GEOLOGIC TIME

What is geologic time?

Geologic time is the immense span of time that has elapsed since Earth first formed almost 4.5 billion years ago to recent times.

What is the geologic time scale?

The geologic time scale is a way of putting Earths vast history into an orderly fashion, giving a better perspective of events. At the turn of the nineteenth century, William Smith (1769-1839), an English canal engineer, observed that certain types of rocks, along with certain groups of fossils, always occurred in a predictable order in relation to each other. In 1815, he published a map of England and Wales geology, establishing a practical system of stratigraphy, or the study of geologic history layer-by-layer. Simply put, Smith proposed that the lowest rocks in a cliff or quarry are the oldest, while the highest are the youngest.

By observing fossils and rock type in the various layers, it was possible to correlate the rocks at one location with those at other locations. Smiths work, combined with the first discoveries of dinosaur fossils in the early 1800s, led to a framework that scientists still use today to divide Earths long history into the geologic time scale, with its various, arbitrary divisions of time including eras, periods, and epochs. Established between 1820 and 1870, the time divisions are a relative means of dating; that is, rocks and fossils are dated relative to each other as to which are older and younger. It was not until radiometric dating was invented in the 1920s that absolute dates were applied to rocks and fossils and to the geologic time scale.

What are the divisions of the geologic time scale?

The geologic time scale divisions have changed significantly over time, mainly because of new fossil discoveries and better radiometric dating techniques and it will no doubt continue to change. The following table is a general listing of the geologic time table based on current interpretations of rocks and fossils.

The Geologic Time Scale (in millions of years ago)

Eon

Era

Period

Sub-Period

Epoch

Precambrian

4,500-543

Hadean

4,500-3,800

Archaean

3,800-2,500

Proterozoic

2,500-543

Paleoproterozoic

2,500-1,600

Mesoproterozoic

1,600-900

Neoproterozoic

900-543

Phanerozoic 543 to present

Paleozoic

543-248

Cambrian

543-90

Ordovician

490-43

Silurian

443-17

Devonian

417-354

Carboniferous

354-290

Mississippian

354-323

Pennsylvanian

323-290

Permian

290-248

Mesozoic

248-65

Triassic

248-206

Jurassic

206-144

Cretaceous

144-65

Cenozoic 65 to present

Tertiary

65-1.8

Paleogene

65-23.8

Paleocene

65-54.8

Eocene

54.8-33.7

Oligocene

33.7-23.8

Neogene

23.8-1.8

Miocene

23.8-5.3

Pliocene 5.3-1.8

Quaternary 1.8 to present

Pleistocene 1.8 to 10,000 years ago

Holocene 10,000 years ago to present

How are the divisions on the geologic time scale named?

Most of the major divisions on the geologic time scale are based on Latin names, or areas in which the rocks were first found. For example, the Carboniferous period gets its name from the Latin words for carbon-bearing, in reference to the coal- rich rocks found in England; the Jurassic period is named after the Jura Mountains along the border of France and Switzerland. The names of the stages or ages most often depend on city and regions where the rocks were found; this is why division names frequently vary on geologic time scale charts from different countries.

What are the major time units used in the geologic time scale?

There are five major time units on the geologic time scale. The units are in order of descending size eons, eras, periods, epochs, and stages (although some list this division as ages and subages). The eon represents the longest geologic unit on the scale; an era is a division of time smaller than the eon, and is normally subdivided into two or more periods. An epoch is a subdivision of a period; a stage is a subdivision of an epoch.

What do the divisions on the geologic time scale represent?

The geologic time scale is not an arbitrary listing of Earths natural history, nor are the divisions merely fanciful. Each boundary between divisions represents a change or an event that delineates it from the other divisions. In most cases, a boundary is drawn to represent a time when a major catastrophe or evolutionary change in animals or plants (including the evolution of specific species) occurred.

Natural erosion clearly reveals the layers of Earths crust, such as seen here in Badlands National Park in South Dakota. Observing these layers is like taking a trip back in time, with each lower level representing a different time period in the planets history (iStock).

What is relative time in relationship to geologic time?

Relative time is a way to establish the relative age of rocks and fossils. It is based on the location of a rock layer in comparison to the location of other rock layers; that is, it is only relative, not absolute, time. In many cases, rock layers are laid down in order, the older layers being below the younger layers. For example, a fossil found in a higher rock layer is usually younger than a fossil found in a rock layer below it. During the nineteenth century, scientists used this method to date rock layers relative to each other and to establish and construct the first geologic time scale.

What is absolute time in relationship to geologic time?

Absolute geologic time is the (approximate) true age of the rock; that is, the absolute time that the rock layer formed. Typically, radiometric techniques, which measure the amount of radioactive decay in rocks, are used to determine absolute time.

When were radiometric dating techniques discovered?

The basic principles and techniques of radiometric dating were not discovered until the turn of the twentieth century. In 1896, French physicist Antoine-Henri Becquerel (1852-1908) accidentally discovered radioactivity when a photographic plate left next to some uranium-containing mineral salts blackened, proving that uranium gave off its own energy. In 1902, British physicist Lord Ernest Rutherford (1871-1937) collaborated with British chemist Frederic Soddy (1877-1966) to discover that the atoms of radioactive elements are unstable, giving off particles and decaying to more stable forms. These findings led United States chemist Bertram Borden Boltwood (1870-1927) to argue that, by knowing the decay rate of uranium and thorium into lead, the dating of rock would be possible. In 1905, Boltwood and John William Strutt dated various rocks, obtaining ages of 400 to 2,000 million years for various rock samples and proving such dating could be done.

Who first developed an absolute geologic time scale using radiometric dating?

In 1911, British geologist Arthur Holmes (1890-1965) began to formulate a geologic time scale based on absolute time, using the uranium-lead dating method to determine the age of rocks. In 1913, he published The Age of Earth, in which he outlined how radioactive decay methods, in conjunction with geological data, could be used to construct an absolute geologic time scale. In 1927, Holmes estimated that the age of Earths crust, based on his radiometric techniques, is approximately 3.6 billion years old.

 
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