By Sabine Stanley, Ph.D., Johns Hopkins University
The Earth is a ‘Goldilocks planet’ in more than one way. It is quite different from other planets as it possesses a number of special attributes. A closer look reveals that the unique features of the Earth are dependent on a typical characteristic of its surface, namely plate tectonics. Tectonic plates are a series of large blocks of rocks that make the Earth’s surface appear like a cracked eggshell.
The tectonic plates are formed on the Earth’s outermost rigid shell, called the lithosphere. While every other planet also experiences tectonics that results in craters, volcanoes, mountains, and faults on their surfaces, plate tectonics is unique to Earth.
Structure of the Earth
The interior of the Earth is layered like a series of spherical shells. The three main layers include an iron core at the center, the viscous mantle, and the crust. The movements inside the iron core are responsible for the protective magnetic field around the Earth. The rocky mantle comes next, and the upper layer of this rocky mantle is the weak asthenosphere. The asthenosphere aids in plate tectonics on the lithosphere. Apart from these, there are also shells such as the cryosphere, hydrosphere, and biosphere. While the cryosphere represents the frozen water portions of the Earth in the near-surface region, the hydrosphere represents the regions of the Earth with liquid water.
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Origin of Plate Tectonics
Around 3 to 4.4 billion years ago, the Earth’s crust was formed almost at a constant rate. However, around 3 billion years ago, this rate began to drop. It is believed that around this time, a weak zone would have developed on the crust. The weak zone would have pulled back some of the crust and the first subduction might have happened. Hence, while the speed of the crust formation remained the same, some of the crust must have been pulled inward, and this would have led to the plate tectonic cycle. Plate tectonics has left no direct evidence as to when it started. This is because the Earth’s surface is being continuously recycled, and the old history is constantly erased. While the oldest rocks on Earth are about 4 billion years old, there are minerals that are older.
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What are Terrestrial Planets?
Terrestrial planets have a dense metallic iron core and silicate rocks in the mantle. These planets were formed in the same inner region of the solar system and hence their overall compositions are similar. Apparently, these planets were very hot when they were formed, and, in effect, the denser iron on these planets sank to the center or core. They are also similar in size and include Mercury, Venus, Earth, and Mars.
Smaller terrestrial planets such as Mercury and Mars cool faster, resulting in thick and strong lithospheres. Since these lithospheres are sturdy and act as a single plate, they do not subduct into the mantle below. However, the convection process does take place in the mantle and it cools without the participation of the lid or lithosphere. This type of cooling, where the surface acts as a single plate and the lid is not pulled back to the mantle, is known as Stagnant Lid Convection.
Large terrestrial planets such as Venus have higher surface temperatures as they do not cool as rapidly as the smaller ones. Sometimes, water is released to the surface due to the eruption of volcanoes. Nonetheless, the higher surface temperatures lead to loss of water, and the lack of plate tectonics does not allow water to be recycled. The interiors of these planets are also hot as they have higher pressures. These terrestrial planets also undergo radioactive decay, which again generates heat. The higher temperatures lower the viscosity of the mantle and can lead to sudden recycling of surfaces. This type of mantle convection is called Episodic Lid Convection where the entire surface overturns into the mantle. Scientists believe that Venus had its last overturn between 200 million and a billion years ago and this is the reason for its surface appearing young.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
The Earth: Similar in Composition Yet Unique
The Earth being the biggest and heaviest of these terrestrial planets has its own unique characteristics. The major difference between the Earth and other terrestrial planets is the fact that the Earth’s surface never stops moving. This large scale movement of a series of plates of the lithosphere is known as plate tectonics.
- Size—The optimum size of the Earth helps in the creation of subduction zones. The smaller planets like Mercury and Mars cool faster. Cooling faster results in stronger and thicker crusts. As a result, their crusts cannot be broken easily nor can they subduct into the mantle. However, on Earth, subduction lubricates the upper mantle or asthenosphere, enabling the movement of the plates.
- Viscosity of the mantle—The Earth’s mantle contains a fair amount of water, mostly locked in minerals. The subduction process recycles water back into the mantle. As a result, the asthenosphere is lubricated, making it less viscous and weaker. This is one of the key features of the Earth that enables the plates to move and float like icebergs. Interestingly, Venus, in spite of being a bigger planet like Earth, has no plate tectonics. This is because the mantle on Venus has less water due to the higher surface temperature of that planet.
- Distance from the Sun and liquid water—The Earth is at the right distance from the sun to allow for the stability of liquid water. This ability of the Earth to retain water allows carbon dioxide to dissolve in water, creating carbonates. Plate tectonics plays a major role in the Earth’s carbon cycle and helps to maintain the correct amount of carbon dioxide in the Earth’s atmosphere. As a result, the composition of the Earth’s atmosphere is unique and unusual in that it supports various life forms. It has the optimum composition of nitrogen, oxygen, and carbon dioxide with a protective layer of ozone. Other planets like Mercury have no atmosphere at all, while Venus and Mars have higher levels of carbon dioxide in their atmospheres.
- Gravity—The strong gravitational force holds on to the atmosphere. The insulating blanket provided by the atmosphere protects the Earth from the harmful charging particles of the Sun and deep space.
Common Questions About Plate Tectonics and Its Influence on Earth
Subduction is a geological term that refers to the bending and sliding of one crustal plate below another. Subduction allows the recycling of rocks and is an important element to plate tectonics. We may also experience earthquakes and volcanoes when subduction happens.
A Goldilocks planet has a habitable zone. The Earth is a Goldilocks planet as it is at the right distance from the Sun to allow the right temperatures for liquid water, a key ingredient for life.
A cryosphere is a shell in the near-surface region and represents the frozen water portion of the Earth. It comprises the continental ice sheets on Greenland and Antarctica as well as the icecaps, glaciers, and permafrost.