Geology Bites

With most of Greenland buried by kilometers of ice, obtaining direct information about its geology is challenging. But we can learn a lot from measurements of the island’s geophysical properties — seismic, gravity, magnetic from airborne and satellite surveys and from its topography, which we can see relatively well through the ice using radar. In the podcast, Joe MacGregor explains how he created a new map of Greenland’s geology and speculates on what we can learn from it. MacGregor is a Research Physical Scientist at NASA’s Goddard Space Flight Center.

As we wean ourselves away from fossil fuels and ramp up our reliance on alternatives, batteries become ever more important for two main reasons. First, we need grid-scale batteries to store excess electricity from time-varying sources such as wind and solar. Second, we use them to power electric vehicles, which we are now producing at the rate of about 15 million a year worldwide. So far, the battery of choice is the lithium-ion battery. In addition to lithium, these rely on four metals — copper, nickel, cobalt, and manganese. In the podcast, Adam Simon explains the role these metals play in a battery. He then describes the geological context and origin of the economically viable deposits from which we extract these metals. Simon is a professor of economic geology at the University of Michigan.

Knowing exactly where faults are located is important both for scientific reasons and for assessing how much damage a fault could inflict if it ruptured and caused an earthquake. In the podcast, Rufus Catchings describes how we can use natural and artificial sources of seismic waves to create high-resolution images of fault profiles. He also explains how faults can act as seismic waveguides, an effect that enables us to determine whether faults are connected to each other. In Napa, a famous wine-growing area near San Francisco, he used guided waves to determine that an active fault is actually ten times longer than previously thought. Rufus Catchings is a Research Geophysicist at the US Geological Survey (USGS). Over the past 40 years, he has studied many dozens of faults in California and elsewhere to pin down their precise locations and help assess the risks they pose.

During the past couple of decades, we have discovered that stars with planetary systems are not rare, exceptional cases, as we once assumed, but actually quite commonplace. However, because exoplanets are like fireflies next to blinding searchlights, they are incredibly difficult to study. Yet, as Sara Seager explains, we are making astonishing progress. Various ingenious methods and the use of powerful space telescopes enable us to learn about exoplanet atmospheres and even, in some cases, what their surfaces consist of. Sara Seager’s research concentrates on the detection and analysis of exoplanet atmospheres, and she has just won the prestigious Kavli Prize for this work. She has had leadership roles in space missions designed to discover new exoplanets and find Earth analogs orbiting a sun-like star. She is a Professor of Aeronautics and Astronautics, Professor of Planetary Science, and Professor of Physics at the Massachusetts Institute of Technology.

We have only a tantalizingly small number of sources of information about the Earth’s deep mantle. One of these comes from the rare diamonds that form at depths of about 650 km and make their way up to the base of the lithosphere, and then later to the surface via rare volcanic eruptions of kimberlite magma. In the podcast, Evan Smith talks about a new class of large gem-quality deep-mantle diamonds that he and his coworkers discovered in 2016. Inclusions within these diamonds serve as messenger capsules from the deep mantle. They show an unmistakable genetic link to subducted oceanic slabs, and thus give us clues as to what happens to subducted slabs as the pass through the lower mantle transition zone. Evan Smith is a Senior Research Scientist at the Gemological Institute of America, New York.

Continental crust is derived from magmas that come from the mantle. So, naively, one might expect it to mirror the composition of the mantle. But our measurements indicate that it does not. Continental crust contains significantly more silica and less magnesium and iron than the mantle. How can we be sure this discrepancy is real, and what do we think explains it? In the podcast, Roberta Rudnick presents our current thinking about these questions. Surprisingly, more than 30 years after she and others first identified the so-called continental crustal composition paradox, there is still no consensus among geologists as to which of the many proposed hypotheses most convincingly solves the paradox. Rudnick is a Distinguished Professor in the Department of Earth Science at the University of California Santa Barbara.

We tend to think of continental tectonic plates as rigid caps that float on the asthenospheric mantle, much like oceanic plates. But while some continental regions have the most rigid rocks on the planet, wide swathes of the continents are not rigid at all. In the podcast, Alex Copley explains how this differentiation comes about and points to evidence that the responsible processes have been operating since the Archean. Copley is Professor of Tectonics in the Department of Earth Sciences at the University of Cambridge.

Shanan Peters believes we need to assemble a global record of sedimentary rock coverage over geological time. As he explains in the podcast, such a record enables us to disentangle real changes in the long-term evolution of the Earth-life system from biases introduced by the unevenness and incompleteness of the sedimentary record. To this end, he and his team have established Macrostrat, a platform for the aggregation and distribution of our knowledge about the spatial and temporal distribution of sedimentary rocks. In the podcast, he describes some important findings made possible by Macrostrat. One of them is that gaps in the record are often as revealing about the underlying processes involved as the rocks preserved above and below the gaps. Peters is a Professor in the Department of Geoscience at the University of Wisconsin-Madison.

