1. Earth’s History & Radioactivity

2. When did all of this Get here??

3. Geologic Time
Relative - Ordinal. One event occurred before or after a different event. Not a quantitative assessment.
“Absolute” Scalar - A date is assigned to an event - relative to a fixed measure (clock, yardstick, time scale)
- interval of Earth's geologic history – proposed to start approx. 4.6 billion years ago (corresponding to the age of the oldest known rocks) to the present day
- that segment of Earth history that is represented by and recorded in rock strata.

5. The fossil forms that occur in these rocks provide the chief means of establishing a geologic time scale. Because living things have undergone “evolutionary” changes over geologic time, particular kinds of organisms are characteristic of particular parts of the geologic record.
By correlating the strata in which certain types of fossils are found, the geologic history of various regions (and of the Earth as a whole) can be reconstructed.
The relative geologic time scale developed from the fossil record has been numerically quantified by means of absolute dates obtained with radiometric dating methods.

6. “Relative” Geologic Time
Original Horizontality - Steno
Superposition - Steno
Faunal Succession - Smith
Cross-cutting Relationships
Inclusions
Deformation
Most of these are important for correlating rock units (and events) from different regions

7. “Original Horizontality”
Layers of sediment are generally deposited in a horizontal position.
So - if we observe rock layers that are flat - they have not been disturbed and still have their original horizontality.
But - if they are folded or inclined at a steep angle, they must have been moved into position by crustal disturbances sometime after their deposition

8. Figure 1-5, Stanley

10. Various dating techniques use geological principles to place events in a chronological order. The first one is:
Principle of Superposition - Older beds are covered by younger beds, so that in a sedimentary sequence, the youngest bed is on the top - unless the sequence has been tectonically overturned.
These beds are also assumed to be part of a continuous layer across the canyon

11. Faunal Succession - Organisms change through time so that fossil forms can be used as age markers wherever they are found. Index Fossils = fossils that are widespread in extent, yet occurred within a restricted interval of time.
Second sampled area
First sampled area

12. Principle of Cross-Cutting Relationships
This Principle states that cross-cutting rocks (e.g. igneous intrusions) are younger than the rocks they cut across. This principle is also useful for determining age of movement along faults.
Younger Igneous Rocks

14. Granite Dike Gabbro Inclusion
Illustration of principle of cross-cutting relationships, as seen in an outcrop containing three different igneous rocks. The thin pink (aplite) dike cuts across or intrudes the other two rocks, hence it is the youngest of the three units present. The black (gabbroic) rocks are also dikes, intruding the granite, which must then be the oldest unit. The evidence for this is given by a small inclusion of the granite enclosed in the basalt.

15. Inclusions
Pebbles and fragments included in sediments and lavas must be older than the host rock containing them. Basal Conglomerates have pebbles from the unit below them. Rock fragments included in magmas are called Xenoliths, and may represent walls of magma chambers, conduits, etc.
Sediment with igneous and metamorphic rock fragments
Sediments with sedimentary pebbles
Metamorphic Rocks
Igneous Intrusion with Xenoliths

16. By using these tools/principles - We should be able to make sense of rather complicated geological stories

18. “Absolute” Geologic Time
Radiometric Dating Techniques
Radioactive parent elements decay (break down) to produce stable daughter elements, at known rates, in the process of Radioactive Decay. Because decay rates are nearly constant for these different reactions, the length of time over which decay has been occurring may be estimated by measuring the amount of radioactive parent element and the amount of stable daughter element.

19. ____________
The nucleii of certain elements spontaneously emit new particles and are changing in size and form - producing new elements = Decay
238U Pb + He
At any instant, there is a unique ratio of 238/206 as a consequence of the length of time it has been decaying

20. Isotopes and Mass Number
Each element may be characterized by a specific Atomic Number, representing the number of Protons.
However, the Mass Number (number of p+ plus the number of no) may change, as the result of differing numbers of neutrons.
Elements with various numbers of neutrons are called Isotopes of that element.

