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6 & 7-1
Geologic Time
The Earth is very old.
Current estimates place its age at 4.6 billion years.
Difficult to fathom.
All of human history (~5000 years) is but 0.0001%
of this age.
Condense all of Earth history into one year and
human history would occupy the last 30 seconds.
While difficult, it is important to try and keep
geologic time in perspective.
Why do we think the Earth is 4.6 billion years old?
How do we measure such large periods of time?
How can we tell the ages of various landforms?
This is what we will discuss the next couple of days.
Two types of dating are used:
Relative dating—here we try to determine which of
two objects is younger, which
older.
However, we do not attempt to
determine the actual ages.
In the following chapters (and some we have seen)
we will see what geologic forces can do over such
immense periods of time.
Absolute dating—here we try to determine the actual
age of a feature.
Relative Dating
2. Principle of Original Horizontality
We can attempt to determine the relative ages of
features by making use of some common sense
assumptions and principles.
When we see sedimentary layers being layed down
today they are always at or nearly horizontal.
Thus if a layer is tilted what must have happened and
when?
1. Principle of Uniformitarianism
3. Principle of Superposition
“The present is the key to the past”
In general, new sedimentary layers are deposited on
top of preexisting layers.
We assume that processes which occur today also
occured in the past.
Where would the youngest layers be seen? Older?
Physical laws of the Universe do not change in time.
But what if a layer has been deformed or overturned?
6 & 7-2
4. Principle of Cross-Cutting Relationships
6. Principle of Faunal Succession
Sometimes igneous rocks intrude into country rock
or a fault cuts through various rock layers.
Sometimes rocks contain fossils.
Which is older, the igneous rock (or fault) or the
country rock?
Typically, organisms only appear within a certain
time frame on Earth.
The presence of a fossil constrains the age of the
rock to the period when the organism existed.
5. Principle of Inclusions
Sometimes rocks contain inclusions within
themselves.
Which is older, the inclusion or rock layer?
Organisms which make only a brief appearance are
particularly useful—if present they can tie the age
down well (index fossils).
Thus, the types of fossils seen can be used to gauge
the age of a rock.
(And vice-versa, using our previous rules,
paleontologists use rocks containing fossils to
determine the relative ages of fossils.)
With all these tools we can use to date features it
should be easy, right?
Now suppose the environment changes to one
favoring erosion over deposition—
Alas, mother nature throws a few curves our way.
Erosion removes some of the upper (younger)
layers.
Not all (or most) sedimentary rock layers present a
nice clean history.
Then the environment changes again to one favoring
more deposition.
What would we see in this case?
Consider the following sequence of events:
Material is initially deposited in a region.
As material is deposited the layers are built up nicely,
young on top, older underneath.
When this occurs we have an unconformity.
Unfortunately, this sequence of events is more the
rule than the exception.
Several types of unconformities exist.
6 & 7-3
Nonconformity
Disconformity
When sedimentary layers are observed overlying an
unlayered body of plutonic igneous rock or
metamorphic rock.
In a disconformity, the layers are still horizontal but
a definite gap exists in the record.
Intrusive igneous rock and metamorphic rocks do
not get produced on the Earth’s surface.
==> Must have been some erosion exposing the
rock before the new deposition took place.
Angular unconformity
Sometimes sedimentary layers are layed on top of
tilted or deformed sedimentary layers.
Perhaps the fossils contained in adjacent layers
suggest quite different ages.
Sometimes evidence of erosion and weathering can
be seen in the underlying layer.
Unfortunately, virtually all outcrops contain
unconformities.
Most contain information on only a small fraction
the Earth’s history.
What must the sequence of events have been in this
case?
Correlation
Absolute Dating Techniques:
If most of the information is missing in a given
outcrop, how do we try to get a coherent history of a
region?
Radioactive dating
The main technique used to determine the absolute
age of features makes use of radioactive elements.
Most atoms are stable.
