Geologic Time

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Geologic TimeOpening QuestionRelative and absolute dating - Class HandoutsUnconformitiesThe Great UnconformityOrigin of the Great Unconformity?PaleontologySlide SetMiscellaneous / In Progress

 

Opening Question

Although most people take it as given that the earth is very old (approximately 4.54 billion years old) , how can we make a plausibility argument that the earth is more than several thousand years old, using the tool/methods available to us in the mid-1700s (ala James Hutton)? Putting yourself in the mid-1700s, with the scientific knowledge base and tools available it is more difficult than it may sound. But when you try to make an argument (again, putting yourself in that place and time), it is difficult.

Background reading on James Hutton: article 1 and article 2 with original website source 1, and original website source 2, should one or the other set of links not work for some reason.

So, what types of geologic observations could be made at that time that would suggest the age of the earth more than the approximately 6,000 year old estimate given by some religious texts and arguments?

Make sure to ponder the above question - remember, you are in the mid-1700s!

Handout on geologic time

Geologic time scale:

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Relative and absolute dating - Class Handouts

Relative dating handout

Absolute dating handout. Note that the numerical example is done in the slide set.

Unconformities

Unconformities are missing time in the geologic record.

Types: disconformity, nonconformity, angular unconformity.

See the slide set below for sequences of figures illustrating the formation of disconformities and angular unconformities.

Here are some additional illustrations of angular unconformities:

angunc

au

 

The Great Unconformity

There are many unconformities in the geologic record of varying (missing) time intervals and spatial extent (the above image from Death Valley, for example). The Great Unconformity (GU) is thought to be a worldwide event where ~ 1.2 billion years of time is missing ... think about that for a minute!!!

The GU is perhaps most famously exposed in the Grand Canyon:

photo credit

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Closer up: (the red arrows point to the GU contact between Vishnu Schist ~ 1.7 billion yers old and Tapeats Sandstone ~ 500 million years old)

photo credit

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And a real close up!! photo credit

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Here is a more detailed view of the stratigraphy (sequence of sedimentary layers) in the Grand Canyon: credit

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An illustration of even more Grand Canyon stratigraphy: credit

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The Grand Canyon in northern Arizona of course gets a lot of play when talking about geologic history, since so much geologic history is recorded in the sedimentary rocks that are now exposed. Obviously the geologic history of the Grand Canyon is complex, but here is a wonderful distillation of Grand Canyon history in mostly chart form along with one page of descriptive text, and a supplement - they are taken from this website.

A couple of GU videos on YouTube:

The GU can also been seen in Wyoming in Wind river Canyon. Here is a "GigaPan" image allowing you to zoom in and pan around:

Great Unconformity, Wind River Canyon, Wyoming - by Callan Bentley

Origin of the Great Unconformity?

The first few links below are short summary articles, after that are more technical articles. Ideas for the cause of the GU vary - it is not yet a settled question.

Did Global Glaciation Cause the Great Unconformity? - Eos

The Great Unconformities? - Eos

The Great Unconformity | The UCSB Current

What could have wiped 3km of rock off the entire Earth? | Ars Technica

Neoproterozoic glacial origin of the Great Unconformity | PNAS

Thermochronologic constraints on the origin of the Great Unconformity

Origin of Great Unconformity Obscured by Thermochronometric Uncertainty

Paleontology

A related field to general concept of geologic time is paleontology.

Here is an article about the first known female paleontologist: Mary Anning: Life and discoveries of the first female paleontologist and more interesting historical aspects of paleontology - Women in the Golden Age of Geology in Britain.

Also see this article on Katherine Van Winkle Palmer for early work on fossils.

There is a class at Cal Poly called Fossils and the History of Life, GEOL 203 if the area of fossils and what is called "historical geology" interests you.

Also see: Bones: Why Utah’s desert is a paleontologist’s playground, or the Internet Archive Page

Slide Set

Here is a slide set.

Miscellaneous / In Progress