Complex life did not start in the Cambrian - it was there in the Ediacaran, the period that preceded the Cambrian. And the physical and chemical environment that prevailed in the early to middle Cambrian may well have arisen at earlier times in Earth history. So what exactly was the Cambrian explosion? And what made it happen when it did, between 541 and 530 million years ago? Many explanations have been proposed, but, as Paul Smith explains in the podcast, they tend to rely on single lines of evidence, such as geological, geochemical, or biological. He favors explanations that involve interaction and feedback among processes that stem from multiple disciplines. His own research includes extensive study of a site where Cambrian fossils are exceptionally well preserved in the far north of Greenland. Smith is Director of the Oxford University Museum of Natural History and Professor of Natural History at the University of Oxford.

Jupiter's innermost Galilean moon, Io, is peppered with volcanos that are erupting almost all the time. In this episode, Scott Bolton, Principal Investigator of NASA's Juno mission to Jupiter, describes what we're learning from this space probe. Since its arrival in 2017, its orbit around the giant planet has progressively shifted to take it close to Jupiter’s moons and rings. In December 2023 and February 2024, it flew by Io, approaching within a distance of only 1,500 km. This enabled Juno to capture high-resolution imagery of its constantly changing surface, including hitherto unseen regions near its poles. As discussed in the podcast, Juno is equipped with a microwave instrument that enables it to look slightly below the moon’s surface into its lava lakes, as well as a suite of magnetometers to study Jupiter’s giant magnetosphere and its remarkable interaction with Io. Bolton’s research focuses on Jupiter and Saturn and the formation and evolution of the solar system. Prior to the Juno mission, he led a number of science investigations on the Cassini, Galileo, Voyager, and Magellan missions. He is Director of the Space Sciences Department at Southwest Research Institute in San Antonio, Texas.

We know that most magma originates in the Earth’s mantle. As it pushes up through the many kilometers of lithosphere to the surface, it pauses in one or more magma chambers or partially melted mush zones for periods of up to a few millennia before erupting. But while we have seismic evidence and models and support this picture, we have not hitherto been able to watch how magma actually moves in the upper mantle and crust. Bob White has set out to change that. Using a dense array of seismometers, he has been able to pinpoint thousands of tiny earthquakes that reveal the detailed movement of melt through the thick crust of Iceland just before it erupted. White combines this seismic data with geochemical analyses of the lava that can tell us about the depths at which the melt is formed. White is Emeritus Professor of Geophysics in the Department of Earth Sciences at the University of Cambridge.

At roughly 15-25-million-year intervals since the Archean, huge volumes of lava have spewed onto the Earth’s surface. These form the large igneous provinces, which are called flood basalts when they occur on continents. As Richard Ernst explains in the podcast, the eruption of a large igneous province can initiate the rifting of continents, disrupt the environment enough to cause a mass extinction, and promote mineralization that produces valuable mineral resources. Richard Ernst studies the huge volcanic events called Large Igneous Provinces (LIPs) — their structure, distribution, and origin as well as their connection with mineral, metal, and hydrocarbon resources; supercontinent breakup; and mass extinctions. He has also been studying LIP planetary analogues, especially on Venus and Mars. He has written the definitive textbook on the subject. Ernst is Scientist in Residence in the Department of Earth Sciences, Carleton University, Ottawa, Canada, and Professor in the Faculty of Geology and Geography at Tomsk State University, Tomsk, Russia.

Perhaps as many as five times over the course of Earth history, most of the continents gathered together to form a supercontinent. The supercontinents lasted on the order of a hundred million years before breaking apart and dispersing the continents. For decades, we theorized that this cycle of amalgamation and breakup was caused by near-surface tectonic processes such as subduction that swallowed the oceans between the continents and upper mantle convection that triggered the rifting that split the supercontinents apart. As Damian Nance explains in the podcast, newly acquired evidence suggests a very different picture in which the supercontinent cycle is the surface manifestation of a process that involves the entire mantle all the way to the core-mantle boundary. Damian Nance draws on a wide range of geological evidence to formulate theories about the large-scale dynamics of the lithosphere and mantle spanning a period going back to the Archean. A major focus of his research is the supercontinent cycle. He is Distinguished Professor Emeritus of Geological Sciences at Ohio University.

The Earth’s tectonic plates float on top of the ductile portion of the Earth’s mantle called the asthenosphere. The properties of the asthenosphere, in particular its viscosity, are thought to play a key role in determining how plates move, subduct, and how melt is produced and accumulates. We would like to know what the viscosity of the the asthenosphere is, and how it depends on temperature, pressure, and the proportion of melt and water it contains. Few mantle rocks ever reach the Earth’s surface, and those that do are altered by weathering. So, as he explains in the podcast, David Kohlstedt and his team have tried to replicate the rock compositions and physical conditions of the mantle in the lab. Using specially-built apparatus, he has been able to determine the viscosity of the asthenosphere to within an order of magnitude, which is an enormous improvement on what was known before. David Kohlstedt is Professor Emeritus at the School of Earth and Environmental Science at the University of Minnesota.

