Geologic Time Lesson Outline / Study Guide

Geologic Time ¨C Lesson Outline / Study Guide

1. Geologic Time

? Geologic time is vast.

? Age of the Earth is 4.6 billion years.

? The Geologic Time Scale divides Earth¡¯s history into Eons, Eras, Periods, and

Epochs. These divisions on the Geologic Time Scale are largely based on dominant

life forms which lived at that time.

2. Dating Methods for Determining the Age of Rocks

? Relative age dating establishes the relationship of rock units and geologic events to

each other (which one is older or younger; which occurred first and which occurred

second, etc.) while absolute age dating uses an element¡¯s rate of radioactive decay

to determine the numeric (or absolute) age of rocks and rock units.

3. Relative Age Dating Method Uses a Scientific Law and Several Basic Principles

? Law of Superposition ¨C states the oldest rocks are on the bottom of a sequence of

undisturbed sedimentary rocks (or layered igneous rocks).

? Principle of Original Horizontality ¨C states layers of sediment are generally deposited

in horizontal, flat layers.

? Principle of Original Lateral Continuity ¨C states that layers of sediment are initially

deposited laterally in all directions. Therefore, in general, strata are laterally

continuous over large areas.

? Principle of Cross-Cutting Relationships ¨C states that younger geologic features cut

across older geologic features.

? Principle of Inclusions ¨C states that fragments of other rocks (inclusions) contained

within another rock must be older than the host rock.

4. Geologic Assumptions Used in Relative Dating

? Virtually all sedimentary beds (strata) are marine in origin (virtually all sediments

forming sedimentary rocks were originally deposited in the ocean).

? Eustatic (global) sea level changes have occurred numerous times through geologic

history.

5. Unconformities

? Unconformities are surfaces of erosion or nondeposition. The types of unconformities

include:

- Angular Unconformity ¨C Tilted rock layers (strata) are overlain by flat-lying rock

layers.

- Disconformity ¨C Strata on each side of the unconformity are parallel.

- Nonconformity ¨C Metamorphic or igneous rocks are overlain by sedimentary

rocks.

6. Using the Relative Dating Method to Age Date Geologic Features

? Apply law and principles described in #3 above to place geologic units and events in

their proper chronologic order.

7. Fossils

? Fossils are traces of past life that are preserved in rocks.

? Fossils are found almost exclusively in sedimentary rocks.

? Paleontology is the study of fossils.

? Fossils help geologists interpret the geologic past by 1) being time indicators and 2)

allowing correlation of rock units from one area to the next.

? Principle of Faunal Succession states that fossil organisms succeed one another in

definite and determinable order. Therefore, any time period can be recognized by the

fossils it contains.

? Index Fossils are valuable for determining the age of geologic rock formations. They

occur over a wide geographic area and represent a narrow band of time in the

geologic record.

8. Life Form Extinction Events

? The extinction event 65.5 million years ago killed off the dinosaurs.

? Many extinction events have occurred throughout geologic time.

? The largest extinction event occurred at the Permian/Triassic boundary when 90-95%

of all species died.

9. Absolute Age Dating Method

? Radioactive decay rates of elements are used to determine absolute ages of rocks.

? Parent ¨C original unstable radioactive isotope.

? Daughter ¨C stable product (element) resulting from the decay of the parent element.

? Half-life of an element is the length of time required for half of a volume of a parent

isotope to decay to its stable daughter product.

? Age dating using radioactive isotopes is called Radiometric Dating.

? Radiometric dating involves 3 types of radioactive particles:

- Alpha particle emission (loss of 4He2 atom).

- Beta particle emission (loss of an electron (e-)).

- Electron capture (gain of an electron (e-)).

? Half-life of Uranium 238 (U-238) is 4.5 billion years. The decay of this isotope is used

to age date the Earth.

- It takes 14 steps for U-238 to decay to its stable daughter product of Pb-206.

- The first 3 steps in the decay sequence of U-238 involves the loss of an Alpha

particle, then the loss of a Beta particle, and then the loss of another Beta particle.

10. Radiocarbon Dating (Carbon 14 dating method)

? Uses the decay of radioactive Carbon 14 (C-14) to stable daughter product of

Nitrogen 14 (N-14) by the process of electron capture.

? Half-life of C-14 is 5,730 years.

? The C-14 age dating method can be used to age date organic materials as old as

70,000 years.

11. Geologic Time Scale

? Geologic Time Scale divides the Earth¡¯s history into Eons, Eras, Periods, and Epochs

based on the dominant life forms which lived at that time.

? The latest eon (Phanerozoic) comprises only 11% of all of Earth¡¯s history.

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