1. Earth in Time The Rock Record and Geologic Time
GEOLOGY TODAY
Barbara W. Murck
Brian J. Skinner
Chapter 3
Maroon Bells, Colorado
N. Lindsley-Griffin,

2. Relative Age - Principles
Original horizontality:
Sediments are deposited on a horizontal surface
Sedimentary rocks form in horizontal layers
Sedimentary rocks not horizontal were disturbed after they formed
Folded sedimentary strata, Crete
N. Lindsley-Griffin, 1999

3. Relative Age - Principles
Stratigraphic superposition:
Each layer of sedimentary rocks is deposited over the previous layer
Lower layers are always older than upper layers
Sandstone, limestone, and shale strata, Grand Canyon N.P., AZ
N. Lindsley-Griffin, 1999

4. Relative Age - Principles
Cross-cutting relationships:
A rock unit is always older than any feature which cuts across or disrupts it.
Fractures cutting sandstone layers, Merseyside, UK
N. Lindsley-Griffin, 1999

5. Relative Age - Principles
Cross-cutting relationships:
A rock unit is always older than any feature which cuts across or disrupts it.
Fractures cutting sandstone layers, Merseyside, UK
N. Lindsley-Griffin, 1999

6. Relative Age - Principles
Correlation:
Lateral continuity --
Strata can be traced from one location to another
Physical similarity --
Same characteristics
= same strata
Similar sequence of strata in two sections
= same sequence
Coal and sandstone strata, Badlands N.P., SD
N. Lindsley-Griffin, 1999

7. Relative Age - Principles
Faunal succession:
Each formation contains a unique fossil assemblage
Assemblages succeed one another in orderly, predictable sequence
Same everywhere in the world
In general, simpler organisms precede more complex ones in the same group
Dinosaur tracks
N. Lindsley-Griffin, 1999

8. Relative Age - Principles
Faunal succession used for correlation:
Look for same fossils in same sequence of similar strata.
Fig. 3.6, p. 63
N. Lindsley-Griffin, 1999

9. Absolute Age - Principles
Radioactive Decay - Release of particles from nucleus
ALPHA EMISSION:
Two protons + two neutrons
Mass Number - 2
BETA EMISSION:
Neutron decays to a proton
and gives off an electron
Mass - no change Number + 1
BETA CAPTURE:
Proton captures electron and
becomes a neutron
Mass - no change Number - 1
© Houghton Mifflin 1998; N. Lindsley-Griffin, All rights reserved

10. Absolute Age - Principles
Half-life:
Time needed for the number of parent atoms to be reduced by one-half
At time zero, 100% P.A.
After one half-life, 50% P.A. and 50% D.A.
After two half-lives, 25% P.A. and 75% D.A.
Figure 3.12, p. 70
N. Lindsley-Griffin, 1999

11. Carbon-14 Dating Neutrons in atmosphere change
nitrogen-14 to carbon-14
C-14 incorporated into tissue of
living organisms
Ratio of C-14 to other C-isotopes
remains constant in living tissue
At death, C-14 not replenished,
ratio of C-14 to other C-isotopes
decreases
Amount of C-14 remaining
determines time since death
of organism
© Houghton Mifflin All rights reserved