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, #4a. 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 - aa. 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 - ba. 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 - ca. 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 -da. 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 - ea. 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 - fa. 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 - Answersa. Is fault A o/y than the ss?

– Y – fault cuts the ssb. Is dike A o/y than the ss?

– Y – dike cuts ssc. Was the conglom.

deposited b/a fault A? – after – conglom not cut

d. Was the conglom deposited b/a fault B? –before – fault cuts it

e. Which fault is older?-A – conglom older than B but younger than A

f. Is dike A o/y than the batholith? – Y – Dike A cuts Dike B, which in turn cuts the batholith.

– 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

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

Figure 18.18

Assume volcano F occurred before Fault GE occurred last

U

Correlation #2

D and K are plutonsM 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. C-

12, 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 half-life, 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 C-14 formation

– Nuclear testing (see 1963 graph)

– Use of dendochronology to create calibration curves

Conversions of Dates and Agesinto 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.