Geologic Time and Earth History - Elmhurst College

Geologic Time and Earth History
Dr. R. B. Schultz

Geologic Time
How do geologists determine how old rocks are?
Relative age dating -- determine whether the rock is older or younger than
other rocks relative to one another
Absolute age dating -- use radiometric dating techniques to determine how
old rock is in the exact number of years
Not all rocks can be dated absolutely, so combinations of techniques are used.
By examining layers of sedimentary rock, geologists developed a time scale for
dividing up earth history.
Early in the 20 th century, radiometric-dating techniques allowed scientists to
put absolute dates on divisions in the geologic time scale.
In this segment, we will learn how geologists:
Determine the relative ages of rock units,
Determined and named the divisions of the geologic time scale, and
Use radiometric techniques can be used to date some rocks.

So, what is Absolute Age Dating?
Uses radiometric Dating Techniques
Use naturally-occurring radioactive isotopes
Isotope: form of an element that has additional neutrons
Radioisotope -- isotope that spontaneously decays, giving off radiation
Rate of Radioactive Decay is important:
Radioisotopes decay at a constant rate.
Rate of decay is measured by half-life
Half-life: Half life: time it takes for one-half of the radioactive material to decay.
Decay products
Radioisotopes may decay to form a different isotope or a stable isotope.
Stable isotope is called the "daughter daughter" formed from decay of radioactive "parent parent"
Exactly how is this accomplished?
Radioisotopes are trapped in minerals when they crystallize.
Radioisotopes decay through time, and stable isotopes are formed.
Scientists determine the ratio of parent isotope to daughter product which reveals the
number of half-lives that has elapsed.
Common isotopes used in age dating
U-Pb -- half-life of U-238 is 4.5 b.y.
K-Ar -- half-life of K-40 is 1.3 b.y.
Rb-Sr -- half-life of Rb-87 is 47 b.y.
Carbon 14 -- half-life of C-14 is 5730 yrs

Absolute Dating Example
If one knows the half-life of a given isotope, and can accurately measure the
quantity of the isotope present in the rock, one can deduce how long it has
taken to decay down to the present amount in the rock.
Example: All living things have carbon, including carbon-14 (radioactive
isotope). In that way, there is a known ratio of carbon-14 to carbon-12 in all
living things.
If a rock has 0.5 (one-half) of the original carbon 14 material in it, one can
deduce that, knowing the half-life of carbon-14 is 5730 years, the rock must
have decayed (lost) 50% of its original carbon-14 material and is now 5730
years old.
In a period of 5730 years from now, the rock will contain .25 (25%) of its
original carbon 14-material, and will be 11,460 years old.
Theoretically, there will always be some trace of carbon 14 present in the
rock…it will never decay totally.

Gradualists vs. Catastrophists
Interpreting the rock record:
James Ussher (1581-1656), Archbishop of Armagh, Primate of All Ireland, and
Vice-Chancellor of Trinity College in Dublin was highly regarded in his day as a
churchman and as a scholar.
Having established the first day of creation as Sunday 26 October 4004 BC at
9:00 a.m., Ussher calculated the dates of other biblical events, concluding,
for example, that Adam and Eve were driven from Paradise on Monday 10
November 4004 BC, and that the ark touched down on Mt Ararat on 5 May 1491
BC `on a Wednesday'.
In the 1600’s, prior to geologic principles, Archbishop James Ussher calculated
the age of the Earth at 6006 years old and was not questioned until nearly 100
years later.
Principle of Uniformitarianism:
James Hutton, late 1700s – (considered to be "Father of Geology") questioned
Ussher’s calculations and debated his “scientific methods”.
Hutton realized that most sedimentary layers were deposited from gradual,
day-to-day processes. He realized that it took a long time to form these rocks.
This was far different from what others believed prior to this time.
"Present is the key to the past" -- whatever processes are occurring today
(plate tectonics, volcanism, mountain building, earthquakes, sedimentation)
also occurred in the past and probably at the same (or very comparable) rates.
Hutton wrote “Theory of the Earth, or an Investigation of the Laws Observable
in the Composition, Dissolution and Restoration of Land upon the Globe” and
began a major controversy.

What is Relative Age Dating?
The comparing of rock units to decipher their age relative to one another
James Hutton used many of the following principles which underlie modern geology:
Principle of Superposition
Rock layer above is younger than the ones below it.
Oldest on bottom, youngest on top
Principle of Original Horizontality
Sedimentary layers are deposited in approximately horizontal sheets.
If layers are folded, episode of deformation must have occurred after rocks formed.
Principle of Crosscutting Relationships
Any feature (e.g. fault or intrusion) that cuts across rocks is younger than the youngest
rock that is cut.
Principle of Faunal Succession
Organisms have evolved and gone extinct through time
Fossil content of rock changes in a systematic way, reflecting evolutionary changes
Fossil content can be used to help determine age of rock and correlate rocks.
Paraphrased as "Organisms within rock units change with time".

