Proportions of Exposed Igneous, Metamorphic, and Sedimentary Rocks
Of the rocks exposed on the Earth9s surface, 66 percent (±3.5 percent) are sedimentary and 34 percent are crystalline, at the 95 percent confidence level. Extrusive igneous rocks average about one-fourth of all crystalline rock outcrops, with the highest percentages in Asia and South America. Less than 5 percent of all Precambrian rocks are mapped as sedimentary. The relationship between the geologic age of a sedimentary rock and its outcrop area is lognormal and is well described by a decay curve with a half-life of 130 × 10 6 yr. That is, one-half of all existing sedimentary rocks are younger than Jurassic. Such a short half-life indicates that the rate of sedimentary recycling must be very rapid.