How Old Is the Earths Surface

The discovery of radioactivity and its application to determining the age of rocks all happened in the 20th century. Before then, geologists spent a couple of centuries working out the relative ages of rocks, that is, which ones were formed first, and in what order the others came. Between the years of 1785 and 1800, James Hutton, a Scottish geologist considered by many to be the founder of the modern field, and William Smith, a surveyor of limited education but great curiosity and intellect, introduced and labored over the idea of geologic time—that the rock record describes events that happened over a long time period.

Fossils were the best and easiest way to correlate between rocks that did not touch each other directly. Some species of fossil life can be found in many locations around the world, and so form important markers in the geologic record; when fossils of one species occur in rock layers all over the world, all those rock layers must have formed before that species went extinct. Relative time was broken into sections divided by changes in the rock record—for example, times when many species apparently went extinct, since their fossils were not found in younger rocks. This is why the extinction of the dinosaurs lies directly on the Cretaceous-Tertiary boundary; the boundary was set to mark their loss. The largest sections of geologic history were further divided into small sections, and so on, from epochs, to eras, to periods. For centuries a debate raged in the scientific community over how much time was represented by these geologic divisions.

With the development of radioactive dating methods, those relative time markers could be converted to absolute time. On Earth the age of a rock can often be exactly determined by measuring its radioactive isotopes and their daughter products, and thereby determining how long the radioactive elements have been in the rock, decaying to form their daughters (see the sidebar "Determining Age from Radioactive Isotopes" on page 76). Through radiodating it is now known that the Cretaceous-Tertiary boundary lies at about 66.5 million years ago, and that the greatest extinction in Earth history, at the Permian-Triassic boundary, occurred at 251 million years ago. The rocks that make up the Earth's surface are largely younger than 250 million years old. Oceanic crust is necessarily young, since it is made constantly and just as constantly lost into the mantle as it dives under continental crust at subduction zones.The oldest oceanic crust still in ocean basins (some slivers have been crushed into the continents during continental collisions) is about 200 million years old.

Because most of the rocks on the surface of the Earth were formed within the last 250 million years (only 5 percent of all of Earth's history!), scientists have difficulty in fully understanding and describing the state of the early Earth. How soon after its formation did the Earth form a crust, when did oceans form, and how long after that did plate tectonics begin? There is hardly a vestige of the early Earth's surface left for us to examine. Because of their importance to understanding the formation of the Earth and its early evolution, geologists are on a perpetual search to find the oldest remaining rocks on Earth. The obvious places to look are the ancient cratons, the stable interiors of the continents. Rocks older than 3 billion years have been found in Greenland; with each successive older find, the scientist responsible gained sudden fame, and the search went on. In the early 1990s, the geologist Sam Bowring from the Massachusetts Institute of Technology was working in the Slave craton in Canada's Northwest Territories. At the end of a stormy day of lake-hopping in a small plane across the tundra of northern Canada, Bowring used a sledgehammer to break off some promising samples from a rock outcrop near a remote lake.These rocks lay awaiting analysis for some time in his lab back in Cambridge, Massachusetts, before they were found to be the oldest known rocks on Earth. Known as the Acasta gneiss, they date to 3.96 billion years, and contain a few stray mineral grains that are even older.

More recently, John Valley, a geologist from the University of Wisconsin at Madison, and his colleagues found a few tiny grains of a mineral called zircon in a sedimentary rock in the Jack Hills of Australia that date as far back as 4.4 billion years. These tiny, purple zircons were not formed in the sedimentary rock they were extracted from, which is much younger than 4.4 billion years old; they were weathered out of their original host rock and later laid down with the sediment that became the rocks found in central Australia today. By measuring the oxygen isotopic ratios in the zircons, some scientists have come to the conclusion that there had to be liquid water on the Earth when these zircons formed. If there was water on the Earth's surface 4.4 billion years ago, then the Earth must have cooled and solidified to the point that the crust was strong and cold enough to support oceans. Aside from the oxygen isotopes found in these zircons, the only evidence for early water are rocks made from sediments that were deposited in water between 3.8 and 3.6 billion years ago. There are only these tiny, tiny clues to what the surface of the Earth was like in the distant past.

The idea that the early Earth cooled fast and developed a crust and oceans much as there is today by as early as 100 million years after formation is a heretical idea to some geologists. In that early time radiogenic heat is estimated to have been five times higher than today's output, and some scientists think that much of the heat of early formation of the planet still existed. This early era in Earth

Geological Time Line for the Earth, Moon, and Mars

Present

1 billion years

2 billion years

3 billion years

4 billion years

4.56 billion years '

Earth

Relative timescale (in eons, the largest units)

Moon Relative timescale

Mars Relative timescale

1 billion years

2 billion years

3 billion years

4 billion years

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Oldest-known rock

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Tycho crater

Copernicus crater

Mare Orientale Mare Imbrium

3-8 Mare Nectaris 3.9

Mare Nubium Mare Fecunditatis Mare Tranquillitatis Mare Procellarum

Continued flowing surface water?

Water flowed on Martian surface

Tharsis begins to form

Magnetic field active

The geologic time line for the Earth compared with the Moon and Mars, as they are understood without further samples to date using radioisotopes history is actually named the Hadean, meaning "hell-like." The research of John Valley and his colleagues indicates that, far from being hell-like, the Earth cooled quickly and formed oceans.This theory is in agreement with the modeling efforts of the author and her colleagues at Brown University, which indicate that the Earth, the Moon, and Mars would all have formed a cool surface and a crust within about 60 million years of formation.

With the possible exception of the Moon, the Earth is the only body for which scientists can develop a detailed relative timescale because there has never been a field geologist on any other planet to do the necessary careful mapping of geologic units. A number of absolute radioisotope dates have been made for lunar rocks returned from the Apollo missions, forming the beginning of an absolute timescale for the Moon. Scientists cannot develop an absolute timescale for Mars because the only rocks available are the Martian meteorites, whose original locations on Mars are unknown. For other bodies there are no absolute dates. Using detailed images of other planets, though, it is possible to work out the relative ages of many of the crustal features. By carefully examining images researchers can determine "superposition," that is, which rock unit was formed first, and which later came to lie on top of it. Impact craters and canyons are very helpful in determining superposition. Through this sort of meticulous photogeology, scientists have developed relative timescales for other planets, as shown in the figure on page 74.

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