Relative Ages of Rocks - Earth Science in Maine
Before applying the Law of Superposition to a set of rock layers, it must be established But if the layers are made of metamorphic or intrusive igneous rocks, then the age . 'Y' and 'Z') may be used to establish the relative ages of those rocks. Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence. The method of reading The Grand Canyon. Principle of are igneous rock layers that form on the surface when lava hardens. Extrusions . The eroded cliffs reveal billion years of fossils, volcanic activity, and geologic history. When one's objective is simply to learn how old a rock layer is, ages, rely on relative dating, correlation, and the use of index fossils. rocks of the Grand Canyon Supergroup, and (3) the layered sedimentary rocks.
A third observer, who has not been to locations A or B, sees the vertical layers and cannot decide which layer was originally 'topmost' and which 'bottommost' and draws no conclusion about their relative ages.
At location D the layers have undergone extreme deformation. The layers within the circled area have actually been inverted.
What now appears to be the 'topmost' layer was originally the 'bottommost' compare with the order of the layers in Diagram A. A fourth observer, who has not been to locations A, B or C, sees the almost horizontal layers and assumes incorrectly that the layers have not been significantly deformed.
Applying the Law of Superposition to determine the relative ages of the layers, the observer gets the relative ages of the layers reversed. Fortunately, many depositional layers both sedimentary layers and lava flows contain features that indicate original orientation. There are hundreds of such features called primary structures. Here are some examples of primary structures: The points of the ripples point upward.
The crater basins are convex down; the crater rims point up. The branches of tree roots point downward. Another primary structure that may be used to determine 'tops' and 'bottoms' of layers is the tilt or lack of tilt of the layers. If the layers are horizontal and traceable over considerable distances, the geologist will conclude unless evidence to the contrary turns up that there is a very high probability that the layers are right-side-up.
Justification for this conclusion is that where obviously deformed rock layers can be observed, the places where complete overturning has been achieved are quite local. This not surprising since it is harder takes more energy for lengthy portions of layers to be 'turned over' than for local portions. Diagram A illustrates an extensive outcrop of horizontal layers exposed over a great distance. The layers have a high probability of being 'right-side-up'. Diagram B illustrates several separated local outcrops in which horizontal layers are exposed.
The layers in the separate outcrops 'line up' with one another. The geologist assumes dashed lines that if the grass and soil were removed, the layers would be continuous over the whole area.
Diagram C illustrates a single local outcrop of horizontal layers. Because completely inverted layers are rare layers turned right over to become horizontal againthe geologist assumes, in the absence of contrary evidence, that the layers are probably 'right-side-up'.
Telling Time at Grand Canyon National Park
That is, the geologist infers that graded bedding, ripple marks, vesicles, etc. Sedimentary rocks frequently contain objects that have been interpreted as evidence that life existed at the time the sediment accumulated. These 'objects in rocks' are exceedingly diverse, including many whose shapes resemble organisms alive today. Shells and bones or their imprints, or impressions such as tracks or burrows are amongst the most common objects.
Others are quite different from any life form that exists today, but seem to have an organization or shape that seems somehow suggestive of life. These life-related objects in rocks have come to be called fossils. The modern interpretation of fossils is that they actually are remains or artifacts of once living organisms.
Normally, after living organisms die, their remains are quickly scattered and decayed and the record of their existence is rapidly obliterated. On rare occasions, quick burial of the remains by mud, sand or volcanic ash prevents their destruction and they become preserved as the loose material in which they are embedded is lithified.
The preservation of soft parts of organisms is extremely rare. Preserved hard parts are commonly mineralized turned into rocky substances. By the early 19th century, through observation of fossils in rocks, it was accepted that through time, the nature of life on Earth has changed. That is, individual species appear in the rock record, exist for a certain period of time, and then disappear forever from the rock record.
Consider the diagram opposite. A sequence of rock layers numbered 52 to 63 exposed at location 'X'. One of the rock layers, 55exhibits graded bedding, indicating the layers are 'right-side-up'. Hence, layer 52 is oldest, layer 63 is youngest. Each layer formed during a certain period of time and represents what happened at location 'X' during that time. A series of colored dots that represent the levels within the rocks where specimens of fossil species A, B, C, and D have been found at location 'X'.
Each level represents a moment in time. A series of colored double-headed arrows indicating the range of time spanned between the lowest and highest levels of the occurrence of each fossil species at location 'X'. A series of black double-headed arrows indicating the range of time spanned between the lowest and highest levels of the occurrence of each fossil species found in rocks throughout the world. It may be seen that the ranges of the different fossils species overlap, so that in some layers, more than one fossil species may occur.
