Convection Currents
Convection currents in the upper mantle provide one mechanism for plate tectonics. When
something is heated, it usually expands. This means that the same amount of matter now
occupies a larger volume.
Role of density in convection currents
You may recall the following relationship;
Density = mass / volume
This states that as the volume increases, the density decreases. Just as an example, let’s
compare the densities of gold and quartz. Both these minerals can be found as sediments in
(some) streams.
One kilogram of quartz has a volume of 370 cm³.
One kilogram of gold has a volume of only 52 cm³.
Calculate their densities, in grams per cubic centimetre (g/cm³), using the equation above.
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You can see why gold sinks to the bottom of the stream bed while quartz sand is easily washed
downstream. Gold is more than seven times as dense as quartz.
Back to the plates! Hot new basalt produced at the mid-oceanic ridge is less dense than the cold
old basalt at the trench. The cold basaltic lithosphere is so dense that it sinks down into the
mantle. The pulling force of this cold dense plate is referred to as trench pull. This sets up a
current moving downwards into the upper mantle. This material is warmed by the hot interior of
the Earth, the density decreases. The material rises. Because of the increased volume the
material exerts pressure and erupts at the mid-oceanic ridge. This hot material creates an
upward convection current completing what is referred to as a convection cell.
There are several processes operating all at once to explain what moves plates. The injection of
basalt at a mid-oceanic ridge causes ridge push by low density basalt. Today most geologists
believe that ridge push is less important than trench pull for producing the forces that cause
plate motion. The oldest part of the oceanic plate sinks because of trench pull where gravity
draws down the dense lithosphere. This subducting plate also pulls the plate behind it assisting
to produce continental drift. A good analogy for this process is the idea of a towel with equal
weights attached to either end draped over a chair. If you suddenly cut the towel in the middle,
the towel would slide in two different directions off the chair. The weighed ends of the towel
are like subducting slabs.
Observing sea floor spreading
This model of how the outer layer of Earth’s surface moves has been tested in various ways.
One of the most spectacular lines of investigation involves deep sea submersibles. These are
like mini-submarines, specially designed to withstand the crushing pressures of the deep sea.
One of these submersibles is Alvin. Alvin was one of a number of research vessels that explored
the deep sea-floor in the 1970s. Beginning in 1972, American and French geologists co-operated
in an exploration program called Project FAMOUS (French–American mid-ocean undersea
study). The ‘team’ photographed the rift valleys along the East Pacific rise and the MidAtlantic ridge, and brought tonnes of rock samples to the surface.
Geologists in submersibles have brought back plenty of evidence to support the idea of sea
floor spreading. Perhaps the most exciting discovery has been finding areas where living things,
never seen before, live in large numbers. These communities depend on very hot water coming
out of the sea-floor, to bring dissolved chemicals which provide the base of the food chain to
chemosynthetic bacteria. You can clearly sea the snails in the photographs over the page. Apart
from the living organisms, these hot water springs bring up several minerals which are deposited
on the sea-floor. The colour of the water is very dark because of the dissolved lead, iron,
copper and zinc minerals that precipitate in sea water. These springs are called black smokers.
French and American scientists have co-operated in studying these black smokers. Black
smokers are common on the sides of mid-oceanic ridges confirming the high heat which results
from the magma and subsequent basaltic eruptions onto the sea-floor. Black smokers can
produce such large amounts of minerals that they can be economic to mine! In December 1997,
the first licence to explore black smoker deposits on the sea-floor was granted to a team of
Australian businessmen.
Experiment: Heating and Cooling Gas
Aim: To see if gas expands when heated and contracts when cooled.
Materials: Balloon, Conical Flask, Ice, Metal Water Bath, String, Ruler
Method:
1. Blow up balloon to the size of a large grapefruit.
2. Place the end of the balloon over the opening of the conical flask.
3. Measure the circumference of the balloon using the string and ruler. Record
results.
4. Place flask in ice for 5mins and then remeasure balloon. Record results.
5. Place flask in hot water for 5 mins and remeasure balloon. Record results.
Results:
Balloon
Circumference (cm)
At Start
After Heating
After Cooling
Discussion:
1. What happened to the balloon when the flask was heated and cooled?
2. Relate the expanding and contraction of gas to plate tectonics.
Conclusion: