in the
demonstration is a convection current. Similar currents exist in the
asthenosphere, and this is what drives the motion of the Earth’s
tectonic plates. In the demonstration, the fluid above the flame is
heated, which causes expansion and a decrease in density. The hot,
less dense fluid rises to the surface. The rising fluid pushes the fluid
above it to the sides (horizontal motion of the thyme at the top of the
container). The fluid begins to cool as it moves away from the flame
toward the edges of the container. This cooling increases the density
of the fluid and causes it to sink. The sinking fluid pushes the fluid
below it back toward the center of the dish and the cycle begins again.
o Add three pieces of balsa wood as shown in Figure 2 to the pan to
demonstrate plate movements. The two plates in the middle above the
heat source should separate (diverge). If surface tension causes the
plates to stick together, use a pencil to slightly separate the two pieces.
You might need to re-place the balsa wood pieces a few times to
enable all the students an opportunity to observe their motion.
o Explain to the students that the pieces of balsa wood represent
different tectonic plates and their job is to observe and describe the
way the plates move due to the convection currents in the oil. Have
the students consider question 3 on the Convection Current and Plate
Tectonics Worksheet. “Describe how the balsa wood “plates” are
moving. Add arrows to the diagram to help you explain the plate
motions. Make note of where they are moving away from each other
and where they are colliding.”
o The two plates in the middle above the heat source separate (diverge).
This is analogous to two plates moving away from each other at a
divergent plate boundary. Divergent plate boundaries occur at midocean ridges (spreading ridges). Magma moves upward into the space
between the diverging plates to create new lithosphere. This young
lithosphere in the oceans is rich in iron and magnesium and is
therefore denser than continental lithosphere, which has less iron. As
time progresses, one of the middle plates should collide with the plate
at the edge of the pan. This is analogous to a convergent plate
boundary. Explain that, although not represented in this model, this
type of collision usually results in the denser oceanic plate being
forced beneath the younger continental plate in a process called
subduction. In addition to the collision creating earthquakes, the
subducting plates can reach a depth where temperatures are hot enough
to cause it to melt. The resulting magma can rise to the surface and
create volcanoes. Point out that this is the reason the student’s
observed that most earthquakes and volcanoes occur along plate
boundaries on the Egg Earth Worksheet they just turned in. Although
they were not asked to identify divergent vs. convergent plate
boundaries, it turns out that the majority of major earthquakes and
volcanoes are associated with convergent plate boundaries.
o Tell the students to finish up the questions about the convection
current demonstration on the Convection Current and Plate Tectonics
worksheet as homework.
Loaf Dish (View from above)
Oil
Wood
Figure 2. – configuration of balsa wood on the surface of the oil to demonstrate plate
movements. From, http://web.ics.purdue.edu/~braile/edumod/convect/convect.doc
•
Plate tectonics online investigation: Students will work either individually or
in pairs on computers with internet access for this part of the lesson.
o Tell the students that they are now going to read some online material
and answer the rest of the questions on the Convection Current and
Plate Tectonics worksheet. Tell them that the online material
summarizes a lot of what they have already learned about plate
tectonics and earthquakes and it also provides a little bit of background
about how scientists figured out that the lithospheric plates are
moving.
o Direct students to
http://www.sio.ucsd.edu/voyager/earth_puzzle/index.html (last
accessed 3/3/10) and have them read through all four sections of the
site. Emphasize that they should click on the images to see the
animations. Depending on how long the convection current