How do we relate one region to another?
i.e how can we tell which layers were formed at the
same time?
However, a few are not stable but decay in time into
different atoms by emitting various particles.
These atoms are said to be radioactive.
The initial atom is known as the parent isotope while
the decay product is known as the daughter isotope.
How can we use radioactive elements to date objects?
6 & 7-4
Radioactive elements decay spontaneously.
However, when a number of atoms are examined a
statistical rate of decay can be determined.
The time it takes for half of the atoms to decay is
known as the isotopes half-life.
Occurring in the nucleus, the half-life is essentially
independent of the environment.
Because of this radioactivity from radioactive
elements can be used to date objects.
Example: Potassium – Argon
Dating
One radioactive isotope often used in dating rocks is
Potassium 40.
Potassium 40 decays into Argon 40 via electron
capture:
K40 + e– —> Ar40
To see how let us consider an example...
The half-life for this decay is 1.3 billion years.
How might we use this decay to measure the age of a
rock?
Difficulties with radiometric
dating:
1. Age of rock is much different than 1/2 life.
Suppose a rock is much younger than the 1/2 life of
a radioactive element.
How many daughter nuclei will there be?
e.g., suppose the 1/2 life is a billion years but a rock
is only 100,000 years old (1/10,000 of a 1/2 life).
Then we would have 1 daughter isotope for every
14,427 parent isotopes.
6 & 7-5
What if the age is much older than a 1/2-life?
3. Atoms may move into or out of rocks.
This typically occurs at about 12 1/2-lives or so.
For example, if an isotope decays into a gaseous
element it may escape from the rock.
Thus, a radioactive isotope is most useful for
determining ages near its 1/2 life.
e.g.: it can be a problem with K->Ar dating
2. When does the radiometric “clock” start?
How would the age determination be affected if
Argon had excaped from a rock?
Radioactive dating techniques are most useful with
igneous rocks.
Their formation resets the radiometric “clock.”
Clock may not be reset by formation of sedimentary
or metamorphic rocks.
4. Contamination to begin with, what if a rock
began with some daughter isotopes?
Major Isotope Systems Used in Dating:
An example where this might occur is the
Rubidium–Strontium decay system.
1. Rubidium-Strontium (Rb87 -> Sr87)
What would happen to the age determination if some
daughter isotopes were present and this was not
accounted for?
1/2-life: 47 billion years—useful for dating old
rocks (10 million – 4.6 billion years)
Strontium is a solid, thus not likely to escape.
Occurs in potassium rich rocks—useful check on
potassium ages.
How can one tell what the initial concentration was?
2. Uranium, thorium to lead
Long 1/2-lives (713 million to 14 billion
years)—date old rocks (10 million years to 4.6
billion years).
6 & 7-6
3. Potassium-Argon (K40 -> Ar40)
Carbon-14 Dating (C14 -> N14):
1/2 life of 1.3 billion years—useful for dating rocks
of 100,000 years to 4.6 billion years.
1/2 life of 5730 years—can be used to date young
objects (100 to 70,000 years old).
Potassium is abundant making this system quite
useful.
Often used to date artifacts.
Argon is a noble gas which does not normally bond
with other atoms—rarely seen in a rock except as a
decay product.
However, this is also a bane—gas can easily escape
from a rock.
One must be careful especially with old, severely
weathered, or metamorphic rocks.
If carbon-14 has a such a short 1/2-life, why is it still
around?
Carbon-14 is continuously created by cosmic rays
impacting atoms in the atmosphere.
As long as an organism lives it continually recycles
its carbon.
When it dies it no longer does and the Carbon-14
clock starts to tick.
Note that carbon-14 dating assumes that the rate of
production of carbon-14 by cosmic rays has been
constant.
This may not be the case!
Certainly isn’t today—atomic bomb tests have
substantially increased the amount of carbon-14 seen
currently.
Other Absolute Dating Techniques
While radiometric dating is the most widely used
technique for absolute dating, other techniques are
available.
