Dynamic Earth
Class 2
12 January 2006
Any Questions?
Uniformitarianism
The present is the key to the past
— James Hutton
Geologic processes that we see in
operation today have worked
much the same way over geologic
time — however, rates and
intensity of processes may have
changed.
Early estimates of Earth’s age

1650: James Ussher -- Age: 6000 years
(From reconstruction of the Bible)

By the mid-19th Century, the age of the
Earth was still only an educated guess,
but the general feeling was that the
Earth was at least 500 million years old,
and probably much older.
Radiometric Dating
Use of radioactive decay to determine
the age of a rock
 First proposed ~1896-1902

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
Production and Decay
of Radiocarbon
Beta Decay
Electron Capture
Alpha Decay
Radioactive
Decay of
Rubidium to
Strontium
Radiometric Dating

Key principle: Half Life = time required for
1/2 of the nuclei in a sample to decay
Radioactivity and Absolute Time
Decay of
parent atoms
Growth of
daughter atoms
Radioactive
Decay of
Uranium 238
to
Lead 206
Isotopic dating

Radioactive elements (parents) decay to
nonradioactive (stable) elements
(daughters)

The rate at which this decay occurs is
constant and known

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
Major Radioactive Elements Used
in Isotopic Dating
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
Radiometric Dating
Shows that the earth is much older than
people had previously suspected
 Earth formed about 4.6 billion years
ago

Relative vs Absolute Age
Usually geologists first establish
relative ages then try to get absolute
age dates
 Determining relative age relies on a
number of geologic principles that
were developed during the 17th to
early 19th centuries

Principle of Superposition

In a sequence of undisturbed layered
rocks, the oldest rocks are on the
bottom
Principle of Superposition
Sedimentary rocks are deposited in a
layer-cake fashion:
Layer 4
Layer 3
Layer 2
Layer 1
Each layer is older than the one
above and younger than the one
below
Youngest rocks
Oldest rocks
Principle of Superposition
Principle of Superposition
Although this is really obvious, it was
not stated until 1669
 This principle generally applies to
volcanic rocks as well as sedimentary
rocks

Principle of Original Horizontality

Layers of sediment are deposited in a
horizontal or nearly horizontal
position parallel to the Earth’s surface
Principles of original
horizontality and superposition
Principle of Original Horizontality

Note that original horizontality is not
strictly applicable to volcanic rocks
because they are often deposited on
slopes
Principle of Cross-cutting
Relationships

Something (such as a dike or fault)
that cuts across a layer must be
younger than the layer
Layers of rock are said to be
conformable when they are
found to have been deposited
essentially without
interruption
Unconformity
Results from interruption of deposition
 Represents a long period of time during
which there either was no deposition, or
earlier deposited material was eroded
away

Sedimentation of Beds A-D
Beneath the Sea
Uplift and Exposure of D to
Erosion
Continued Erosion Removes
D and Exposes C to Erosion
Subsidence and
Sedimentation of E over C
Unconformity:
a buried surface of erosion
Formation of a Unconformity
Unconformity
Often not easy to recognize if the
layers are all parallel
 Much easier to recognize when there is
a period of folding of the rocks before a
period of erosion and renewed
deposition

South rim of the Grand Canyon
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
Unconformity
1.7 billion years old
The Great Unconformity of the Grand Canyon
Siccar Point, Scotland: Hutton’s Classic
Unconformity -- Old Red Sandstone (~345 my)
overlies rocks that are ~425 million years old
Siccar Point, Scotland
Buried and tilted erosional surface
Summary of Geologic Events
in a small area
Relative Geologic Dating



These methods work well in small areas
where we can see the relationships
between rock layers.
What happens when we want to tell the
relative ages of the strata on Oahu with
respect to strata on Maui?
We have to figure out some way to
correlate the layers of interest.
Correlation

Process used to tie separated strata together

Based on matching physical features such as
 Physical continuity - trace of rock unit
 Similar rock types - marker beds, coal
seams, rare minerals, odd color
Correlation
Within sedimentary layers there are
often the remains of small animals
(fossils)
 Fossils are quite useful for correlating
between two sections that are not
laterally continuous

Principle of Lateral Continuity
Layered rocks are deposited in
continuous contact
Correlation

Fossils have evolved through time, so
when we find a fossil of the same type in
two different areas, we are pretty sure
that the rocks are about the same age
Correlation

This technique is not very useful in
Hawaii -- Why?
 Fossils helpful in sedimentary rocks,
but usually no fossils in volcanic rocks
 Sometimes small amounts of sediment
between layers of volcanic rock (such
layers might have fossils), but most
rocks in Hawaii do not have fossils
Radiometric
dates provide
absolute ages
to the
Geologic
Column
Bracketing ages
Magnetostratigraphy
Technique that works best in volcanic
rocks
 Time scale based on polarity reversal of
Earth's magnetic field
 Major problem is that Earth's magnetic
field has been constant for the past
700,000 yrs (no reversals), so this
doesn't work for very young rocks

Earth’s
Magnetic
Field
Magnetization of
Magnetite
Lavas
record
magnetic
reversals
Magnetic
reversals
over the past
20 million
years
Magnetic time scale
0-700,000 -- Normal
 700,000 - 2.5 my -- Reversed
 > 2.5 my -- Normal
 Ko`olau lavas mostly reversed in
polarity, so they must be older than
700,000 yrs, but younger than 2.5 my
 Lavas on Kaua`i and in Wai`anae
Range show normal polarity, so they
must be older than 2.5 my

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
Tuesday
Geologic Time Scale (continued)
 Homework #1 is due next Thursday,
January 19
