Earth
Science, 10th edition
Chapter
10: Geologic Time
I. Historical notes
A. Catastrophism
1. Landscape developed by
catastrophes
2. James Ussher,
mid-1600s, concluded Earth was only a few thousand years old
B. Modern geology
1. Uniformitarianism
a. Fundamental principle of
geology
b. "The present is the
key to the past"
2. James Hutton
a. Theory of the Earth
b. Late 1700s
II. Relative dating
A. Placing rocks and events in sequence
B. Principles and rules of
1. Law of superposition – oldest
rocks are on the bottom
2. Principle of original
horizontality – sediment is deposited horizontally
3. Principle of cross-cutting
relationships – younger feature cuts through an older feature
4. Inclusions – one rock contained
with in another – rock containing the inclusions is younger
5. Unconformities
a. An unconformity is a break
in the rock record
III. Correlation of rock layers
A. Matching rocks of similar age in
different regions
B. Often relies upon fossils
IV. Fossils: evidence of past life
A. Remains or traces of prehistoric life
C. Conditions favoring preservation
1. Rapid burial
2. Possession of hard parts
D. Fossils and
correlation
1. Principle of
fossil succession
a. Fossils
succeed one another in a definite and determinable order
b. Proposed
by William Smith – late 1700s and early 1800s
2. Index fossils
a.
Widespread geographically
b. Existed
for a short range of geologic time
V. Radioactivity and
radiometric dating
A. Atomic structure
reviewed
1. Nucleus
a. Protons –
positively charged
b. Neutrons
1.
Neutral charge
2.
Protons and electrons combined
2. Orbiting the
nucleus are electrons – negative electrical charges
3. Atomic number
a. An
element's identifying number
b. Number of
protons in the atom's nucleus
4. Mass number
a. Number of
protons plus (added to) the number of neutrons in an atom's nucleus
b. Isotope
1.
Variant of the same parent atom
2.
Different number of neutrons
3.
Different mass number than the parent atom
B. Radioactivity
1. Spontaneous
breaking apart (decay) of atomic nuclei
2. Radioactive decay
a. Parent –
an unstable isotope
b. Daughter
products – isotopes formed from the decay of a parent
c. Types of
radioactive decay
1. Alpha
emission
2. Beta
emission
3.
Electron capture
C. Radiometric dating
1. Half-life –
the time for one-half of the radioactive nuclei to decay
2. Requires a
closed system
3. Cross-checks
are used for accuracy
4. Complex
procedure
5. Yields
numerical dates
D. Carbon-14 dating
1. Half-life of
only 5730 years
2. Used to date
very recent events
3. Carbon-14
produced in upper atmosphere
a.
Incorporated into carbon dioxide
b. Absorbed
by living matter
4. Useful tool
for anthropologists, archeologists, historians, and geologists who study
very recent Earth
history
E. Importance of
radiometric dating
1. Radiometric
dating is a complex procedure that requires precise measurement
2. Rocks from
several localities have been dated at more than 3 billion years
3. Confirms the idea that geologic time is immense
VI. Geologic time scale
A. Divides geologic
history into units
B. Originally created
using relative dates
C. Subdivisions
1. Eon
a. Greatest
expanse of time
b. Four eons
1. Phanerozoic ("visible life") – the most recent
eon
2. Proterozoic
3. Archean
4.
Hadean – the oldest eon
2. Era
a.
Subdivision of an eon
b. Eras of
the Phanerozoic eon
1.
Cenozoic ("recent life")
2.
Mesozoic ("middle life")
3.
Paleozoic ("ancient life")
3. Eras are
subdivided into periods
4. Periods are subdivided into
epochs
D. Difficulties in dating the time scale
1. Not all rocks are datable
(sedimentary ages are rarely reliable)
2. Materials are often used to
bracket events and arrive at ages