That is not surprising since more than one type of organism lives at the same time. Different fossils species that occur together constitute a fossil assemblage. The time interval between the first and last appearance anywhere in the world of a fossil species is known as its 'geologic range'.
With continued investigation, The geologic ranges of individual species are subject to revision as investigation of rocks continues. Newly discovered occurrences may place the introduction and extinction of species respectively earlier and later in time. As the geologic ranges of species are adjusted, the geologic ranges of fossil assemblages are also revised.
Although the mechanisms that brought species into existence and then caused their extinction is debated for example, evolution vs. The 'fact' of change in life through time is referred to as the Law of Biotal Succession. Some holdouts who do not accept the Law of Biotal Succession are people who claim that all rocks were created by God at the same time; therefore, rocks do not record history.
They consider any appearance of history to be an illusion. Since this claim cannot be tested, it falls outside the realm of scientific discussion.Rock & Fossil Correlation
Fossil assemblages found in rocks at other locations where no primary structures are present such as locations 'Y' and 'Z' may be used to establish the relative ages of those rocks. The black double-headed arrows shown in the diagram for location 'X' represent the geologic world-wide time ranges of fossil species A, B, C and D. Geologists therefore are keenly interested in working out equivalency of age of rocks in different locations.
Rocks that have the same age to the best of geologists' ability to determine their ages are said to correlate. Correlating rocks in some cases is simple. In others, it can be a complex process involving many observations, hypotheses and tests of hypotheses.
As will be seen, fossils frequently play a vital role in correlation. First, consider a relatively simple case. A grassy slope displays three outcrops of horizontally layered rocks diagram A. The geologist notes that the sequence and characteristics thickness, color, texture, mineralogy of the layers in the three outcrops are the same.
The geologist therefore infers that the three outcrops reveal separate parts of the same continuous sequence of horizontal layers diagram B. That is, the geologist believes that if all the material covering the bedrock was removed, the continuity of the layers would be revealed diagram C. Another inference the geologist makes is that rocks at the same level within each outcrop are the same age and correlate with each other.
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This argument is based upon two important assumptions. It is assumed that it is highly probable that the layers observed in each outcrop continue laterally underneath the grass beyond each outcrop. This assumption is known as the Law of Lateral Continuity: Most sediments are laid down as layers on flat surfaces and have considerable extent in all directions compared to the thickness of the layer.
For example, pulses of sediment flushed into in a lake may come to rest as inch-thick layers deposited over a considerable portion of the lake floor. Thus it is reasonable to assume that the layers seen in the separated outcrops are actually joined. It is assumed that it is highly probable that each layer has the same age throughout its length and breadth. In the lake example, all parts of each pulse of sediment brought to the lake settle to the lake floor at roughly the same time. Correlation becomes more difficult when rocks forming at the same time do so in different environments.
There is no reason for a layer of sediment being deposited on the floor of a lake to be similar in thickness, texture or composition to sediment being deposited by waves and currents along the shore of an ocean, by wind in the desert, by melting glacial ice, or by streams over a floodplain.
Correlation in these instances is less straight forward but may be accomplished with the aid of fossils. Recall that the black double-headed arrows represent the worldwide geologic ranges of fossil species A, B, C and D. That is, no specimens of these fossil species have been found anywhere in the world in rocks older or younger than the indicated ranges. In constructing the ranges of the fossils, hundreds of localities including location 'X' have been examined and the accuracy of the geologic ranges is considered established: The following conclusions may be drawn: The rock colored blue at location 'X' formed before fossil B disappeared from the rock record and after fossil C appeared in the fossil record.
Rocks at location 'Z' have not been examined before.
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The rock colored blue at location 'Z' has the same fossils as those in the 'blue' rock at location 'X'. Because the geologic ranges of fossils B and C are considered well-established, it is judged highly likely that this rock formed within the same time interval as that at 'X': Although both 'blue' rocks formed within the same time interval - between 'p' and 'q' - they did not necessarily form at precisely the same time.
It may be that the rock at 'X' formed towards the middle of the time interval, whereas the rock at 'Z' formed towards the end of the time interval. Therefore, our project first required identifying the rock units for which numeric ages are important.
We limited our project to the three overall sets of rocks and those rock formations or groups that interpreters and resource managers routinely discuss.
