functional until about 3 billion years ago, when the formation of the crust
was nearly complete.
Much information about the early crust is provided by some of the most
ancient rocks that survived intact. They formed deep within the crust a few
hundred million years after the formation of the planet and now outcrop at
the surface. Zircon crystals (Fig. 6) found in granite are enormously resistant
Figure 6Zircons from
the rare-earth zone, Jasper
Cuts area, Gilpin
County, Colorado.
(Photo by E. J.Young,
courtesy USGS)PLANET EARTH