Immunology: A helping hand against autoimmunity


convincing observations from volcanics and xenoliths. But relatively little is known about how much slab-derived hydration and magmatic infiltration would actually be required to counteract the initial buoyancy and strength of the ancient lithosphere, shaking it loose from its overlying crust and producing uplift. Erosion of the Colorado plateau occurs not only from below by dripping, but also from above by surface processes such as those that carved the Grand Canyon. Because surface erosion is driven in large part by elevation and topographic relief, its spatial–temporal patterns provide clues about past uplift events. Levander et al. use evidence for increased erosion rates about 6 million years ago in the Grand Canyon area — the approximate epicentre of the drip — to suggest that their drip is actually a delamination peeling off from northeast to southwest. But the history and spatial pattern of elevation gain are probably more complex than this, and much of it may have occurred much earlier. Evidence from regional drainage patterns, stable isotopes and palaeohydrology, as well as a growing body of thermochronological data, provide strong indications that much of the Colorado plateau was quite high much earlier. Although still controversial, several lines of evidence indicate that, by 20 million to 60 million years ago, at least parts of the Grand Canyon were already almost as deep as they are today, as rivers flowed into what is now the Colorado plateau from highlands to the southwest and the Rockies to the northeast. Also complicating a simple model of a single recent drip is evidence that large increases in the erosion rate over the past 6 million years or so are widespread and most pronounced tens to hundreds of kilometres upstream, in the Colorado River drainage basin. Whether this requires multiple drips, broader astheno spheric upwelling or geomorphic effects of recent river integration, it is likely that the story of one of geologists’ favourite natural laboratories and playgrounds is far from fully told. ■ George Zandt and Peter Reiners are in the Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. e-mail:


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