Transcript Unit 2 - Coach Frei Science

Unit 2
Lecture: Geologic Time
Concepts for Relative & Absolute
Dating of Geologic Strata
How do we measure time?
 Easiest solution – look at the rate of a
convenient natural process. If the rate is
constant, then use it as a timer.
Examples: Revolution of the Earth (years)
Rotation of the Earth (days)
 But what about geologic time?
 Same answer.
Useful “Timing” Processes:
 Biological – Use tree rings “dendrochronology”
Bristle-Cone Pine Tree: ~ 3 -4000 years
 Geological – Counting “varves” (annual sedimentation
layers (Baron de Geer, 1878; Bradley, 1929).
 Geophysical- Measuring cooling rates for magmas,
then
extrapolating for the entire Earth. (Lord Kelvin, 1899)
 Geochemical - Measuring thickness of sedimentary
layers and estimating erosion rates
 Nuclear – Measure rates of radioactive decay and
proportions of parent & daughter isotopes.
Two Kinds of Ages
Relative - Know Order of Events But Not
Dates
 Civil War Happened Before W.W.II
 Bedrock in Wisconsin Formed Before The
Glaciers Came
Absolute - Know Dates
 Civil War 1861-1865
 World War II 1939-1945
 Glaciers Left Wisconsin About 11,000 Years
Ago
18.2 The beginnings of geology
 In 1666, Nicholas Steno, a
Danish anatomist, studied
a shark’s head and
noticed that the shark’s
teeth resembled
mysterious stones called
“tonguestones”.
18.2 The beginnings of geology
 Steno theorized that
tonguestones looked like
shark’s teeth because
they actually were shark’s
teeth that had been
buried and became
fossils.
Relative Dating - placing the geologic occurrence
in the proper sequence
Which came first and WHY-----
To establish a “relative” time scale, rules were discovered
(principles of relative dating) – Nicholas Steno (1636-1686)
o Principle of Original Horizontality
o Law of Superposition
o Principle of Cross-Cutting relations
o Principle of Inclusions
RELATIVE DATING & AGE
 Relative Dating: putting
rocks and geological
events in correct
chronological order
 Relative Age: how old
something is in
comparison to something
else
 HOW?



Use of sedimentary rocks
Use of fossils
Study of strata
Let’s unravel some geologic history from observations of various formations
and their contacts
Nicholas Steno – 1669 proposed the following relative dating principles
The principle of Original Horizontality:
• Sedimentary rock layers are deposited as horizontal strata
• Any observed non-horizontal strata has been disturbed
Sediment input
C
B
A
basin
LAW OF HORIZONTALITY
Sediments are originally deposited in horizontal
layers
 Folds or inclines:
layers must have
been deformed
after they were
deposited
The principle of Superposition
In any undisturbed sequence of strata, the oldest
stratum is at the bottom of the sequence and the
youngest stratum
is on top.
Unit 1 = old
Unit 5 = young
5
4
3
2
1
LAW OF SUPERPOSITION
For undisturbed rocks, the oldest layer is on
the bottom and the youngest is on top
(Supai is oldest)
Superposition
:
Mindoro Cut,
Wisconsin
The principle of Cross Cutting relationships
• Any geologic feature that cuts across another geologic
feature is younger
5
Unit 1 = older
Unit 6 = youngest
4
3
2
Which came first
unit 5 or Unit 6 ?
1
6
The principle of Inclusions
• A piece of rock (clast) that has become “included”
in another rock body is older than the rock body
it has become part of – why?
Rock body A
A
A
Intrusion of pluton B
A
Older (rock A was there first)
LAW OF INCLUSIONS
If a rock body (Rock B)
contained fragments of
another rock body
(Rock A),
then Rock B must be
younger than the
fragments of rock it
contained
Which “granites” are older and younger?
OLDER
YOUNGER
Original Horizontality
Youngest
Superposition
Oldest
principle of
inclusions
Cross-cutting relationship
Which granite is older?
Older
Younger
A B C Asp Vn
The principle of Unconformities
• Rock surface that represents a period of erosion or non-deposition
• Often represent a “gap” in time
• Three major types of unconformities
o Disconformity
o Angular unconformity
o Non-conformity
Disconformity – unconformity in non-disturbed sedimentary
layers
Angular unconformity – unconformity lies between
angled strata and overlying horizontal strata
Non-conformity – sedimentary strata
Unconformity
overlies crystalline rocks (ign and meta)
Igneous or metamorphic rock
Layers are formed according to superposition.
Something happens to uplift the area
folding
faulting, etc.
Erosion wears away the uppermost layers
Area submerges and deposition begins again.
Here’s the
unconformity
Disconformity
Angular
Unconformity
Sedimentary rocks
Nonconformity
Xln rocks
Sequence 1: Uplift & Erosion
1. Limestone deposited
2. Sandstone deposited
3. Shale Deposited
4. Uplift
5. Erosion
Sequence 2: Faulting
1. Limestone
deposited
2. Sandstone
deposited
3. Shale
deposited
4. Faulting
Sequence 3: Folding
1. Limestone
deposited
2. Sandstone
deposited
3. Shale deposited
4. Folding
What happened here? Deciphering Earth’s
rock record…
Start by listing the events ,such as deposition of ..
,erosion, intrusion of.., faulting of, etc. in order to
piece together the story..
1.Deposition of rock layer O
2.Deposition of rock layer N
3.Deposition of rock layer L
4.Intrusion of M (law of inclusions)
5.Erosion of surface(unconformity)
6.Depositionof H,I,J
7.Erosion (unconformity ) above J
8.Deposition of K
9.Erosion to present day surface
Let’s practice “Reading “ the rocks!!
Determine the sequence of events in this geologic cross section:
The sequence of events is as follows:
1. Deposition of sedimentary rocks D
2. Fault B
3. Intrusion of igneous rock C
4. Erosion, forming the unconformity
5. Deposition of sedimentary rocks E

