Jyotirmoy Paul, Indian Institute of Science Contact: Tweet @GeophyJo The top ~100 km of the Earth is rigid and elastic. This part is known as the lithosphere. Below the lithosphere, there is the mantle. The lithosphere is broken into several fragments, just like the crack on an eggshell. Each of these fragments is known as a lithospheric plate or simply, plate. At present, it has been well established that plates are not stagnant; rather, they move in certain velocities with respect to each other. Their velocities are well constrained from the satellite observations. This relative mobility of the plates is known as plate tectonics: the unifying theory of geoscience. Figure 1 showing a schematic diagram of present-day plate tectonic mode. White arrows indicate relative movement of the lithosphere, and the black arrows represent the movement of the lithosphere-mantle coupled system. Hence, due to plate tectonics, older lithosphere sinks into the mantle along the subduction zone, and newer lithosphere is generated along the mid-oceanic ridges. This lithosphere recycling is the key feature of plate tectonics. It is the fundamental curiosity of science to trace the origin of a natural phenomenon. As biologists are interested in understanding the origin of life, physicists are interested in the origin of the universe; geoscientists are often interested in pinpointing the initiation of plate tectonics on the Earth. Using geochemical analysis, the solar system's origin is precisely dated at around ~4.56 Ga (billion years ago). Nevertheless, Exactly when and how plate tectonics started operating has remained a primary question. Several studies have suggested the initiation of plate tectonics over a range of time, starting from ~4.2 Ga to ~0.85 Ga. However, most studies consider ~3 Ga to be the onset of plate tectonics. Hence, plate-tectonics was not active for the first ~1.5 billion years since the inception of our planet. So, what was there before plate tectonics? There are several hypotheses for early Earth tectonics. It is believed that the early Earth had one unbroken lithosphere like a complete eggshell without any crack. The surface did not interact with the interior of the Earth. This type of tectonism is called the "stagnant-lid" tectonics. Many planetary bodies of the solar system (e.g., Io, a moon of Jupiter) possess stagnant-lid tectonics. Rising the mantle-derived melt from Earth's interior to the surface is the only heat transfer mechanism in this tectonic mode (Figure 2). That is why this is also called the heat pipe mode of tectonism. The second mode of tectonism is called "episodic-lid" tectonics. In this tectonic mode, the rigid planetary lithosphere sinks into the interior mantle, and a completely new lithosphere is formed. In Figure 3, it is shown that a part of the lithosphere is destroyed, and a newly formed lithosphere is being resurfaced (green patch) at the place of the previously destroyed lithosphere. Such tectonic mode is probably active on Venus. Plate-tectonics is the third mode of convection, which is also known as mobile-lid tectonics. Earth is the only planet in the solar system that shows active mobile-lid tectonics (Figure 1 ). Recently, another tectonic mode has been discovered, which is called "squishy plume-lid" tectonics. In such mode, the planetary lithosphere is broken into plates only along the divergent boundaries or ridges. Destruction of the lithosphere happens through the dripping of the thick lithosphere. Lithospheric drip occurs when an old and colder part of the lithosphere becomes thick and dense. Due to gravitational instability, this thicker lithosphere gets detached. Figure 4 shows a schematic diagram of this tectonic mode. But how would we check which of these mechanisms were actually active on the early Earth? The answer lies in the oldest rocks of the Earth, known as craton. Due to plate tectonic recycling, most rocks have been destroyed. However, very strong cratons older than 3 billion years have survived against the tectonic recycling force. They comprise only 5% of the Earth's surface area. Scientists develop numerical models of different tectonic modes and investigate under which mode they could obtain rocks equivalent to the cratonic composition and how these newly formed rocks could remain stable for billions of years. These numerical models show that rocks similar to cratons could be generated under the squishy plume-lid tectonic regimes. But to stabilize them, a transition from the heat pipe tectonics to plate tectonics could be more efficient. More research on the early Earth geodynamics integrated with geochemical observations will give us a better idea of the exact mode of tectonism in the early Earth. The early tectonic modes were efficient enough to cool down the earth significantly, that could produce cold and denser slabs - a prime driver of present-day plate tectonics. Further reading:
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