Relative Dating
Younger Than, Older Than
Students often view teachers as old (I am really not that old). In turn, young teachers
often view veteran teachers as old (we don’t use this word though). Furthermore, veteran
teachers often view those individuals who have retired as old. How is this so? Well, ages
are often times relative. In other words, depending on your perspective, the ages of
people may seem quite different. Believe it or not, this ideology also holds true for rocks
(although older rocks never tell younger rocks to turn the music down).
As you have researched, prior to the late 1700’s and a man named Hutton, the age of the
Earth was believed to be “only” (refer to above) 6000 years old. Hutton and other
geologists began to analyze rocks and their changes, albeit slow. These observations lead
to the qualitative belief that the earth is very, very old (much older than 6000 years). In
addition, the model of uniformitarianism was adopted and the dawn of procrastinators
began (I am joking about the second part).
Beginning in the 1800’s and early 1900’s, scientists began to determine quantitative ages
for the Earth. Lord Kelvin attempted to determine the age of the Earth by calculating the
cooling rate of the Earth from its molten beginnings (Plutonian). Imagine melting candle
wax and timing how long it would take for the wax to solidify after the flame went out;
this is similar to Kelvin’s calculations. Next, John Joly calculated the age of the Earth
using the concentration of salt in the oceans. Joly assumed that oceans began as
freshwater and attempted to determine how long it took for the oceans to contain the
present concentration of salt. Finally, with the onset of the 20th century, Bertram
Boltwood discovered how radioactive materials could be used to determine the age of the
Earth. With the advent of radiometric dating, scientists determined the age of the Earth to
be roughly 4.5 billion years old (a little older than a teacher). However, this age was not
determined using any rocks found on Earth. Scientists used rocks from the moon and
space. Why? (Hint: think about what processes occur on Earth that don’t occur in space).
Let’s begin to discuss relative (qualitative) dating techniques for analyzing the history of
the Earth. For those of you familiar with the science fiction author, Jules Verne, we are
about to journey to the center of the Earth. We begin our journey on Earth’s surface, a
part of the Earth that is currently eroding. Whenever a part of the geologic record is
missing or eroded away, this is called an unconformity. Currently, the surface of the
Earth is eroding, thus you are part of a future unconformity.
Unconformity: period of erosion recorded in the geologic record (“missing time”)
Unconformity
http://geoinfo.nmt.edu/staff/scholle/graphics/Unconf.jpg
Enough about what is missing in Earth’s record, let’s begin to analyze what we still have.
As we begin to dig to the center of the Earth, we cut through various layers of rock.
Imagine we were digging in a garbage dump (ignore the smell). Where would we find the
youngest/newest garbage? Hopefully you answered, near the top. This same principle
holds true for strata. The youngest rocks, in an undisturbed sequence, are found near the
surface. This is known as the law of superposition. So, the deeper we dig, the older the
rocks.
Law of Superposition: in an undisturbed sequence of strata, the oldest rock will be
found deeper underground
Younger
Oldest
http://www.calstatela.edu/faculty/acolvil/sediment/superposition.jpg
As we continue to the center of the Earth, we notice not all strata are flat (horizontal). In
theory, all sedimentary strata begin horizontal. This is called the principle of original
horizontality (creative, don’t you think). Think of a dump truck dumping dirt into a
swimming pool. As the sediment settles to the bottom, the layer will flatten out. Over
time, the sediment will solidify and become a rock. The rock will appear to be horizontal.
If the strata we come across is no longer horizontal, we can assume that the layer has
been tilted after the rock has formed. This tells us that the rocks have been shifted by an
event younger than the rocks that have been moved. Possibilities for the shift include
plate collisions, volcanic activity, and movement of faults.
Principle of original horizontality: strata begin horizontal, any shifting occurs after the
formation of the strata.
Strata that began horizontal
but have been tilted.
http://www.uwsp.edu/geo/projects/geoweb/participants/dutch/GeoPhotoWis/WI-PZ-NE/BrusselsHill/BRHILL07.JPG
Venturing further into the Earth (ignore the increasing heat) we also see rocks crisscrossing other rocks. Rocks that cut-through other rocks are younger. This is known as
the principle of cross-cutting relationships. Imagine you and your friends are driving
down a busy interstate. The cars are bumper to bumper and some jerk cuts in front of
you. Who was there first, you or the other driver? If you were not there, would he have
cut you off? The same idea holds true for rocks. When igneous rocks (rocks that form
from magma) cut through other rocks, the rock being cut-off must have been there first.
You will also notice that the intrusion (the rock cutting through) will “cook” the rock it
cuts through, evidence that the “host” rock must have been there first. After all, you can’t
cook something if it is not in the oven.
Principle of cross-cutting relationships: rocks cutting through other rocks (intrusions)
are younger than the host (rock being cut through) rock.
Intrusion: younger than
the host rock.
http://geology.sdsmt.edu/geolgeoe/igpet.jpg
Considering that it is hard to fit all of us in one hole, a group decides to begin digging
another hole on the other side of the Earth, hoping we will meet in the middle. As they
are digging, they notice the rocks are in the same order as ours. This is because strata
form over extensive areas (hundreds of miles). This is known as the principle of lateral
continuity. Because of this principle, we can correlate, or match, rocks found in different
areas. For example, the rock that forms High Cliff State Park is the same rock that forms
Niagara Falls. As such, the rocks are the same age.
Correlations: matching layers of rocks with similar characteristics or fossils
Same strata separated
because of erosion.
http://geology.asu.edu/~sreynolds/geologic_scenery/layers_mesa5_multimesa.jpg
Let’s continue with the idea of matching strata and begin using fossils to help us. Fossils
that live during a specific time period are known as index fossils. We can match layers if
they contain the same index fossils because we assume they formed at the same time.
Imagine this, there are one billion Mr. Choudoirs walking the Earth at this time (man, that
would be sweet). A major geologic event causes all Mr. Choudoirs to go extinct and
become fossils. One million years in the future, you find strata that contains a fossil of
Mr. Choudoir’s. What could you say about the age of the rock? What could you say about
the age of any rock that contains a Mr. Choudoir fossil? This is the idea of correlating the
age of strata using index fossils. Rocks containing the same fossils are probably the same
age.
Index Fossils: fossils that live during specific time periods used to match strata. Strata
containing similar fossils are probably the same age.
http://www3.interscience.wiley.com:8100/legacy/college/levin/0470000201/chap_tutorial/ch01/images/le01_21.jpg
Finally, the climactic conclusion. As both groups meet at the center of the Earth, one
student says to another, “man, this rocks.”
Analysis Activity
Analyze the following cross-section using relative dating principles. Create a geologic
profile and explain the profile in a paragraph. Remember to justify your decisions with
the principles we have discussed.
Key Words Not Defined in Text
Uniformitarianism: belief that the Earth Changes slowly. Natural events changing the
Earth today also occurred in the past.
Plutonian: belief that the Earth originated as a molten sphere and has since cooled.
Radiometric Dating: use of radioactive material and half-life to determine a quantitative
age for igneous and some metamorphic rocks.
Relative Dating: qualitative analysis of ages of strata (younger than or older than)
Strata: layer of sedimentary rock