Example of Relative Age Dating and Correlation

Relative Age Dating

Original
Horizontality
Illustration of Relative Age Principles
Superposition
Cross Cutting
Relations

Correlation
Correlation is determining
that rocks are within the
same formation in a
different geographic
location (may mean rocks
are the same age)

Illustration of Principle of Faunal Succession

James Hutton also noticed that not only were the rock layers that were
present important, but rock layers that were missing were also important.
Unconformities
Unconformities are surfaces in rock that represent periods of erosion or nondeposition.
In other words, time has been left out of the physical geologic rock record.
There are three (3) principal types of unconformities:
Angular Unconformity
Rocks above and below unconformity have different orientations.
Rocks are at an angle and truncate at a horizontal layer.
Easiest of the three types to recognize because the units are at an angle
truncated with the units above them.
Nonconformity
Rocks in a horizontal fashion were eroded down to igneous bedrock material
at which time subsequent deposition of sedimentary layers commenced.
Represents the greatest amount of time left out of the geologic rock record.
Disconformity
Rocks in a nearly horizontal fashion were eroded and an erosional profile
remains covered by subsequent sedimentary deposition.
Most difficult to recognize because the units are nearly horizontal and only a
small discontinuous layer can be observed (rubble zone or soil profile).
Represents the least amount of time left out of the geologic rock record.

Unconformity Types Using Grand Canyon as Example

Grand Canyon Flyby

What is the Geologic Time Scale?
A global scale constructed to correlate world-wide rock layers
Developed in 1800s from relative dating of rocks
More recently, radiometric techniques have allowed us to determine
ages of units in years before present.
Many of the names relate back to localities in England (Ex: Devonian
from Devonshire) or ancient peoples (Ordovcian from Olduvai)
See this website for details of naming:
http://www.geotech.org/survey/geotech/geotech6.html
Divisions of Geologic Time Scale:
Eons �Eras � Periods � Epochs

The Geologic Time Scale
Most recent
“Ice Age”
“Humans”
arrive
Major Mass
Extinction
Age of
Dinosaurs
Major Mass
Extinction
Age of Coal
Formation
Age of Fishes
First multicelled
organisms
Origin of the Earth
4.55 Billion years

How Do We Know What Killed the Dinosaurs?
Investigating a small clay layer in Gubbio, Italy, Dr. Walter L. Alvarez discovered a layer
with an unusually high concentration of iridium.
This layer was found at the Cretaceous-Tertiary (K-T) Boundary. The Cretaceous period
ended 65 million years ago. With the end of the Cretaceous period, so can the end of the
dinosaurs and most other forms of life.
Iridium is a dense and rare metal, and it is the most corrosion-resistant metal known to
man. Iridium can be found in the core of the earth, but the levels found at the K-T
Boundary were too high. Normally, iridium is found in concentrations of 0.3 parts per
billion. The clay layer at Gubbio had concentration 30 times higher. Iridium is also found,
in much higher levels, in asteroids.
Alvarez and his team, which included his father Dr. Luis Alvarez, proposed that an
asteroid hit the earth, throwing up a dust layer that encircled the earth and lead to the
extinction of the dinosaurs.
Since Alvarez’s proposal, more than 100 iridium rich deposits at the K-T Boundary have
been found around the world. These additional sites support the theory set forth by Dr.
Alvarez and his team.
Additionally, a site for the asteroid impact has been proposed. It is believed that the 180mile
crater in Chicxulub, on the Yucatan Peninsula, is the site of the asteroid that brought
an end to the dinosaurs and most life forms at the end of the Cretaceous Period. This site
was discovered in 1960, but it was not revealed until 1990, where it was met with very
little interest.
Today however, many scientists accept the buried 180-mile crater as the impact site of
the asteroid, and many scientists also believe that this asteroid caused the extinction of
the dinosaurs.

What Killed the Dinosaurs?

Can these meteorites hit anywhere on Earth?
YES! The probability that the Earth will be struck
again is 100%! We just don’t don t know when.

Key Terminology
Relative age dating Absolute age dating
Isotope Radioisotope
Half-life Half life Daughter product
Parent Material Archbishop Ussher
James Hutton Uniformitarianism (Uniformity)
Superposition Cross cutting relationships
Original horizontality Faunal succession
Correlation Unconformity
Angular unconformity Disconformity
Nonconformity Geologic time scale
Eon Era
Period Epoch
Mass extinction Walter Alvarez