Fission-track dating—the decay of isotopes create
high energy particles
==> upon leaving a rock tend to leave a path of
destruction behind.
The more fission tracks seen, the older the rock.
6 & 7-7
Dendrochronology—due to the variations between
winter and summer, trees create alternating dark and
light rings, one set per year.
Ice core samples
Similar to varves, glacial ice shows a seasonal cycle.
These can be used to date trees and events during a
tree’s lifetime.
Can be used to date climate variations on Earth.
Varve chronology—inflow into lakes may also
follow a seasonal cycle leading to sets of sediments
layed down each year.
Lichenometry—lichen tends to grow on rocks at a
relatively constant rate. (Though it depends on the
rock and climate).
By counting the varves we can determine the timing
of various events (e.g. the formation of the lake,
times of drought...)
Can be used to date relatively young rocks
(~9000years).
The larger the lichen colony the older the rock.
Surface Exposure Dating
Sometimes we are not only interested in the age of a
rock but how long it has been in its current
environment.
For example: How long has a rock been on the
surface of the planet?
Cosmis radiation is constantly bombarding the
surface of the planet.
This radiation can create new isotopes not normally
seen on Earth (cosmogenic isotopes).
It can also leave tracks of destruction (much like
fission tracks).
A technique known as surface exposure dating can
be used to determine this:
Such particles can only penetrate the near surface
(upper meter or two or so).
Thus the abundance of these cosmogenic elements
and/or tracks is an indication of how long the rock
has been within a meter or so of the surface.
6 & 7-8
Geologic Time Scale
The largest blocks of time are the eons.
Geologic history is organized into different blocks
of time.
The earliest eon is the hadean which extends from
the planets formation to ~3.8 billion years ago.
The blocks of time are divided on the basis of major
events which have occured in the past.
Period before origin of life—very few rocks are seen
at the surface from this age.
The major divisions are split on the basis of
biological events (e.g. the appearance of multicelled
organisms).
The archean marks the first appearance of life and
extends to about 2.5 billion years ago.
However nonbiologic events do divide some of the
finer time periods.
The proterozoic marked the beginning of multicelled
organisms and runs to ~545million years ago.
Together, the hadean, archean, and proterozoic are
sometimes referred to as the precambrian.
The phanerozoic (visible life) eon marks a great
diversification in life and runs to the present.
The eons are further subdivided into eras.
The phanerozoic is split into 3 eras:
The paleozoic (ancient life) 245 – 545 million years
ago.
The mesozoic (middle life) 65 – 245 million years
ago.
The cenozoic (recent life) 65 million years ago to the
present.
The age of the Earth
I’ve said the age of the Earth is 4.6 billion years.
How do we determine this age?
Oldest rocks on Earth are dated at 3.96 billion years
by radiometric dating techniques.
But the Earth exhibits active geology—thus we
believe the oldest rocks have been destroyed.
Lunar rocks have been dated at ~4.55 billion years
Meteorites give a similar age.
Eras are then further subdivided into periods.
Astronomy suggests the sun is about this age.
Periods are divided into epochs.
6 & 7-9
Earthquakes
Unfortunately, faults blocks do not slide smoothly
by each other.
Friction between the blocks will lock them into place
for a time.
Rocks on either side of the fault continue to move
and deform elastically.
Even if there isn’t a fault initially in place one can
still get an earthquake:
Stress on rock may become large enough to fracture
it.
Rock suddenly loses strength.
==> slippage along new fault.
As they deform they build up strain energy.
Stress along the fault builds.
Eventually, the stress along the fault becomes large
enough to overcome the friction.
Fault slips ==> Earthquake!
Measuring the Strength of
Earthquakes: Richter Scale
In terms of the energy released by an earthquake, the
difference is even more severe.
When earthquakes are reported, one will often be
given the earthquake’s “magnitude.”
A gain of one in magnitude indicates a 33× increase
in energy released.
