A wonderful satellite photo taken over Alaska illustrates well two geological phenomena, rock flour and Katabatic winds. The former is the finely ground mulch left behind by melting glaciers that colours glacial lakes blue (seen in the photo where the river estuaries meet the sea and the flour diffuses)and braided streams running out of the melt front an ashy grey brown. It is a very light powder consisting of tiny rock particles, the final product of the grinding and scouring of the Earth’s surface by many tons of ice. The dust plume in the image is made of this flour, carried by a particular kind of wind that blows off the world’s high places, including plateaus, mountain chains and ice sheets.
