AGE DATING THE
EARTH
Geologic Techniques and
The Geologic Time Scale
INTRODUCTION
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Speculations about the ‘nature’ of the Earth as well as the Age of the
Earth inspired much of the lore and legend of early civilizations.
In the 3rd century B.C., Eratosthenes depicted a spherical Earth and
even calculated its diameter and circumference
– The concept of a spherical Earth was beyond the imagination of
most men.
– Only 500 years ago, sailors aboard the Santa Maria begged
Columbus to turn back lest they sail off the Earth's "edge."
Herodotus, ancient historian, made one of the earliest recorded
geological observations in the 5th century B.C
– Fossil shells found inland in Egypt and Libya
• He suggested Mediterranean Sea had once extended farther
south
– Few believed him, however, and this idea did not catch on.
Similar opinions and prejudices about the nature and age of the Earth
have waxed and waned through the centuries
Even today - traditional beliefs among certain religious groups
suggest the Earth is quite young--that its age might be measured in
terms of thousands of years, but certainly not in millions.
• Scientists have established the age of the Earth as
4.54 billion years old.
HOW DO GEOLOGISTS KNOW
THE AGE OF THE EARTH?
• The evidence to age-date the Earth is
concealed in the rocks that form the
Earth's crust and surface.
• The rocks are not all the same age -- or
even nearly so -- but, like the pages in a
long and complicated history, they
record the Earth-shaping events and
life of the past.
WHAT DO GEOLOGISTS USE?
• Two time-measuring systems are used
to date past Earth-shaping events and
to measure the age of the Earth
– A ‘relative time’ measuring system
• based on the sequence of layering of the rocks
and the evolution of life as ‘recorded’ in the
rocks
– A ‘radiometric time’ (“Absolute”)
measuring system
• based on the natural radioactivity of chemical
elements in some of the rocks
RELATIVE DATING: NICOLA STENO’S
LAWS and STRATIGRAPHY
Principle of Original Horizontality
- Bedrock layers formed from
sedimentary material are deposited in a
horizontal position, and any deviations
from this position are due to the rocks
being disturbed later.
• Law of Superposition
– Wherever uncontorted layers are
exposed, the bottom layer was
deposited first and is the oldest layer
exposed
– Each succeeding layer, up to the
topmost one, is progressively
younger.
– In any rock layer, each layer
represents a specific interval of
geologic time.
Nicola Steno – 1638 - 1686
THE BEGINNING – JAMES HUTTON
“FATHER” OF MODERN GEOLOGY
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In the late 18th century, James
Hutton, a Scottish geologist,
proposed a fundamental
principle in Geology:
• Uniformitarianism
– A theory that natural agents now
at work on and within the Earth
have operated with general
uniformity through immensely
long periods of time
– Picture at right was Hutton’s first
clue as to age of material,
assuming that materials on
bottom were originally deposited
horizontally and then uplifted
much later
Charles Lyell – “Principles of
Geology”
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Charles Lyell, in the 1830s, followed up on
James Hutton’s work.
Lyell viewed the history of the Earth as vast and
directionless.
Changes to the Earth’s surface were gradual and
over time, great changes could be effected: the
Earth was vastly old.
Lyell worked from the theory of
“Uniformitarianism” not “Catastrophism”
– Lyell found evidence that valleys were formed
through the slow process of erosion, not by
catastrophic floods.
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Darwin used Lyell’s “Principles of Geology” to
decipher the volcanic rocks on the Canary
Islands
THE BEGINNING – WILLIAM
‘STRATA’ SMITH
• GEOLOGIC TIME SCALE – RELATIVE TIME
• William "Strata" Smith, a civil engineer and
surveyor, collected and cataloged fossil
shells from rocks
• Discovered that certain layers contained
fossils unlike those in other layers – Index
Fossils
– Index Fossils existed in limited periods of
geologic time, but were widespread
geographically
– They can be used as guides to age of rocks
PUTTING IT ALL TOGETHER
• RELATIVE TIME SCALE
• Studying origin of rocks (petrology),
combined with studies of rock layers
(stratigraphy) and studies of the evolution of
life (paleontology), allow geologists to
reconstruct the Earth using four basic
principles.
