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Yellowstone, USA, and Iceland7. Reimink et al.2 suggest that the ancient Canadian gneisses formed in a setting similar to that of present-day Iceland. There, a stream of ascending mantle material billows into the rift between two diverging tectonic plates, causing outpouring of large magma volumes. Zones of upwelling mantle in a hotter early Earth could have generated just enough silicic crust to withstand later pummelling by meteorites and any return flow into the mantle. Conceivably, these small nuclei, as now represented by the Canadian gneisses, seeded growth of the continental masses that began to be preserved from around 3.8 to 3.6 billion years ago.

The Canadian gneisses are a valuable piece in the Early Earth puzzle. This picture is, however, far from complete. For example, if upwelling zones were common in a vigorously convecting primordial mantle8, then it is unclear why iron-rich gneisses do not form large parts of the Archaean crust today. Perhaps these shallow-formed rocks were eroded away. Alternatively, rocks formed above ancient mantle upwellings may still exist, but have had their tell-tale chemical signatures disguised by later metamorphism. Small flakes of ancient crust are hard to pinpoint in a sea of younger gneiss, and soil, ice and vegetation are notorious conspirators against roving geologists. But the search is warranted because these small iron-rich flakes may signpost the oldest cores of the Archaean cratons.

Reimink and colleagues2 identified ancient rocks in Canada that preserve a record of early continental crust forming at shallow depths above upwelling mantle material, in a setting similar to present-day Iceland. Whether voluminous, but now vanished, continents surfaced the earliest Earth is an enduring controversy. Future research could test whether the Canadian gneisses retain

any memory of yet more ancient crustal relics. The hafnium isotope ratios of dated zircons in these samples might answer this question. Further geochemical analyses will improve our knowledge of exactly when the continental distillation process started on Earth, and how we might recognize its incipient traces on other planets. ❐

Anthony I. S. Kemp is in the School of Earth and Environment, The University of Western Australia, Crawley, Australia. e-mail: tony.kemp@uwa.edu.au

References1. Tatsumi, Y. GSA Today 15, 4–10 (2005).2. Reimink, J. R., Chacko, C., Stern, R. A. & Heaman, L. M.

Nature Geosci. 7, 529–533 (2014).3. Wilde S. A., Valley, J. W., Peck, W. H. & Graham, C. M. Nature

409, 175–178 (2001).4. Defant, M. J. & Drummond, M. S. Nature 347, 662–665 (1990).5. Campbell, I. H. & Taylor, S. R. Geophys. Res. Lett.

10, 1061–1064 (1983).6. Martin, H. & Moyen, J-F. Geology 30, 319–322 (2002).7. Muehlenbachs, K., Anderson, A. T. & Sigvaldason, G. E.

Geochim. Cosmochim. Acta 38, 577–588 (1974).8. O’Neill, C., Debaille, V. & Griffin, W. Am. J. Sci.

313, 912–932 (2013).

Published online: 25 May 2014

Figure 1 | Volcanic eruption at Fimmvorduhals, Iceland. Beneath Iceland, ascending mantle material streams into the rift between two diverging tectonic plates, causing voluminous eruptions of magma at the surface and the formation of thick crust. Reimink et al.2 show that 4-billion-year-old continental crustal rocks discovered in the Northwest Territories of Canada have a distinctive geochemical signature that is similar to some Icelandic rocks, implying that Earth’s early continental crust may have formed in an Iceland-like setting.

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Tales of topographyThe origins of topographic relief are challenging to disentangle. Modelling shows that differential isostatic rebound due to erosion of rocks of variable density may influence topography, inspiring a fresh look at topographic highs in landscapes.

Rebecca M. Flowers

The Earth’s surface is not flat and uniform. Rather, it is characterized by extraordinary variability, from

the soaring peaks of our planet’s greatest mountain belts to the subdued morphologies

of continental interiors. Key controls on this topographic diversity include deformation associated with the movement of tectonic plates, differences in rock strength, climatic variations, and perhaps

flow within Earth’s deeper mantle. Writing in Nature Geoscience, Braun et al.1 reach the counter-intuitive conclusion that in some circumstances the isostatic rebound caused by the erosion of rocks with different

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densities is also an important but previously overlooked mechanism for the generation of topographic relief.

