Geology Bites

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...

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...

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 C...

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 sideway...

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 entirel...

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...

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...

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...

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...

A key question about the early history of the Solar System is whether the giant planets formed roughly at the distances from the Sun they presently occupy, or, as some theories predict, much closer to the Sun. The discovery of other solar systems with radically different configurations of planets has made this question more pressing, since it appears that the configuration of the Solar System migh...

The first multicellular animals to build reefs lived in the Early Cambrian around the time of the Cambrian explosion. They were sponges called archaeocyaths. In the podcast, Sara Pruss suggests that the rise of the archaeocyaths fostered an increase in animal diversity. But they were relatively short-lived, and when they died out in the Middle Cambrian, the diversity declined. Over geological time...

Water can have a dramatic effect on the style of an eruption. In the podcast, Michael Manga explains how the most powerful eruptions, such as the 2022 Hunga Tonga eruption, occur when hot magma comes into contact with water and suddenly generates vast quantities of steam. Water dissolved in magma as it rises to the surface and depressurizes can also drive destructive volcanic eruptions. Manga also...

The Amazon Basin is the most biodiverse region on Earth, being the home of one in five of all bird species, one in five of all fish species, and over 40,000 plant species.  In the podcast Carina Hoorn explains how the rise of the Andes and marine incursions drove an increase in biodiversity in the Early Miocene. This involved the arrival of fresh river-borne sediments from the eroding mountains an...

Over 6,000 exoplanets have now been found, and the number is constantly rising.  This has galvanized research into whether one of them might host life. Since all forms of life on Earth require liquid water, at least at some stage in their life cycle, it is natural to suppose that in order to be habitable, an exoplanet should also have liquid water. While much of the public discussion has focussed...

Plutons are bodies of igneous rock that crystallize from magma at depth below the Earth’s surface.  But even though this magma never makes it to the surface, it still has to travel many kilometers up from its source near the base of the crust to the upper crust where plutons form.  In the podcast, Keith Klepeis explains how it makes that journey and describes the shape of the resulting structures....

There are three main types of geodetic measurement systems — satellite-based systems such as GPS, very long baseline interferometry (VLBI), and interferometric synthetic-aperture radar (InSAR). While each type of systems has its particular strengths, the cost of satellite-based receivers has plummeted. Millimeter-level accuracy will soon be incorporated into phones. This has broadened the kinds of...

In this episode, Jiří Žák describes the two main orogenies whose remnants figure prominently in central European geology: the Cadomian orogeny that lasted from the late Neoproterozoic to the early Cambrian (c. 700 Ma to c. 425 Ma) and the Variscan orogeny that occurred in the late Paleozoic (c. 380 Ma to 280 Ma). The Cadomian took place on the northern margins of Gondwana, only later to rift and t...

Subduction zones can be very long-lived, persisting for tens of even hundreds of millions of years. During that time they rarely stay still, but instead retreat, advance, move laterally, or reverse direction. In the podcast, Claudio Faccenna discusses the processes that govern these movements. It turns out that they depend not only on the properties of the subducting slab, but also on the environm...

In the podcast, Cees Van Staal tells us about the Paleozoic tectonic events that led to the formation of the Appalachians. The events are closely related to those involved in the Caledonian orogeny and the mountains it created in what is now Ireland, Scotland, east Greenland, and Norway, as discussed in the episode with Rob Strachan. However, the Appalachians that we see today are not the worn-dow...

In previous episodes of Geology Bites , Barbara Romanowicz gave an introduction to seismic tomography and Ana Fereira talked about using seismic anisotropy to reveal flows within the mantle. In this episode, Andreas Fichtner explains how, despite the many fiendish obstacles that stand in our way, we are making steady improvements in our ability to image the Earth on both regional and global scales...

When the Earth formed, it was covered by a hot magma ocean. So when and how did thick, silica-rich continental lithosphere form? Were the first, ancient continents similar to the present-day continents? And did the continents form in a burst of activity at a certain point, or was it a gradual build-up over Earth history? In the podcast, Renée Tamblyn addresses these questions, as well as how early...

From East Africa to southwest USA, many regions of the Earth’s continental lithosphere are rifting. We see evidence of past rifting along the passive margins of continents that were once contiguous but are now separated by wide oceans. How does something as apparently solid and durable as a continent break apart? In the podcast, Folarin Kolawole describes the various phases of rifting, from initia...

Most of Earth’s salt is dissolved in the oceans.  But there is also a significant amount of solid salt among continental rocks.  And because of their mechanical properties, salt formations can have a dramatic effect on the structure and evolution of the rocks that surround them.  This gives rise to what we call salt tectonics – at first sight, a rather surprising juxtaposition of a soft, powdery s...

Megafloods are cataclysmic floods that are qualitatively different from weather-related floods. In the podcast, Vic Baker explains our ideas as to what causes megafloods and describes the striking evidence for such floods in the Channeled Scablands of Washington State and in the Mediterranean. Vic Baker has been studying megafloods for over 50 years.  He is a Professor of Hydrology and Atmospheric...

The planets formed out of a cloud of gas and dust around the nascent Sun. Within the so-called snow line, it was too hot for liquid water to exist. Since the Earth lies well within this line, why does it have water? Did it somehow manage to retain water from the outset or did it acquire its water later? In the podcast, Lindy Elkins-Tanton explains how these two scenarios might have played out but...