Transcript Chapter 18

Geologic Time
Determining geological ages
• Relative dating – placing rocks and
events in their proper sequence of
formation, without actual dates.
• Numerical dating – specifying the
actual number of years that have
passed since an event occurred (also
known as absolute dating)
Principles of Relative Dating:
Law of Superposition
In an undeformed sequence of
surface-deposited rocks, the
oldest rocks are on the bottom.
• Includes sedimentary rocks, lava flows,
ash deposits and pyroclastic strata.
• Does not include intrusive rocks, which
intrude from below.
Law of Superposition – Grand Canyon
Principles of Relative Dating
• Principle of original horizontality
– Layers of sediment are generally
deposited in a horizontal position
– Rock layers that are flat have not been
disturbed
• Principle of cross-cutting relationships
– Younger features cut across older features
(faults, intrusions etc)
Figure 18.3
Figure 18.4, #4
a. Is Fault A o/y than the
ss layer?
b. Is Dike A o/y than the
ss?
c. Was the conglom.
deposited b/a fault A?
d. Was the cong.
deposited b/a fault B?
e. Which fault is older, A
or B?
f. Is dike A o/y than the
batholith?
Figure 18.4 - a
a. Is Fault A o/y than the
ss layer? -Y
b. Is Dike A o/y than the
ss?
c. Was the conglom.
deposited b/a fault A?
d. Was the cong.
deposited b/a fault B?
e. Which fault is older, A
or B?
f. Is dike A o/y than the
batholith?
Figure 18.4 - b
a. Is Fault A o/y than the
ss layer? -Y
b. Is Dike A o/y than the
ss? - Y
c. Was the conglom.
deposited b/a fault A?
d. Was the cong.
deposited b/a fault B?
e. Which fault is older, A
or B?
f. Is dike A o/y than the
batholith?
Figure 18.4 - c
a. Is Fault A o/y than the ss
layer? -Y
b. Is Dike A o/y than the
ss? - Y
c. Was the conglom.
deposited b/a fault A? After
d. Was the cong. deposited
b/a fault B?
e. Which fault is older, A or
B?
f. Is dike A o/y than the
batholith?
Figure 18.4 -d
a. Is Fault A o/y than the ss
layer? -Y
b. Is Dike A o/y than the
ss? - Y
c. Was the conglom.
deposited b/a fault A? After
d. Was the cong. deposited
b/a fault B? - Before
e. Which fault is older, A or
B?
f. Is dike A o/y than the
batholith?
Figure 18.4 - e
a. Is Fault A o/y than the ss
layer? -Y
b. Is Dike A o/y than the
ss? - Y
c. Was the conglom.
deposited b/a fault A? After
d. Was the cong. deposited
b/a fault B? - Before
e. Which fault is older, A or
B? - A
f. Is dike A o/y than the
batholith?
Figure 18.4 - f
a. Is Fault A o/y than the ss
layer? -Y
b. Is Dike A o/y than the
ss? - Y
c. Was the conglom.
deposited b/a fault A? After
d. Was the cong. deposited
b/a fault B? - Before
e. Which fault is older, A or
B? - A
f. Is dike A o/y than the
batholith? - Y
Figure 18.4 - Answers
a.
b.
c.
d.
e.
f.
Is fault A o/y than the ss?
– Y – fault cuts the ss
Is dike A o/y than the ss?
– Y – dike cuts ss
Was the conglom.
deposited b/a fault A? –
after – conglom not cut
Was the conglom
deposited b/a fault B? –
before – fault cuts it
Which fault is older?-A –
conglom older than B but
younger than A
Is dike A o/y than the
batholith? – Y – Dike A
cuts Dike B, which in turn
cuts the batholith.
Inclusions
– An inclusion is a piece of rock that is
enclosed within another rock.
– Principle of cross-cutting relationships
tells us rock containing the inclusion is
younger than the inclusion itself.
– The presence of inclusions allow us to
determine whether a intrusive igneous
rock is older or younger than the rock
above it.
– Let’s see how
Inclusions
• Magma intrudes
into an existing rock
formation,
surrounding small
pieces of it.
• The magma
becomes an
intrusive igneous
rock (e.g. granite).