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Original Horizontality
Superposition
Lateral continuity
Cross-cutting relationships
GEOLOGIC TIME AND DATING – 4 BASIC
PRINCIPLES OF RELATIVE DATING
(1) Principle of Original Horizontality
Beds of sediment deposited in water form as horizontal
or nearly horizontal layers.
(2) Principle of Superposition
Within a sequence of undisturbed sedimentary or
volcanic rocks, the layers get younger going from
bottom to top.
(3) Lateral Continuity
An original sedimentary layer extends laterally until it
tapers or thins at its edges
(4) Cross-cutting Relationships
A disrupted pattern is older than the cause of the disruption.
PRINCIPLE OF ORIGINAL HORZONTALITY
and SUPERPOSITION
Principles of Dating
RELATIVE DATING - How it works
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Physical Continuity
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Similarity of Rock Types
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Physically tracing the course of a rock unit to correlate rocks between
two different places
Correlation of two regions by assumption that similar rock types in two
regions formed at same time, under same circumstances
Correlation by Fossils
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Plants and animals that lived at the time rock formed were
buried by sediment
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If there are fossil remains preserved in the layers of sedimentary
rock: fossils nearer the bottom (in older rock) are more unlike
those near the top (in younger rock)
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Observations formalized into Principle of Faunal Succession –
fossil species succeed one another in a definite and
recognizable order.
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Index Fossil – a fossil from a short-lived, geographically
widespread species known to exist during a specific period of
geologic time.
CORRELATION OF ROCK UNITS
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Each column represents the
sequence of sedimentary
beds at a specific locality
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The same beds are
bracketed within the lines
connecting the three
columns.
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Adjoining beds that possess
similar or related features
(including fossils) are
grouped into a single, more
conspicuous unit called a
formation
Faunal Succession
INDEX FOSSIL CHART
Relative Dating:
“Reading” the Sedimentary Layers
• When ‘reading’ the sedimentary layers, geologists
not only look at the layers and the fossils within
them; geologists also look for an Unconformity:
– A surface representing a ‘gap’ in the geologic record
• Types of Unconformities:
– Disconformity – parallel strata missing a layer
– Angular Unconformity – younger horizontal layer overlying
a folded or tilted layer
– Nonconformity – a plutonic/metamorphic rock layer covered
by younger sedimentary or volcanic rock
Formation of An Unconformity
Disconformity
-unconformity
Discnformity
Angular
unconformity
Angular Unconformity
Examples of angular
unconformities
DISCONFORMITY
• The upper 2/3 of the
cliff is Redwall
Limestone, whereas the
lower part is Cambrian
carbonate rock
• The age difference
between these units is
roughly 150 Ma.
• The contact between
the two rock units
represents a significant
span of geologic time
and is termed a
disconformity
Disconformity
NON CONFORMITY
Stratified rocks upon
unstratified rocks
(sedimentary rocks
overlying metamorphic
or plutonic rocks).
DATING – RADIOMETRIC OR “ABSOLUTE”
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Radiometric dating –“Absolute Dating” - based on radioactive
decay of ‘isotopes’
–An isotope is a form of an element containing different atomic
mass (Carbon-12 vs Carbon-14, for example)
• Same number of protons, but different number of neutrons
• Most isotopes are radio-active
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Radioactive decay:
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any number of processes by which unstable isotopes emit
radioactive particles and eventually become stable elements
Decay rate can be quantified because it occurs at a constant rate
for each known isotope and is measured in ‘half-life’
– ‘Half-life’ is the time required for a quantity of radio-active material to
decay to half of its initial value
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“Parent” (unstable radioactive) isotope to “daughter” (stable, non-radioactive) isotope
– The half-lives of isotopes have all been measured directly
• Using a radiation detector to count the number of atoms decaying in a
given amount of time from a known amount of the parent material
• Measuring the ratio of daughter to parent atoms in a sample that
originally consisted completely of parent atoms
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Measuring ratio of parent to daughter isotopes determines
absolute ages of some rocks.
RADIOMETRIC DATING
• The decay of Radio-active atoms compares to sand
grains falling in an hourglass.
