The effect of rotational evolution on the surface and interior of the early Earth

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Among the unsolved mysteries of Earth’s history are the age of the first continental crust and whether processes akin to the modern plate tectonic cycle are required to form continental crust. The oldest preserved minerals found on Earth are zircons up to 4.4 billion years old, which is only a few tens of million years younger than the Earth itself. Some geochemists interpret these zircons as evidence that there was already, at that time, crust similar to continents, but it is very unclear how and whether plate tectonics could have started so early. In this work the investigators will explore an alternative series of processes that operated only in this early era of Earth history and determine whether these processes could have led to the formation of continental crust and could explain the zircon evidence. The process to be studied is rapid changes in the Earth’s rotation rate, or length of day. This is motivated by the most successful theories of the origin of the Moon, which all involve a giant impact, perhaps with a Mars-sized object, that left the Earth molten and rapidly spinning and ejected debris that condensed to form a very close-in Moon. It only takes about a million years for the outer shell of the Earth to freeze and become rigid, forming a crust and lithosphere (a crust that would be, at this stage, nothing like continents). It is well-understood that, over the next ten million years or so (and continuing at an ever-decreasing rate all the way to the present), the enormous tides raised by such a close moon cause a gravitational interaction that slows the Earth’s rotation and boosts the Moon to a higher orbit. What has not been studied in detail is the consequence of this for the shape of the Earth itself and for the deformation experienced by the rocky outer shell of the Earth as it changes shape. A rapidly spinning planet flattens significantly into a shape with a large equatorial bulge so that just after the Moon formed, the radius of the Earth through the equator may have been twice as large as that through the poles. As the length of day increases and the equatorial diameter decreases, this puts the entire equatorial region into a state of compressive stress that will thicken the lithosphere, driving possibly water-bearing material (that has interacted with the steam-rich early atmosphere) to depths where it undergoes another stage of melting, the products of which should resemble continental crust. In order to study these phenomena in detail, investigators will create a series of computer codes necessary to describe rapidly rotating planets and the melting and crystallization processes they experience. The team will document and release these codes to the scientific community for use in numerous other studies. They also intend to develop educational and outreach resources based on the unique and dramatic series of events that our planet experienced at its birth. This work will involve advanced scientific and professional training for a postdoctoral scholar as well as undergraduate research experiences. The numerical models that will be used to explore the consequences of rotational evolution on the early Earth include HERCULES, alphaMELTS, Perple_X, and custom advection-diffusion codes. HERCULES solves the hydrostatic equations of planetary structure in a frame that does not assume spherical symmetry or small perturbations thereto. It is able to accurately describe pressure, density, and gravity in a planet with arbitrarily fast rotation rate up to its stability limit (where angular acceleration at the equator exactly cancels gravity). HERCULES will be improved for this project to incorporate a wider range of equations of state. Extents of melting as a function of latitude and the composition and thickness of proto-crust will then be computed in the structures predicted by HERCULES using the pMELTS calibration, implemented in alphaMELTS for MATLAB or Python (tools developed with NSF geoinformatics support). The next stage is to move from statics to dynamics and calculate deformation rates and mechanisms using tectonophysics codes that incorporate elastic-plastic rheologies, brittle failure and viscous flow. Knowing the type of faults that are predicted and the amplitude of motion, the investigators can describe the pressure-temperature paths that protocrustal material will experience using advection-diffusion models. Finally, along those P-T paths Perplex_X pseudosections will be utilized to predict where felsic melts will form and whether the resulting crust will be buoyantly stable and likely to survive long enough to be eroded at the surface and create detrital zircons. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.