Landscape in Carina Nebula [Courtesy NASA] Bronze pseudo-magic square on exterior of La Sagrada Familia cathedral, Barcelona, Spain [Photo by DHB, (c) 2011]

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

David H. Bailey
1 Jan 2012 (c) 2012

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, namely 4.56 billion years. Since the earth was formed out of this same material, this figure is taken to be the age of the earth.

Some of the oldest rocks on the earth are in western Greenland. Here is a summary of the ages of some specimens from a single formation, measured using five different radiometric techniques [Dalrymple1991; Wiens2002]:

(Byr)
Age Error
Technique range
uranium-lead 3.60 0.05
lead-lead 3.56 0.10
lead-lead 3.74 0.12
lead-lead 3.62 0.13
rubidium-strontium 3.64 0.06
rubidium-strontium 3.62 0.14
rubidium-strontium 3.67 0.09
rubidium-strontium 3.66 0.10
rubidium-strontium 3.61 0.22
rubidium-strontium 3.56 0.14
lutetium-hafnium 3.55 0.22
samarium-neodymium 3.56 0.20

Note that all of the ages and ranges overlap, agreeing that the true age of this rock formation is between 3.62 and 3.65 billion years.

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 has established dates for these layers that extend back from the present to the formation of the earth 4.56 billion years ago. The following table shows these various ages (down to the "epoch" level, and abbreviated after the Triassic), together with the current best estimates (and error range) of how long ago each age started, in millions of years:

Eon Era Period Epoch Start (Myr) Characteristics
Phanerzoic Cenozoic Quaternary Pleistocene 1.806 ± 0.005 Prehuman primates, first humans
Tertiary Neogene Pliocene 2.588 ± 0.005 First prehuman primates, first modern whales
Miocene23.03 ± 0.05 Apes, horses, dogs, bears, modern birds, modern flowering plants
Paleogene Oligocene 33.9 ± 0.1 Deer, cats, pigs, rhinos, horses, rabbits
Eocene 55.8 ± 0.2 Many early mammals, rodents
Paleocene 65.5 ± 0.3 First large mammals; primitive primates
Mesozoic Cretaceous Upper 99.6 ± 0.9 Primitive marsupials, birds; end of dinosaurs
Lower 145.5 ± 4.0 First crocodiles, snakes and bees; feathered dinosaurs
Jurassic Upper 161.2 ± 4.0 First flowering plants; large and numerous dinosaurs
Middle 175.6 ± 2.0
Lower 199.6 ± 2.0
Triassic Upper228.0 ± 2.0 First mammals, dinosaurs; many reptiles, mollusks
Middle245.0 ± 1.5
Lower251.0 ± 0.7
Paleozoic Permian 299.0 ± 0.8 Reptiles, amphibians abundant, end of many species
Carboniferous
Pennsylvanian		 318.1 ± 1.3 Coal swamps, first reptiles and cockroaches; many ferns
Carboniferous
Mississippian		 359.2 ± 2.5 First winged insects; early sharks
Devonian 416.0 ± 2.8 First amphibians, tetrapods, sharks, bony fish, many coral reefs
Silurian 443.7 ± 1.5 First insects, jawed fishes, centipedes; first vascular plants on land
Ordovician 488.3 ± 1.7 First land plants, corals, primitive fishes, seaweed, fungi
Cambrian 542.0 ± 1.0 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 ~3800 Simple single-celled life (probably bacteria)
Hadean ~4550 Solidification of continents begins; oldest known mineral

A more detailed version is available at [Geologic2011].

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.

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].