•Even though it is
underneath the pink
rock, it is younger
• The contact
between the two
layers is not an
unconformity,
because it was
never exposed at
the surface.
Inclusions
• First the
“country rock”
(the pink stuff)
weathers away,
exposing the
granite (gray) at
the surface.
• The granite
also weathers
away, leaving
an erosional
surface.
Inclusions
• Conditions change and
the erosional surface
becomes a depositional
environment.
• The lower layers of the
sedimentary formation
contain inclusions of
granite.
• This shows the granite is
older than the sedimentary
layers.
• The contact between the
older igneous and younger
sedimentary rocks is a type
of unconformity, because it
was at one time exposed at
the surface.
Unconformity
• a break in the rock record produced by
erosion of rock units and/or
nondeposition of sediments
– Between sedimentary rocks and
crystalline (non-layered) bedrock
– Between two sets of layered sedimentary
rocks deposited at two different times
– Angular unconformity – tilted rocks are
overlain by flat-lying rocks
Formation of an
angular unconformity
Figure 18.7
Unconformity Types
Unconformities in the Grand Canyon
Unconformities, especially between sedimentary strata, are hard to distinguish.
Figure 18.6
Fossils: the remains or traces of
living organisms
• Conditions favoring preservation
• Rapid burial
• Possession of hard parts (shells or
bones
• Correlation: Matching of rocks of
similar ages in different regions
• Correlation often relies upon
fossils
Principle of Fossil Succession:
Fossil organisms succeed one
another in a definite and
determinable order, so any time
period can be recognized by its
fossil content.
Principle of Fossil Succession:
• Although developed over 50 years
before Darwin’s work, it is now
known that the reason this
principle is valid is due to
evolution.
• Fossil organisms become more
similar to modern organisms with
geologic time
• Extinct fossils organisms never
reappear in the fossil record
Index Fossils
– Widespread geographically
– Limited to short span of geologic
time
– Valuable for correlation: use of index
fossils can often provide numerical
dates for rock units and events
– Similar accuracy to radiometric
dating techniques.
Using fossil groups to determine the
age of rock strata
Geologic time scale: a “calendar” of
Earth history
• Subdivides geologic history into units based
on appearance and disappearance of fossils
from the geologic record
• Structure of the geologic time scale
• Eon – the greatest expanse of time
• Era – subdivision of an eon
• Eras are subdivided into periods
• Periods are subdivided into epochs
The “Precambrian”
• Used to refer to all
geologic time before the
Phanerozoic (Visible Life)
Eon
• Represents almost 88%
of geologic time
• Originally it was thought
that no life existed before
the Phanerozoic Eon
• Now we know that the
lack of fossil evidence in
the Precambrian rocks is
partially due to the lack of
organisms with
exoskeletons
Eras of the Phanerozoic eon
– Cenozoic (“recent life”)
– Mesozoic (“middle life”)
– Paleozoic (“ancient life”)
Notable divisions between the Eras
• Paleozoic-Mesozoic – 248 mya
– Mass extinction of trilobites and many other marine
organisms
– Possibly due to climate change that occurred with the
formation of Pangaea
• Mesozoic-Cenozoic – 65 mya
– Mass extinction of dinosaurs and many other species
– Probably caused by meteor impact
– Made way for the domination of mammals
• Cenozoic- ????
Figure 18.16
Correlation #1
U
Correlation #2
Figure 18.18
U
Assume volcano F
occurred before Fault G
E occurred last
D and K are plutons
M is metamorphic
Radioactivity activity 2 - rhyolite
Radioactivity 3 – felsic ash
Basic atomic structure
• Proton – positively charged particle
found in nucleus.
• Neutron – neutral particle, which is a
combination of a proton and an electron,
found in nucleus.
• Electrons – very small, negatively
charged particle that orbits the nucleus.
Also, an elementary charged particle that
can be be absorbed by a proton or
emitted by a neutron to change one into
the other.
Basic atomic structure
• Atomic number
– An element’s identifying number
– Equal to the number of protons in the atom’s
nucleus
– Carbon’s atomic number is always 6.
• Mass number (formerly “atomic weight”)
– Sum of the number of protons and neutrons
in an atom’s nucleus
– Indicates the isotope of the element (e.g. C12, C-14).
