Transcript Earth in Time The Rock Record and Geologic Time

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, 1999.
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
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
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
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
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
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
N. Lindsley-Griffin, 1999
Dinosaur tracks
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
Absolute Age - Principles
Radioactive Decay - Release of particles from nucleus
ALPHA EMISSION:
Two protons + two neutrons
Mass - 4 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, 1999. All rights reserved
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.
N. Lindsley-Griffin, 1999
Figure 3.12, p. 70
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 1998. All rights reserved