chapter 13 Flashcards
(15 cards)
Discuss and describe the geological time scale.
The geologic time scale was developed based on the extensive fossil record available. Boundaries between various geologic time units are based on either the formation or extinction (appearance or disappearance) of particular life forms. It is divided (in order of decreasing length) into four major time units: Eons, Eras, Periods, and Epochs.
Eon is the largest division of geological history.
How old is the oldest rock found on Earth?
4 Ga
What is the age of meteorites and lunar rocks?
~4.5 to 4.6 Ga
Draw the Geological Time Scale including all the major dates.
Picture 2 in folder
List the type of unconformities and explain what they are.
Unconformities are a substantial break (or gap) in the geologic record of a stratigraphic sequence and represent a period of erosion or non-deposition.
Three majors types of unconformity occur:
1. Angular unconformity: angular discontinuity between older and younger strata
2. Disconformity: irregular surface of erosion between parallel strata
3. Nonconformity: represents a surface between sedimentary strata and overly igneous or metamorphic rocks
Describe the formation of an angular unconformity with an example. Draw a diagram to show angular unconformity.
Picture 3.
Beneath the sea, sediments accumulated in beds. Later, tectonic forces caused uplift, folding, and deformation of the sedimentary layers during mountain building. Erosion stripped away the tops of the folded laers, leaving an uneven plain with exposed portions of several folded layers. Subsidence below the sea allowed new sediments to be deposited on the former erosion surfaces. The surface where the folded layers and the new sediments meet is preserved as an angular unconformity.
Example: Grand Canyon
How does disconformity form? Give an example and a drawing.
Picture 4. Disconformity is an irregular surface of erosion between parallel strata. Beneath the sea, sediments accumulated in beds A to D. Later, tectonic forces caused uplift of the beds above sea level, exposing them to erosion. Erosion stripped away bed D and part of C, leaving an irregular surface of hills and valleys. Subsidence below the sea allowed a new layer, E, to be deposited over C. The irregular surface of C is preserved as an unconformity.
Example: Grand Canyon at the south rim
List the names of the five general geological concepts
- Principle of Uniformitarianism
- Principles of the Stratigraphic Record
a. Principle of Original Horizontality
b. Principle of Superposition
c. Principle of Law of Crosscutting Relationships
d. Principle of Lateral Continuity - Principle of Faunal Succession
- Relative Ages
- Correlation
Explain the principle of uniformitarianism.
Sedimentation processes (and other geological processes) are the same now as in the past. Natural laws don’t change, but the rates and intensity of the processes may.
Explain the principle of the stratigraphic record.
a) Principle of Original Horizontality: sediments are deposited as horizontal or nearly horizontal beds or strata
b) Principle of Superposition: the topmost layer is always the youngest and the lowermost layer is always the oldest in a series of sedimentary strata that have NOT been disturbed or overturned by tectonic processes.
c) Principle of Law of Crosscutting Relationships: any structure (fault, dike or vein) is younger than the rock that it cuts across.
d) Principle of Lateral Continuity: layered rocks are deposited continuously and don’t suddenly end. Used for correlation of outcrops through distanced based on rock similarities.
is the fault or fold younger or older than sedimentary strata?
Tectonic/ structural events are younger than the rock layers.
Describe the principle of faunal succession.
Group of fossils [fauna (animals) and flora (plants)] have succeeded one another in a definite and recognizable order and each time period can be recognized by the fossils found in rocks of that age. Fossils tend to evolve from simpler to more complex species with time.
What are the principles that allow the interpretation of successive vertical strata as a chronological record of the geologic history of a region (Geologic Time)? Provide examples.
Principles 1, 2, 3 (listed before). Strata having dinosaur fossils must be of the Mesozoic age. Those having Trilobite fossils are of Cambrian age etc.
How can the relative ages be obtained? List the types of age dating.
There are two types of age dating:
a) Relative age dating (fossils, structures)
Relative ages can be obtained by applying the principle of superposition (in a depositional sequence older strata occur at the bottom and recent or younger ones occur at the top) and the principle of cross-cutting relationships (a dyke that cuts across strata is younger than the strata). Accordingly, a relative age can be assigned to any rock in the sequence by relationship or correlation.
b) Absolute age dating (radiometric (isotopic) age dating) where the precise age dating is done through using radiometric (radioactive) isotopes. Isotopes are a species of the same element (same atomic number) but with different atomic mass number. Some isotopes are stable such as H,O,C isotopes; C12, C13, C14 but C14 is radioactive and is useful for dating young rocks and archeological materials (ex. deposits formed during the quaternary period). Radioactive isotopes include U, Th, Rb, K, Sm, etc.
Half-life is the time required for half of the initial number of atoms to decay. The rate at which this decay occurs is constant and known. Parent (P) such as U decay giving daughter isotope such as Pb.
List the types of radioactive decay schemes.
Beta-decay, electron-capture, alfa decay
