Stories set in stones: Acasta gneiss
The Acasta Gneiss Complex in Canada has a shiny title next to its name: it contains the oldest rocks on Earth. At an impressive 4.03 billion years old, the gneisses from Acasta mark a new chapter in the Earth’s (geo)biography: the beginning of the Archean Eon. For billions of years, these rocks have been silent witnesses to most of the geological history of our planet. The Late Heavy Bombardment, when meteorites still frequently struck the Earth. The rise of the modern atmosphere. The evolution of life and the numerous extinctions that followed. Continents coming together and drifting apart. Mountains rising and eroding away, glaciers advancing and retreating, rivers changing courses… But what about the genesis of the gneisses? These rocks have a story to tell; perhaps they are not so silent after all.
When we think of the deep-time history of our planet, it’s natural to imagine that all the biogeochemical processes we are familiar with today have always been happening. Humans like consistency. However, this thinking is very present-centric: the Earth is always changing. After all, the early Earth didn’t even have the same tectonic regime we have now (that is, one dominated by the subduction of old oceanic crust). We need to forget the familiar. For starters, the mantle was way hotter during the Hadean and Archean Eons (up to 2.5 billion years ago) than it is today. Subduction was unthinkable. With no mechanism to move the thin crust around, the surface of the Earth probably remained more-or-less stagnant, like a lid on a pot of boiling magma. Sections of hot basaltic crust may have stacked themselves on top of one another, melting the magnesium-rich rock in the process. This partial melting gave rise to the first continents made of tonalite, trondhjemite and granodiorite (felsic rocks that occur so often together that they are commonly referred to as the TTG suite). Any melt unused in their formation sank in blobs towards the cooling core, ready to be recycled. Instead of plate tectonics and subduction, we had drip tectonics and sagduction (I swear I didn’t make those words up). This is all very speculative, though; it’s a mystery geologists are still trying to get to the bottom of.
The composition of the absolutely oldest Acasta gneisses differs from the younger ones derived from the TTG suite. Their chemistry suggests that they formed when the basalt of the upper crust was partially melted by very high temperatures – which is unusual because the upper crust is not that hot. How did the temperature rise all of a sudden? One of the explanations is literally shocking (for the rocks): they were melted when meteorites struck the Earth during the Late Heavy Bombardment. The incredible heat we needed was delivered like a bolt out of the blue. What’s more, it may have been these constant meteorite impacts that eventually kickstarted modern plate tectonics.
Years of being exposed to the numerous environments of deep time take their toll. Most of the original felsic rocks, plus the few volcanic and sedimentary rocks that managed to form on the ancient land, have been metamorphosed into gneisses and greenstones. All that is left of the earliest continents today are cratons: isolated vestiges that form the oldest, most stable interiors of modern continents. They are few and far between: most were eroded away and destroyed by tectonic movements. But there is another tantalising explanation that adds to the mystery surrounding the Earth’s earliest lithologies: the volume of rocks produced during the Archean was tiny. We may never know for sure. The rocks like to keep some stories to themselves.
Hungry for more? Here are some scientific articles that describe it in more detail:
Brown, M., Johnson, T. & Gardiner, N. (2020). Plate Tectonics and the Archean Earth. Annual review of earth and planetary sciences 48.1: 291–320.
Johnson, T., Brown, M., Kaus, B., VanTongeren, J. (2014). Delamination and recycling of Archaean crust caused by gravitational instabilities. Nature Geoscience 7, 47–52. https://doi.org/10.1038/ngeo2019.
Johnson, T., Gardiner, N., Miljkovic, K., Spencer, C., Kirkland, C., Bland, P. & Smithies, H. (2018). An impact melt origin for Earth’s oldest known evolved rocks. Nature Geoscience 11, 795–799. https://doi.org/10.1038/s41561-018-0206-5.