In many countries, nuclear power is a significant part of the energy mix being planned as part of the drive to achieve net-zero greenhouse-gas emissions. This means that we will be producing a lot more radioactive waste, some of it with half-lives that approach geological timescales, which are orders of magnitude greater than timescales associated with human civilizations. In the podcast, Claire Corkhill discusses the geology such storage sites require, some new materials that can confine radioactive isotopes over extremely long timescales, and the kind of hazards, including human, we need to guard against. Claire Corkhill is Professor of Mineralogy and Radioactive Waste Management in the School of Earth Sciences at the University of Bristol, UK.

We have learned a great deal about the geology of the Moon from remote sensing instruments aboard lunar orbiters, from robot landers, from the Apollo landings, and from samples returned to the Earth by Apollo and robot landings. But in 2025, when NASA plans to land humans on the Moon for the first time since 1972, a new phase of lunar exploration is expected to begin. What will this mean for our understanding of the origin, evolution, and present structure of the Moon? A lot, according to Mahesh Anand. For example, as he explains in the podcast, satellite imagery suggests that volcanism continued for much longer than was previously thought, perhaps until as recently as 100 million years ago. In-situ inspection and sample return should help us explain this surprising finding. Mahesh Anand is Professor of Planetary Science and Exploration at the Open University, UK.

At the core of Earth’s geological thermostat is the dissolution of silicate minerals in the presence of atmospheric carbon dioxide and liquid water. But at large scales, the effectiveness and temperature sensitivity of this reaction depends on geomorphological, climatic, and tectonic factors that vary greatly from place to place. As described in the podcast, to predict watershed-scale or global temperature sensitivity, Susan Brantley characterizes these factors using the standard formula for the temperature dependence of chemical reaction rates using an empirically-determined activation energy for each process. Overall, her results suggest a doubling of the weathering rate for each 10-degree rise in temperature, but this value changes with the spatial scale of the analysis. Susan Brantley is a Professor in the Department of Geosciences at Pennsylvania State University.

Banded Iron Formations (BIFs) are a visually striking group of sedimentary rocks that are iron rich and almost exclusively deposited in the Precambrian. Their existence points to a major marine iron cycle that does not operate today. Several theories have been proposed to explain how the BIFs formed. While they all involve the precipitation of ferric (Fe3+) iron hydroxides from the seawater via oxidation of dissolved ferrous (Fe2+) iron that was abundant when the oceans contained very low levels of free oxygen, they disagree as to how this oxidation occurred. In the podcast, Clark Johnson describes how oxidation could have occurred without the presence of abundant free oxygen in the oceans. Clark Johnson is a Professor Emeritus in the Department of Geoscience at the University of Wisconsin-Madison.

The geological history of most regions is shaped by a whole range of processes that occur at temperatures ranging from above 800°C to as low as 100°C. The timing of events occurring over a particular temperature range can be recorded by a mineral which crystallizes over that range. The mineral calcite is suitable for recording low-temperature processes such as fossilization, sedimentation, and fluid flow, and it is especially useful as it is virtually ubiquitous. But using uranium-lead radiometric dating in calcite is very challenging as it often contains very little uranium and the ragiogenically-produced lead isotopes can be swamped by common lead within a calcite crystal. In the podcast, Catherine Mottram explains how these challenges are being overcome and shares some of her findings based on radiometric dating of calcite. Mottram is an Associate Professor of Geology at the University of Portsmouth. 

In this episode, Martin Van Kranendonk lays out a convincing case for life on Earth going back to at least 3.48 billion years ago. To find evidence for very ancient life, we need to look at rocks that have been largely undisturbed over billions of years of Earth history. Such rocks have been found in the Pilbara region of northwest Australia. As explained in the podcast, the 3.48-billion-year-old (Ga) rocks of the Pilbara's Dresser Formation contain exceptionally well-preserved features that show unmistakeable physical and chemical signatures of life. While older 3.7 Ga rocks in west Greenland may also prove to have harbored life, the Dresser Formation rocks represent the oldest widely accepted evidence for life on Earth. Martin Van Kranendonk has devoted his long and prolific research career to the study of the early Earth. One major theme of his work has been to use detailed mapping and lab research to develop geological models for the environments of Earth’s oldest fossils. This has helped establish the biological origin of many ancient fossils. His recent work on a newly discovered find of exceptionally well-preserved 3.5-billion-year-old sedimentary rocks in the Pilbara Craton of Western Australia has provided the strongest evidence to date that structures of this great age were produced by the earliest forms of life. Martin Van Kranendonk is a Professor in the School of Biological, Earth, & Environmental Sciences at the University of New South Wales in Sydney.