Relative dating - Wikipedia
sums it all up very well: negeriku.info http:// negeriku.info Cross-Cutting Relationships. • sequence of events record in rock relationships. G Development of the Global Environment. Principles of Stratigraphy. Cross-cutting relationships is a principle of geology that states that the geologic feature which cuts another is the younger of the two features. It is a relative dating technique in geology. Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures. Explanations: A – folded.Relative Age Dating I
A third key principle--faunal succession--is reviewed in Section 2. Principle of superposition Just as uniformitarianism is the key underlying assumption of geology, the science's most fundamental principle is superposition, developed by Danish anatomist Nicholas Steno in the 17th century.
Portrait of Nicholas Steno public domain; Wikimedia Commons.
Relative age dating | Digital Atlas of Ancient Life
The principle of superposition is simple, intuitive, and is the basis for relative age dating. It states that rocks positioned below other rocks are older than the rocks above. The rocks near the bottom of the waterfall were deposited first and the rocks above are subsequently younger and younger. Image by Jonathan R.
Superposition is observed not only in rocks, but also in our daily lives. Consider the trash in your kitchen garbage can. The trash at the bottom was thrown out earlier than the trash that lies above it; the trash at the bottom is therefore older and likely smellier!
Or, think about a stack of old magazines or newspapers that might be sitting in your home or garage: Use superposition to determine which is older: How do you know?
Principle of cross-cutting relationships The principle of cross-cutting relationships states that a rock unit or other geological feature, such as a fault that is cut by another rock unit or feature must be older than the rock unit or feature that does the cutting.
And, the mud layer is older than the forest layer. When scientists look at sedimentary rock strata, they essentially see a timeline stretching backwards through history. The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago. How do we use the Law of Superposition to establish relative dates?
Let's look at these rock strata here: Example of rock with five layers We have five layers total. Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old.
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We're not so sure about the next layer down, but the one below it is million years old. Can we tell how old this middle layer is? Not exactly, but we do know that it's somewhere between 70 and million years old. Geologists use this type of method all the time to establish relative ages of rocks. Now, what if instead of being horizontal, this rock layer was found in a tilted position?
Whatever caused this formation to tilt happened after the strata was formed. What could a geologist say about that section of rock? Following the Principle of Original Horizontality, he could say that whatever forces caused the deformation, like an earthquake, must have occurred after the formation of all the rock strata.
Principles of relative dating[ edit ] Methods for relative dating were developed when geology first emerged as a natural science in the 18th century. Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events. Uniformitarianism[ edit ] The principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time.
In geology, when an igneous intrusion cuts across a formation of sedimentary rockit can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccolithsbatholithssills and dikes. Cross-cutting relationships[ edit ] Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures. The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.
Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault.
Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.
Cross-cutting relationships - Wikipedia
As a result, xenoliths are older than the rock which contains them. Original horizontality[ edit ] The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal.
This is because it is not possible for a younger layer to slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed. As organisms exist at the same time period throughout the world, their presence or sometimes absence may be used to provide a relative age of the formations in which they are found.
Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin 's theory of evolutionthe principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat facies change in sedimentary strataand that not all fossils may be found globally at the same time.