Geology & Geophysics Editors' Vox

Understanding the Formation and Primordial Evolution of the Earth

The processes that formed the infant Earth set the stage for its subsequent evolution into the dynamic and habitable planet we know today.

By and

A recent AGU Geophysical Monograph, The Early Earth: Accretion and Differentiation, provides a multidisciplinary overview of the state of the art in understanding the formation and primordial evolution of the Earth.  The evolution of the Earth from a molten ball of metal and magma to the tectonically active, dynamic, habitable planet that we know today is unique among the terrestrial planets. AGU asked the editors of the book to highlight some of the important results that have emerged from a wide range of disciplines and some of the important questions that remain.

Q1: What was the motivation for this book?

For several consecutive years we organized topical sessions at the AGU Fall Meeting aimed at bringing together scientists across a range of disciplines investigating aspects of the origin and earliest evolution of Earth. At some point we realized that there has not been a collection of papers on the subject for decades, and that the field has progressed in so many areas that the time seemed right to put together a book that would synthesize the state-of-the-art.

Q2: The book brings together authors and work using a variety of recent techniques and methods on the formation of the solar system and Earth and follows on recent sample returns and other new data.  Is there a particular new view that is emerging from this synthesis and combination?

Yes, the book is wide in scope and we aimed for a series of papers that would bring into focus the main questions around how our Earth formed and evolved in the first few hundred million years, starting from condensation and accretion of primitive objects in the solar nebula, right through to core formation and magma ocean evolution and crystallization.

We think there are lots of new views since the last monograph on this subject in the 90’s, although many unknowns and healthy debates remain. There has been tremendous progress in better understanding the timing of events through the advent of short-lived isotopic tracers like 182W and 142Nd, and these have revolutionized our understanding of core formation and early magmatic differentiation in terrestrial objects, sparked fantastic debates about early differentiation, and challenged the long-held ‘chondrite’ model for bulk planetary composition.

For instance, while the old school of thought was that Earth accreted from chondrites, we now know thanks to planetary dynamical modeling that the Earth and most probably the other terrestrial planets accreted a range of larger already differentiated objects, such as planetesimals or even proto-planets. While the chondritic model is still by and large accepted as a starting point for planetary building blocks, we all have come to terms with the fact that no terrestrial planet was built from chondrites per se, and that there are a series of steps leading from chondrites to planets that are still being elucidated.

Core formation has greatly benefited from experimental and theoretical advances, resulting in coordinated efforts for comprehensive models that explain all aspects of the problem, rather than just focusing on one part and disregarding the others. In that sense, the most recent models attempt to explain simultaneously core formation in terms of (1) the depletion of siderophile elements in the mantle, (2) the isotopic fractionation of elements impacted by core formation, and (3) the density-deficit related to the light element content of the core.

Great strides have also occurred in geodynamic modeling of deep magma ocean processes, with entirely new models for how the magma ocean crystallized and evolved, the fact that the early molten Earth may not have frozen from the bottom up, but maybe from the top down, or from the center outwards, and these may explain some of the large scale features we see today at the base of the mantle.

Lastly, our better understanding of the Moon and the Earth-Moon system is allowing us to make strong statements about Moon-forming scenarios and those obviously allow understanding the formation of the Earth in a planetary perspective.

Q3: What would you say are the two or three major outstanding questions (among many) and how might we go about addressing them?

We need to understand how the Earth became a solid planet, what geochemical reservoirs formed early on, and whether they survived or not to this day. We need better experiments in the laser-heated diamond anvil cell on the detailed phase relations of crystallization of the magma ocean in the deep mantle and trace element partitioning at extreme conditions. We also need a new class of geodynamical models that allow us to simulate magma ocean crystallization, by bridging the tremendous gap between solid and liquid convection models.

Another area of active research is in accretion models that account for erosion and loss of material – accretionary differentiation – as well as those that link volatilization and core formation. How much material was eroded and lost from the Earth or Earth-Moon system by early impacts? How much was scavenged by the core? How (and how much) was the “Late Veneer” delivered to Earth, and what was it made of?

We also really need to nail down the composition of the core, especially in terms of light elements. Is the core exsolving mantle components or dissolving them at present, and what exactly is occurring at the core-mantle boundary? To what extent are the core and mantle in equilibrium and what exactly are they exchanging?

