Introduction

If the age of the earth were represented as the length of one's arm with the shoulder representing the beginning of geologic time, the dinosaurs would have died out about where wrist would be and all of human existence could be measured by the tip of a fingernail, the part that typically would be trimmed by a manicurist.

Click here to display a time scale showing how geologists typically divide the duration of time since the earth came into being presumably out of the solar nebula. People might wonder though how this time scale was developed, how you can tell geologic time.

Please take note of the periods and eras, since we will refer to some of them again. Also, please refer to this Geologic Glossary for definitions of unfamiliar terms.

The Law of Superposition

To answer the question, first there's Superposition. Superposition applies to strata or layers of material deposited in low places on the Earth's surface by natural processes over time. The Principle of Superposition simply states that the lower strata or layers are older than the upper layers because they were deposited first. This means we can date a stratum or layer relative to the strata above and below it.

Strata is almost always deposited in horizontal layers, at least initially. This also means that if, for example, we find an exposure of strata that is tilted, then we can conclude that the event that caused the tilting is younger than the strata that was tilted.

This is how we know that a tilted exposure of the Fountain Formation on the eastside of the Front Range of the Southern Rocky Mountains is not material eroded from that range but from a much older range that preceeded it. The Fountain Formation then is one of the few pieces of the evidence that we have that the former Ancestral Rockies ever existed.

Igneous intrusions into strata can also be dated as younger than the strata. Such intrusions are magma or molten rock from deep in the earth's crust or mantle. When they extrude on the surface we call them lava flows or volcanic eruptions.

Radiometric Dating

Such igneous intrusions or extrusions for that matter are important for dating geologic time intervals as well because they give us the opportunity to assign absolute as well as relative dates. Igneous rock often contains radioactive elements with known half-lives. By measuring the proportion of the element to its daughter element or elements, the products of its radioactive decay, we can determine the number of years since the rock containing the element was molten and hence intruded into or extruded onto the strata.

Here is a link to a more comprehensive explanation of Radiometric Dating techniques.

Chronometric Techniques–Part II

Paleomagnetism

There is another way to obtain absolute dates in conjunction with the radioactive method that can be applied worldwide.

The earth has a magnetic field, as anyone can tell who has used a magnetic compass to determine which way is North. It turns out that the direction that the north end of the compass' needle points has changed 180 degrees periodically during the earth's long history. These magnetic reversals have occurred more or less randomly and persisted for different durations so that any given sequence of reversals will differ from other sequences.

It's possible then to associate magnetic reversal sequences worldwide with different geologic eras and in conjunction with radiometric studies assign absolute dates to those eras.

Here is a link to an image of paleomagnetic data associated with the world's tectonic spread zones.

Digital Isochrons of the Ocean Floor

Molten rock is continually upwelling on these zones. As the rock solidifies it preserves a record of the direction of the earth's magnetic at the time. That record is then pushed outwards from the spread zone as more molten rock is extruded and the cycle is repeated. The resulting records represent the state of the magnetic field over time. These are the sequences referred to above.

The Principle of Faunal and Floral Succession

Previous to the introduction of paleomagnetic studies, such worldwide dating was accomplished by the use of fossils under the Principle of Faunal and Floral Succession. In the evolution of life on earth certain assemblages of creatures have followed others. Formations in which dinosaurs predominate are earlier than formations in which mammals predominate, for example. Signature fossils become associated with certain sequences of strata or sedimentary layers. If those same fossils are found in other strata, it is assumed that the sequences are approximately contemporary and can, therefore, be calibrated in time.

Another example relates to well established fossil records, such as that associated with Mesozoic Ammonites, described in the excellent, linked Guide Entry by researcher Frogbit and relatives of the modern Nautilus. Because these creatures changed over time, their different forms were represented in different strata or layers. These could be used to resolve the relative dates of horizons or layers in marine deposits all over the world. This in turn could be used to date shoreline deposits that interleaved with the marine deposits.

Other Methods

Dendrochronology, the analysis of tree rings to determine time spans, can be utilized to resolve yearly changes within a specific era. It isn't much good though for assigning absolute dates earlier than a few thousand years ago. However, since tree trunks sometimes fossilize, the method is not limited to the Holocene but has been applied as far back as the Mesozoic to resolve shorter time spans. It is sometimes used for calibration of Carbon 14 methods, which are radiometric.

Varves, the seasonal layers of sediment deposited in lakes, are discernable in Precambian formations and, therefore, are somewhat more useful than dendrochronology because they can apply to eras when trees didn't yet exist.

Presumed varve couplets located in ancient Lake Florissant, in the Southern Rocky Mountains yield estimates for the life of the lake of 2500-5000 years, however, the lake deposits date from the upper or latest Eocene, about 34 million years ago. As we go back further in geologic time, such fine resolution in formations is not easily attained.

Conclusion

With geologic time, we are dealing with durations of millions and sometimes billions of years, not just thousands. The average horizon resolution in this immense span of time would still be measured in hundreds of thousands of years until we approach relatively recent times.

Radiometric and paleomagnetic studies in conjunction with applications of the faunal and floral succession allow us to devise a pretty precise time scale for what has happened on earth over eons. Various stratigraphies can be correlated with these dating techniques, as well as dendrochronology and varve analysis, to determine such things as how the earth's climate has changed, how the continents have drifted around the earth, how seas have inundated continents, how mountain ranges have risen and been eroded away, and how life has evolved.

Some Further Reading

U.S. Geological Survey article on geologic time - http://pubs.usgs.gov/gip/geotime/contents.html

The Rock Record and the Geologic Time Scale - http://www.sfu.ca/earth-sciences/courses/101web/Chap9.html

Relevant Links Kindly Provided by The Pencil Queen

"Another geological timeline, not as detailed but pretty."

"The main page is also worth a look> (Not really relevant to dating but the experiments are nice.)"