Absolute geologic age dating

When rocks are formed, small amounts of radioactive elements usually get included.

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The base layer must come first, then the second and then the third.

So I can say the second layer is younger than the first and older than the third but I have no idea whether the cake is 5 minutes or 5 million years old.

According to absolute-age measurements, an accurate representation of the lengths of the major geologic time periods is shown in the time bar at right.

To better understand these concepts, let's look at an archeological example: Imagine we are a group of archeologists studying two different trash pits recently discovered.

Thus rocks that we now see inclined or folded have been disturbed since their original deposition.

Stratigraphic Superposition - Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.Thus, the older a rock is, the larger the number of daughter elements and the smaller the number of parent elements are found in the rock.A common "parent-daughter" combination that geologists use is radioactive uranium and non-radioactive lead.When sediment is not being deposited, or when erosion is removing previously deposited sediment, there will not be a continuous record of sedimentation preserved in the rocks.We call such a break in the stratigraphic record a hiatus (a hiatus was identified in our trash pit example by the non-occurrence of the Ceramic Cups layer at the UNO site).The age of the rock in years can be found by measuring the rate at which a parent element decays and then measuring the ratio of parent element to daughter element in the rock.