• You cannot predict when the individual sand grain
will fall, but you can predict from one time to the
next how long the whole pile of sand takes to fall to
the bottom.
• Similarly, you can predict how long it takes for all the
radio-active atoms in a given amount of rock to
decay to a non-radioactive form.
RADIOMETRIC DATING
In exponential decay the amount of material
decreases by half during each half-life ; rapidly
at first, then slowly with each succeeding
half-life.
The daughter element or isotope amount
increases rapidly at first and more slowly with
each succeeding half life
ABSOLUTE DATING ISOTOPES
• URANIUM–LEAD (U238→Pb206)
– Half-life: 4.5 billion years
– Dating range: 10 million – 4.6 billion years
• URANIUM–LEAD (U235 →Pb207)
– Half-life: 713 million years
– Dating Range: 10 million – 4.6 billion years
• POTASSIUM-ARGON (K40→Ar40)
– Half-life: 1.3 billion years
– Dating Range: 100,000 – 4.6 billion years
• CARBON-14 (C14→N14)
– Half-life: 5730 years
– Dating Range: 100 – 40,000 years
Absolute Dating – Half-life
Uranium half-life
Radio-carbon half-life
Radio Carbon – Carbon 14
• All living organisms absorb radiocarbon, an
unstable form of carbon.
• After death and fossilization, C14 continues
to decay without being replaced (half-life of
about 5,730 years).
• Radiation counters are used to detect the
electrons given off by decaying C14 as it
turns into nitrogen (N14).
• Remaining amount of C14 is compared to the
amount of C12, the stable form of carbon, to
determine how much radiocarbon has
decayed to date the fossil.
Radiocarbon Dating
Relative and Radiometric Dating
Using Relative
and
Radiometric
Dating together
gives the most
accurate timescale for
geologic time
Absolute Dating Dendrochronology
• Annual growth of tree
rings
– Dating back 11,500
years – Holocene
Epoch
Principles of Dendrochronology
• The dating of past
events (climatic
changes) through
study of tree ring
growth
• A chronology
(arrangement of
events in time) can
be made by
comparing different
samples
Cross-dating in Dendrochronology
• Process of matching rings of trees in
an area based on patterns of ring
widths produced by regional climate.
• More accurate age than ring counting
Cross-dating techniques
Absolute Dating -Varve Chronology
• Parallel strata deposited in deep
oceans or lakes
• Varves are a pair of sedimentary layers
deposited on seasonal cycles
– Winter/summer
• Date back to over 200 million years
Varves
Geologic Time Scale
• Fossils in rock used to age date rocks
• Time scale consists of periods of time broken into
smaller and smaller units: eons (100s of millions of
years), eras, periods, epochs (millions to thousands
of years)
– Eons, eras, periods and epochs are listed with
oldest at the bottom of the scale and youngest at
the top
• Names of eras, periods and epochs based on global
location
– PreCambrian: from rocks near Wales “Cambria”
– Jurassic: from limestone found in Jura
Mountains, France
ERAS IN GEOLOGIC TIME SCALE
• Paleozoic Era – appearance of complex
life (oldest time)
– Approximately 600 million years ago to 250
million years ago)
• Mesozoic Era – Age of Dinosaurs
– Approximately 250 million years ago to 65
million years ago)
• Cenozoic Era – Age of Mammals
– Approximately 65 million years ago to
present
TERTIARY
Red Arrows point to mass extinction
dates
The Great Permian Extinction
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At the end of many large
time units on the Geologic
Time Scale, mass
extinctions took place.
The end of the Permian
approximately 250 million
years ago (also the end of
the Paleozoic era), was
marked by the greatest
extinction of the
Phanerozoic eon.
During the Permian
extinction event, whose
cause(s) remain
controversial, over 95% of
marine species became
extinct, while 70% of
terrestrial taxonomic
families suffered the same
fate of extinction!
PERMIAN EXTINCTION – CAUSES?
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Climate change, possibly caused by glaciation and/or volcanic activity, has
been associated with many mass extinctions. It seems likely that climate
change is a consequence of the cause of extinction rather than the root cause
itself.