Earth is a habitable planet because, among other reasons, it has an active dynamo generating a magnetic field. Geodynamicists know how to produce a field up to 1-1.5 billion years ago, but not earlier than that. So what made the early dynamo go, and how can we produce a magnetic field as early as 4.2 billion years ago?

The structure of the deep mantle is becoming more complicated by the day, as seismologists user ever-evolving tools to discover new patches, piles, plumes, and slabs in the deep mantle. How can we interpret those in terms of composition or temperature? How can we interpret complex anisotropy them in terms of dynamics? What are LLSVPs? Or ULVZs? What exactly is the D’’ layer; is it compositionally distinct? Or dynamically/thermally different from the rest of the mantle? With all these new results, the debate about whole mantle vs. layered mantle convection is long forgotten: but what new view is emerging in terms of mixing scales, times, and model for the mantle?

—James Badro and Michael Walter, Editors, The Early Earth: Accretion and Differentiation; email: [email protected] and [email protected]

  • aedgeworth

    First, maybe you should try to figure out how this massive universe of space, time, matter, and energy came out of a state of nothingness violating the Law of Inertia, the First Law of Thermodynamics, and the Law of Cause and Effect. And how exactly did this amazingly fine-tuned planet with about 72 parameters that allow life on this planet come into existence from about where the center of the explosion supposedly took place?

    Of course Alan Guth has it all figured out. “In a moment of time (when time didn’t exist yet), in a spot no bigger than a dime (when space didn’t exist yet), a vacuum, (the absence of matter – which also requires the existence of matter when none existed) fluctuated (which requires energy when none existed).”

    You know what though, considering how the earth formed into it’s present state, have you ever considered the fact that it almost looks like it was formed to be inhabited? I’m sorry, that would require purpose and direction, I forgot we can’t consider that. Carry on with your speculation.

    • bookspeople

      For goodness sake, give us some time. We only scampered onto the grasslands a relatively short time ago and most of our existence revolved around avoiding being something’s snack or avoiding other carnivorous apes waving sharp sticks. I think the funny stories and songs our ancestors made up, the gadgets that let us stroll fearlessly through the grasslands or lunar landscapes, the idea of science which produced the laws you cite are evidence that, though we don’t have all the answers now, we will someday, if we don’t lose heart. We’ll come up with even better questions, too, and we’ll solve them, perhaps with the help of new robotic co-seekers or even friends from out there. It’s way too soon to call the game on account of a little darkness.

    • BEN

      I don’t think that figuring out where the universe came from first is the best place to start. 13 some billion years ago is a long time ago, and to take measurements of that is very difficult and possibly quite speculative. This article is about the formation of the earth during its first billion years of existence that happened less than 5 billion years ago. Still a difficult problem to theorize about, but at least the evidence is under our feet as opposed to far over our heads.

      You, me, and everything on this planet did not come from just one center of an explosion, but a whole series of them. The first one was the big bang, but lets not get into that or any thing that came before. Then came a series of stars, forming and collapsing, forming and collapsing forging the heavy elements we are made of. Lots of theory and way over my head. Ah but when we finally arrive at the formation of our planet, 8 or so billion years after the first explosion, some of the evidence of how we came to be can be physically touched and handled by even you. There is no need to speculate on the big bang to try and understand how our planet formed.

      Its all very complicated, lots of math, theory, and supercomputer number crunching, studied by a million people in thousands of specializations, all coming together, ever moving closer to the truth. I’m not a genious by any means, and so I do need to have a bit of faith in what the scientists say. I can fathom some of the basics, and can see for myself that some is valid, facts that I can touch with my own hands. But when I consider that our planets formation was guided by the hands of an all powerful god to be inhabitable, there are no facts that I can grasp. To believe in that would take a leap of faith into uncharted territory devoid of evidence. I’ll take my chances that we are all here by chance and carry on with speculating, grounded in the facts and evidence upon which we all stand.

    • Mark B

      What a load of total nonsense. If it was “designed” to be inhabited why is most of it uninhabitable? It hasn’t even finished forming or cooling and kills people on a regular basis. Life in the ocean doesn’t require the parameters you suggest – some can live in toxic hydrothermal vents. You are just ignorant and wanting to support the fiction that is creationism – and of course you present no science at all – you just throw rocks.