MicroRNA ’ s influence on leukemia

Continental crust rises higher than oceanic crust because it is less dense: It is rich in Si and poor in Mg. However, basalt, which is derived from magma, is denser and has a higher ratio of Mg to Si than the rock that makes up mature continental crust. Cin-Ty Lee et al. argue that chemical weathering alters the composition of continental crust as it ages and helps give it its more buoyant character. Both chemical weathering and ‘‘delamination’’ of Mg-rich lower crust, which occurs as it peels off and reenters the mantle, have been proposed as mechanisms that remove Mg from continents. However, quantifying the relative strength of chemical weathering versus delamination has long been a challenge. The problem is essentially that two equations arise to explain the removal of Mg, but the equations contain three unknowns and therefore cannot be solved. The authors observe that the weathering of Li and Mg is coupled, which allows the system to be solved. The authors report that chemical weathering accounts for a loss of 20% of the original basalt Mg compared with a 40% loss by other routes. From this perspective, it seems possible that life, by altering crustal chemistry via weathering, can affect the evolution of continents. — K.M.