Isostasy is the condition of gravitational equilibrium towards which the Earth tends, where the elevation of the Earth’s surface — its height above sea level — adjusts to the density and thickness of rocks beneath. This fundamental concept underpins much of our understanding about the nature of the crust and the processes that cause deviations from its equilibrium state. In the simplest characterization of isostasy, Earth’s buoyant outermost shell, the crust, is considered to be floating on the denser, underlying mantle2. Thickening of the crust, deposition of sediments, removal of rocks via erosion, or even the emplacement of large ice sheets, can perturb the crust from isostatic equilibrium and cause the surface elevation to alter gradually, over thousands to millions of years, to re-achieve the ideal isostatic state. Erosion continuously removes rocks and sediments at the Earth’s surface through water, glaciers or other natural processes. The long-recognized consequence of isostasy in an eroding landscape is that a given magnitude of erosion does not reduce the surface elevation by the equivalent amount. Rather, the surface is typically only lowered by 30 to 40% of the eroded extent because isostatic rebound restores the remainder of the elevation.

Braun et al.1 take this notion one step further by using analytical and numerical models to show that the erosion of denser

surface rocks will lead to greater isostatic rebound and therefore less surface lowering than erosion of the equivalent volume of more buoyant rocks. Consequently, a granitic rock body that is surrounded by less-dense sedimentary rocks should stand as a topographic high in the landscape. Although such a relationship is commonly observed, it is traditionally attributed to the relative erodabilities of these rock types, due to lower erosion rates in the harder and less erodable granite compared with the weaker surrounding rocks.

The effect of density-dependent isostatic rebound is diminished by the flexural strength of the continents. Continental interiors are typically composed of strong lithosphere — contrary to, for example, recently rifted continental margins2,3. The mechanism of differential isostatic rebound for generating topographic relief should therefore be most strongly manifested in areas of weak continental lithosphere and for rock bodies larger than ~50 km in diameter, narrowing the range of settings where this effect is likely to be the dominant control on the landscape.

This mechanism is, in theory, testable. In a steady-state landscape — one that is eroding but undergoing no topographic change — the differential isostatic rebound postulated by Braun and colleagues1 requires that the denser rocks are eroded faster than the less-dense rocks. This erosion-rate pattern is opposite to that predicted by rock hardness alone. Thermochronologic data, which record the

history and rates of rock cooling, can be used to infer rock erosion rates. So, for a cross-section through a sizeable dense granitic body enclosed by less-dense units in a region of weak continental lithosphere, the density-dependent isostatic rebound model predicts that thermochronologic dates will dip to their youngest values in the middle of the dense intrusion and get older towards the edges. The thermochronologic data pattern therefore provides a powerful test of this model, assuming that the landscape is in topographic steady state.

Braun et al.1 suggest a number of settings where low-temperature thermochronologic data may record the spatial pattern predicted by this mechanism. For example, Mount Kinabalu in Indonesia is underlain by an approximately 10-km-wide granitic intrusion surrounded by less-dense sedimentary rocks. Braun and colleagues show that this region is characterized by thermochronologic dates that are younger towards the centre of the granitic intrusion, when corrected for the effects of topography on subsurface temperatures1. However, without this correction the thermochronologic dates are broadly uniform across the Mount Kinabalu intrusion4, thus lending some uncertainty to the interpretation of the inferred data pattern.

In a second setting, the approximately 30,000 km2 Shakhdara gneiss dome in the Pamir Mountains of central Asia juxtaposes gneissic rocks with lower-density sedimentary and volcanic units in an area of relatively low lithospheric strength, therefore providing the appropriate density structure for topographic relief to be influenced by differential isostatic rebound. Braun and colleagues argue that the thermochronologic data pattern across the region supports this mechanism as the main cause of relief. However, the gneiss dome is structurally complex and characterized by substantial spatial and temporal variability in geothermal gradients that could influence the thermochronologic results5. It is unclear whether tectonics and faulting or the proposed isostatic effects impose the dominant controls on the thermochronologic data and regional topography. This ambiguity reflects the difficulties associated with definitively isolating the primary cause of topographic relief in a geologically complex setting.

Braun et al.1 provide an important step in identifying density-dependent isostatic rebound as a potentially significant and previously unrecognized influence on topography. Future studies aimed at discovering the extent to which density-dependent isostatic rebound influences topographic relief should focus on identifying localities that have low lithospheric strength

Figure 1 | Mount Kinabalu, Borneo. Mount Kinabalu is underlain by an approximately 10-km-wide granitic intrusion that was emplaced into less-dense host sedimentary rocks. Braun et al.1 use a numerical model to show that the topographic relief of this region could be generated by the density-dependent differential isostatic rebound of these different rock types.

