Dating of Rocks Dinosaur national monument October 2012

Layers of the Grand Canyon

Age Relative age: establishes order from youngest to oldest, but does not give exact time frames Absolute age: establishes the date of an event First, a couple of terms about age. What is relative age? It is establishing the order from youngest to oldest, but without giving exact time frames. You were born and are now listening to this video. How long apart were these events? [advance] absoluate age establishes the date of an event, within the limits of the technology used at least.

We Place Geologic Events in Relative Order By application of a set of geological “rules” Uniformitarianism Superposition Original Horizonality Cross-cutting Relationships Inclusions We apply these principles to determine the relative age of the layers of a sedimentary rock formation.

Uniformitarianism Uniformitarianism: the processes we can observe today are the same processes that happened millions or billions of years ago Waves crashing against the shore Wind blowing sand eroding stone

Superposition For an undisturbed rock sequence, the oldest rocks are at the bottom and each successive layer is younger than the layer beneath

Original Horizontality Original horizontality: sedimentary rocks are deposited in nearly horizontal or horizontal layers (just like your laundry) The principle of original horizonality points out that sedimentary layers are, more or less, horizontal when they form. By the time we see them, they may no longer be horizontal – perhaps do to an earthquake or two, but when they formed, at least, they were flat.

Original horizontality: as it was formed – it was flat! The principle of original horizonality points out that sedimentary layers are, more or less, horizontal when they form. By the time we see them, they may no longer be horizontal – perhaps do to an earthquake or two, but when they formed, at least, they were flat. Of course, it may no longer be flat!

Cross-cutting Relationships Cross-cutting relationships: an intrusion or fault is younger than the rock it cuts through You have to bake the cake before you can cut through it, right?

Cross-Cutting Relationships Which is older? Which is younger? Why?

Cross-Cutting Relationships Which is older? Which is younger? Why?

Inclusions Any material in a layer has to be younger than the material it is embedded in When particles of loose rock are incorporated into the newly deposited top layer, these clasts are called inclusions.

Unconformity Unconformity: some of the rock record is missing

Correlation Matching of geologic or fossil record in different places in the world http://www.geology.ohio-state.edu/~vonfrese/gs100/lect29/index.html

But that’s another story… http://scimathmn.org/stemtc/frameworks/9311-plate-tectonics

Absolute Dating Radioactive decay- the process in which a radioactive isotope breaks down into a stable isotope of a different atom Radiometric dating- a method of determining age by estimating percentages of a radioactive (parent) isotope and a stable (daughter) isotope Half-life- the time needed for half of a sample of a radioactive substance to decay