The Age of the Earth
The Earth is very old—four and a half billion years or more according to recent estimates. Most of the evidence for an ancient Earth is contained in the rocks that form the Earth’s crust. The rock layers themselves—like pages in a long and complicated History—record the surface-shaping events of the past, and buried within them are traces of life—the plants and animals that evolved from organic structures that existed perhaps three billion years ago.
Also contained in rocks once molten are radioactive elements whose isotopes provide earth scientists with an atomic clock. Within these rocks, “parent” isotopes decay at a predictable rate to form “daughter” isotopes. By determining the relative amounts of parent and daughter isotopes, the age of these rocks can be calculated.
Thus the results of studies of rock layers (stratigraphy), and of the progressive development of life (paleontology), coupled with the ages of certain rocks as measured by atomic clocks (geochronology), attest to a very old Earth!
Summary of Geologic History
Having finished our geologic ramble through Canyonlands National Park, let us see how this pile of eroded rocks fit into the bigger scheme of things—the geologic age and events of the earth as a whole, as depicted in [figure 80]. As shown in [figure 9], the rock strata still preserved in the park range in age from Pennsylvanian to Jurassic, or from about 300 to 175 million years ago, a span of about 125 million years. This seems an incredibly long time, until you note that the earth is some 4.5 billion years old and that our rock pile is but one twenty-fifth, or 4 percent, of the age of the earth as a whole. Thus, in [figure 80] the rocks exposed in the park occupy only about the left-hand third of the top whorl of the spiral.
But this is not the whole story. As indicated earlier, about 10,000 feet of younger Mesozoic and Tertiary rocks that once covered the area have been carried away by erosion, and if we include these, the span is increased to about 250 million years, or nearly a full whorl of the spiral.
Deep tests for oil and gas tell us that much older rocks underlie the area, and we have seen that some of these rocks played a part in shaping the park we see today—note the breaks in the deep-seated Precambrian rocks and the salt in the Paradox Member. In addition to the Precambrian igneous and metamorphic rocks, there are about 2,000 feet of Paleozoic sedimentary rocks older than the Pennsylvanian Paradox Member. Most of these sedimentary rocks were laid down in ancient seas during Cambrian, Ordovician, Devonian, Mississippian, and Pennsylvanian times ([fig. 80]). There are some gaps in the rock record caused by temporary emergence of the land above sea level and erosion of the land surface before the land again subsided below sea level so that deposition could resume. Silurian rocks are absent altogether, presumably because here the Silurian Period was dominated by erosion rather than deposition.
While Pennsylvanian and Permian sediments were being deposited in and southwest of the park, a large area to the northeast—called by geologists the Uncompahgre highland, because it occupied the same general area as the present Uncompahgre Plateau—rose slowly above sea level. Whatever Paleozoic rocks there were on this rising land, plus part of the underlying Precambrian rocks, were eroded and carried by streams into deep basins to the northeast and southwest. Thus, while mostly marine or nearshore deposits were being laid down in and near the park, thousands of feet of red beds were being laid down by streams in an area between the park and the Uncompahgre Plateau. During part of Middle Pennsylvanian time a large area including the park and known as the Paradox Basin was alternately connected to or cut off from the sea, so the water evaporated during cutoff periods and was replenished during periods when connection with the sea resumed. In this huge evaporation basin were deposited the layers of salt and gypsum plus some potash salts and shale that now make up the Paradox Member.