The Richter scale is based on the amount a
seismograph needle will be deflected and is
logarithmic:
A 2 magnitude earthquake makes 10 times the
displacement of a magnitude 1 earthquake.
A magnitude 3 ten times that of a magnitude 2 (100×
that of a magnitude 1)
A magnitude 4 is ...?
e.g. A magnitude 2 earthquake releases 33× the
energy of a magnitude 1 earthquake.
Magnitude 3 earthquake releases 33× the energy (or
33×33 ≅ 1100× that of a magnitude 1).
And so on...
6 & 7-10
Relieving Stress for the “big one”
The Moment-Magnitude Scale
“Maybe this will relieve stress for the big one”
While the Richter Scale is often cited it does have
limitations:
An oft heard refrain when spoken about a moderate
earthquake.
1. It was calibrated using crustal rocks in California
and thus is less accurate elsewhere.
Is it true?
2. While theoretically the scale doesn’t have an
upper limit, it cannot effectively measure earthquakes
of more than magnitude 7.0 or so.
Suppose we look at a magnitude 6 earthquake.
Compare this to the big one: 8 magnitude.
How many “small” magnitude 6 earthquakes would
need to occur to releasr the same energy as one
magnitude 8 earthquake?
3. It is based on the deflection of a needle on a
Wood-Anderson seismograph—more modern
seismographs are now available.
This has led to the adoption of another scale:
The Moment-Magnitude Scale
Seismic Waves
The energy released by an earthquake generally
depends on:
Before an earthquake occurs, the rocks are stressed.
Those nearest to the fault are pulling on those a bit
further out.
These then pull on those still further out and so on.
When the fault goes, stress is released all along the
line.
Rocks fall back, then rebound pushing/pulling on
rocks further away causing them to react.
In this way the energy from the earthquake is
propagated away from the earthquake site.
The motion that occurs as rocks move in reaction to
the earthquake are called seismic waves.
6 & 7-11
Body Waves
Body waves are waves that are transmitted through
the interior of the Earth.
There are two varieties:
The motion of particles in a P-wave are in the
direction of travel of the wave.
Particles are continually compressed then extended
then compressed...
Because of this P-waves are sometimes called pushpull waves.
P-Waves
P (or primary)-waves are the fastest travelling body
wave (~7km/s).
==> they are the first to arrive at a location.
S-waves
Surface Waves
The second type of body wave is called an S (or
secondary) wave.
Surface waves travel on or near the Earth’s surface.
Similar to water waves travelling on the surface.
S-waves travel more slowly than P-waves (3.5km/s).
Hence they arrive later.
As with water waves, surface seismic waves die out
quickly with depth.
Surface waves are slower than P- or S-waves.
Unlike P-waves, in S-waves the particles move
perpendicular to the direction of travel of the wave.
However, they tend to result in larger motions of the
ground.
S-waves are also sometimes called shear waves.
Further, because they are slower, they take longer to
pass a given location.
For these two reasons surface waves tend to be more
destructive than body waves.
6 & 7-12
Where do Earthquakes Occur?
Around the pacific we have transform boundaries
and convergent boundaries leading to widespread
seismic activity.
Earthquakes do not occur at random locations.
Most (~80%) earthquakes occur in a ring around the
Pacific plate.
At the midoceanic ridge we see mostly normal
faulting.
All evidence of plate tectonics.
Another group occurs in the
Mediterranean–Himalayan region.
A final group of earthquakes occurs along the
midoceanic ridge.
Each of these zones indicate a region on plate
boundaries.
Depth of Earthquakes
Most earthquakes are shallow (within 70km of the
Earth’s surface).
Though some (~15%) occur at greater depths.
Coping with Earthquakes
Earthquakes are a fact of life, or are they?
People have proposed setting earthquakes off
artificially before they can build up large energies.
All large earthquakes occur near the surface.
For example, by pumping water into the ground.
