Early Earth
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Continental crust is derived from magmas that come from the mantle. So, naively, one might expect it to mirror the composition of those magmas. But our measurements indicate that it does not. Continental crust contains significantly more silica and less magnesium and iron than mantle-derived magmas. How can we be sure this discrepancy is real, and what do we think explains it? In the podcast, Roberta Rudnick presents our current thinking about these questions. Surprisingly, more than 30 years after she and others first identified the so-called continental crustal composition paradox, there is still no consensus among geologists as to which of the many proposed hypotheses most convincingly solves the paradox.
In the early Proterozoic, about 2.5 billion years ago, enormous thicknesses of iron-rich rocks were deposited on continental shelves. Striking cliffs of these rocks form the walls of many canyons in Western Australia and South Africa. They are called the banded iron formations (BIFs) because they show vivid banding between the reddest, most iron-rich layers and ochre-colored layers containing siliceous material such as chert. What is the origin of the BIFs? For many years, the prevailing theory was that large amounts of dissolved iron were present in the early oceans, and it was the oxygen from the first oxygen-producing life forms in the water that oxidized this iron into an insoluble form, which then precipitated out of the oceans to form the BIFs. In the podcast, Clark Johnson explains how biological processes may have been responsible for oxidation of iron in seawater under oxic or anoxic conditions, as well as later reduction as the BIF sediments compacted on the seafloor.
Stromatolites are the layered structures left behind by microbial communities. They are the fossil evidence for far and away the earliest life on Earth. But the interpretation of the very earliest stromatolite-like structures as being biogenic in origin is controversial. Martin Van Kranendonk tells the story of the discovery of almost pristine stromatolites in 3.48-billion-year rocks of the Pilbara of Western Australia. By meticulous study of their structure, relationship to their geological environment, and chemical and isotopic composition, he establishes a convincing case for life on Earth going back at least as far as the age of these Pilbara rocks.
In the image, Martin Van Kranendonk is on one of the main stromatolite outcrops in the Pilbara , looking down on the tops of multiple cone-shaped stromatolites of the 3.4 Ga Strelley Pool Formation, only slightly younger than the 3.48 Ga Dresser Formation.
In a recent episode, Nadja Drabon spoke about newly discovered zircon crystals from the late Hadean and early Archean. The zircons revealed information about processes occurring in the Earth’s nascent crust, casting light on when and how modern-day plate tectonics may have started. In this episode, we talk about a very different source of information about the early Earth, namely the abundances of noble gases occurring within present-day basalts. It turns out that these can probe the Earth’s mantle and atmosphere even further back in time – to the first 100 million years of Earth history.
Sujoy Mukhopadhyay leads a team of researchers who have developed new techniques for measuring the abundances of noble gas isotopes in a variety of Earth materials. He used his results to identify mantle reservoirs that formed during the first 100 million years of Earth history and, amazingly, survive to the present day.
Vanishingly few traces of the early Earth are known, so when a new source of zircon crystals of Hadean age is discovered, it makes a big difference to what we can infer about that eon. In the podcast, Nadja Drabon describes how she analyzed the new zircons she and her colleagues discovered and what they reveal about the Earth’s crust between about 4 and 3.6 billion years ago.
Nadja Drabon in the Green Sandstone Bed in South Africa
Rivers can seem very ephemeral, often changing course or drying up entirely. Yet some rivers have persisted for tens or even hundreds of millions of years, even testifying to the breakup of Pangea, the most recent supercontinent, about 200 million years ago. In the podcast, Martin Gibling talks about rivers old and new, and how they are affected by tectonic processes over tens of millions of years, as well as by human activities over a few thousand years.
During the course of Earth history, new types of rock have appeared on the scene as the Earth cooled and plates formed and started to move and interact in accordance with plate tectonics. The different rock types reflect the particular assemblage that composed them, and each tectonic environment favors the formation of a particular rock type, such as granite or basalt. But what about the minerals themselves? Have they been present since the Earth formed, or did they, too, only appear when certain conditions were met? In the podcast, Bob Hazen discusses mineral evolution, which turns out to be fertile ground for probing the deep history of Earth.
Almost all the evidence about the nascent solar system has been erased by processes accompanying the formation of the Sun and the bodies that formed out of the circumsolar disk about 4.6 billion years ago. But some meteorites and the tiny dust grains contained within them have anomalous compositions that can be understood only by invoking a history going back to the giant molecular cloud progenitor of the solar system, and to the stars that ejected the material that formed the cloud. Rick Carlson is an isotope geochemist who measures such anomalies and uses them as clues to the birth of the solar system.
We know that the Earth was formed as a ball of molten material and quickly differentiated into a metallic core and a rocky mantle. We also know that at some point, the Earth’s surface formed itself into the rigid blocks we call plates, and that these plates started moving and interacting as part of a global process we call plate tectonics. But when was this point reached? Peter Cawood explains how five quite different types of evidence preserved in the geological record all point to the same answer.
The Earth is losing heat. Claude Jaupart wants to know how much, and so he has measured the heat flowing up through the top layers of the crust by making measurements down hundreds of boreholes. But the Earth continues to generate heat through radioactive decay of elements such as potassium, uranium and thorium. He calculates that for the past two billion years at least, the radioactive heat generation has not kept up with heat loss, and that the Earth is therefore cooling down - though at the extremely slow rate of 100 °C over a billion years.
Sarah Stewart uses computer-based dynamical simulations and lab experiments to create scenarios for the collision of a massive body with the Earth that can reproduce the composition, orbits, and spins of the Earth and Moon today. She believes a new kind of object called a synestia formed in the immediate aftermath of such a collision. Here she is in her lab with an instrument that can generate very high temperatures and pressures in a target to simulate conditions after the impact.
David Evans is a tectonic jigsaw puzzle-master. Using a wide diversity of geological clues, he reconstructs the supercontinents into which almost all of the Earth’s landmasses were joined at various points in Earth history. His own observational work focuses on magnetism imprinted in ancient rocks, which can tell us the latitude at which these rocks formed. To measure these magnetic signals, he has a specialized paleomagnetism lab at Yale.
Courtesy of Saran Morgan
John Valley describes how he analyzes mineral crystals from the earliest eon - the Hadean - to pin down the conditions that prevailed soon after the Earth formed. The picture shows him using the ion microprobe at the University of Wisconsin in Madison to analyze a Hadean zircon crystal (with Takayuki Ushikubo and Noriko Kita).
The asteroid belt is a region lying between the orbits of Mars and Jupiter that contains billions of asteroids, ranging from some as small as pebbles to as large as nearly 1000 km across. Sara Russell explains how we can use asteroids to help us unravel how the solar system formed and to cast light on questions such as how the Earth got its water and organic materials.
Photo courtesy of Tammana Begum
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