Geology Bites: An Earth and Planetary Science Podcast

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