Tuesday, December 23, 2008

The evolution of rocks

This wonderfully diverse planet we live on began as a rather homogeneous conglomeration of minerals in the pre-solar nebula, an inventory not unlike what falls from the sky as meteorites. Inanimate, almost certainly, although I suppose it is not impossible that some sort of living spore arrived on or with the early Earth from elsewhere.

The primeval Earth was hot from the energy of its gravitational formation and radioactivity, hot enough that its minerals were molten. As the planet cooled, only a few basic minerals condensed from the magma, such as feldspar and olivine. As the Earth continued to cool, elements formed more "picky" mineral structures and combinations, such as clay and zeolites. All of this is pretty straight forward: elements find their way into chemical combination and crystalline structures depending on the ambient temperature, pressure, and so on. So, in a certain sense, minerals "evolved" as the planet cooled.

This inanimate "evolution" was discussed recently by Robert M. Hazen, et. all. in American Mineralogist, and summarized by Minik Rosing in Nature (November 27). The next big step was the origin of life, which may have required a mineral template such as clay. As living organisms evolved, still other minerals appeared on the scene as by byproducts of life, such as aragonite (in animal skeletons), gypsum (drywalls) and hematite (red paint). As life diversified, so did the mineral inventory of the planet.

All those years ago when I lived next door to the Geological Museum in London, I spent many happy hours drifting among the cases of minerals -- the inanimate substance of the Earth -- as even now I am browsing my Larousse Guide to Minerals, Rocks and Fossils. Gorgeous stuff! Native elements -- gold, silver, copper, iron. Combinations of elements in crystalline arrays, glittering in their jewel-like shapes. Gemstones. Geodes. Agates. And the colors! Erythrite. Turquoise. Jasper. Jade.

I hadn't thought much about it before, how the mineral composition of the Earth diversified according to temperature, pressure, and chemical and biological environment from the relative humdrum uniformity of meteorites. Whether evolution is the right word for this process of diversification I'll leave to the geologists to debate (Rosing would reserve the term for its biological meaning). Gnomic artificers in their underground halls could not arrange a more splendidly beautiful stage for the drama of life.