Teacher Notes that Accompany the Power Point Slides on Convection
Density and
Mantle Convection
1. Title. Students should take notes when type is black,
but read, listen, and think when type is blue.
2. Notes on density are optional (teacher discretion), but
it is suggested that you review the density lesson before
getting into convection notes. Understanding shock
waves is not part of the lesson – this is here for interest.
A shock wave is cone of high density air (sound wave)
that builds up due to the Doppler effect in front of the
plane. When a plane is traveling faster than the speed of
sound in the air, the shock wave cone trails behind the
plane. It continues to spread in a cone shape, and the
sonic boom you hear occurs when the high density air
has reached your position on the ground.
http://commons.wikimedia.org/wiki/File:FA18_Hornet_breaking_sound_barrier_(7_July_1999)__filtered.jpg
3. Again, students don’t need to understand population
density, but this illustration might help them understand
that density is a ratio. Density is the measure of the
amount of mass in a given volume (number of kilograms
in a Liter for example). It is a ratio of mass to volume.
Population density is similar in that it is a ratio of the
number of people in a given area (population/square
mile, for example).
http://commons.wikimedia.org/wiki/File:World_populat
ion_density_1994.png
4. Students should understand this concept if they
understand density. The smoke is hot and therefore less
dense than the cooler air around it. The cooler air sinks
(is pulled down by gravity more than the smoke) and the
smoke goes to the “empty” place left by the air. In a
sense, the cooler air pushes the smoke up. Nothing rises
without a force.
http://commons.wikimedia.org/wiki/File:Smoke_from_c
ontrolled_burn_of_house_containing_explosives.jpg
5. The answer is B. One quick way to handle a question
like this in the middle of a lecture is to have students
make the letter shape with their fingers OR hold up 1
finger for A, two for B, and so on. Please let us know if
you have a better distractor for D!
http://commons.wikimedia.org/wiki/File:Decorative_bri
ck_wall.jpg
6. Convection – We say less dense matter rises, but in
truth, more dense matter is pulled down by gravity
making room for less dense matter and pushing it out of
the way. Nevertheless, the basic principle of convection
is movement of matter due to differential heating that
causes density differences. Here, the spreading occurs
when the material in the pan strikes the boundary with
the air above it. If the material in the pan is water, then
even low density water will be more dense than the air
above it. The air behaves like a “cap” and keeps the
water from rising further. It spreads as more rising water
pushes it out of the way.
http://commons.wikimedia.org/wiki/File:Convection_ca
sserole.png
7. (Optional) Click on picture (in slide show mode) to
access the youtube video at
https://www.youtube.com/watch?v=7xWWowXtuvA
This video shows warm red dye and cold blue dye
dropped into a container of water. Convection currents
are established and can be seen due to the colors of the
dye.
8. For convection to occur there must be
A) A type of matter that can flow. A fluid like a gas or
liquid can flow, for example the air in this room, water in
the ocean, and the atmosphere. A soft, squishy solid, like
Silly putty, or bread dough can also flow. A solid in this
condition is referred to has “having plasticity” or as in a
plastic state. Mantle material can flow.
B) Convection will occur in a material whose density
changes inversely with temperature. The density of
room temperature or hot water, air, and the material in
the mantle of the earth all decrease in density as their
temperatures rise. Water at 4°C is one material whose
density and temperature do not follow this relationship.
C) There must be room for convection to occur. For
example, convection might occur in porous rock if
materials can flow through the pores, but does not occur
in solid rock where no material can move. House
insulation is designed to keep air from moving around,
so little convection occurs in house insulation. This keeps
the warm air inside the house from moving to the
outside of the walls of the house.
D) Convection only works because of gravity! In space,
denser air would not be lifted above less dense air.
http://commons.wikimedia.org/wiki/File:Cumulonimbus
_Cloud_over_Africa_(color).jpg This is a retouched
picture, which means that it has been digitally altered
from its original version (for clarity and color)
9. The most heat is released at the fast-spreading East
Pacific Rise off the western coast of Mexico and South
America.
Earth interior
http://commons.wikimedia.org/wiki/File:Terrestrial_Pla
net_Interiors_(Earth,_Mars_and_Moon)__Artist's_Concept.jpg
Mid-ocean ridge
http://commons.wikimedia.org/wiki/File:Midocean_ridge_topography.gif
10. Convection in the mantle – warm plumes of less
dense mantle are pushed up by colder, denser, mantle
materials. These plumes rise until they strike the less
dense boundary at the crust, where they are pushed
aside. As the mantle is pushed aside, the crust flows with
it. Some hot mantle materials is ejected between the
plates and form a volcanic ridge. The lithosphere is the
ridged part of the crust and the asthenosphere is the
plastic, moveable part of the mantle that moves along
with the crust. As the asthenosphere and crust move
away from the hot ridge, they cool and both mantle
material and crust sink into the earth under other plates.
http://commons.wikimedia.org/wiki/File:Oceanic_sprea
ding.svg
11. Scientist now study the boundaries between layers
to advance the field of geophysics leading to more
accurate predictions and a better understanding of the
Earth.
http://commons.wikimedia.org/wiki/File:2004tsunami_balanced.jpg
12. The answer is B. One quick way to handle a question
like this in the middle of a lecture is to have students
make the letter shape with their fingers OR hold up 1
finger for A, two for B, and so on. Please change this and
let me know if you have a better distractor for D!
http://commons.wikimedia.org/wiki/File:Decorative_bri
ck_wall.jpg