Deepest earthquakes occur at ~700km depth—occur
at subduction zone boundaries within the subducting
plate.
Given our current knowledge, such an effort might
be foolhardy...
Why?
What if we set off the “big one” by accident?
Think of the lawsuit!
6 & 7-13
Thus, we are not likely to control earthquakes in this
way in the near future, but maybe someday.
In the mean time, what can we do to minimize their
destructive effects?
Earthquake Prediction
We can then be careful when building in those areas:
Strict building codes.
Avoid placing critical facilities in such areas (e.g.
nuclear power plants).
Unfortunately, unlike volcanoes, geologists have not
been very successful in predicting when earthquakes
will occur.
While we cannot predict earthquakes in the shortterm we can make long term predictions.
How?
“Seismic Zoning”
Interior of the Earth: Seismology
Some types of land are safer than others.
Most of what we know about the Earth’s interior
comes from seismology—the study of how seismic
waves propogate within the Earth.
Loosely consolidated sediment is not a good place to
build.
Such sediments may suffer liquefaction during an
earthquake:
Loose sediments also tend to focus seismic waves.
Seismic waves can be used to “sound out” the
Earth’s interior.
Much like how sonar is used to look for underwater
objects (submarines) or examine the ocean bottom.
The velocity of seismic waves depends on the
physical properties of the rocks they are moving
through.
Strength of rocks:
Solid bedrock is a better place to build.
Density:
6 & 7-14
Reflection and Refraction of Waves
When a seismic wave reaches a boundary between
regions of significantly different properties it will be
reflected and refracted.
As a familiar example consider the reflection and
refraction of light waves.
Same holds for seismic waves.
Crust
Velocity of seismic waves is seen to increase
significantly through the crust.
What does this suggest about how the properties of
rocks vary with depth in the crust?
Crust-Mantle boundary
Seismic waves are reflected and refracted from a
zone at some depth below the surface.
What does this suggest?
Mantle
As the temperature of a rock approaches the melting
point what happens to its strength?
The mantle is the largest segment within the Earth
accounting for 80% of the Earth’s volume.
As one continues into the surface, seismic wave
velocities continue to increase.
However, at about 100 – 350 km depth the velocities
of seismic waves are seen to decrease.
What does this suggest about the strength of rocks
at this depth?
Why does this occur?
What happens to the melting point as the pressure
increases?
6 & 7-15
Below the asthenosphere the pressure again
dominates, and the rocks’ temperatures become less
than the melting point.
What does this suggest about the rigidity of rocks
here and thus the velocity of seismic waves?
Lower Mantle
The lower mantle extends from a depth of 700km to
2900km.
Compression within this region increases the density
of rocks from ~4g/cm3 to 5.5g/cm3.
How would you expect this to affect the seismic
wave velocities?
Rock rigidity also increased by phase transitions
which occur at depths of 400 – 700 km depth.
Yet, the opposite is seen!
What does this indicate about the rocks?
These phase transitions mark the boundary between
the upper and lower mantle.
Core-Mantle Boundary
Indicates a region of substantially reduced seismic
wave velocities.
At a depth of 2900km we see evidence of the
reflection and refraction of seismic waves indicating
another major boundary.
In fact, S-waves can’t go through this region at all.
Liquids can’t support shearing motion of S-waves.
This boundary has major effects on the seismic
waves we see.
==> suggests the outer core is liquid!
P-waves are not seen at angles between 103° and
143° from an earthquake.
Obviously the region also has much lower rigidity
==> reduced P-wave velocities.
P-wave shadow zone.
Analysis of seismic data also indicate this region to
be one of substantially increased density.
S-waves are not seen at all beyond 103° from an
earthquake.
How do we explain this?
==> Metal, mostly iron and nickel.
6 & 7-16
Solid Inner Core
One final boundary is seen within the core.
At this boundary (at a depth of ~5100km) the
velocity of P-waves is seen to suddenly increase.
What does this suggest about the rigidity of the
material here?