Geology

Fighting for Exposure

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Science  29 May 2009:
Vol. 324, Issue 5931, pp. 1118
DOI: 10.1126/science.324_1118a

Geologic maps provide information on the distribution of rocks and deposits currently exposed on Earth's surface. It has been recognized for some time that fewer older rocks are exposed than younger ones; only a few outcrops remain from Earth's earliest history, more than 3.5 billion years ago. Wilkinson et al. separate out rocks deposited at the surface (volcanic and sedimentary rocks) from those that form in the crust (metamorphic and intrusive rocks representing chilled magma bodies) and show how the exposures today of rocks of different ages and origination depth reflect geologic history and processes. On average, about 6.5% of continental land is covered by new sediments or volcanic deposits every million years. Young metamorphic and plutonic rocks are rare; the average age of metamorphic rocks (nearly 1 billion years) is older than that of intrusive rocks (about 200 million years), reflecting the greater difficulty in exposing deeper rocks. Together, the data imply that long-term rates of burial and uplift are about equal at roughly 0.5 km per million years, in reasonable agreement with rates measured by other techniques in many specific orogenic belts.

Geol. Soc. Am. Bull. 121, 760 (2009).

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