Periodic Table
Isotopes and Radioactivity
– Isotope: Variety of an atom with a different
number of neutrons and mass number
– Some isotopes (not all!) are inherently
unstable, which means the forces binding
nuclear particles together are not enough
to hold the nucleus together. These are
called radioactive isotopes.
– Examples of isotopes include O-16, O-18,
C-12, C-13, and C-14. Only the last is
radioactive.
Comparison of C-12 with C-14
Radioactivity
• Many common radioactive isotopes are
naturally occurring.
• Most radioactive processes release
energy; formation of C-14 by neutron
capture is an exception. It requires
cosmic (solar) radiation.
• They also often release energy and
sometimes eject atomic particles as
they “decay” or change into a more
stable substance.
From Parent to Daughter
• In many cases atomic particle are
ejected during radioactive decay
– Protons and/or neutrons ejected from
nucleus
– Protons become neutrons or vice verse
• Atomic number changes so a new
daughter element results.
• How long does a radioactive parent take
to turn into a stable daughter?
Figure 2.4
Half-life
• the time required for one-half of the
radioactive nuclei in a sample to change
from parent isotope to daughter isotope.
• Decay occurs at random. Can’t predict
when an individual atom will decay.
• However, decay is statistically
predictable.
• Comparison with coin toss
Half Life (cont’d)
• After one half-life, 50% of the parent
isotope will have become daughter
isotope, regardless of the sample size.
• After 2 half-lives, 50% of the remaining
parent isotope will have become
daughter isotope. This means 75% of
the original parent isotope will have
changed.
• This is an exponential relationship.
Fig. 4-19, p.86
Using half-lives of radioactive isotopes in
an object to determine the numerical age
• Zircon (zirconium silicate)
is a common accessory
mineral in igneous,
sedimentary and
metamorphic rocks which
contains traces of uranium
and thorium
• Potassium-40 is a
radioactive isotope which
occurs in K-spars and
other minerals containing
potassium.
Zircon crystal
Using half-lives of radioactive isotopes in
an object to determine the numerical age
• Every radioactive isotope has a unique halflife, which can be determined by experiment
• For radioactive isotopes other than C-14, the
ratio of parent to daughter product in a
sample determines the age of the sample
• C-14 is compared to atmospheric
concentration to determine age of organic
material.
• After approximately 10 half-lives, the method
is no longer effective as the amount of parent
material is too small to measure.
Table 18.1
Figure 18.14
Importance of Radiometric Dating
• Radiometric dating is
a complex procedure
that requires precise
measurement
• Rocks from several
localities have been
dated at just under 4
billion years
• Confirms the idea
that geologic time is
immense.
Formation and radioactive decay of
Carbon-14
C-14 is created in the upper
atmosphere when
bombardment of (N-14)2 gas
with high energy cosmic rays
results in neutron capture.
C-14 is unstable and
eventually will turn back into
N-14, by ejecting a negative ß
(beta) particle. The half-life is
5730 years.
Radiometric dating with Carbon-14
• The % C-14 is equal to atmospheric C-14 in a
living object, but decreases after death
• To determine the age of the sample, compare
% C-14 in sample with % atmospheric C-14.
• Due to relatively small half-life, C-14 is used
to date recent events only (10 half-lives is
less than 60, 000 years)
• Most useful in the fields of archeology and
anthropology, also for climate change
studies
Corrections for C-14 dating
The % C-14 in our
atmosphere has
changed over time
– solar flare activity
determine cosmic ray
activity, which causes C14 formation
– Nuclear testing (see
1963 graph)
– Use of dendochronology
to create calibration
curves
Conversions of Dates and Ages
into years BP
If the age of the object is given as a date:
• AD (“year of the lord”): This is the same as a calendar
date. The years BP value is how old the sample was in
1950. Ex: If an object is dated at 5 AD, the BP age is
1950-5 or 1945 years BP
• BC/BCE: This is also a date, indicating how many years
before “the birth of Christ”. Ex: If an object is dated at 5
BC, it was already 5 when the AD numbering system
began. In 1950, it was 1950 + 5 or 1955 years BP
•
If an age is given (ex: the object is 2000 years
old, that’s 2000 years older than today.
Assuming today is in 1999 (!) simply subtract 49
years from the age.