21. Alpha Decay/Emission nucleus loses an alpha particle
(nucleus of a He atom (2p+ + 2no))
Mass decreases by 4 (2p+ + 2no)
Atomic Number decreases by 2 (2p+)
Alpha particles are rather large and can be stopped by a piece of paper

22. Beta Emission
element loses a beta particle (  e-) from the neutrons in the nucleus.
the neutron becomes positively charged ( a proton)
Atomic Mass stays the same
Atomic Number increases by 1
Beta particles penetrate 100s of times farther than alpha particles, but are easily stopped compared with neutrons & gamma rays

23. Electron Capture nucleus gains an electron from the orbital electrons.
Therefore, a proton becomes neutral (no), and
Atomic Mass stays the same
Atomic Number decreases by 1
neutrons are highly penetrating = nasty

24. The time for 50% of the atoms in a sample to decay is called the half-life. Each radioactive Isotope has a unique half-life.
After one half-life, the ratio of parent to product is 1:1 (one). After two half-lifes the ratio is 1:3 - after three half-lives 1:7 and so on until after ten half-lives the ratio is 1:1023 and less than 0.1% of the original parent isotope remains.
The Rate of Decay is proportional to the number of parent atoms present.

25. Radioactive Decay curve - showing exponential change.
Half of the radioactive parent remains after one half-life. After a second half-life. One-quarter of the parent remains, and so forth….

26. All radiogenic isotopes have characteristic decay series
All radiogenic isotopes have characteristic decay series. The most useful series for dating rocks are those involving elements found in common silicate minerals.
Potassium-Argon dating and Rubidium-Strontium dating are most useful in this respect.
Parent
Daughter

27. Radioactive decay of Uranium (U238)
Many steps to get to the most stable isotope Pb206

28. Potassium-40 may be found in: Uranium may be found in:
Minerals You Can Date !!!
Potassium-40 may be found in:
Potassium Feldspar (Orthoclase)
Muscovite
Amphibole
Glauconite
Uranium may be found in:
Zircon
Uraninite
Monazite
Apatite
Sphene

29. The commonly used decay series and their half-lives are given in the table:
Parent Isotope Daughter Isotope Half-Life (Years)
Carbon Nitrogen ,730
Uranium Lead ,000,000
Potassium Argon ,300,000,000
Uranium Lead ,500,000,000
Thorium Lead ,000,000,000
Rubidium Strontium ,000,000,000
The age given by a mineral separated from a rock is the age of
crystallization. Metamorphic recrystallization will reset the radiometric clock. Sediments are difficult to date radiometrically because few minerals crystallize in sediments at the time they are formed.

30. Carbon-14 (14C) Dating
Cosmic Rays (no) from the sun bombard 14N in outer atmosphere
knock p+ from 14N nucleus C
14C combines with oxygen CO2
14CO2 circulates and is absorbed and used by plants and the radioactive C gets into the food chain (C cycle)

31. Production of Carbon-14
Decay of Carbon-14

32. Decay of Carbon-14
All Living Things contain a constant ratio of 12C to 14C (1 in a trillion)
At death, 14C exchange ceases and any 14C in the tissues begins to decay to 14N and is not replenished by new 14C
Compare the amount of 14C present in tissue to current 14C/12C ratio
Calculate amount of time elapsed from death to the present = “age”

33. Calibrate technique by comparison to bristlecone pines
Problems with 14C Dating
Assumes the rate of 14C production has been constant over the past 70,000 years
Since amount of nonradioactive 12C has increased dramatically as a result of pollution from combusting fossil fuels
Since C has been created as a by-product of nuclear explosions
Calibrate technique by comparison to bristlecone pines
(ca yrs) considered the oldest living organism named “Methuselah” and closely tied to our solar calendar

34. so, rocks with different polarity from the present may be “dated”
Other Techniques
Fission Track Dating
charged particles from radioactive decay pass through a mineral’s crystal lattice, leaving trails of damage = Fission Tracks which may be counted
Magnetostratigraphy
Earth’s magnetic field has switched from Normal (as in the present) to Reversed and back and forth
so, rocks with different polarity from the present may be “dated”