•
A temperature increase produces a
decrease in the density of a substance
• Density is defined as mass divided by
volume (D=M/V)
• Less dense substances float on more
dense substances
• Convection is defined as the circulation
of a fluid at a non-uniform temperature
due to the variation of its density and
the action of gravity
COLD, HIGHER DENSITY
Cool
descending
current
Hot
rising
current
HOT, LOWER DENSITY
Cool
descending
current
Circular Paths of Food Coloring Gel Created by
Convection Currents in Heated Water
• Water at the bottom is warmer and less dense;
food coloring gel rises
• Water at the top is cooler and denser; food
coloring gel sinks
A radiator in your home or
classroom uses the principle of
convection currents as a heat
source.
•
A bulb at the bottom of the lamp heats up the two liquids in the lamp
•
One liquid expands more than the other, becomes less dense and rises
•
The liquid cools at the top; becomes denser; and sinks
•
As long as the lamp is on, this cycle continues
•
Lava lamp serves a model for internal processes occurring
in the Earth’s mantle producing convection currents causing
tectonic plate movement
HOW DO TECTONIC PLATES MOVE?
• Heat for convection comes from deep within the earth
• Two main sources: radioactive decay and residual heat
• Primary mechanism for mantle convection is seafloor spreading
• “Ridge Push” caused by magma intrusion also provides additional force
• “Slab Pull” caused by subduction now considered to be major driving
force
Convection Current
crust
mantle
Hot material rises, cooler material sinks,
creating a Convection Current or Cell
RIDGE
“RIDGE PUSH” and “SLAB PULL”
LET’S REVIEW
Continents are continuously slowly moving on the Earth’s crust --- only
a few cm a year. Convection currents are what move the Earth’s continents.
THE END