Class Notes:

Telling Geologic Time

Relative Dating: determining the __________________ of geologic events.

Absolute Dating: determining the _______________ of geologic events.

Relative Dating: Rules of the Game

-

Principle of ___________________:

In an un-deformed sequence of stratified rocks (sedimentary and volcanic), the oldest rocks are at the bottom and the youngest are at the top. Also known as: “The Principle of the Messy Desk”

Principle of _____________________:

Stratified rocks are assumed to have been originally deposited horizontal. Any deviation from horizontality would then be the result of post-depositional deformation.

The Grand Canyon – 500 million years and still horizontal!

But….not all beds remain horizontal!

Principle of _______________________:

A fault, intrusion, or erosional surface is younger than any feature it cuts. You can’t cut something that didn’t already exist!

Examples of determining sequence of events (c.f. extra credit)

Significance of _________________:

If a fragment of one rock is included within another rock, the included fragment (inclusion) is the older of the two rocks. (e.g. figure 9.5)

You can’t include something that didn’t already exist!

Significance of ______________________-

an ______________ is a “buried surface of erosion”. It records a period of missing time (and sediment) within the geologic record.

Types include:

*

*

*

:

an unconformity where the beds below the erosional surface are parallel to the beds above the erosional surface.

1)Sedimentation of Beds A-D Beneath the Sea

2) Uplift and Exposure of D to Erosion

3)Continued Erosion Removes D and Exposes C to Erosion

4)Subsidence and Sedimentation of E over C

Example of a disconformity in Central Texas

:

an unconformity where the beds below the erosional surface are at an angle to thebeds above the erosional surface.

Siccar Point – where it all began! (figure 9.7

Examples, including the Grand Canyon.

:

An unconformity where the buried erosional surface is developed on exposed plutonic or metamorphic rocks.

The Great Unconformity of the Grand Canyon

Some of the Geologic Units Exposed in the Grand Canyon

Types of Unconformities Exposed in the Grand Canyon (figure 9.6)

Putting it all together (fig. 9.8) (c.f. Extra Credit Assignment)

Q. How do we extend the geologic record beyond one location?

A.

Methods:

*

*

Correlating Formations from the Grand Canyon to Zion N.P. (fig. 9.9)

Can these principles be used beyond the earth?

See Box 9

______________: Evidence of Prehistoric Life

________________: the study of ancient life based on the fossil record

Fossils preserved in sedimentary rocks are used to determine:

*

*

*

Fossils that are good time indicators (Index Fossils) are:

*

*

*

Why?

Fossils that are good environmental indicators are:

*

*

*

Why?

Principle of ___________________: Fossil organisms succeed one another in a definite and determinable order Thus any time period can be recognized by its fossil content.

A horse’s family tree.

Q. What is the significance of overlapping Index Fossils?

A.

The Geologic Time Scale (fig.9.17)

The largest sub-division of geologic time is an .

% of geologic time falls within the Precambrian.

The remaining % falls within the _______ Eon.

Q. What major event approximately 543 m.y. ago marks the end of the Precambrian and beginning of the Phanerozoic? Listen to Stephen Gould’s interview on the topic.

A.

The Phanerozoic is divided into three major Eras:

* (the youngest)

* (the middle era)

* (the oldest)

The Phanerozoic is subdivided into three major Eras on the basis of two major that occurred at about 245 and 66 m.y. ago.

Eras are further subdivided into Periods marked mainly by

Some major stages in the evolution of life:

Age of (Early Paleozoic)

Age of (Mid-Paleozoic)

Age of (Late Paleozoic)

The Late Paleozoic was also the time of the great coal swamps of the world!

Age of (Mesozoic)

Age of (Cenozoic)

Evolution as both Fact and Theory! An essay by Stephen Gould.

