The Geological Time Scale

 

The Immensity of the Time Scale:

The time scale of geology is immense and almost impossible for us to comprehend. It is difficult to wrap your mind around the concept of 4,600,000,000 years - 4.6 billion years, or 4600 million years, or however you write it.

The Divisions and Subdivisions:

The divisions and subdivisons of the time scale are presented below along with a discussion of some of the events that occurred during each of the time periods. Notice that the names of the periods sometimes correspond evolutionary events.
 
The Entire Geological Time Scale

Geological Time Scale

A reasonable estimate is that the earth coalesced into a more or less solid sphere about 4.6 billion years ago. This is the beginning of the time scale. The time scale is divided into the Precambrian and Phanerozoic periods.

Precambrian The first subdivision is the Precambrian which stretches across 90% of geological time. This period is subdivided into:

HADEAN (means "below"): The geological records from this period have been almost entirely lost, covered over by subsequent events and are "below" the rocks we have access to. The earliest known rocks date at about 4400 million years ago (MYA). The Hadean extends from 4600 to 3700 MYA.
 
ARCHEAN (means "ancient"): Begins at 3700 MYA with the first fossil bacteria and blue green algae. Extends from 3700 to 2500 MYA.
 
PROTEROZOIC (means "first life"): Begins at 2500 MYA with the first fossil evidence of organisms with modern (eukaryotic) cells. Later during this period organisms that reproduced sexually evolved as did organisms formed by colonies of cells. By 660 MYA organisms very similar to modern jellyfish evolved. The Proterozoic extends from 2500 to 540 MYA.

PHANEROZOIC: This subdivison comes after the Precambrian. Extends from 540 MYA to the present.

The Phanerozoic Time Scale

Phanerozoic

At the beginning of the Phanerozoic a staggering abundance and diversity of living organisms suddenly appeared in the fossil record. (The word Phanerozoic means "open" - i.e. not hidden - "life") This is thought to be due to the earth's primitive atmosphere gaining enough oxygen that an ozone layer could be formed in the upper atmosphere. This ozone layer protected surface dwelling organisms from excessive ultraviolet radiation. The Phanerozoic is divided into the Paleozoic, the Mesozoic, and the Cenozoic.

 

PALEOZOIC (means "old life"): Begins at 540 million years ago (MYA) and is subdivided into:

Cambrian: At the beginning of this period the earliest marine animals with shells appear in the fossil record. During this period the now extinct trilobite arthropods became dominant. The earliest chordates, then the earliest fish appear. The Cambrian extends from 540 to 505 MYA.
 
Ordovician: A now extinct animal called a graptolite becomes abundant. Echinoderms, then molluscs also become abundant. The first corals evolve. Heavily armored fish evolve. Extends from 505 to 438 MYA.
 
Silurian: The now extinct eurypterid arthropods become dominant. The earliest land plants and animals appear. The earliest insects appear. Extends from 438 to 408 MYA.
 
Devonian: In the seas shelled forms called brachiopods are diverse and abundant. The heavily armored fish are replaced by more lightly armored forms, then sharks and more modern fish evolve. A type of mollusc called an ammonoid evolves. The Devonian extends from 408 to 360 MYA.
 
Mississippian: In the seas sharks become dominant. On land numerous amphibians dominate in forests formed of scale trees and seed ferns. Extends from 360 to 320 MYA.
 
Pennsylvanian: In the seas little changes. On land the great coal forming forests dominate the landscape. The amphibians still dominate but the earliest reptiles evolve. Extends from 320 to 286 MYA.
 
Permian: Little changes in the seas or on land until the very end of the period when many of the most common types of marine invertebrates, including the trilobites, eurypterids, and graptolites, become extinct. This massive extinction may have been related to the formation of the supercontinent Pangaea and a resulting loss of marine habitat. On land the early reptiles replace the amphibians as the dominant large form of life. The Permian extends from 286 to 245 MYA.

MESOZOIC (means "middle life"): Begins 245 MYA and is subdivided into:

Triassic: On land the earliest dinosaurs and flying reptiles evolve and replace the early reptiles. Cycads and conifers evolve and form abundant forests. In the sea modern coral groups evolve. The Triassic extends from 245 to 190 MYA.
 
Jurrasic: On land the dinosaurs dominate all ecosystems. The earliest birds and mammals evolve. In the sea the ammonoid molluscs are very abundant. Extends from 190 to 144 MYA.
 
Cretaceous: In the sea modern fish evolve. On land the dinosaurs reach their greatest development. Placental mammals and flowering plants evolve. At the end of this period the dinosaurs abruptly become extinct. This may have been caused by the collision of the earth with a giant meteorite. At the same time the ammonoids in the sea become extinct. Extends from 144 to 66 MYA.

