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 the wrist would be and all of human existence could be measured by the tip of a fingernail.
However, geologists typically divide the duration of time since the Earth came into being in different terms. The names of periods and eras they use will be referred to during this entry; please refer to this Geologic Glossary for definitions of unfamiliar terms.
The Law of Superposition
Superposition is a term applied 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 are 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 were 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 preceded 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.
Such igneous intrusions (or extrusions, for that matter) are important for dating geologic time intervals because they give us the opportunity to assign absolute as well as relative dates. Igneous rock often contains radioactive elements with a known half-life. By measuring the proportion of the element to 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.
There is, however, another way to obtain absolute dates.
The Earth has a magnetic field, as anyone who has used a magnetic compass to determine which way is North will know. The direction that the north end of the compass needle points has actually changed 180° periodically during the Earth's 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 is possible, therefore, to associate magnetic reversal sequences worldwide with different geologic eras and, in conjunction with radiometric studies, assign absolute dates to those eras.
Molten rock is continually upwelling on these zones. As the rock solidifies, it preserves a record of the direction of the Earth's magnetic North. 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 palaeomagnetic 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, 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.
Dendrochronology, the analysis of tree rings to determine time spans, can be utilised to resolve yearly changes within a specific era. It isn't particularly satisfactory for assigning absolute dates earlier than a few thousand years ago. However, since tree trunks sometimes fossilise, 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, somewhat more useful than dendrochronology because they can apply to periods before trees existed.
Presumed varve couplets located in ancient Lake Florissant yield estimates for the life of the lake of 2,500 - 5,000 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.
Geologic time deals with durations of millions and sometimes billions of years. The average horizon resolution1 in this immense span of time would still be measured in hundreds of thousands of years until we approach relatively recent times.
Radiometric and palaeomagnetic 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 aeons. 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
- A US Geological Survey article on geologic time.
- The Rock Record and the Geologic Time Scale