Vishnu Basement Rocks We established the informal name Vishnu Basement Rocks for all of the ancient crystalline rocks at the bottom of the Grand Canyon because no formal nomenclature encompasses all the metamorphic units and individual igneous plutons exposed there. The many reliable radiometric age determinations of the igneous and metamorphic Vishnu Basement Rocks e. The challenge was to interpret the geologic significance of the dates in a meaningful context for interpreters and resource managers.
The Elves Chasm is significantly older, at least 90 million years, than any other basement rock. It formed before the main tectonic collisions that produced most of the other rocks comprising the Vishnu Basement Rocks 1,—1, million years ago.
We also chose to exclude a few younger plutons, which formed about 1, million years ago, from the overall age of the Vishnu Basement Rocks. These rocks postdate the main tectonic events that formed this set and, though interesting, are a detail better left to the advanced study of Grand Canyon geology Grand Canyon Supergroup Rocks Grand Canyon Supergroup Rocks are primarily sedimentary.
However, radiometric age determinations of the Cardenas Basalt, ash beds, and other datable material within the sedimentary rocks provide age constraints for this set.
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We included some dates from paleomagnetic studThese numeric ages are an important translation for park managers and the public. The Supergroup rocks predate the Cambrian Period, when hard-shelled organisms first appeared in the fossil record, so they have few identifiable index fossils.
Our dates are bracketed by the ages of the basal Unkar Group at 1,—1, million years ago Arizona Geological Survey, M. Timmons, personal communications, — and the Chuar Group at — million years ago Dehler et al.
No datable material has been found in the uppermost Sixtymile Formation see table 1. The Supergroup is the focus of active geologic investigation, so these ages may change as new information becomes available. Because no single stratigraphic name exists for this set, Layered Paleozoic Rocks is also an informal term; nevertheless, their rock type, age, and overall geologic setting naturally package them together.
No reliable radiometric dates exist for these sedimentary rocks, so their ages are constrained by index fossils. Units with richer fossil records have more precise age constraints. All geologists use the same basic divisions of geologic time e. The International Stratigraphic Chart Grandstein and Ogg ; International Commission on Stratigraphy is the most accurate and up-to-date time scale available for worldwide correlation of rock units.
We used it as our basis for determining the numeric ages for rocks in Grand Canyon National Park. However, investigators have used many local or regional scales, such as the North American Chronostratigraphic Scale, for finer subdivisions.
These other scales work well for describing regional geology but can be difficult to correlate worldwide. The relationship between the North American Chronostratigraphic Scale and the International Stratigraphic Chart is not straightforward.
Hence, we consulted Dr. Ronald Blakey, a stratigrapher at Northern Arizona University, to ensure that we had developed a set of reasonable dates for the Layered Paleozoic Rocks. The other challenge of determining the age of the Layered Paleozoic Rocks was identifying the best single number to represent the age of each unit.
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Sedimentary rocks are usually deposited over long periods of time, and some units exposed in Grand Canyon contain significant gaps in the rock record, called unconformities. Furthermore, many formations, in particular the Tonto Group, record marine transgressions as sea level rose, making the unit older in the west than in the east.
Because most developed areas of Grand Canyon National Park are in the eastern canyon, we targeted our compilation on the age of rocks there. Results and Distribution We completed our original compilation of Grand Canyon rocks in Because of refinements in the geologic time scale and new findings by researchers, we revised it in Further revisions may be necessary as knowledge of Grand Canyon geology improves, new or improved absolute dating techniques are developed, or the geologic time scale is modified.
Given the current knowledge of Grand Canyon geology, table 1 compiles our best numeric ages of its rocks. Both now use the numeric ages in their interpretive programs, publications, exhibits, and resource management reports fig.
These articles, which targeted lay audiences and Colorado River guides, explained geologic dating techniques and summarized the ages of Grand Canyon rocks. These publications further encouraged consistency among park cooperators who interpret and otherwise communicate the ages of Grand Canyon rocks. Conclusions From literature searches, consultations with geologists, and interpretations of scientific data, we compiled the numeric ages of rocks exposed in Grand Canyon National Park.
Our age compilation provides information about the age of Grand Canyon rocks in a form meaningful to interpreters, park managers, and visitors.
The primary outcome of this project is that the ages given for Grand Canyon rocks are more consistent in interpretive media, park documents, and popular GCA publications. While the compilation is our primary product, the interpretive publications based on this work provide additional information about how geologists tell time and why these dates are important. With this broader perspective, the age of Grand Canyon rocks becomes more meaningful. Furthermore, providing a consistent set of reliable ages adds to the credibility of geologic interpretation.
This project is a good example of collaboration among scientists, resource managers, and interpreters.