Alec Brenner on When Tectonic Plates First Moved

Tiny bubbles of air trapped in Antarctic ice are samples of the ancient atmosphere itself, one of the few direct records of how greenhouse gases and climate have changed together. Until recently, the oldest ice known was 2.7 million years old. Now researchers in the Allan Hills, where ice from the interior stalls against the mountains and fierce winds strip away its surface, have found ice about 6 million years old. Ed Brook explains how such old ice survives, how it can be dated with no layers to count, and how noble gases in its bubbles take the temperature of the entire ocean. And he describes a puzzle: over three million years in which the ocean cooled and the ice ages deepened, carbon dioxide fell only modestly. What does that say about how sensitive the climate is to greenhouse gases? Brook directs COLDEX, the US center set up to find and study the oldest ice, and is University Distinguished Professor of Earth, Ocean, and Atmospheric Sciences at Oregon State University.For photos and figures illustrating this episode, visit geologybites.com.

By the end of the 1960s, plate tectonics had been widely accepted, but it rested on an untested assumption: that the plates are strong enough to move as rigid slabs. Tony Watts describes how, as a postdoc at the Lamont-Doherty Earth Observatory in the early 1970s, he helped measure that strength by treating the lithosphere like an elastic beam bending under a load, first at ocean trenches and then at seamounts. The same approach has since been applied to sedimentary basins, where it called into question some estimates of past sea-level rise, as well as to ice sheets and to volcanoes on Mars. He closes with the questions that remain open, including why the plate seems thicker over short timescales than over long ones, and how new subduction zones begin. Watts is Emeritus Professor of Marine Geology and Geophysics at the University of Oxford.Illustrations and full transcript: https://geologybites.com/tony-watts-on-the-mechanics-of-tectonic-plates

Continental crust is buoyant, which is why continents stand high above the ocean floor. Yet rocks in western Norway contain microscopic grains of coesite, a form of quartz that forms only under pressures found about 100 km down. A slab of continent was dragged deep into the mantle, and then came back up. In this episode, Torgeir Andersen, Professor Emeritus at the University of Oslo, explains how the rocks of Norway's Western Gneiss Region made that round trip some 400 million years ago, and why a few isolated diamond-bearing pods may record local overpressure rather than even greater depth.Illustrations and full transcript: https://geologybites.com/torgeir-andersen

Where tectonic plates collide, the result is clear: the edge of a plate crumples and throws up a mountain belt. But where a plate is stretched rather than squeezed, the evidence is usually lost, because the thinned lithosphere subsides and the basins that form fill with sediment.The Basin and Range Province of the western United States and northwestern Mexico is the great exception. Between the Colorado Plateau and the Sierra Nevada, dozens of mountain ranges march north in near-parallel array, in a crust that has been pulled out to about twice its original width — and it is still stretching today.In this episode we look at how we know that: from GPS antennas bolted into bedrock that measure the province widening in real time, to the record of ancient earthquakes, to the dismembered thrust sheets that let the whole region be reassembled as it was 20 million years ago. We also discuss the low-angle detachment faults that have brought rock from fifteen kilometres down up to the surface and why a stretched province still stands a mile high.Brian Wernicke is an Emeritus Professor in the Division of Geological and Planetary Sciences at Caltech.Illustrations and full transcript: https://geologybites.com/brian-wernicke

Volcanic island chains like Hawaii record the passage of a plate over a hotspot — a plume of hot rock rising from deep within the mantle. For decades, hotspots were assumed to be fixed, providing a stationary frame of reference against which to measure the absolute motions of the plates. But are they? Bernhard Steinberger has spent his career modeling how mantle flow carries plume conduits sideways as they rise, setting each hotspot drifting on its own course. In the podcast, he explains how paleomagnetism, the ages of volcanoes along hotspot tracks, and relative plate motions can be combined to disentangle plate motion, hotspot drift, and true polar wander — and how, even in motion, hotspots can still anchor the plates to the deep Earth.Steinberger is a geophysicist at the GFZ Helmholtz Centre for Geosciences in Potsdam.Illustrations and full transcript: https://geologybites.com/bernhard-steinberger

We have known for decades that the Moon once generated a strong magnetic field — comparable in strength to Earth's — throughout the period from about 4.25 to 3.5 billion years ago. Only in the past few years have we learned that the field didn't simply switch off then: it weakened dramatically but lingered on, faintly, until as recently as 1.5 billion years ago, before disappearing entirely. As Sonia Tikoo explains in the podcast, we don't really understand either how the early field grew so strong or how any field could last so long — and no single mechanism seems able to account for both the intense early epoch and the long, weak tail that followed.Sonia Tikoo studies the history of magnetic fields on the Moon and other small solar system bodies using paleomagnetism and fundamental rock magnetism.  She is an Assistant Professor in the Department of Geophysics at Stanford University.Illustrations and full transcript: https://geologybites.com/sonia-tikoo

Birds are the only dinosaurs that survived the asteroid impact 66 million years ago — but not all birds did. In this episode, Steve Brusatte draws on the fossil record to explain which birds came through the extinction, and what set the survivors apart from the many that perished alongside the rest of the dinosaurs. He traces the evolutionary transition from ground-living theropods to modern birds, drawing on the spectacular feathered fossils unearthed over the past three decades in northeastern China. Brusatte is Professor of Palaeontology and Evolution at the University of Edinburgh and author of The Story of Birds, published this year.Illustrations and full transcript: https://geologybites.com/steve-brusatte

A key development in the history of the early Earth is the formation of lithospheric plates that move independently of one another. In this episode, Brenner describes how he used paleomagnetic methods to detect relative motion between two ancient cratons, the East Pilbara and the Kaapvaal, 3.5 billion years ago. This is a full billion years earlier than any previous such detection, and it enables us to narrow down the kind of tectonics operating in the Paleoarchean. Of the candidate regimes, episodic subduction models fit his data best. Brenner is a Postdoctoral Associate in the Department of Earth & Planetary Science at Yale University. Illustrations and full transcript: https://geologybites.com/alec-brenner

Most of the material in the Earth and other planets exists under extremes of pressure and temperature quite unlike those we inhabit on the surface of the Earth. Steve Jacobsen is a mineral physicist who studies how rocks and minerals behave under such alien conditions. In the podcast, we discuss his experiments and what we’ve learned about three extreme environments: the core-mantle boundary, the mantle transition zone, and the surface of the Moon.Jacobsen is a Professor of Geological Sciences at the University of Colorado Boulder. The image shows him in his optical spectroscopy lab, where extreme conditions found throughout the solar system are re-created.Illustrations and full transcript: https://geologybites.com/steve-jacobsen

Though turbidity currents are massive and frequent underwater events, we have rarely observed them directly. Esther Sumner is one of the few researchers who has. In the podcast, she describes what it's like to instrument an active submarine canyon, what these flows have revealed about the way sediment moves across the seafloor — and the day her team accidentally flew an underwater robot into a live turbidity current in the Mendocino canyon off the coast of California. She is an Associate Professor of geology and geophysics at the University of Southampton. Illustrations and full transcript: https://geologybites.com/esther-sumner