August GEOLOGY and GSA TODAY Media Highlights

Mountains form by competing processes of tectonic uplift and erosion, whereby rivers cut valleys and transport the eroded sediments. One theory suggests that when mountainous slopes reach a critical steepness angle, their evolution will occur predominantly through landsliding. This critical, or ‘threshold,’ angle will exist where the stream erosion in the bottom of valleys keeps pace with the rate of tectonic uplift, and has important implications for geologic understanding of how mountains are formed and how they respond to changing climatic and tectonic conditions. However, this theory has been little studied, and the operation of ‘threshold hillslopes’ has not been not adequately shown to exist in nature. Binnie et al. test whether such a process can be observed in the San Bernardino Mountains, California, by comparing erosion rates and hillslope gradients. Their data show that increases in gradients are matched by increases of erosion rates, until slope angles reach 30 degrees. Above this angle, increasing erosion rates do not correspond to increasing hillslope gradients, suggesting they cannot steepen further, and thus marking the emergence of slopes at their threshold angle. Binnie et al. use these results to illustrate how mountainous topography evolves over time in response to tectonic uplift.

Glacial isostatic adjustment as a control on coastal processes: An example from the Siberian Arctic
Pippa L. Whitehouse et al., University of Missouri, Columbia, Department of Geological Sciences, 101 Geological Sciences Building, Columbia, MO 65211-1380, USA. Pages 747-750.

The vast majority of the world's major rivers terminate at extensive coastal deltas. The rivers of western Siberia, however, do not. The Ob' and Yenisei rivers, most notably, flow into the Arctic Ocean via estuaries several hundreds of kilometers long. Whitehouse et al. investigated the reason for this departure from the rule. During the last glaciation, the land in central Scandinavia was depressed beneath the weight of the Fennoscandian ice sheet, and consequently the surrounding land, such as that in western Siberia, was elevated above its present height. Since the ice sheets melted, the Earth has been slowly returning to its original shape, and continues to do so in the present day. Whitehouse et al. demonstrate via geophysical modeling that the present-day subsidence of the land in western Siberia is still sufficiently rapid to cause apparent sea-level rise at rates that prohibit the development of coastal features such as river deltas.

Vapor segregation and loss in basaltic melts
Marie Edmonds and Terrence M. Gerlach, University of East Anglia, School of Environmental Sciences, Norwich, NR4 7TJ, UK. Pages 751-754.