Carina Nebula [Courtesy NASA] Interior columns of La Sagrada Familia cathedral, Barcelona, Spain [Photo by DHB, (c) 2011]

How old is the earth? How old are the geologic ages? How are these dates determined?

David H. Bailey
21 Jun 2014 (c) 2014

The age of the earth

One challenge in assessing the age of the earth is the fact that virtually all rocks that were originally on the face of the earth when it first formed and solidified have subsequently been subducted into the earth's mantle and are no longer available for analysis. However, scientists who have dated meteorites have noted that almost all of them have the same age, roughly 4.567 billion years. Since the earth was formed out of this same material, this figure is taken to be the age of the earth. Consistent with these measurements, the oldest mineral ever found on earth, a zircon specimen found in in the Jack Hills region of Western Australia, has been measured to be 4.374 billion years old in a 2014 study [Oskin2014].

The geologic time scale

Modern geology has observed a continuous sequence of rock layers, throughout the world, each with a unique set of fossils and a sequence of dates that extend back from the present to the formation of the earth 4.567 billion years ago. The following table shows these various ages (down to the "epoch" level, and abbreviated following the Mezozoic era in the table), together with the current best estimates of when each age started, in millions of years before the present:

Eon Era Period Epoch Start (Myr) Characteristics
Phanerzoic Cenozoic Quaternary Holocene 0.0117 Start of human civilization
Pleistocene 2.588 Evolution of modern humans and other modern mammals
Tertiary Neogene Pliocene 5.333 Many modern mammals; Australopithecines and Homo habilis
Miocene23.03 Apes, horses, mastodons, modern birds, ubiquitous grasses
Paleogene Oligocene 33.9 Early evolution of mammals; proliferation of flowering plants
Eocene 56.0 Archaic mammals, primitive whales, first grasses
Paleocene 66.0 First modern plants; mammals diversify; first large mammals
Mesozoic Cretaceous Late 100.5 Flowering plants, insects, fish proliferate; many new dinosaur species
Early 145.0
Jurassic Late 163.5 First flowering plants, large and numerous dinosaurs, some small mammals
Middle 174.1
Early 201.3
Triassic Late235.0 Early mammals, dinosaurs, reptiles, mollusks
Middle247.2
Early252.2
Paleozoic Permian 258.9 Reptiles, amphibians abundant, extinction of many species
Carboniferous 323.2 Many winged insects, amphibians; first land vertebrates, reptiles and fern forests
Devonian 419.2 First ferns, seed-bearing plants; early sharks, amphibians
Silurian 443.4 First insects, jawed fishes, centipedes
Ordovician 485.4 Many invertebrates, early corals, first green plants and fungi on land
Cambrian 541.0 Cambrian explosion -- first shellfish, primitive fish, trilobites, corals
PrecambrianProterozoic Neo-proterozoic 1000 Trace fossils of simple multi-celled eukaryotes
Meso-proterozoic 1600 Green algae colonies in the oceans
Paleo-proterozoic 2500 Atmosphere flooded with oxygen; first complex single-celled life
Archean 4000 Simple single-celled life (probably bacteria)
Hadean 4567 Solidification of continents begins; oldest known mineral

A much more detailed version is available in the Wikipedia article on the geologic time scale [Geologic2011], or in any of a number of modern geology textbooks.

How are these dates determined?

The ages of various geologic epochs and specimens are determined by radiometric dating, which in turn is based on known rates of radioactivity, a phenomenon that is rooted in fundamental laws of physics and follows simple mathematical formulas. Radiometric dating methods have been developed, refined and scrutinized over a period of several decades. The latest high-tech equipment permits reliable results to be obtained even with microscopic samples. Potential difficulties can be dealt with by using well-honed statistical and experimental techniques.

As a result, these dates are now regarded as extremely reliable. There is not the slightest chance that they will ever be revised downward to just a few thousand years, as proposed by various young-earth creationists (see Creationism). This would require us to believe that hundreds of thousands of careful measurements, performed by thousands of scientists worldwide using multiple techniques, the latest technology and highly scrutinized methodologies, are all simultaneously off by factors of thousands or millions. There is no possible way that all of these measurements are so wrong. The earth and its geologic layers really are very old.

The methodology of radiometric dating is explained in a separate article: Radiometric dating. The reliability of radiometric dating is discussed in this article: Reliability. A third article discusses radiocarbon dating: Radiocarbon dating. A fourth related article discusses the "uniformitarian" assumption and how it relates to evolution, radiometric dating and the age of the earth: Uniformitarian.

References

[See Bibliography].