Transcript topic #7 - geologic time

William E. Ferguson
Geologic Time
A major difference between
geologists and most other
scientists is their attitude about
time.
A "long" time may not be
important unless it is > 1 million
years.
Amount of Time
Required for
Some Geologic
Processes and
Events
Some geologic
processes can be
documented
using historical
records
(brown area is new
land from 1887-1988)
Uniformitarianism
The present is the key to the past.
— James Hutton
Natural laws do not change—
however, rates and intensity of
processes may.
Two ways to date
geologic events
1 RELATIVE DATING (relative
position of fossils, structure,
geomagnetics)
2 ABSOLUTE DATING (isotopic,
tree rings, varves, etc.)
RELATIVE GEOLOGIC TIME
Steno Laws (1669) developed to
arrange rock units in time-order
• Principle of Superposition
• Principle of Original Horizontality
• Law of Cross -Cutting Relationships
• Law of Inclusions
Laws apply to both sedimentary and
volcanic rocks.
Principle of Superposition
In a sequence of undisturbed
layered rocks, the oldest rocks
are on the bottom.
Principle of Superposition
Youngest rocks
Oldest rocks
Jim Steinberg/Photo Researchers
Principle of Original Horizontality
Layered strata are
deposited horizontal or
nearly horizontal or nearly
parallel to the Earth’s
surface.
Principles of original
horizontality and superposition
Law of Cross-Cutting
Relationships
• A rock or feature is younger
than any rock or feature it
cuts across.
Law of
Cross-cutting Relationships
LAW OF INCLUSIONS
• Included rocks are older than
surrounding rocks.
PRINCIPLE OF FAUNAL
SUCCESSION
Principle of Faunal Succession - groups of fossil
plants & animals have followed one another in
a definite & discernable order so certain fossil
assemblages characterize a specific time.
INDEX FOSSILS - fossils used to correlate a
specific time period
Based on distinct preservable parts, lived a
short time , in a specific environment with
wide distribution - MICROFOSSILS
Ammonite Fossils
Chip Clark
Petrified Wood
Tom Bean
CORRELATION
• Process used to tie separated strata together
• Based on matching physical or fossil features
such as
– Physical continuity - trace of rock unit
– Similar rock types - marker beds, coal
seams, rare minerals, odd color
– Fossils
Using Fossils to Correlate Rocks
Correlating beds using
index fossils
South rim of the Grand Canyon
Generalized
Stratigraphic
Section of Rocks
Exposed in the
Grand Canyon
after: Beus & Moral (1990)
Some of the Geologic Units
Exposed in the Grand Canyon
Michael Collier
Unconformity
A buried surface of erosion
Separates much older, eroded strata
from younger ones
Hiatus - the time gap or the time lost
in the record
Unconformitites - 3 kinds
• Disconformity - undeformed beds
• Nonconformity - sedimentary
over igneous or metamorphic rx.
• Angular Unconformity - flat
sediments overly tilted beds
Formation of a Disconformity
South rim of the Grand Canyon
250 million years old
Paleozoic Strata
550 million years old
1.7 billion years old
Precambrian
South rim of the Grand Canyon
250 million years old
550 million years old
Nonconformity
1.7 billion years old
Nonconformity in the Grand Canyon
Nonconformity in the Grand Canyon
Tapeats Sandstone
(~550 million years old)
Vishnu Schist
(~1700 million years old)
Angular unconformity, Grand Canyon
The Great Unconformity of the Grand Canyon
Geoscience Features Picture Libraryc
Formation of an
Angular Unconformity
The Geologic Time Scale
• Divisions in the worldwide
stratigraphic column based on
variations in preserved fossils
• Built using a combination of
stratigraphic relationships, crosscutting relationships, and absolute
(isotopic) ages
The
Geologic
Column
and
Time Scale
ice ages
Kaua‘i
dinosaurs out
oldest Emperor
seamount,
~80 Ma
dinosaurs in
plants, fish
animals with
skeletons
Absolute geochronology
• Adds numbers to the
stratigraphic column based on
fossils.
• Based on the regular
radioactive decay of some
chemical elements.
Isotopic dating
• Radioactive elements (parents) decay to
nonradioactive (stable) elements
(daughters).
• The rate at which this decay occurs is
constant and knowable.
• Therefore, if we know the rate of decay
and the amount present of parent and
daughter, we can calculate how long this
reaction has been proceeding.
Isotopes
Different forms of the same
element containing the same
number of protons, but varying
numbers of neutrons.
i.e.:
235U, 238U
87Sr, 86Sr
14C, 12C
Naturally Occurring
Isotopes of Carbon
Beta Decay
Electron Capture
Alpha Decay
Production and Decay
of Radiocarbon
Radioactive
Decay of
Rubidium to
Strontium
Half-life
The half-life of a radioactive
isotope is defined as the time
required for half of it to decay.
Proportion of
Parent Atoms
Remaining as a
Function of
Time
Geologically Useful Decay Schemes
Parent
235U
Daughter
207Pb
Half-life (years)
4.5 x 109
238U
206Pb
0.71 x 109
40K
40Ar
1.25 x 109
87Rb
87Sr
47 x 109
14C
14N
5730
PROBLEMS
• NEED A CLOSED SYSTEM!!!
– MINERAL MAY LEAK PARENT OR
DAUGHTER
– MINERAL MAY BE CONTAMINATED
WITH EITHER PARENT OR DAUGHTER
Another Clock
Paleomagnetism
• Earth’s magnetic field reverses every
half million years
• Reversals are recorded in rocks that are
forming at that time - seafloor
• Time scale calibrated by both relative &
absolute time methods
Earth’s
Magnetic
Field
Lavas
record
magnetic
reversals
magnetically polarized layers in a volcano