19.1: The Beginning of Plate Tectonics and the Beginning of Continents - How One Caused the Other

Introduction

Figure \(\PageIndex{1}\): Diagram summarizing evidence for active volcanism on Venus. (ESA graphic)

In some ways, the first rocks that existed on Earth are fairly straightforward, even though no direct evidence of their existence has ever been found. After coalescing via impacts, the material that would become the Earth was basically a hot ball of magma. The densest (metallic) material sunk down to form the core, and lighter (silicate) material floated upwards, eventually cooling enough to form the first rocks: most likely a very mafic veneer on a magma ocean. At this time, Earth may have more resembled modern Venus, with no significant tectonic movements and volcanism as the driver which shapes the surface. So, how does an Earth-like tectonic system with subduction start on a planet with no plates? Also, since continental material today is made via processes like subduction, how does continental crust evolve on a planet without tectonics? Even on Earth today, the process of how a subduction zone initiates is a bit of an enigma. How do we solve the seemingly unanswerable question of how subduction started on Earth billions of years ago? For details on these beginning steps, please review the case study on Earth’s Oldest Rocks.

Figure \(\PageIndex{2}\): Cartoon showing the general process of accretion in growing continents. (Graphic: BZenith via Wikimedia.)

Why is the start of subduction so important for understanding the start of plate tectonics? There are two main reasons: First, subduction is an important driver of plate movement today. In fact, some would argue it is the main driver. Once subduction begins, other processes, like the building of continents through collisions and the creation of new seafloor at oceanic ridge systems can also begin. Secondly, some models of early Earth focus on subduction, and use it as an assembly process for Earth’s first continental cores. Basically, subduction would create an island arc, and island arcs would accrete to make the first continents. Though this model has fallen somewhat out of favor (e.g. Bedard and Harris, 2014) in place of models discussed below, it is still a possible mechanism for Earth’s early history, and should be considered. The only rocks we have from the Hadean and Archean eons are found in buoyant continental cratons, and thus, are the only rocks you can use to directly surmise the start of plate tectonics. While subduction is a driver of felsic magmatism today (the makings of the continental crust), is it proper to assume that subduction is the main process that made it in the past? Lastly, subduction also causes the initiation of many geochemical cycles (deep water cycle, many chemical and isotopic heterogeneities) which are used to understand the mantle. If the mantle has evolved over time via tectonic movement or other methods, it is important to have a time constraint on this mixing process.

Hot Stuff, Coming Through

Figure \(\PageIndex{3}\): Hypothetical core-mantle differentiation processes: Percolation, diking, and diapirism. After Rubie et al. (2015). (CC BY-SA 4.0; By AlexInMetal – Own work, https://commons.wikimedia.org/w/inde...curid=76607390)

One of the biggest issues with modern tectonics and subduction is the heat the early Earth possessed. It is speculated that the internal heat of the early Earth was much higher than today (as much as 300 degrees Celsius), suggesting both a higher heat flow out of the planet, and thus a warmer lithosphere. Modelling has shown that this heat would have made the lithosphere weaker, more prone to breaking. A trigger, like the extra heat that comes with a mantle plume (Greya et al., 2015) or even an extraterrestrial impact (e.g., O’Neil et al, 2019) may have weakened the lithosphere enough to start the subduction process.

Figure \(\PageIndex{4}\): Block diagram showing the structure of a typical Archean craton, such as the Pilbara. Arrows show relative motion of the granite “domes” relative to the metavolcanic & metasedimentary “keels.” (Callan Bentley cartoon)

Early models for the movement of material within Earth, i.e. a proto-tectonics, have focused on vertical tectonics, found within these greenstone belts: upward moving plumes and diapirs and downward-moving delamination or “drip” tectonics. The extra heat in the mantle could have driven a more-active plume system all over the planet. Each plume could have been a place where the lithosphere was heated and weakened. This is known as plume-lid tectonics–the rising material from plumes was somewhat capped by the young, hot, but continuous lithosphere. Another part of this plume-lid model has to do with a rock known as ecologite. Models predict a thicker oceanic crust during this time; much thicker than today’s. At about 40 km, basalt metamorphoses to eclogite, which is much denser type of rock. This dense lower part of the crust would “peel off” in a process known as delamination,or ‘drip’ downward. This is a possible way material can cycle before the beginning of plate tectonics.