Fossils are the
remains or traces of
prehistoric life. They
are important
components of
sediment and
sedimentary rocks.

Specific conditions are needed for
fossilization.
• Only a tiny percentage of living things
became fossils.


Rocks can tell where
fossils were made
and when
Rocks can tell when
mass extinctions
happened
the study of fossils  remains of ancient
life

Body fossils vs. trace fossils
 Body = remain of organism, like bones;
 Trace = evidence of organism, like
footprints
Hard Parts of Organisms:
 Bones
 Shells
 Hard Parts of Insects
 Woody Material
Soft or Easily Decayed Parts of Organisms:
 Internal Organs
 Skin
 Hair
 Feathers

Permineralization occurs when minerals carried
by water are deposited around a hard structure.

The remains of an organism are
likely to be changed over time.

Molds and casts are another
common type of fossil.

Carbonization is particularly effective
in preserving leaves and delicate
animals. It occurs when an organism
is buried under fine sediment.

A natural cast forms when flowing water removes all
of the original tissue, leaving an impression.

Amber-preserved fossils are organisms that
become trapped in tree resin that hardens after
the tree is buried.
Carbon Film
Amber
Impressions
Dinosaur
Tracks
Petrified Wood
Casts
Fossil that defines and identifies geologic
periods; often in only one layer of rock



Easily recognizable
Short-lived (found only in a few layers of rock
worldwide)
Wide distribution (geographic range)

Ammonite fossils are
found worldwide, but they
existed for only a very
specific period of time

this means ammonites are
found in very specific
layers of rock

when an index fossil is
found, the age of the
rocks it is preserved in
can be determined

The principle of fossil succession means that
fossils can be used to identify the relative
age of the layers of a rock formation.
 The organisms
found in the top
layers appeared
after the
organisms found
in the layers
below them.


Fossils are found in a predictable sequence
Fossils in rock B are older then fossils in rock A
What kind of rocks are
these fossils in?
Which layer is oldest?
Which layer is youngest?
How do you know?
GEOLOGIC TIME SCALE
a series of time
intervals that
divides Earth’s
history
• Each layer of rock
represents specific
interval of time
• Index fossils help
determine specific period
• Time periods divided by
specific events like mass
extinctions
ABSOLUTE DATING
(RADIOMETRIC DATING)
ABSOLUTE DATING
 Absolute Time -
Numerical age determination of strata,
events, and geologic structures from
radiometric dating techniques.
Absolute Time
 Think of an Hourglass timer (the term used by Arthur
Holmes).
 Some initial quantity reduces, while its product
accumulates at a constant rate.
 NO “sand” can be added or removed at any point in
the process (closed system).
 Knowing the rate, and measuring quantities allows
us to calculate the TIME duration for the process.
Absolute Time
Early Attempts
 Bishop James Ussher (Prelate of Ireland)
 (1600s) Used O.T. biblical chronologies
to date the “creation”
 October 22, 4004 B.C. (Sunday)
 Georges Buffon
 (1700s) Used a measured cooling rate
from metal & non-metal balls to
estimate the age for a molten Earth to
cool. Earth’s Age = 75,000 yrs.
And others…
 John Joly (1889) acting upon a
suggestion from Edmund Halley,
estimated the ocean’s salinity &
its rate of increase.
Age: 90 million years
 Lord Kelvin (1899) estimated the
Earth’s thermal gradient.
Comparing this to cooling rates
for known materials he said:
Age : 20 – 100 million years (max)
A few more…
 Various geologists (1800s)
estimated sedimentation & erosion
rates and compared these to
sediment thicknesses.
Age : ~ 3 million to 1.5 billion years
 Arthur Holmes (1900s) first to use
Uranium decay techniques.
Age of Earth: ~ 4 billion years
pЄ boundary: ~ 600 million
Basic Atomic Structures
 Orbiting the nucleus are electrons,
which are negative electrical charges.
 Atomic number is the number of
protons in the atom’s nucleus.
 Mass number is the number of protons
plus the number of neutrons in an
atom’s nucleus
ISOTOPES: Isotopes are atoms of an element
that differ in their number of neutrons.
neutrons
protrons
RADIOMETRIC DATING
 Radioactivity is the spontaneous
decay of certain unstable atomic
nuclei.
 Radiometric dating provides an
accurate way to estimate the age of
fossils.
 Radiometric dating uses the decay of
unstable isotopes.
Radiometric Dating
 Each radioactive isotope has been
decaying at a constant rate since the
formation of the rocks in which it
occurs.
 Radiometric dating is the procedure of
calculating the absolute ages of rocks
and minerals that contain radioactive
isotopes
Radiometric Dating Techniques
 Radioactive elements
decay at constant rates.
 There are various decay
processes. see chart →

