Episodes
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 is Assistant Professor of Earth and Planetary Sciences at Harvard University.
For podcast illustrations...
Published 01/02/23
Over the course of Earth history, many parts of the crust have undergone multiple episodes of metamorphism. Modern methods of dating and measuring trace-element abundances are now able to tease out the timing and conditions of the individual episodes. But new techniques were needed before these methods could be scaled up to unravel regional tectonic events such as the formation of mountain belts and subduction zones and continental rifting. In the podcast, John Cottle describes one such...
Published 12/03/22
This episode is the second of two of my conversation with Martin Gibling. In the first episode, we discuss fluvial deposits in the geological record and we trace the effect that the break-up of Pangea around 200 million years ago had on river systems. In this episode, we address the history of the rivers of Europe and the Americas, as well as the impact of the recent ice ages on today’s rivers. We end by considering how humans have changed rivers and their deposits throughout mankind’s...
Published 11/10/22
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. On the one hand, their courses may be determined by tectonic processes such as the formation of mountain belts. And on the other, they themselves can affect tectonic processes by creating continent-scale features, such as giant...
Published 11/10/22
This episode is a bit of a departure from the objective approach to geology of past episodes in that here we address the subjective nature of various rocks as experienced by a rock climber with a literary bent.
A rock climber’s very survival can depend on the properties of a rock encountered along a climbing route. This engenders a uniquely intense relationship between climber and rock. Anna Fleming has written perceptively about this intense relationship gained from climbing in Britain...
Published 10/10/22
Between 1.3 and 1.1 billion years ago, magma from the Earth's mantle intruded into a continent during the assembly of the supercontinent called Nuna. Through good fortune, the dykes and central complexes that resulted have been preserved in near-pristine condition in what is now the south of Greenland. The dykes are extraordinarily thick, and the central complexes contain an order of magnitude more exotic minerals than otherwise similar complexes around the world. In the podcast, Brian Upton...
Published 09/02/22
Subduction zones are places where a slab of oceanic lithosphere plunges into the mantle below. The slab consists of the sediments on top, crustal rocks in the middle, and the lithospheric mantle on the bottom, all plunging down together as a kind of sandwich. In each of these layers is an ingredient that plays a key role in shaping the evolution of the Earth over geological time – and that is water.
Geoff Abers has conducted extensive research on water in subduction zones. In this...
Published 08/10/22
Popular reconstructions of ancient environments, whether they be in natural history museum dioramas, in movies, or in books, present a world of color. But are those colors just fanciful renderings, perhaps based on the colors we see around us today? Or is there evidence in the fossil record that we can use to determine the actual color of plants and animals that lived in the geological past?
Maria McNamara tries to answer these questions by studying the fossil preservation of soft tissues,...
Published 06/17/22
For many years, efforts to limit climate change have focused on curtailing anthropogenic emissions of greenhouse gases. But it is increasingly clear that such curtailment will not, on its own, be able to prevent the damaging effects of global warming. Therefore, more attention is now directed to mitigating climate change by enhancing the removal or sequestration of greenhouse gases from the atmosphere. As a result, our climate change goals are now often specified in terms of when we plan...
Published 06/01/22
When plate tectonics was adopted in the 1960s and early '70s, researchers quickly mapped out plate movements. It seemed that plates moved as rigid caps about a pole on the Earth's surface. But since then, a lot of evidence has accumulated suggesting that plates are not, in fact, totally rigid. In fact, we can see them flex in response to stresses that are imposed on them. Such stresses can arise on plate boundaries, such as when two plates collide and one plate flexes down to subduct...
Published 05/12/22
Life only emerged from water in the Ordovician. By that time, life had been thriving in oceans and lakes for billions of years. What did the colonization of the land look like, and how did it reshape the Earth’s surface? Neil Davies describes how we can decipher the stratigraphic sedimentary record to address these questions. Perhaps surprisingly, it’s easier to recognize small and fleeting events than to recognize large-scale features such as mountains, valleys, and floodplains. He also...
Published 04/24/22
The asteroid Psyche is probably the most metal-rich body we have discovered. There are two, quite different, theories as to how it may have formed: Either it formed that way, or it originally had a more typical composition, but its rocky outer portion was blasted off during a major collision. To help determine which is most likely, NASA is sending a space probe there, to be launched on August 1, 2022. And if we can unravel the history of Psyche, we will also learn how other planets may...
Published 04/12/22
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...
Published 03/22/22
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...
Published 03/01/22
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...
Published 02/21/22
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...
Published 02/12/22
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...
Published 02/05/22
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...
Published 01/25/22
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...
Published 01/08/22
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...
Published 01/01/22
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...
Published 12/25/21
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...
Published 12/18/21
Some of the most extensive sandstone deposits in the world were deposited by wind. How do such aeolian rocks differ from water or ice-deposited rocks? And what do they reveal about the environments in which they formed? In the podcast she describes the dunes we see in the geological record on Earth, as well as on Mars and on a comet, and explains what we've learned from them.
Mackenzie Day is an Assistant Professor in the Department of Earth, Planetary, & Space Sciences at the...
Published 12/11/21
The best maps we have of Venus were made by Magellan, a space probe that flew in the 1990s. In the summer of 2021, NASA approved a new mapping mission that will produce radically improved maps of the topography, radar reflectivity, and gravity field, and the first ever global map of surface rock type. Sue Smrekar, the mission Principal Investigator, explains why this will revolutionize our understanding of Venus and perhaps also throw light on the early history of Earth when processes...
Published 12/04/21
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 only be understood 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...
Published 11/28/21