Geology Bites: An Earth and Planetary Science Podcast

Latest Episodes:

Roger Bilham on Himalayan Earthquakes Mar 22, 2022

Show notes

We hear about earthquakes in the Himalaya, especially when they claim lives and cause damage. And we understand that, broadly speaking, it is the continued northward movement of India ploughing into Tibet that causes these earthquakes. But where exactly do the earthquakes occur, how do they occur, and what determines how much damage they inflict?

Roger Bilham has conducted a detailed study of the historical record of earthquakes in the Himalaya over the past millennium. He tries to reconcile what we’ve observed with our current understanding of the physical mechanisms at play. This in turn helps us assess future hazard potential. He is Emeritus Professor of Geology at the University of Colorado, Boulder.

For podcast illustrations and more about Geology Bites, go to geologybites.com.

Susannah Porter on Tiny Vampires in Ancient Seas Mar 01, 2022

Show notes

The fossil record of complex life goes back far beyond the Cambrian explosion, to as far back as 1,600 million years ago in the late Paleoproterozoic with the first appearance of eukaryotes. But these creatures only started to diversify much later, around 750 million years ago. What enabled this evolutionary change has been a puzzle, but one idea is that it reflects the appearance of microscopic predators. In the podcast, Susannah Porter tells us how she discovered incontrovertible signs of predation in vase-shaped microfossils dating from this period.

Susannah Porter is a professor in the Department of Earth Sciences at the University of California, Santa Barbara. She studies microfossils of eukaryotic life forms that lived in the Neoproterozoic, about 750 million years ago.

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Ana Ferreira on Seeing Flows in the Mantle Feb 21, 2022

Show notes

Does the pull of a subducting slab drive plate motions? Or is it the upwellings of convection cells in the mantle? We now have a new way to shed light on this question. It's called seismic anisotropy, which is the spreading out of seismic waves according to their direction of polarization. This happens when the mantle through which the waves travel has crystals which are preferentially aligned, and that occurs when there is deformation or flow going on. So we can work backwards to use the observed dispersion of seismic wave arrival times to infer flow patterns in the mantle. In the podcast, Ana Ferreira explains how we do this, and describes our first views of an interaction between a mantle plume pushing up into and flowing around a subducting plate in the southwest Pacific. But, despite the vigor of such plumes, she concludes that it is probably the pull of the sinking slabs that is still the primary driver of plate motions.

Ana Ferreira is Professor of Seismology at University College, London. She collects and analyzes seismic data from around the world, focusing particularly on seismic anisotropy.

See her maps of mantle flow at geologybites.com.

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Phil Gibbard on the Anthropocene Feb 12, 2022

Show notes

There’s a lot of debate about the idea that the global changes brought about by humans define a new geological epoch, dubbed the Anthropocene. Should such an epoch be added to the official geological time scale? If so, what aspect or aspects of anthropogenic change should be used, and exactly where do we place the golden spike that will define the base of the Anthropocene?

Such questions come under the purview of the International Commission on Stratigraphy, whose current secretary general is our guest in this episode. Phil Gibbard is Emeritus Professor of Quaternary Palaeoenvironments at the University of Cambridge. In 2009, he was a founding member of the Anthropocene Working Group tasked by the International Commission on Stratigraphy to examine the status, hierarchical level, and definition of the Anthropocene as a potential new formal division of the Geological Time Scale. He explains that no consensus on the Anthropocene has been reached, and it remains controversial as to whether there is even a need for such an epoch at all.

Go to geologybites.com for podcast illustrations. And if you're enjoying the series, I'd be grateful if you could rate the podcast and leave a review.

David Bercovici on How Plate Subduction Starts Feb 05, 2022

Show notes

Subduction zones are a fundamental aspect of plate tectonics, yet we still don't really understand how subduction initiates. It's a tough problem because as oceanic plates move away from a mid-ocean spreading center and cool, they get stiffer and should become more and more resistant to bending and sinking down into the mantle. But recent work suggests that the clue to this puzzle lies in the physics of grains at the microscale.

David Bercovici is one of the geologists who has pioneered the approach that looks at what happens in the rocks of the oceanic lithosphere on the scale of a few microns. It turns out that stresses in the plate can generate bands of vertically aligned weakness that could make the lithosphere much weaker in precisely the direction in which it sinks into the mantle.

