1. Geologic Time Chapter 8

2. Historical notes Catastrophism Modern geology
Landscapes developed by catastrophes
James Ussher, mid-1600s, concluded Earth was only a few thousand years old (created in 4004 B.C.)
Modern geology
Fundamental principle of geology
Uniformitarianism aka “The present is the key to the past”
James Hutton
Theory of the Earth
Published in the late 1700s

3. “Hutton’s Unconformity” on Siccar Point, Scotland, is a common destination for geologists.

4. Geologic Time There are two ways of dating geological materials.
Relative ages – Based upon order of formation.
Qualitative method developed 100s of years ago.
Permit determination of older vs. younger relationships.
Numerical ages – Actual number of years since an event.
Quantitative method developed recently.
Age is given a number.

5. Relative vs. Absolute Relative ages assign order to events.
Numerical ages
assign exact
dates to events.

6. Relative dating Law of superposition
Developed by Nicolaus Steno in 1669
In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom

7. Superposition is well-illustrated by the strata in the Grand Canyon

8. Principle of original horizontality
Layers of sediment are generally deposited in a horizontal position
Rock layers that are flat have not been disturbed
Principle of cross-cutting relationships
Younger features cut across older features

9. Cross-cutting relationships
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10. Inclusions Unconformity
An inclusion is a piece of rock that is enclosed within another rock
Rock containing the inclusion is younger
Unconformity
An unconformity is a break in the rock record produced by erosion and/or nondeposition of rock units

11. Unconformity Types of unconformities
Angular unconformity—Tilted rocks are overlain by flat-lying rocks
Disconformity—Strata on either side of the unconformity are parallel
Nonconformity—Metamorphic or igneous rocks in contact with sedimentary strata

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16. Angular Unconformity
“Hutton’s Unconformity” on Siccar Point, Scotland, is a common destination for geologists.

17. Fossils: Evidence of past life
Fossil—traces or remains of prehistoric life now preserved in rock
Fossils are generally found in sediment or sedimentary rock (rarely in metamorphic and never in igneous rock)
Paleontology—study of fossils

18. Geologically fossils are important because they
Aid in interpretation of the geologic past
Serve as important time indicators
Allow for correlation of rocks from different places
Conditions favoring preservation
Rapid burial
Possession of hard parts (skeleton, shell, etc.)

19. Types of fossils Petrified - turned into stone
Replacement – cell walls and and solid material replaced with minerals
Mold and cast
Impressions
Amber
Coprolite
Tracks
Burrows

20. Trilobite showing mold and cast preservation

21. Petrified wood from Arizona

22. Fossils and correlation
Matching of rocks of similar ages in different regions is known as correlation
Correlation often relies upon fossils
William Smith (late 1700s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content

23. Principle of fossil succession—Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content
Index fossil—Geographically widespread fossil that is limited to a short span of geologic time

24. Dating rocks using overlapping fossil ranges

25. Dating with radioactivity
Reviewing basic atomic structure
Nucleus
Protons—positively charged particles with mass
Neutrons—neutral particles with mass
Electrons—negatively charged particles that orbit the nucleus
Atomic number
Element’s identifying number
Equal to the number of protons

26. Reviewing basic atomic structure
Mass number
Sum of the number of protons and neutrons
Isotope
Variant of the same parent atom
Differs in the number of neutrons
Results in a different mass number than the parent atom

27. Types of radioactive decay
Radioactivity
Spontaneous changes (decay) in the structure of atomic nuclei
Types of radioactive decay
Alpha emission
Emission of 2 protons and 2 neutrons (an alpha particle)
Mass number is reduced by 4 and the atomic number is lowered by 2

28. Types of radioactive decay
Beta emission
An electron (beta particle) is ejected from the nucleus
Mass number remains unchanged and the atomic number increases by 1
Electron capture
An electron is captured by the nucleus and combines with a proton to form a neutron
Mass number remains unchanged and the atomic number decreases by 1

30. Parent—an unstable radioactive isotope
Daughter product—the isotopes resulting from the decay of a parent
Half-life—the time required for one-half of the radioactive nuclei in a sample to decay

31. Radiometric dating Principle of radioactive dating
The percentage of radioactive atoms that decay during one half-life is always the same (50 percent)
However, the actual number of atoms that decay continually decreases
Comparing the ratio of parent to daughter yields the age of the sample

32. Radioactive decay curve

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34. Dating with Carbon-14 (Radiocarbon dating)
Used to date very recent events (half-life of 5,730 years) and historic events
Continually produced in the upper atmosphere from cosmic-ray bombardment
All living things have carbon-14 in them
Anthropologists, archeologists, and historians use carbon-14 dating

35. Importance of radiometric dating
Sources of error
A closed system is required
To avoid potential problems, only fresh, unweathered rock samples should be used
Importance of radiometric dating
Rocks from several localities have been dated at more than 3 billion years
Confirms the idea that geologic time is immense

36. The geologic time scale
The geologic time scale—A “calendar” of Earth history
Subdivides geologic history into units
Originally created using relative dates
Structure of the geologic time scale
Eon—The greatest expanse of time
Phanerozoic (“visible life”)—The most recent eon, began about 540 million years ago
Proterozoic
Archean
Hadean—The oldest eon

37. Structure of the geologic time scale
Era—Subdivision of an eon
Eras of the Phanerozoic eon
Cenozoic (“recent life”)
Mesozoic (“middle life”)
Paleozoic (“ancient life”)
Eras are subdivided into periods
Periods are subdivided into epochs

38. The geologic time scale

40. Precambrian Nearly 4 billion years prior to the Cambrian period
Not divided into smaller time units because the events of Precambrian history are not known in great enough detail
First abundant fossil evidence does not appear until the beginning of the Cambrian

41. Difficulties in dating the geologic time scale
Not all rocks can be dated by radiometric methods
Grains comprising detrital sedimentary rocks are not the same age as the rock in which they formed
The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed
Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages

42. Bracketing sedimentary ages using igneous rocks

43. Questions?