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and limited structural complexity, and are characterized by large rock bodies of contrasting density, where landscapes can reasonably be considered to be at steady state. Further work that carefully integrates modelling, geochronologic analysis, geologic study and geomorphic observation is required to continue refining our understanding of the relative importance and complex

interplay of isostatic, tectonic, lithologic, climatic and mantle dynamic phenomena in the topographic evolution of Earth’s diverse landscapes. ❐

Rebecca M. Flowers is in the Department of Geological Sciences, University of Colorado, Boulder, Colorado, USA. e-mail: rebecca.flowers@colorado.edu

References1. Braun, J., Simon-Baric, T., Murray, K. & Reiners, P. W.

Nature Geosci. 7, 534–540 (2014).2. Watts, A. Isostasy and Flexure of the Lithosphere (Cambridge

University Press, 2001).3. McKenzie S. & Fairhead, D. J. Geophys. Res.

102, 27523–27552 (1997).4. Cottam, M. et al. J. Geol. Soc. London 170, 805–816 (2013).5. Stübner, K. et al. Tectonics 32, 1404–1431 (2013).

Published online: 1 June 2014

Over the past few million years, the Earth’s climate has oscillated between cold glacial and warm interglacial states. The nature of these climate swings — and the idea of the interactions between solar radiation and the land–sea–air–ice system that govern them — seems incontrovertible. However, it was only a few decades ago that scientists such as Thomas Crowley began working to bring our understanding of these climate fluctuations to the fore. Tom Crowley, it might be said, wrote the book on the subject (Paleoclimatology Oxford Univ. Press, 1991). With this book, for which I served as a minor co-author, he aimed to introduce geologists and geographers to climate modelling, and conversely show climate modellers the types and properties of the data available. The book fulfilled its mission.

Tom received his PhD at Brown University, USA, in marine geology under the direction of John Imbrie in 1976. Afterwards, he taught oceanography to sailors on US Navy ships for a few years, and then joined the University of Missouri–St Louis where he taught introductory geology. His seminal book was founded on a series of lectures on palaeoclimatology delivered at the NASA Goddard Space Flight Center in the summer of 1981. As his career took a leap, he summarized his lectures into a review article published in 1983 in the journal Reviews of Geophysics and Space Physics. The article explored how changes in atmospheric carbon dioxide concentrations and atmospheric and oceanic circulation were required to explain past climate changes. His scholarship led to his appointment at the US National Science Foundation to help develop a programme to fund palaeoclimate research.

After working for NASA and the private sector, Tom Crowley joined the Department of Oceanography at Texas A&M University as a research scientist and soon after as a full professor (1996–2001). It was during

A broad view of climate historyTHOMAS J. CROWLEY

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this time that he married a fellow climate scientist, Gabi Hegerl, with whom he had two sons, Michael (13) and John (11).

In the following years, he made a broad and very significant contribution to our understanding of the history of Earth’s climate, both in the USA and ultimately at the University of Edinburgh in Scotland, from which he retired a few years ago. His retirement did not, however, mark the end of his research: he finished his last paper shortly before his death.

In total, Tom Crowley published more than 100 refereed papers on climate research. He was a man of great integrity, reluctant to publish work he considered marginal. He worked hard and was very dedicated to science. His greatest gift was to see patterns persistent across several different — and often obscure — sources of information. He had a unique combination of creativity and vast but deep knowledge of both the empirical and theoretical bases of climate change. His interests ranged from the deep geological past to the current issues regarding anthropogenic causes of global warming.

He was deeply troubled that society was unable to moderate its desire for near-term growth and acquisition as opposed to preventing or even preparing for serious environmental consequences. In his retirement he frequently wrote criticisms to newspapers for their lack of accuracy or their inconsistency in reporting on matters of climate and environment. He did not write these letters to gain attention for himself; he simply felt it was his responsibility to share his knowledge and express his opinions.

Tom was also a spiritual person — after long periods of hard work, he often took off for a few weeks to be by himself at some remote spot. During his time in the USA, he especially liked to visit the Big Bend Park in West Texas (pictured).

We had been close friends and collaborators for more than thirty years. It was an interesting and complementary partnership venturing into palaeoclimatology together — Tom a marine geologist, learning to be an atmospheric scientist, and myself a theoretical physicist learning to be a climate modeller. Nevertheless, our partnership worked. Our research interests diverged somewhat in later years, but our friendship continued.

Tom Crowley enjoyed good literature, history, movies and sports. During the years we were located near each other, we frequently had lunch, gossiping about friends and colleagues and arguing about science, sports, politics and religion — sometimes to exhaustion, but we always returned the next day to start over. He was a great colleague and friend. I will miss him dearly.

GERALD R. NORTHGerald R. North is in the Department of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843-3150, USA. e-mail: g-north@tamu.edu

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