How can we extrapolate backwards to understand the conditions of the Hadaean Earth?

Submitted by Russell Scott Shapiro, California State University Chico

What we know...

The Hadaean Eon by definition is the span of time from the accretion of the Earth to the oldest rock record. Therein lies the conundrum for a geologist used to interpreting Earth history from rock data and using the well-founded principles of cross-cutting relationships superposition and original horizontality.

On one hand we can apply a paradigm approach tweaking physical chemical and biological principles to fit the conditions most likely for the early Earth, namely increased heat flow, meteoric bombardment, low solar luminosity, etc.

Another method would be to look around the solar system at the planets that have not had as much surficial modification and derive consistencies in lithospheric and atmospheric composition and build up a model for the Hadaean Earth. Also of use would be meteorites available for chemical and mineralogical analysis.

By applying both approaches students and researchers can build many alternate models for the Hadaean while also appreciating the benefits and pitfall of each method. Interestingly a review of any of the Historical Geology texts widely in use in undergraduate courses will reveal a rhetoric that announces the answers are clear and the model as presented is accepted. However current literature shows that many of the fundamental questions are still being hotly debated. What was the original composition of the atmosphere? When did oceans first appear? Was the original continental crust felsic, mafic, or ultramafic? When did lithospheric recycling in a matter recognizable as plate tectonics first appear? Before we can address these questions in a classroom setting we need to be clear on where data ends and how our model is developed.