Relative Dating of Rocks
Relative dating cannot establish absolute age, but it can establish whether establishing the relative ages of faults and igneous intrusions in sedimentary rocks. Standard – Infer the relative age of rocks and fossils from index Absolute Age The age of a rock given as the number of years since the rock formed. ( example – an eruption would put a layer of igneous rock on top of. Relative Dating is when you give the age of a rock or fossil compared to If an igneous intrusion or a fault cuts through existing rocks, the intrusion/fault is.
As sediment weathers and erodes from its source, and as long as it is does not encounter any physical barriers to its movement, the sediment will be deposited in all directions until it thins or fades into a different sediment type. For purposes of relative dating this principle is used to identify faults and erosional features within the rock record.
The principle of cross-cutting states that any geologic feature that crosses other layers or rock must be younger then the material it cuts across. Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses.
Once a rock is lithified no other material can be incorporated within its internal structure. In order for any material to be included within in the rock it must have been present at the time the rock was lithified.
For example, in order to get a pebble inside an igneous rock it must be incorporated when the igneous rock is still molten-- such as when lava flows over the surface. Therefore, the piece, or inclusion, must be older than the material it is included in.
Lastly the Principle of Fossil Succession. Aside from single-celled bacteria, most living organism reside at or very near the Earth's surface either in continental or oceanic environments. It's called the Principle of Original Horizontality, and it just means what it sounds like: Of course, it only applies to sedimentary rocks.
Recall that sedimentary rock is composed of As you can imagine, regular sediments, like sand, silt, and clay, tend to accumulate over a wide area with a generally consistent thickness. It sounds like common sense to you and me, but geologists have to define the Principle of Original Horizontality in order to make assumptions about the relative ages of sedimentary rocks.
Law of Superposition Once we assume that all rock layers were originally horizontal, we can make another assumption: This rule is called the Law of Superposition.
Again, it's pretty obvious if you think about it. Say you have a layer of mud accumulating at the bottom of a lake. Then the lake dries up, and a forest grows in.
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More sediment accumulates from the leaf litter and waste of the forest, until you have a second layer. The forest layer is younger than the mud layer, right?
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.Radiometric or Absolute Rock Dating
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. We're not so sure about the next layer down, but the one below it is million years old.
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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?