Geology Bites

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. For photos, figures, and further reading, see https://www.geologybites.com.

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. Go to geologybites.com to see illustrations supporting this episode.

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.

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.

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.

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.

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.

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 might be atypical. In the podcast, Hal Levison explains why the Trojan asteroids of Jupiter offer us the best opportunity to discriminate between the various models of Solar System evolution. And that is why a spacecraft called Lucy is now well on its way to a rendezvous with these asteroids. Hal Levison is the Principal Investigator of the Lucy mission. He studies the dynamics of astronomical objects and, in particular, the formation and long-term behavior of solar system bodies. He is one of the original proponents of the Nice model (named after the city where it was conceived), a scenario that proposes the migration of the giant planets from an initial compact configuration closer to the Sun to their present positions. He is Chief Scientist in the Department of Space Sciences at the Southwest Research Institute in Boulder, Colorado.

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, reef-building organisms appear and disappear again and again until the corals we have today appeared in the Middle Triassic, about 240 million years ago. Pruss is currently trying to understand why reefs are such a persistent feature of the geological record, despite the environmental stresses imposed on them. She is a Professor of Geosciences at Smith College.

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 talks about water-driven volcanism on Mars and on the icy moons of Jupiter and Saturn. Manga is a Professor in the Earth and Planetary Science department of the University of California, Berkeley.

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 and the diversification of aqueous environments caused by influxes of salt water during the marine incursions. Hoorn is an Associate Professor in the Institute for Biodiversity and Ecosystem Dynamics at the University of Amsterdam and Research Associate at the Negaunee Integrative Research Center, Earth Science Section, Field Museum of Natural History, Chicago.

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 on constraining the so-called Goldilocks zone, i.e., not too hot nor too cold for liquid water to exist, an equally key issue is how a planet would get its water in the first place. In the podcast, Anat Shahar explains how her modeling and experiments predict that plenty of water would form as a result of chemical reactions between the hydrogen atmospheres observed on many exoplanets and the magma ocean with which planets initially form.. Shahar is a Staff Scientist and Deputy for Research Advancement at the Earth and Planets Laboratory at the Carnegie Institution for Science in Washington, DC.

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. Many of his findings come from one region in particular that provides an exceptional window into the origin, evolution, and structure of plutons – the Southern Fiordland region of New Zealand’s South Island. Klepeis is a Professor in the Department of Geography and Geosciences at the University of Vermont.

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 geological questions we can now address with such systems. In the podcast, Tom Herring describes how these systems are giving us new insight into plate motions, slow and fast deformation associated with faults and earthquakes, the Earth’s rotation, as well as applications in civil engineering, such as dams and tall buildings, and agriculture. Herring is a pioneer in high-precision geodetic analytical methods and applications for satellite-based navigation systems to study the Earth’s surface. He is a Professor in the Earth, Atmospheric, and Planetary Sciences department at the Massachusetts Institute of Technology.

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 travel north to form what was to become Europe. The Variscan was caused by the collision of Gondwana with Laurussia in the final stages of the assembly of the supercontinent Pangea. Both orogenies have been heavily eroded, and we see their imprint in the form of metamorphic rocks, volcanic rocks, granites, and deformation structures. These are scattered across Europe, from southern Britain to eastern Europe. Žák has been studying the geology of central Europe for over 25 years using methods ranging from structural studies in the field to detrital zircon geochronology. He is a Professor in the Institute of Geology and Paleontology at Charles University in Prague.

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 environment, including the proximity of other subduction zones. Faccenna has been studying how convergent margins evolve for over 30 years, concentrating particularly on the Mediterranean region. He is Head of the lithospheric dynamics section at the Helmholtz Center for Geosciences at GFZ in Potsdam in Germany and also a Professor at the Department of Science at Roma Tre University.

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-down remnants of the Paleozoic mountains. Instead, they reflect much more a topography that was created during processes associated with rifting and magmatism that accompanied the opening of the Atlantic Ocean as well as the effects of the ice ages as recently as about 10,000 years ago. Van Staal has been studying the Appalachians for over 35 years, focusing especially on the large-scale tectonics of their formation. He is Emeritus scientist at the Geological Survey of Canada and an Adjunct/Research Professor in the Department of Earth and Environmental Sciences at the University of Waterloo in Ontario.

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. These give us confidence that we can make three-dimensional maps of certain structures, such as the plume below Iceland, cold continental interiors, mid-ocean ridges, and the large low shear-velocity provinces. Fichtner is a Professor in the Department of Earth and Planetary Sciences at the Federal Institute of Technology in Zurich.

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 geological processes created molecular hydrogen that may have powered the first forms of life. In her own research, she has focused on the critical role played by water released from hydrous minerals that formed within oceanic lithosphere on the sea floor. Tamblyn is a Postdoctoral Researcher at the University of Bern.

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 initial widespread normal faulting to the localization of stretching along a rift axis, followed by rapid extension and eventual breakup and formation of oceanic lithosphere. Kolawole is especially interested in the early stages of rifting, and in his research he uses field observation, seismic imaging, and mechanical study of rocks. He is Assistant Professor of Earth and Environmental Sciences, Seismology, Geology, and Tectonophysics at the Lamont-Doherty Earth Observatory of Columbia University.