# Geologic Time.

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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 Is Fault A o/y than the ss layer?
Is Dike A o/y than the ss? Was the conglom. deposited b/a fault A? Was the cong. deposited b/a fault B? Which fault is older, A or B? Is dike A o/y than the batholith?

Figure 18.4 - a Is Fault A o/y than the ss layer? -Y
Is Dike A o/y than the ss? Was the conglom. deposited b/a fault A? Was the cong. deposited b/a fault B? Which fault is older, A or B? Is dike A o/y than the batholith?

Figure 18.4 - b Is Fault A o/y than the ss layer? -Y
Is Dike A o/y than the ss? - Y Was the conglom. deposited b/a fault A? Was the cong. deposited b/a fault B? Which fault is older, A or B? Is dike A o/y than the batholith?

Figure 18.4 - c Is Fault A o/y than the ss layer? -Y
Is Dike A o/y than the ss? - Y Was the conglom. deposited b/a fault A? - After Was the cong. deposited b/a fault B? Which fault is older, A or B? Is dike A o/y than the batholith?

Figure 18.4 -d Is Fault A o/y than the ss layer? -Y
Is Dike A o/y than the ss? - Y Was the conglom. deposited b/a fault A? - After Was the cong. deposited b/a fault B? - Before Which fault is older, A or B? Is dike A o/y than the batholith?

Figure 18.4 - e Is Fault A o/y than the ss layer? -Y
Is Dike A o/y than the ss? - Y Was the conglom. deposited b/a fault A? - After Was the cong. deposited b/a fault B? - Before Which fault is older, A or B? - A Is dike A o/y than the batholith?

Figure 18.4 - f Is Fault A o/y than the ss layer? -Y
Is Dike A o/y than the ss? - Y Was the conglom. deposited b/a fault A? - After Was the cong. deposited b/a fault B? - Before Which fault is older, A or B? - A Is dike A o/y than the batholith? - Y

Figure Answers 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 To view this animation, click “View” and then “Slide Show” on the top navigation bar.

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

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 Correlation #2 U Assume volcano F occurred before Fault G
E occurred last D and K are plutons M is metamorphic

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

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.

Figure 4.19 As a radioactive parent isotope decays to a daughter, the proportion of parent decreases (blue line) and the amount of daughter increases (red line). The half-life is the amount of time required for half of the parent to decay to daughter. At time zero, when the radiometric calendar starts, a sample is 100 percent parent. At the end of one half-life, 50 percent of the parent has converted to daughter. At the end of two half-lives, 25 percent of the sample is parent and 75 percent is daughter. Thus, by measuring the proportions of parent and daughter in a rock, the rock’s age in half-lives can be obtained. Because the half-lives of all radioactive isotopes are well known, it is simple to convert age in half-lives to age in years. 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

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.

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 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 Ex: If an object is dated at 5 AD, the BP age is 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 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.

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