sábado, 16 de maio de 2009

Some Like It Hot—Especially the Continents

Some Like It Hot—Especially the Continents

Photo of Jeff ZweerinkI ran across an interesting article in Science Daily a few weeks ago. The article’s authors point to evidence that the vast majority of North America would reside under water if the rocks making up the continental crust were cooled to the same temperature as some of the oldest crust underneath Canada. While it is not surprising that the oceans might cover coastal cities of New York, Miami, New Orleans, and Los Angeles, water hundreds of feet deep would drown even mountain cities such as Denver and Salt Lake City.

Clearly plate tectonics plays a critical role in the formation and sustaining of continental crust. However, the temperature of the rocks figures just as importantly in ensuring that continents remain above sea level. Adding heat to rock makes it expand and, therefore, become less dense. Less-dense rock floats higher above the denser surrounding crust, meaning the surface rock sits at higher elevations. In fact, the authors of the original scientific article note that “temperature differences within the Earth’s crust and upper mantle explain about half of the elevation of any given place in North America” (emphasis added).

The bulk of life’s diversity on Earth is found on the land and shallow seas of the continental crust. Less continental crust provides fewer habitats for life and also limits the capacity of plate tectonics to regulate the global temperature. (Peter D. Ward and Donald Brownlee’s book Rare Earth highlights the importance of plate tectonics in maintaining a global temperature supportive of liquid water oceans.)

While studies for other continents remain to be done, we expect similar conclusions since North American crust exceeds the average density of continental crust across the globe. The heat content of Earth’s continents appears to be finely tuned to ensure Earth’s habitability for human life.

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