Chapter 10 Geologic Time

10-1    A brief history:

• Catastrophism: the earth's feature is creased by unknown forcers, not operating now.

• Uniformitarianism: the laws governing today are also the laws governing in the past. It was proposed by Hutton, successfully interpreted by Lyell. Some animals and plants live in water, indicating they lived in water when their fossils were found.

10-2    Relative dating:

• For a sequence of rock formation, which formed first, which second, etc.

• Doesn't tell exactly when why formed.

1. Law of superporition: Proposed by Nick Steno. In undisturbed formation, the lower the formation, the older the formation (Fig. 10.3B). How do we know that the figure is not upside down? 

2. Principle of original horizontality: states when the rocks formed, the formation was horizontal.

3. Cross-cutting relationship: sequence of formation in Fig 10.5:

   1. Sandstone formation

   2. Fault A

   3. Conglomerate

   4. Fault B

   5. Batholith

   6. Dike B

   7. Dike A

   The one that cuts through other formations is younger.

4. Inclusions: Fig. 10.10.

   

5. Unconformities: represents time lapse between the two formation, indicating weathering occurred.

   • Angular unconformity: the older and younger formation don't have same tilting angles.

   • 

   • Disconformity: time gaps exist, but both formations have same angles.

   • Nonconformity: contact between sedimentary and igneous or metamorphic rocks. Fig. 10.7.

10-3    Correlation of rock layers using fossils:

1. Fossils: 

   • Remains or trace of life.

   • Has to be petrified.

2. Types of fossils: 

   • Bones, teeth, shells, easy to be petrified.

   • Others: impression, tracks, borrows (tubes made by animals), foot prints.

   • Animal fossils vs. plant fossils.

3. Fossil succession: 

   • fossil animals succeed one another in a regular pattern, e.g. trilobite before fish, which before reptile.

4. Index fossils: fossils occurred in larger area but a short period of time. Index fossils are used to match rocks with similar ages.

5. Other information fossils provide: 

   • Temperature: corals ==> shallow water and warm weather.

   • Depth: Broken shells ==> shallow due to wave.

10-4    Absolute dating:

1. Atoms composed of nucleus and electrons, nucleus composed of protons and neutrons. Charges each type of particles provide: protons: +, neutrons: none, electrons: -. For an atom, electric charge is neutral.

   • Different atoms have different numbers of protons, e.g. ¹²₆C, and ¹⁴₇N.

   • If the numbers of p the same and the numbers of n different, they are called isotopes, e.g. ¹²₆C, ¹³₆C, and ¹⁴₆C.

2. Stable vs. radioactive isotopes:

   • Stable: same nucleus with respect to time.

   • Radioactive: spontaneously decay of a nucleus.

3. Types of radioactive decay:

   • alpha-particles, 2n + 2p released, 

   • beta-particles, an electron emitted.

   • electron capture: an electron is gained.

4. Half-life: Time required to decay half of the original nucleus.

5. Common radionuclides for dating geologic rocks: K/Ar, Rb/Sr, and U/Pb for loder rocks and ¹⁴C for younger rocks.

10-5    Geologic time scale:

Used to be on relative scale. Now absolute scale is added.

1. Eon: the largest division: 

• Phanerozoic: visible life. Thus, before 570 Ma no fossils were found. Fig. 10.17.

2. Ears: one scale smaller than Eon.

3. Period: one scale smaller than Eras:

etc.

4. Epoch: Early, middle, and late

Notice the division: The older, the wider. Why? Not enough information to accurately date.

10-6    Difficulties in dating:

1. Absolute dating works better for crystalline rocks, not for sedimentary rocks, e.g. K/Ar in mica. If the mica was in sedimentary rocks, the mica was formed much earlier than the sedimentary rock.

2. Combination of relative sequence with absolute dating works best, e.g. Fig 10.18. The igneous dike was dated 66 Ma, the Mancos shale formation formed should be before 66 Ma.

Homework:

• Read chapter summary on p.291.

• Use your own word to explain the key terms on page 291.

• Answer the review questions on page 292.