GLOBAL MAPS AS GEOLOGIC ODOMETERS

Digital data from Generalized Geology of the World and Linked Databases (Kirkham et al., 1995) yield global data on areas and ages of 7,550 exposures of rock that now surface our planet. Sedimentary, metamorphic, volcanic, and plutonic rocks comprise 68.1, 13.9, 12.3, and 5.6% of the Earth's continental surface, respectively.

Each group yields a characteristic area/age relation that reflects inherent long-term rates of transfer of rock material into or out of each reservoir via erosion, burial, melting, and metamorphism. Duration-normalized areas of sedimentary and volcanic rock exhibit negative area versus age trends; these constitute power-law distributions that reflect a progressive decrease in cycling rate (and increase in preservation potential) with subsequent burial by younger units. Over the long-term, continental surfaces are blanketed by new lava and sediment at rates of about 1.9 and 30.0 million square kilometers per million years, respectively. Owing to preferential accumulation at low elevations, erosional cycling of sedimentary sequences is about one-forth that experienced by volcanic successions. In contrast, duration-normalized areas of metamorphic basement rocks exhibit a positive area/age trend that is interpreted as primarily reflecting the significant amount of time that separates episodes of metamorphism and exposure of crystalline basement at continental surfaces. Somewhat surprisingly, area/age data for global plutons define a lognormal relation, with a mode at about 110 Ma. This distribution reflects some combination of greater Mesozoic pluton emplacement, and the several tens of millions of years of exhumation that separates plutonism from exposure.

Finally, outcrop areas for all of the 7,500 mapped global exposures yield a trend of exponentially decreasing abundance with exponentially increasing outcrop size. This area/frequency distribution of individual rock bodies is the same as that resulting from random geologic segmentation of the Earth's surface. It is the distribution of area sizes that would be anticipated for a population of mosaic elements where lateral extents define an exponential distribution of outcrop sizes. Areas of exposures are almost entirely dependent on the number of outcrops designated over the Earth's continental surfaces.