Geology 15                                                                              West Valley College

Robert Lopez

Study Questions 8: Geological Time


Read Harden Chapter 3, and browse through her discussion on fossils (p. 45-54).


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

2)      Be able to discuss and apply the four principles used to determine relative ages.

3)      Name four primary structures 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.

4)      How are “dips” of inclined beds described (two features)?

5)      What is an unconformity, and what does an angular unconformity indicate?

6)      What is a disconformity?

7)      What is a nonconformity?

8)      Work out the relative ages (youngest to oldest) of the rock units, unconformities, and faults shown in Harden, Fig. 3-1 and 3-2, and the exercise found at this link form more practice on relative time diagrams (I will put at least on relative time diagram on the exam).

9)      Know, be able to use, and arrange in order the following terms from the geological time scale (see Harden Fig. 3-3):  Precambrian (Hadean, Archean, Proterozoic), Phanerozoic,  Paleozoic,  Mesozoic,  and Cenozoic.  Also know the periods of the Mesozoic (Triassic, Jurassic, and Cretaceous) and of the last four epochs of the Cenozoic (Miocene, Pliocene, Pleistocene, Holocene).  Know the six absolute ages for the age of Earth, base of the Paleozoic, base of the Mesozoic, base of the Cenozoic, base of the Miocene, base of the Pliocene, base of the Pleistocene, and base of the Holocene).

10)  What is absolute dating based on?  Be able to define “atomic number of element” and mass number of an isotope.  What is an isotope?  Give some examples of isotopes.

11)  Be able to explain the concept of half-life using the example of some given number of parent atoms with a specific half-life (e.g. one hour).  Be able to calculate the ratio of parent to daughter atoms after some given amount of time has passed.

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

13)  How does the C-14 isotopic system work? See Figure 3-4 page 45.

14)  What is Rancho La Brea?  What is the California State Fossil (Latin name)?  Go to the following the Page Museum for more on Rancho La Brea.


Geology 15                                                                                          West Valley College

Robert Lopez


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.

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