CHAPTER 2 Plate Tectonics and the Sea Floor

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Transcript CHAPTER 2 Plate Tectonics and the Sea Floor

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
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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
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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
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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.
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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
Proterozoic
Quaternary
Cretaceous
The Geologic Column
[Fig. 3.8, p. 64]
N. Lindsley-Griffin, 1999
Dating the Geologic Time Scale
Sedimentary rocks
not easily dated
radiometrically
Radiometric
ages added to
geologic time
scale by:
1. Cross-cutting
relationships
2. Bracketing
© Houghton Mifflin 1998. All rights reserved
Relative Age - Principles
Unconformities:
Gaps in the rock
record - recognized
by an erosional
surface
Signifies a major
break in deposition
Columbia River Gorge, OR-WA
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Relative Age - Principles
Original horizontality
Superposition
Cross-cutting relationships
Horizontal sedimentary rocks
Youngest
sedimentary rock
Dike cuts strata
- dike is younger
Fault cuts dike and strata
- fault is youngest
Oldest
sedimentary rock
© Houghton Mifflin 1998. All rights reserved
Dating the Geologic Time Scale
Radiometric dating and cross-cutting relationships
bracket ages of sedimentary rocks
Radiometric age of
intrusion Y is 350 m.y.
Radiometric age of
intrusion X is 400 m.y.
Devonian strata (B) must be
older than 350 m.y. and
younger than 400 m.y.
Mississippian strata are
younger than 350 m.y.
© Houghton Mifflin 1998. All rights reserved
Silurian strata are older than
both Devonian and 400 m.y.
Angular unconformity - younger layers
over tilted layers - Grand Canyon, AZ
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Angular unconformity - younger layers
over tilted layers - Grand Canyon, AZ
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Disconformity
Time gap without angular relationship
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Angular Unconformity
Time gap after folding, faulting
Disconformity
Time gap without angular relationship
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Angular Unconformity
Time gap after folding, faulting
“Baked zone” - soil formed on older lava
flow before being covered and cooked by a
new lava flow - Disconformity
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“Baked zone” - soil formed on older lava
flow before being covered and cooked by a
new lava flow - Disconformity
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Nonconformity - sedimentary rocks over
crystalline (metamorphic or igneous) rocks
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Nonconformity - sedimentary rocks over
crystalline (metamorphic or igneous) rocks
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Nonconformity - sedimentary gravels over garnet
schist. What is the history of this outcrop?
N. Lindsley-Griffin, 1998
Nonconformity - sedimentary gravels over garnet
schist. What is the history of this outcrop?
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Cross-cutting relationships:
dike cutting gneiss is younger
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Cross-cutting relationships:
dike cutting gneiss is younger
Cross cutting relationships: what is the
sequence of events in this folded marble
with two dikes?
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Cross cutting relationships: what is the
sequence of events in this folded marble
with two dikes?
N. Lindsley-Griffin, 1998
Grand Canyon Formations
Superposition
Disconformity
Original
horizontality
Angular
unconformity
Cross-cutting
relationships
Nonconformity
© Houghton Mifflin 1998. All rights reserved
Grand Canyon Formations
Sequence of events based on geology of the Inner Gorge
1.Deposition of shale in
marine environment
4. Erosion to sea level,
unconformity surface
© Houghton Mifflin 1998. All rights reserved
2. Metamorphism of shale
into Vishnu schist
5. Deposition of Grand
Canyon Supergroup
6. Faulting
3. Intrusion by
Zoroaster granite
7. Erosion of fault blocks,
deposition of Tapeats
sandstone
Geologic Time - Summary
Relative Age -- whether a particular rock or
feature is older or younger than another
Absolute Age -- the age of a rock in years
(m.y. = millions of years)
Geologic Column -- shows succession of all
known strata, in chronological order, based on
fossils and other relative ages
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