Why do the plates move?
By Allysa Orwig
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Last updated over 2 years ago
24 Questions
Think back to the experiment we did with liquids that had different densities.
Experiment Time!
- Place a small aluminum pan on a wire rack. Pour about a 1-cm-thick layer of vegetable oil into the beaker or pan.
- Cut out two small squares about 1 cm on a side from a piece of colored transparency paper.
- Place the pieces of transparency on top of the oil in the center of the beaker or pan.
- Adjust their positions with the tip of a pen or pencil so they are about 5 mm apart, edge to edge. See the diagram below.
5. Light a candle and place it under the center of the pan for 30
seconds. Align the candle so the flame is under the space
between the transparency pieces.
6. Observe the transparency pieces and record your
observations in question 10.
The Flow of Matter and Energy within the Earth
The temperature of the Earth increases with depth. This can be observed directly in mines and in oil wells. At a depth of about 3.5 km below Earth’s surface, the temperature of a mine can reach 55°C. One of the sources of Earth’s internal heat is the decay of radioactive elements. Radioactive decay is the process by which an unstable nucleus of an atom gives off energy. Other sources of Earth’s internal heat include the original heat of Earth’s formation and heating by the impact of meteorites early in Earth’s history. Earth can be thought of as a massive heat engine. The transfer of heat from Earth’s interior to its surface drives the movements of Earth’s crust and mantle.
Temperature affects the density of materials. Hot-air balloons show this effect well. When the air inside a balloon is heated it expands (increases in volume). The mass of the air in the balloon stays the same, but the volume increases. When the ratio of mass to volume decreases, the density decreases. Therefore, heating makes the air in the balloon less dense than the surrounding air. The hot-air balloon begins to rise. Similarly, as rocks in the interior of Earth are heated enough, their density decreases. The less dense rock rises slowly over time, unless the rocks are too rigid to allow flow.
In the experiment, you heated vegetable oil and observed the movement of transparency pieces. Why did the transparency pieces move? The answer lies in the process of thermal convection. Heating lowers the density of the oil at the bottom of the container. The less dense material begins to rise. As the oil approaches the surface, it flows to the side, making room for more oil rising from below. As it moves to the side, it cools. As it cools, it becomes more dense, and it sinks back to the bottom of the container. At the bottom of the container it is heated and rises again. This kind of density-driven circulation is called thermal convection, as shown in the figure below. Thermal convection transfers energy from one place to another by the movement of material.
In 1929, Arthur Holmes proposed the idea that there were convection cells in Earth’s mantle. He suggested that this thermal convection is like a conveyor belt. He reasoned that rising mantle material can break a continent apart. It then forces the two parts of the broken continent in opposite directions. The convection currents would then carry the continents.
Mantle convection cannot be observed directly. You cannot see the convection the way you could have observed convection in the corn syrup if you had put some tiny marker grains in the syrup. Geologists are sure that the mantle is convecting. However, they are still unsure of the patterns of convection. The patterns probably do not look much like what is shown in the diagram above.
According to this hypothesis, material is heated at the core-mantle boundary. It rises upward and spreads out horizontally. The material cools and sinks back into the interior. These convection cells are very slow moving. They might provide the driving force that moves the lithospheric plates. Material rises to the surface at places where the plates spread apart from one another. Material sinks back into Earth where plates converge. The idea of convection cells was not widely recognized during Holmes’s time. However, mantle convection cells were important in the development of the plate tectonic theory.
Geologists now think that the lithospheric plates are not just passive riders on the convection cells. Instead, they think the plates themselves play a major part in driving the convection. You will learn more about how the plates themselves play a role in driving convection in a later lesson.