CENOZOIC (means "new life"): The third subdivision of the Phanerozoic. Extends from 66 MYA to the present.

The Cenozoic Time Scale

Cenozoic

This period is begins with the extinction of the dinosaurs and the end of the Age of Reptiles. It is sometimes called the Age of Mammals and is divided into the Tertiary and Quaternary.

 

TERTIARY: Begins at 66 million years ago (MYA) and is subdivided into:

Paleocene: Early groups of mammals evolve. Extends from 66 t0 55 MYA.
 
Eocene: Most modern groups of mammals evolve. Extends from 57 to 37 MYA.
 
Oligocene: Large running mammals evolve. Earliest grasses evolve. Extends from 37 to 24 MYA.
 
Miocene: Abundant grazing animals dominate many areas. The Adirondack Dome begins to rise. Extends from 24 to 5 MYA.
 
Pliocene: Large carnivores become abundant. The first hominids (the group which will contain humans) appears in the fossil record. The first of the recent glacial ages occurs during the end of this period. Extends from 5.0 to 1.6 MYA.

QUATERNARY: The "ice ages." Extend from 1,600,000 years ago to the present.

The Quaternary Time Scale

Quaternary

The subdivisions of this period are based on the ebb and flow of the continental glaciers. Notice that the first glacial period in the current round occured during the end of the Pliocene.

We are now in the Holocene or Recent period, which is probably an interglacial period.

The Unevenness of the Subdivisions:

Aside from the number of years involved there are two other factors that skew our appreciation of the geological time scale. First, there is the fact that the closer the geological period is to the present day, the more likely it is to be subdivided. In the first figure above time is divided into two unequal parts. The smaller, the more recent period (the Phanerozoic) is only about 10% of the total, but it is the part that gets the most attention and is divided and subdivided again. You will see that the same pattern is found in turn within each of these subdivisions.

There is a logical reason for this. As time progresses the older parts of the record are progressively erased. The closer the time is to the present, the more we know about what happened and the more we are able to meaningfully subdivide the time periods. This is not to say that similar events didn't occur in these older periods. They did. We've just lost (or have not yet found) the records.

Because of this unevenness we tend not to appreciate the immense amount of time in these earlier periods.

The Somewhat Arbitrary Nature of the Subdivisions:

Second, the figures above show the geological ages as if there were precise beginnings and endings for each period. Geological processes tend to move slowly and uniformly. To imply that you would go to bed one night in the Jurrasic and wake up next morning in the Cretaceous is misleading. The events of the Jurrasic gradually segued into those of the Cretaceous. There is only a hazy and somewhat arbitrary line between them.

Most geological periods do not begin nor end with an eye-opening bang. There may, however, be exceptions to this. For example, the meteor crash hypothesized to end the Age of the Dinosaurs (Cretaceous).

 

A "Gedanken" Experiment

Here is a thought experiment which may help you comprehend the immensity of the geological time scale:

Imagine: Take a fresh roll of toilet paper and start to unroll it. Let each sheet represent 100,000 years. As the toilet paper unrolls from the starting point we go back in time.

The first sheet, the first 100,000 years, represents about how long modern man, Homo sapiens as it is today, has been around. The end of the last ice age comes less than one-half inch from the end of the roll. The beginning of human civilization is less than one-fourth inch from the end. The last 2000 years about one-sixteenth of an inch from the end. Not enough paper to be of any use.

Now let us start back in time. Time is millions of years ago (MYA). Better bring along a few more rolls of toilet paper.
beginning of the last glacial period (Wisconsinian) 0.16 MYA 1.6 sheets 0.5 feet away
hominids appear in the fossil record 5 MYA 50 sheets 17 feet away
age of mammals begins 66 MYA 660 sheets 220 feet away
age of the great dinosaurs begins (Cretaceous) 144 MYA 1440 sheets 480 feet away
beginning of the Phanerozoic 540 MYA 5400 sheets 1800 feet away = 0.35 miles
modern celled organisms appear (Proterozoic) 2500 MYA 25,000 sheets 8300 feet away = 1.6 miles
bacteria and blue-green algae apper (Archean) 3700 MYA 37,000 sheets 12,300 feet away = 2.3 miles
the earth forms 4600 MYA 46,000 sheets 15,300 feet away = 2.9 miles

So we've festooned the landscape with a 3 mile strip of toilet paper. Our recorded history represents only the last one-eight inch of that 3 miles.

 

An Aside:

In a way the evolution of life that is recorded in the geological time scale is a swelling progression reminiscent of the orchestral piece "Bolero" by Ravel.

One can only hope that we as individuals and as a species will have the wisdom and ethical forbearance not to bring the music to a halt.