TELLING TIME GEOLOGICALLY

3. TELLING TIME GEOLOGICALLY UNCONFORMITIES Not all the rocks that ever formed are preserved. Many rocks are subjected to weathering and erosion. Gaps in the geologic record exist. These gaps are termed UNCONFORMITIES. They occur when erosion has removed rocks or none were deposited. Some are small gaps in time. Some are extensive amounts of time. They exist in practically every sequence of sed. rocks.

4. TELLING TIME GEOLOGICALLY UNCONFORMITIES NONCONFORMITY Cambrian Sawatch Sandstone overlying the Precambrian Pikes Peak Granite 1.6 billion years missing

5. TELLING TIME GEOLOGICALLY UNCONFORMITIES ANGULAR UNCONFORMITY Siccar Point, Scotland Birthplace of Unconformities

6. TELLING TIME GEOLOGICALLY UNCONFORMITIES DISCONFORMITY Wingate Sandstone, overlying Chinle Formation Utah

7. TELLING TIME GEOLOGICALLY CORRELATION In geology, we try to relate all the rocks on Earth into a relative age scheme. Consider sequences of sedimentary rocks from all over the Earth and fit them together in the proper order. Process is called CORRELATION. CORRELATION is the determination of equivalence of age between geographically distant rock units using paleontologic (fossils) or lithologic (rock) similarities.

8. TELLING TIME GEOLOGICALLY CORRELATION The farther apart the units, the harder it is to correlate the units. With distance depositional environments change, resulting in different facies.

9. TELLING TIME GEOLOGICALLY CORRELATION Fossils help in correlation. KEY BEDS are also used. KEY BEDS record a geological event of short duration that affected a large area.

10. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE Relative age dating provides valuable information. Puts rocks in proper sequence. But….. It is important to know in years, how long ago an event happened or when a rock formed. NUMERICAL or ABSOLUTE DATING can do this to a point. Generally depends on some type of “natural clock”. Depends on a process that occurs at a known, constant rate.

11. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE ISOTOPE DATING Depends on the decay of radioactive isotopes. Isotopes are varieties of elements that differ by the number of neutrons in the nucleus. Radioactive isotopes have nuclei that spontaneously decay by emitting or capturing a variety of subatomic particles. The decaying isotope is known as the parent isotope. By decay, the parent isotope forms a daughter isotope.

12. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE ISOTOPE DATING Loss or gain of neutrons converts a parent isotope into a daughter isotope of the same element. Loss or gain of protons changes the parent isotope into a daughter isotope of a completely different element. Through this process, unstable radioactive isotopes decay to form stable, non-radioactive daughter isotopes.

13. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE ALPHA () DECAY Alpha () particles are composed of two protons and two neutrons (He nucleus) By expulsion of  particles, the atomic mass decreases by 4 and the atomic number decreases by 2. Produces a daughter isotope that is a completely new element.

14. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE ALPHA () DECAY 238U92 decays by alpha ()decay to form 234Th90

15. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE BETA () DECAY Beta () particles are essentially electrons. These electrons are released from the nucleus of the parent isotope. Neutrons are composed of a proton and an electron. Neutron decays, releasing an electron, while at the same time produces a proton. Beta () decay increases the atomic number by 1. No change in the atomic mass.

16. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE BETA () DECAY 40K19 decays by beta() decay to form 40Ca20

17. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE ELECTRON OR BETA () CAPTURE Electron or Beta () capture involves capture of an electron from the surrounding orbiting cloud by the nucleus. These electrons join with a proton and form a neutron. Electron or Beta () capture decreases the atomic number by 1. No change in the atomic mass.

18. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE ELECTRON OR BETA () CAPTURE 40K19 decays by beta() capture to form 40Ar18

20. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE Radioactive isotopes are incorporated in minerals and rocks in a variety of ways. As minerals crystallize from magma, radioactive isotopes are included in mineral crystal structure. At the time of crystallization, only parent isotopes are included in the mineral. Radioactive parent isotopes then begin to decay producing daughter isotopes.

21. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE ISOTOPE DATING uses this process to measure the amount of time elapsed since the mineral’s formation. With time, the amount of parent isotope will decrease and the amount of daughter isotope will increase. The DECAY RATE is constant and acts like a “clock”. Decay rates are not affected by temperature, pressure, or chemical reaction with the parent isotope. By measuring the ratio of parent to daughter isotopes in the mineral and comparing it with the rate of radioactive decay, we can determine the numerical age of a rock.

22. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE The time it takes for HALF of the atoms of the parent isotope to decay into daughter isotopes is known as the isotope’s HALF-LIFE (t½).

23. 1:1 parent to daughter 1:3 1:7

24. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE To calculate the numerical age of a rock, mineral, bone, etc., we determine the number of half-lives or fraction thereof and multiply the number of half-lives gone by by the known half-life (in years). Simply put: In a rock we find 23 atoms of 235U and 161 atoms of 207Pb Half-life (t½) is 713 million years. Age of the rock is 2.139 billion years.