Geology 001                                                                                        West Valley College

Physical Geology                                                                                   R. Lopez

Study Questions 13: Geological Time

1)      Geologists deal with two types of ages (i.e. time).  What are they and how do they differ?

2)      What are the five principles used to determine relative ages. State a definition or give an example of each on.

3)      Who is credited for the first three relative time principles?

4)      Who is credited for the last two relative time principles?

5)      Who was Charles Lyell? What book and when did he publish this work?

6)      What is the connection between Darwin’s On The Origin of Species and the work done by Lyell?

7)      What is the principle of uniformitarianism and who is credited for this principle?

8)      Name three of the four primary structures (paleo-up indicators) in rocks that can be used to determine if beds have been overturned (three found in sedimentary rocks and one in lavas)?  Be able to recognize these structures in slides.

9)      What is an unconformity? 

10)  Distinguish between disconformity, nonconformity, and angular unconformity?  Draw sketches of each below.

11)  Work-out the relative ages (youngest to oldest) of the rock units, unconformities, and faults on Pamala Gore’s Historical Geology Lab (I always use one of her diagrams on our exam): relative time exercises.

12)  What experiment did Lord Kelvin do to claim that he destroyed the uniformitarian foundation?

13)  What did Henri Becquerel discover to level a heavy blow to Lord Kelvin?

14)  Explain the Pb-Pb or U/Pb evolution curve for the age of Earth (the one that used meteroites and well mixed terrestrial sediment).

15)  What is absolute dating based on?  Be able to define “atomic number” and atomic mass number of an isotope. 

16)  What are good isotopes for dating old rocks?

17)  What is a half life (T½)?

18)  Radioactive C-14 decays to stable daughter isotope N-14.  The T½ = 5730 years.  If you measure a P/D ratio of 6.25/93.75, how many half-lives have elapsed?

19)  What is the age of the sample measured in the problem above.

20)  Name examples of parent-daughter pairs used in absolute dating, and know the approximate half-life of the parent isotope.

21)  Read “The Asteroid Impact Theory Becomes Testable” at the following url: (  You will have to scroll down to get the outline numberal II.  Most geologists now accept that the dinosaurs were wiped out by a cataclysmic asteroid impact.  What are some of the lines of evidence that support Dr. Alvarez’s theory? 

22)  In the Asteroid article, what is the significance of the element iridium? 

23)  What could have caused the spike in iridium in the clay layer and what two hypotheses did the Alvarez’s propose? 

24)  What is a supernova? 

25)  What was their argument against the supernova hypothesis for the spike in iridium? 

26)  Where is the location of the K-T impact crater?

27)  What is the age of the Earth?

28)  Using our decay equation, determine the age of the following rocks using the 235U-207Pb isotopic system (l = 9.8485x10-10/yr): Sample 1 – D = 2722, N = 85; Sample 2 – D = 998, N = 555; Sample 3 – D = 122, N = 111; Sample 4 – D = 1555, N = 1001.


Definitions Related to Study Questions 8: Geological Time


1)      Relative time simply deals with the order of events; no information on how long the events lasted or the length of time between events.

2)      Absolute time assigns absolute age to events, usually in “years before present”.           1 Ma = one million years before present.

3)      Superposition: in any sequence of strata not overturned, the order of deposition is bottom to top.

4)      Original horizontality:  water-laid sediments are deposited in strata that are horizontal. (Note:  cross beds are within beds or strata.)

5)      Use of inclusions:  a rock or sediment containing fragments or other rocks must be younger than the rocks it contains.

6)      Unconformity:  a break or gap in the stratigraphic record.  Also, a hiatus; i.e. a lapse in time.

7)      The nucleus of an atom contains protons (+ charge) and neutrons (no charge, or you can think of a neutron as having both a + and – charge which balance each other).

8)      Atomic number:  an element is defined by its atomic number, that is the number of protons in the nucleus of the atom.  All atoms of an element have the same atomic number.

9)      Atomic weight (mass number) of an atom is the sum of its protons and neutrons.  For example, the most common atoms of oxygen contain eight protons and eight neutrons, and thus have an atomic weight of 16.

10)  Isotopes: atoms of an element that contain different numbers of neutrons, and consequently, have different atomic weights or mass numbers.

11)  Radioactive Isotopes: isotopes that spontaneously transform into either a more stable isotope of the same element or an isotope of a different element.  Parent isotopes decay to daughter isotopes. Energy is produced with radiometric decay.

12)  Half life: time needed for the number of parent atoms to be reduced by one-half.