Earth Day

12:00 am Formation of the Earth

03:21 am Oldest Known Rock

06:15 am Oldest Known Life

07:18 pm Global “Ice House”

07:30 pm Multicellular Invertebrates

09:10 pm Explosion of “Hard Parts” (begining of Phanerozoic)

09:20 pm Fish

09:35 pm Land Plants

10:00 pm Amphibians

10:15 pm Reptiles

10:45 pm Global Extinction Event (end of Paleozoic)

11:00 pm Mammals

11:10 pm Birds

11:40 pm Major Extinction Event (end of Mesozoic)

11:52 pm Grasses

11:59:22 Present Ice Age Begins

11:59:58 Homo sapiens

The rates at which different geologic processes and events proceed vary greatly. Therefore, so do the kinds of “clocks” necessary to record those events!

What Clock can we use to tell Absolute Geologic Time?

*Historical Records (~4000 yrs?)

* Tree Rings (~ 2800 yrs)

* Coral Bands (~50,000 yrs)

* Glacial Ice Layers (~250,000 yrs)

* Isotopic Age Dating(~4.6 billion yrs)

Historical Photos can be used to measure change (or lack thereof), e.g. the Grand Canyon from 1871 -1968 and surging glaciers in Alaska.

Historical Maps (brown is new land formed from 1887-1988) can be also used to document some geologic processes

Satellite Data can verify rates of plate motion previously determined using other techniques.

Tree Rings (Box 9.4)

Coral Growth Rings:

Ice Cores:

AtmosphericCO₂ over the past 140 years.

Global temperature over the past 600 years.

Global temperature over the past 1000 years.

Atmospheric CO₂ and global temperature over the past 150.000 years.

But……How do we obtain the age of an igneous rock?

Send it to the Laboratory!

Atomic Number?

Atomic Mass?

______________________: Elements with the same atomic number but different atomic masses.

e.g.

²³⁵U,²³⁸U

⁸⁷Sr,⁸⁶Sr

¹⁴C,¹²C

²H,³H, ⁴H

Figure 9.13

_______________________:

The spontaneous decay of certain unstable (radioactive) atoms to form stable (non-radioactive) daughter atoms

Types

* _____________________

* _____________________

* _____________________

_______________________: an unstable nucleus emits an __________ particle, which consists of two protons and two neutrons.

Q. How does the atomic number and mass change as the result of this type of decay?

A.

____________________: an unstable nucleus emits a _________ particle, which consists of a single electron. The electron came from the breakdown of a neutron into a proton and electron.

Q. How does the atomic number and mass change as the result of this type of decay?

A.

___________________: an unstable nucleus captures a beta particle (an electron) which combines with a proton to form a neutron.

Q. How does the atomic number and mass change as the result of this type of decay?

A.

Q. How does Carbon-14 form, how does it decay, and why is it important? (figure 9.16)

A.

Principles of Isotopic Age Dating:

*Unstable radioactive elements (parents) decay to stable, non-radioactive elements (daughters).

* The rate at which this decay occurs is constant and knowable.

*Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding.

_______________________:

The time it takes for half the unstable (radioactive) parent atoms to decay to more stable (non-radioactive) daughter atoms

% of the parent remains after 1 half life.

% of the parent remains after 2 half lives.

% of the parent remains after 3 half lives.

% of the parent remains after 4 half lives.

Graph showing Proportion of Parent Atoms Remaining as a Function of Time (fig. 9.15)

Selected radioactive elements used in age dating (c.f. table 9.1)

Parent Isotope	Daughter Isotope	1/2 life (yrs)	Effective Dating Range (yrs)
^235U	^207Pb	4.5 x 10^9	1.0 x 10^7 – 4.6 x 10^9
^40K	^40Ar	1.25 x 10^9	5.0 x 10^4 – 4.6 x 10^9
^14C	^214N	5730	1.0 x 10^2 – 7.0 x 10^4

Q. How can isotopic dating of igneous rocks tell us anything about the age of sedimentary rocks and the fossils they contain?

A.

For a look at a Creationist Geologic Time Scale check out this website.