Geologic Principles for Defining Relative Age


Building from the work of Steno, Hutton, and others, the British geologist Charles Lyell (1797–1875) laid out a set of formal, usable geologic principles. These principles continue to provide the basic framework within which geologists read the record of Earth history and determine relative ages.

Uniformitarianism: The principle of uniformitarianism states that physical processes we observe operating today also operated in the past, at roughly comparable rates, so the present is the key to the past.

Original horizontality: The principle of original horizontality states that layers of sediment, when first deposited, are fairly horizontal because sediments accumulate on surfaces of low relief (such as floodplains or the sea floor) in a gravitational field. If sediments were deposited on a steep slope, they would likely slide downslope before they could be buried and lithified. With this principle in mind, geologists conclude that examples of folds and tilted beds represent the consequences of deformation after deposition.



Law of Superposition: The principle of superposition states that in a sequence of sedimentary rock layers, each layer must be younger than the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect. Thus, the layer at the bottom of a sequence is the oldest, and the layer at the top is the youngest. 



Lateral continuity: The principle of lateral continuity states that sediments generally accumulate in continuous sheets within a given region. If today you find a sedimentary layer cut by a canyon, then you can assume that the layer once spanned the area that was later eroded by the river that formed the canyon. 


Cross cutting relations: The principle of cross-cutting relations states that if one geologic feature cuts across another, the feature that has been cut is older. For example, if an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the dike. If a fault cuts across and displaces layers of sedimentary rock, then the fault must be younger than the layers. But if a layer of sediment buries a fault, the sediment must be younger than the fault. 

 

Principle of baked contacts: The principle of baked contacts states that an igneous intrusion “bakes” (metamorphoses) surrounding rocks, so the rock that has been baked must be older than the intrusion.



Principle of inclusions: The principle of inclusions states that a rock containing an inclusion (fragment of another rock) must be younger than the inclusion. For example, a conglomerate containing pebbles of basalt is younger than the basalt, and a sill containing fragments of sandstone must be younger than the sandstone. 



Geologists apply geologic principles to determine the relative ages of rocks, structures, and other geologic features at a given location. They then go further by interpreting the formation of each feature to be the consequence of a specific geologic event.

Examples of geologic events include: Deposition of sedimentary beds; erosion of the land surface; intrusion or extrusion of igneous rocks; deformation (folding and/or faulting); and episodes of metamorphism. The succession of events in order of relative age that have produced the rock, structure, and landscape of a region is called the geologic history of the region. We can use these principles to determine relative ages of the features. We develop a geologic history of the region, defining the relative ages of events that took place there.

Fossil Succession
As Britain entered the industrial revolution in the late 18th and early 19th centuries, new factories demanded coal to fire their steam engines and needed an inexpensive means to transport goods. Investors decided to construct a network of canals to transport coal and iron, and hired an engineer named William Smith (1769–1839) to survey some of the excavations. Canal digging provided fresh exposures of bedrock, which previously had been covered by vegetation. Smith learned to recognize distinctive layers of sedimentary rock and to identify the fossil assemblage (the group of fossil species) that they contained. He also realized that a particular assemblage can be found only in a limited interval of strata, and not above or below this interval. Thus, once a fossil species disappears at a horizon in a sequence of strata, it never reappears higher in the sequence or, put another way, extinction is forever. Smith’s observation has been repeated at millions of locations around the world, and has been codified as the principle of fossil succession. It provides the geologic underpinning for the theory of evolution.
Example: Bed 1 at the base contains fossil species A, Bed 2 contains fossil species A and B, Bed 3 contains B and C, Bed 4 contains C, and so on. From these data, we can define the range of specific fossils in the sequence, meaning the interval in the sequence in which the fossils occur. The sequence contains a definable succession of fossils (A, B, C, D, E, F), that the range in which a particular species occurs may overlap with the range of other species, and that once a species vanishes, it does not reappear higher in the sequence. Once the relative ages of a number of fossils have been determined, the fossils can be used to determine the relative age of the beds containing them. For example, if a bed contains Fossil F (from the succession specified above), geologists can say the bed is older than a bed containing Fossil A, even if the two beds do not crop out in the same area. As we will see, painstaking work over many years eventually allowed geologists to assign numerical age ranges to fossil species. Of note, some fossil species are widespread, but survived only for a relatively short interval of geologic time. Such species are called index fossils (or guide fossils), because they can be used by geologists to associate the strata with the specific time interval.


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