19.2: The Chicken and Egg Problem of Making Continents

Figure \(\PageIndex{1}\): Partial melting makes magmas that are more felsic than their source rocks, and leaves behind solid rocks that are more mafic. (Cartoon by Callan Bentley.)

The process of making continental material on modern Earth with plate tectonics has one major requirement: continental material. So, how can you make a continent before you have a continent? With modern tectonics, the making of felsic continental rock that composes the thicker continental plates, involves the differentiation of partially melted mantle material through the subduction process and partial melting. This results in diapirs (magma chambers) of more felsic magma rising through the pre-existing crust, becoming increasingly more felsic as the buoyant magma melts surrounding rock in its journey toward the surface. Models that predict a thicker mafic crust early in Earth’s history can allow for more magmatic differentiation as magma rises in a plume (or otherwise) than would happen within oceanic plates on Earth today. Another factor is felsic input directly into magmas, a major part of today’s formation of continents. Through partial melting of host rock, which pulls more felsic components out of the host than mafic components, and assimilation, in which felsic material can enter a melt and drive the overall composition of it towards being more felsic (less mafic), large portions of recent continental material has been made. The problem is both of these methods require felsic host rocks present in the beginning of the process to work properly in large quantities. Even without plate tectonics as we know it today, there certainly is a propensity for making some felsic material, or at least material that is more felsic than the average crust at the time, in a variety of ways. And perhaps that is all that is needed; a slight increase in felsic material in a section of the crust might have been enough at the time to make it buoyant and start the more complex processes described above and make full-fledged continents. It seems that if this is the case, plume-lid volcanism or early subduction or both may have seeded the first continents. One final piece of the modern subduction story may come down to something as simple as sediment (Sobolev and Brown, 2019). It turns out the sediment is a great lubricator of slab movement, and the amount of sediment even affects modern slab descent. The problem is this: without a continent to supply sediment to lubricate subduction, it may be slow or inhibited. Sobolev and Brown suggest only after continents form and provide sediment as lubrication (such as with the post-Archean, 2.4-2.1 billion year old Huronian Glaciation), does subduction really take off.

Figure \(\PageIndex{2}\): A fragment of the Acasta Gneiss, the oldest known rock on our planet. In exhibition at the Natural History Museum in Vienna.
(CC BY-SA 3.0; By Pedroalexandrade on Wikimedia.)

Archean continental rocks may be hinting at a different process in their formation. This is due to a high concentration of rocks which are only rarely produced in tectonic settings today: the tonalite-trondhjemite-granodiorite (TTG) suite of intrusive igneous rocks and their metamorphosed descendants/equivalents, like the Acasta Gneiss. TTGs are superficially similar to normal crustal granites, but typically have a much lower amount of potassium feldspar from a lower overall elemental potassium content. According to modeling, TTGs are only formed via partial melting of metamorphically altered mafic rocks, which seems to be less common today than in the past. One thing TTGs cannot tell us is the source of this melting metamorphic mafic rock. It could come from a melting subducting slab, or it could come from lower crust delamination. It is helpful to remember that slabs today do not really melt to produce magma, but rather introduce volatiles to produce flux melting in the mantle wedge. If slabs were melting in the past this would be a clear difference between early Earth and modern tectonics. Water is also an important factor in TTG formation according to geochemical models, which does lend some credence to an early but not fully modern subduction-like process. Since water is needed to form TTGs, some process like subduction should have brought water down into the zone of formation of TTG rocks. Trace elements also support the argument that mantle mixing (with water also helping mantle convection by making it more ductile) happened with the formation of TTGs (Hastie et al, 2016). Again, a plume could also be creating weaknesses, inducing subduction (Beas et al, 2020), but either way, it appears material was moving from the surface to depth.