8. Plate Tectonics – Thermostat of the Earth

Earth’s lithosphere is composed of six large and a dozen small plates. Plate tectonics is a theory that describes large scale motions of these plates. The surface temperature of Earth is governed by many factors such as intensity of solar energy received, amount of energy actually absorbed, and amount of greenhouse gas in the atmosphere.

Fig. 1.14 – Six large and about a dozen small plates that compose Earth’s crust

The most important greenhouse gas on the Earth is carbon dioxide. Depending on the amount of carbon dioxide in the atmosphere, Earth can be too hot or too cold. Most of the carbon dioxide in the atmosphere was originated from volcanic activity. Carbon dioxide turns into limestone by chemical reaction with silicate minerals. This process removes carbon dioxide in the atmosphere and deposit the limestone onto the ocean floor. Plate tectonics transport the ocean floor deep into the magma region by subduction and melts the limestone. The carbon dioxide separated from molten limestone is released into the atmosphere through the volcanic activity. Thus, the plate tectonics and volcanic activity work together to recycle the carbon dioxide in the atmosphere. If the crust of Earth does not move by plate tectonics, the carbon dioxide will just be accumulated onto the ocean floor and won't be recycled. If this happens, temperature of the Earth will drop rapidly and our Earth will no longer be a habitable planet.

Fig. 1.15 – Correlation between the amount of carbon dioxide (middle graph) and Earth's temperature (top graph)

If there’s too much carbon dioxide in the atmosphere, temperature of the Earth rises because of excessive greenhouse effect. If the temperature of the Earth rises, formation rate of silicate minerals increases and more carbon dioxide will be removed from the the atmosphere. If there's less carbon in the atmosphere, greenhouse effect will be decreased and the temperature of the Earth will decrease too. On the other hand, if too much carbon dioxide is removed from the atmosphere, temperature of the Earth decreases and as a result the formation rate of silicate minerals will also be decreased. If there is less silicate minerals, less carbon dioxide will be removed from the atmosphere and will gradually be accumulated. If the amount of carbon dioxide increases, greenhouse effect also increases and raises the temperature of the Earth again. This cycle repeats and keeps the Earth’s temperature constant just as the thermostat in air conditioner keeps the room temperature constant.

Fig. 1.16 – Recycling of carbon dioxide by plate tectonics

The total amount of carbon dioxide in the Earth’s atmosphere is only 0.03%. Such a small fraction of carbon dioxide in the atmosphere, together with the plate tectonics regulates the temperate of the Earth just the right range we can live comfortably.

Is it a coincidence or God's design?