 If we can measure:
number of Parent &
Daughter isotopes, and
the decay rate, then we
can calculate an age
Radiometric Dating
 As a radioactive isotope decays, atoms
of the daughter product are formed and
accumulate.
 Each radioactive isotope has its own
unique half-life. A half-life is the time it
takes for half of the parent
radioactive element to decay to a
daughter product.
The key is the radioactive “Half
Life”
 The idea is: Parents decay into Daughters
P radioactive → D stable
 The rate of this decay is constant.
 A period of time exists during which ½ of the P
isotopes will decay into D’s. This is called the half
life, t ½ . Since the rate is constant, so is the t ½ .
 Isotopes used for geologic dating are called:
“Geochronometer Isotopes”
Okay, so does this work?
 Let’s not get too
technical. What we do is
use a radioactive
isotope’s “half life”.
 If we know how long a
half-life is, then all we
need to do is measure
the number of half-lives
that have elapsed for a
particular sample.
An example would be nice…
 Okay. We measure P & D in





a rock sample. The ratio of
P:D is 1:3.
Or…25% of P remains
Look at chart.
25% P corresponds to 2 half
lives.
If a half life is 200,000 yrs,
then this sample is:
2 hl x 200,000 yr/hl
400,000 years old
“hl” – half life
Radiometric dating uses decay of unstable isotopes.
– Isotopes are atoms of an element that differ in their number of neutrons.
– A half-life is the amount of time it takes for half of the isotope to decay.
Radiometric Dating: Half-Life
What if there’s been 2.4 or 1/3
of t ½ ?
 Okay. In the “real world” of geochronology things
can get a bit more tricky. We have equations that
we use to calculate ages that don’t really use the t ½
approach…directly.
Like: Age = 1/λ ln (D/P +1)
 This is good because we can then use statistics to
evaluate the reliability of the age we’ve found. If an
age passes the test, its called an isochron age. If it fails,
then it’s called a errorchron, and isn’t used.
Dating with Carbon-14
 Radiocarbon dating is the method for
determining age by comparing the amount
of carbon-14 to the amount of carbon-12
in a sample.
 When an organism dies, the amount of
carbon-14 it contains gradually decreases
as it decays. By comparing the ratio of
carbon-14 to carbon-12 in a sample,
radiocarbon dates can be determined.
Present Radiometric Dating
Methods
Cosmogenic
 C-14 5700 Yr.
Primordial
 K-Ar (K-40) 1.25 B.Y.
 Rb-Sr (Rb-87) 48.8 by
 U-235 704 M.Y.
One last thing
 “Radiocarbon” dating is rarely used in geology.
 The t ½ of 14C is only 5730 yrs. After 4 - 5 t ½’s it’s
reliability becomes questionable.
 Also,14C is created in the atmosphere at uneven rates.
 14C decays into 12C leaving 14N behind…so the P/D ratio
only tells you the “age since death” for living things. It is
useless for rocks…but absolutely great for archaeologists,
who use it as far back as ~ 50,000 yrs
 Don’t confuse radiocarbon with geologic dating!
 In practice, both relative and absolute dating
are combined, following a procedure like this:
Igneous rocks, such as lava flows, volcanic
ash beds, and intrusions) are dated
radiometrically.
The dates of fossil-bearing sedimentary
rocks are in a certain area are bracketed
using the dates of associated igneous rocks
which have been dated radiometrically.
The fossil-bearing sedimentary rocks are
correlated with sedimentary rocks in other
areas which contain the same fossils.
The age of the rocks in other areas is
determined indirectly, from the ages of the
fossils they contain..