The Plate Tectonic Theory: The Structure Of The Earth's Crust And Its Associated Phenomena

The Plate Tectonic theory explains how the structure of the earth’s crust and its associated phenomena, result from the interaction of lithospheric plates, and is integrally linked to the landforms that exist, animals that thrive and the oxygen that we breathe. It was, and remains, fundamental in the formation and development of the Earth as we know it today. This blog aims to hypothesize the differences between the World we are familiar with, and one that would have developed without plate tectonics.

The Earth’s Structure

Fundamentally, if not for plate tectonics, the Earth’s structure and topography would remain consistent with its original form. The original Earth, explained by the nebula hypothesis, was a molten ball of rock which overtime developed new topographic features and changes via plate tectonic processes. However, with little seismic activity, there would be no opportunity for these developments. The only major topographic features potentially recognisable from life as we know it today would be hotspot-formed volcanoes and islands – such as the Hawaiian Islands. The formation of continents, due to the accretion of volcanic islands, brought together by oceanic subduction would no longer be possible.

Topography and erosion

Further standardising the Earth’s surface, major variances in topography would be broken down over time by wind erosion, leading to a flattened Earth. The breakdown of geographical features would create a positive feedback loop, in which as features were eroded, there would be less shelter, resulting in more erosion. To begin with, this would result in a water world, uniformly covering the Earth’s surface area, although as the atmosphere was gradually stripped away by solar winds, the Earth would heat up and boil the oceans, leaving a flat, waterless surface – discussed later in the article.

Lithosphere composition and the effects of this on carbon concentrations

Without plate tectonics, the only rock variety would be from asteroids, comets and meteorites, which are primarily peridotite or iron, both of which have been present since the Earth’s formation – iron in the inner core, and peridotite in the mantle. While arguably meteorites and asteroids could be classified as sedimentary during their flight (if their home planet was sedimentary, or they had simply aggregated overtime) the heat of atmospheric re-entry, and the subsequent impact when it reached the surface would metamorphosis all minerals contained. As a result of this, it is likely that there would be no sedimentary rocks present on Earth at all, they would be unable to form on the surface due to the constant wind erosion preventing compression overtime. A further effect of standardised rock types would be on the CO2 balance in the atmosphere. Continuous tectonic replenishment of un-weathered surface silicate allows for the creation of bicarbonate ions, which are precipitated into the oceans, forming carbonate sediments (solid rocks), concluding the process by which carbon dioxide is removed from the atmosphere (Waltham, 2019). Without this process, the carbon balance would be altered, and would eventually cause global desiccation, long before the water would be boiled by solar radiation.

Global resurfacing events

A fundamental process of plate tectonics is the release of the heat contained in the mantle of the Earth, present due to the molten central core, and because of the decay of long-lived radioactive isotopes in the continental crust. Due to plate tectonics, this heat is released by volcanoes or breaks in the crust allowing for magma to escape periodically. Without these methods, hotspots and magma plumes would be the only way for heat to escape, resulting in a temperature build-up beneath the mantle. The Earth would experience a similar process to the ones observed on Venus (Robert Herrick, 1994), where large sections of the mantle would melt entirely, generating and releasing huge eruptions of magma. This magma would cool overtime, creating a new layer on the surface. Internally, the mantle would lose some heat, and the process would continue until it cooled to an extent where the long-life radioactive decay in the crust no longer pushed the mantle temperature gradient above its melting point.

The Magnetosphere and Atmosphere

The Earth has a magnetic field due to the convection of electrically conducting liquid in the outer core, which successfully diverts the solar wind around our planet(Nola Taylor Redd, 2019), but would no longer be possible without plate tectonics. Without this protection – our atmosphere would gradually be stripped away over time, until eventually we were left in the vacuum-like state, with a Mars-like atmosphere. The influx of cosmic radiation would result in heating, causing the leftover cryosphere to evaporate (with no atmosphere to contain the water vapour), which would then cause further heating – a positive feedback loop. The effects of this would be even further exaggerated by the fact that a lack of atmospheric pressure would cause the boiling point of water to drop significantly (Robin Mei, 2018)

Meteorite Impacts and their effects

We estimate that between 90-95% of meteors (Samantha Matthewson, 2016) that enter our atmosphere burn up entirely before hitting the surface. Without this process, the earth would be very vulnerable to damage from external debris. There would be little to no burning of the objects as they travelled towards our Earth’s surface, resulting in frequent, powerful impacts on the planet. This would create a positive feedback loop, where impacts would release energy, blasting away parts of our atmosphere, which in turn would make the atmosphere less effective at reducing meteorite impact velocity and size. While the Earth would likely remain intact, because meteorites would lack sufficient energy to prevent recreation, impact sites would be visible all over the Earth’s surface. While the frequency and strength of meteoric impacts has waned overtime, steadily decreasing since the end of the bombardment period (3.8 Ga ago), the lack of atmosphere would mean that any rock on course for the Earth would have significant mass and impact velocity. This would ultimately combine with the high erosion rates to create a reasonably shallow topography, with low mountains, but deep impact craters.

The development of the Biosphere

A further effect of the lack of atmosphere is the devastating effect it would have on the biosphere. We currently believe that the first organisms on Earth were formed by the combination of non-gaseous minerals with an energy source, potentially lightning, which allowed for the formation of the first organic molecules. Without oceans or the volcanoes that produce hydrothermal vents, it is very possible that life on Earth would simply never exist. Furthermore, the fact that all creatures on Earth would have to be able to survive in a vacuum, makes the possibility of life developing extremely slim.

Conclusion

Overall, evidence suggests that a plate tectonic-free Earth would be unrecognizable, with no aspect of the lithosphere, biosphere, and hydrosphere as we know it today being untouched. Both Mars and Venus (planets that do not currently experience tectonic activity) provide evidence for several of the points made, with a reduced atmosphere, minimal hydrosphere and no biosphere, all of which I believe would be the same on Earth. Despite not having conclusive knowledge of the absolute effects and benefits of plate tectonics, we can make educated guesses, backed by evidence and science about how the Earth would be unrecognizable without them.