April GEOLOGY and GSA TODAY media highlights

Many volcanic ocean islands, like Hawaii, occur at locations that are far from plate boundaries, and are therefore not easily explained by plate tectonics. Morgan suggested that such volcanoes are the result of high-temperature thermal upwellings, called mantle plumes; these plumes presumably derive their heat from the core-mantle boundary. A consequence of this idea is that ocean islands, like Hawaii, tap a deep mantle source, bringing up material that lies at the base of Earth’s mantle. A common assumption is that a high 3He/4He ratio at ocean islands is a geochemical signal of this deep mantle source. These ideas have been controversial, but find support in new estimates of mantle temperatures at 28 ocean islands by Putirka. These temperature estimates show that the mantle beneath 27 of 28 volcanic ocean islands is hotter than ambient mantle by 115–290 °C. Mantle plumes are thus common, not rare. In addition, though, hotter plumes tend to have higher 3He/4He, which suggests that high 3He/4He is indeed a signal from the deep mantle; this means that Earth’s mantle, despite active convection, is compositionally layered. Finally, a comparison of mantle temperatures to Pb isotope ratios indicates that even the deepest mantle is not primitive or undifferentiated, but rather, it has been depleted by prior partial melting events, probably as a result of formation of the continental crust. Putirka’s paper thus indicates that plumes are common, they tap a deep mantle source, and the deep mantle is depleted.

Controls on sinuosity evolution within submarine channels

Ian A. Kane et al., Earth Sciences, University of Leeds, Leeds LS29JT, UK. Pages 283-286.

River channels have been the focus of study for centuries; however, processes within river channels familiar to schoolchildren, such as meandering and ox-bow lakes, are still very poorly understood when it comes to submarine channels. Submarine channels extend from the continental slope into deep-ocean basins and are built by the passage of turbidity currents carrying large volumes of terriginous sediment. The study of submarine channels has advanced greatly over the last 50 years or so with new techniques allowing us to visualize the sea floor. One thing that has struck scientists is the relative stability of submarine channels, in comparison to fluvial channels. To investigate this phenomenon, Kane et al. used experimental modeling to attempt to create a process-similarity with submarine channels. The results of the modeling suggest that submarine channels may be subject to deposition on either the inner or the outer bend of meander, which is very different from river channels where deposition occurs primarily at the inner bend, inducing meander-loop expansion. This mechanism may explain why submarine channels generally lack ox-bow lake–type features and may remain stable over large time periods.

Microtektites from Victoria Land Transantarctic Mountains

L. Folco et al., Museo Nazionale dell'Antartide, Universita' di Siena, Via Laterina 8, 53100 Siena, Italy. Pages 287-290.