Relative vs. Absolute Dating. Important Facts Uniformitarianism The Present is the KEY to the PAST. Laws of nature are the same now as they have always been. Past events explained and estimated by modern processes. Charles Lyell credited with this concept. Explained in his book “The Principles of Geology.” Other Important Words… Lithification: the consolidation of a loosely deposited sediment into a hard sedimentary rock. Strata: rock layers Original Horizontality When sedimentary rock layers form, they are “created” in a horizontal fashion. And, most importantly…. Relative vs. Absolute Age of Rock Relative Dating: determination of the age of a rock or geologic event by comparing with other rocks/geologic events Can you infer who is older? She is.
Why is the "Law of Superposition" critical to the study of fossil life forms? It states that heavier rocks are on the bottom and light rocks rise to the top.
It specifies that each layer of rock is a different type of mineral. It allows us to time-sequence fossils It tells us that organisms are usually replaced by more highly-adapted ones. Rock layers tell a story about their past. What is the basic assumption that is made when reading the story? Chemical reactions in the past were slower than they are today The Earth\'s crust cooled and them remelted before the rock layers began to form The composition of the Earth\'s atmosphere has been constant The processes in the rock cycle were the same in the past as they are today The absolute age of a rock is the approximate number of years ago that the rock formed.
The absolute age of an igneous rock can best be determined by examining the environment in which the rock is found comparing the sizes of the crystals found in the upper and lower parts of the rock comparing the amounts of decayed and undecayed radioactive isotopes in the rock examining the rock's relative position in a rock outcrop
best absolute dating vs relative dating geology practice - Relative dating. Relative dating is the process of determining the order that things happened in geologic time.
By: Laura Wormuth Geologists can employ two basic methods to dating rocks and fossils. Relative and absolute geologic dating practices have their strengths and weaknesses, which are inherent to their process. Although this article highlights a brief overview of two basic types of geologic dating, it is not meant to represent all methods of dating, and it is also not meant to be a comprehensive review of relative and absolute methods.
Rather, this is a simplified explanation paired with their apparent weaknesses. Relative dating simply orders stratigraphic units, or layers of rock, from oldest to youngest. There are several rules or laws in place that provide scientists with the means to determine which layer is oldest, youngest and the relative ages of the layers in between. The rules are a fundamental frame, and they are based in logic.
For example, the Principle of Original Horizontality states that all layers of sedimentary rocks are deposited in flat, parallel layers. The Law of Superposition indicates that the oldest layers will be on the bottom, the youngest on the top. Likewise, the Law of Cross-Cutting Relationships holds that any igneous intrusion or physical change such as faulting, must have occurred after the deposition of the strata that it cuts through, and thus, must be older than those particular rocks.
These simple parameters provide the fundamental basis for relatively dating geologic strata. The weakness of relative dating is inherent to its very nature. The primary issue is the lack of "real" dates, as in assigning a numerical value to the number of years that have passed since the formation of the rock sample.
Stating that one layer is older or younger than another provides invaluable information regarding the environmental changes represented in the geologic record, but it does not provide a date for the bed. For example, a scientists will be able to say Layer A is older than Layer B, and Layer C cuts across Layer A; therefore, Layer C must be younger than A.
However, they will not be able to conclude that Layer A is X number of years older than Layer B. Absolute Dating is the method by which an actual numerical value is determined for a layer of rock. The most reliable way to accomplish this is through radiometric dating. When an unstable radioactive isotope, or parent, decays or loses radiation such as a beta particle, antineutrino or a gamma ray, a daughter product is left behind.
The time it takes for an unstable nucleus to decay to the daughter product is called a half-life. The half-life of many isotopes has been consistently tested and measured precisely. The half-life of the isotope is the number that determines the age of a rock. For instance, potassium-40 decays to argon-40 with a half-life of 1.3 million years. Therefore, if there is a 50-50 ratio of parent to daughter, 1.3 million years have passed since the formation of the rock containing the elements.
There are several elements with radioactive isotopes, including carbon-14 and uranium-238, common isotopes used for geologic dating. Accurate data can only be relied on if the mineral used for radiometric dating was in a closed system for the duration of its life -- from formation to discovery.
Exposure to the elements can create an addition or loss of parent or daughter isotope, skewing the results. Using the previous example of potassium-argon isotopes, argon is a gas, which, when exposed, can be released into the atmosphere, thus losing some of the measurable daughter element. This can also occur when isotopes are exposed to extreme temperatures, weathering or chemical processes.
To avoid this simple pitfall, a geologist must be sure to take mineral and rock samples from freshly broken pieces, rather than exposed or weathered surfaces.
Slide 1Relative dating Slide 2 Relative dating is the process of determining the order that things happened in geologic time. Slide 3 Principles of Relative Dating Stenos Laws The Principle of Superposition The Principle of Original Horizontality The Principle of Original Lateral Continuity The Principle of Intrusive Relationships The Principle of Cross-Cutting Relationships Slide 4 Stenos laws Principle of superposition- in a sedimentary sequence, the older beds are on the bottom, and the younger beds are on the top.
Slide 5 The Principle of Original Horizontality states that sediments are deposited in flat, horizontal layers. Slide 6 The Principle of Original Lateral Continuity When sediment is deposited on the seafloor, the sediment will not only be deposited in a flat layer, it will be a layer that extends for a considerable distance in all directions.
In other words, the layer is laterally continuous. Slide 7 The Principle of Intrusive Relationships Where an igneous intrusion cuts across a sequence of sedimentary rock, the relative ages of these two units can be determined.
The sedimentary rocks are older than the igneous rock which intrudes them. (In other words, the sedimentary rocks had to be there first, so that the igneous rocks would have something to intrude.) Or, you could say, the intrusion is younger than the rocks it cuts. Slide 8 The Principle of Intrusive Relationships continued Diagram (1): Dike B is younger than Sedimentary Rock A. Erosion surface C is younger than Dike B. Sedimentary Rock D is younger than Erosion Surface C. Diagram (2) Sill B is younger than Sedimentary Rock A.
Dike C is younger than sill B. Diagram (3) Stock B is younger than Sedimentary Rock A. Dike C is the youngest. Slide 9 The Principle of Cross-Cutting Relationships Where a fault cuts across a sequence of sedimentary rock, the relative ages of the fault and the sedimentary sequence can be determined.
The fault is younger than the rocks it cuts. The sedimentary rocks are older than the fault which cuts them, because they had to be there first, before they could be faulted.
Slide 10 The Principle of Cross-Cutting Relationships continued Normal fault Reverse fault Examples of faults to illustrate cross-cutting relationships. (1) Unit A is the oldest, followed by B and C. Fault D is the youngest. (2) Unit A is the oldest, followed by B and C. Fault D is younger than C, but older than unit E Slide 11 Using these basic principles, you may determine the order in which different events happened during geologic time.
Laws of Relative Rock Dating