David Bercovici is a professor of Earth and Planetary Science at Yale University. In the podcast, he explains how the behavior of stressed mineral grains at the microscale can radically affect the strength of oceanic lithosphere on the plate scale. The model is consistent with lab experiments and the contrasting seismic observations of the Pacific and Atlantic plates.

Bob Hazen on the Evolution of Minerals Jan 25, 2022

Show notes

New rock types emerge during the history of the Earth. For example, the silica-rich felsic rocks such as granite that characterize continental crust, accumulated during the course of Earth history. Granite only forms in certain specific tectonic settings, such as above subduction zones and when lower crustal rocks melt in mountain belts. But what about the minerals themselves? Have they been around since the Earth formed, or did they too only appear on the scene later as a result of some geological process?

The question of how and when the minerals evolved is a relatively new subject, and was, and continues to be, pioneered by this episode's guest. Bob Hazen is Senior Staff Scientist at the Earth and Planets Laboratory of the Carnegie Institution for Science and Professor of Earth Sciences at George Mason University. At a Christmas party in 2006, a well-known biophysicist asked him the question: “Were there clay minerals in the Archean?” Apparently, nobody had given this much thought prior to 2006. The topic quickly became the focus of his research, rapidly blossoming into a whole new branch of mineralogy.

Matt Jackson on the Heterogeneity of the Mantle Jan 08, 2022

Show notes

Matt Jackson is a Professor of Earth Science at the University of California, Santa Barbara. He probes the chemical composition of the mantle by analyzing trace elements and isotopes in hot-spot lavas from around the world. In the podcast, he describes the intriguing heterogeneity among the hot-spots of the so-called “hot-spot highway” in the western Pacific. The heterogeneity there, as well as on larger spatial scales is challenging our ideas about the motions of the mantle over the billions of years of Earth history.

Carmie Garzione on Reconstructing Land Elevation Over Geological Time Jan 01, 2022

Show notes

Throughout geological history, various points on the Earth’s surface have been lifted up to great elevations and worn down into low, flat-lying regions. Determining surface elevation histories is difficult because rocks that were once on the surface are usually eroded away or buried. Furthermore, most rock-forming processes are not directly affected by elevation. But it turns out that we can overcome these challenges, as Carmie Garzione explains in the podcast.

Carmie Garzione is Dean of the College of Science at the University of Arizona. She has managed to pin down the history of elevation changes by analyzing stable isotopes of carbon and oxygen in carbonate rocks. She describes how the method works, and presents her findings for the Tibetan plateau and the Andes. They show pulses of very rapid (geologically speaking) uplift. What might this be telling us about what has been going on in the lower crust and upper mantle in these regions?

Chuck DeMets on High-Resolution Plate Motions Dec 25, 2021

Show notes

The magnetic stripes frozen into the sea floor as it forms at mid-ocean ridges record the Earth’s magnetic field at the time of formation. Reversals in the Earth’s magnetic field define the edges of these stripes, in effect time-stamping the sea floor position.

Chuck DeMets is Emeritus Professor of Geoscience at the University of Wisconsin, Madison. He studies the magnetic anomalies in seafloor rocks to reconstruct plate motions at a temporal resolution five times better than has been done hitherto. This has revealed unexpected speed-ups and slow-downs in plate motions that provide juicy puzzles for geodynamicists. In the podcast he focuses on the detailed motions of the Indian plate that show, among other things, that its northward movement actually sped up for a period after the collision with Eurasia before it settled down to a steady slower speed.

Go to geologybites.com for podcast illustrations and to learn more about Geology Bites.

Mike Searle on Ophiolite Dec 18, 2021

Show notes

As the name implies, oceanic lithosphere underlies the oceans of the world. Except when they are ophiolites, when oceanic lithosphere is thrust on top of a continental margin. Are ophiolites a special kind of oceanic lithosphere? Or are there peculiar tectonic circumstances that emplace denser oceanic rocks on top of lighter continental ones? Mike Searle addresses these questions, and reveals the sequence of events that created the world's most extensive and best-preserved ophiolite - the Semail ophiolite in Oman.

Mike Searle is Emeritus Professor of Earth Sciences at the University of Oxford and at the Camborne School of Mines in Cornwall.

For podcast illustrations, go to geologybites.com.