– The Siberian Traps triggered a massive, sudden glaciation as well as other
environmental consequences of volcanic eruptions.
– The opening of the Atlantic Ocean basin as the result of sustained volcanic
eruptions (the Central Atlantic Magmatic Province) led to the release of
toxic fumes, greenhouse gases, and ultimately, global climate change –
perhaps triggering an ice age
– Formation of Pangaea has been invoked as a cause for the extinction.
• Pangaea's presence may have led to extreme environments with hotter
interior areas of the continent and colder polar areas, possibly
producing glaciation.
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Poisoning of the ocean has been suggested due to an apparent drop in carbon
isotope data obtained from marine sediments formed at the time of the
extinction.
– The cause of this apparent drop off in the photosynthetic rate in the seas
has not yet been determined
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Extraterrestrial Objects:
– Evidence of a large impact at the close of the Permian is not strongly
supported, although some indirect evidence suggests an impact did occur
during the Permian, although possibly not at the time of the extinction
crisis.
CRETACEOUS-TERTIARY EXTINCTION
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Impact Theory:
– 1980: L.W. Alvarez and
colleagues published a paper
proposing that, approximately
65 million years ago, the earth
was struck by an asteroidsized object on Yucatan
Peninsula – Chicxulub, Mexico
– Evidence:
• Boundary Clay with high
levels of iridium very rare
in terrestrial rocks, but are
much more common in
extraterrestrial rock
samples
• Microtektites: hollow,
microscopic, glass-like
spheres that form when a
violent explosive event
occurs in association with
molten rock
• "Shocked" quartz grains,
where the regular,
crystalline structure has
been distorted by the
application of large forces
ALTERNATE THEORY
• VOLCANISM:
• The eruption of the Deccan Traps
approximately 65-64 million years ago
is the largest volcanic event since the
Permian-Triassic event at 245 Ma
• The impact at Chicxulub, Mexico
predates Dinosaur extinction by
300,000 years.
• Selective extinction: only dinosaurs
EARTH STRUCTURES
• The Earth is composed of four layers:
– Inner Core
– Outer Core
– Mantle
– Crust
EVIDENCE OF EARTH’S
INTERIOR LAYERS
• DIRECT EVIDENCE OF LAYERING
– Mantle rock brought up to surface through volcanism
– Intrusion and erosion of diamond-bearing kimberlite pipes
– Lower layers of oceanic lithosphere brought to surface at
subduction zones
• Ophiolite Suite
• INDIRECT EVIDENCE OF LAYERING
– Seismic reflection: return of energy from seismic waves
‘bouncing’ off rock boundaries.
• Similar to light off a mirror, rock boundaries of differing
densities set up a reflection of seismic waves
– Seismic refraction: bending of seismic waves as they pass
through rock layers of differing densities
• Seismic waves change speed or direction passing through
different rock boundaries
SEISMIC EVIDENCE
INNER AND OUTER CORES
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INNER AND OUTER CORE – represent
approximately 31% of Earth’s mass
– Inner core
• The inner core is under such
extreme pressure that it remains
solid
– Composed mostly of iron (Fe)
and Nickel (Ni)
– Temperatures over 7,0000 F
(4,3000 C)
– ~1200 Km radius
– Outer core
• The outer core is under less
pressure and is molten
– Composed mostly of iron (Fe)
and Nickel (Ni)
– Temperatures of 6,700-7,7000F
(3,700 – 4,3000C)
– ~4000 Km across
MANTLE AND CRUST
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MANTLE– represents 68% of Earth’s
mass
• At over 1000 degrees C, the
mantle is solid but can deform
slowly in a plastic manner
– Composed of iron (Fe),
magnesium (Mg), aluminum
(Al), silicon (Si), and oxygen
(O)
– ~2900 km thick
– LITHOSPHERE
• Upper mantle more
rigid, bonded to Crust:
• 100 km thick
– ASTHENOSPHERE
• Mantle below
Lithosphere more
plastic, weaker and
more molten:
• 100-200 km thick
CRUST – represents 1% of Earth’s
mass
• The crust thinnest of the layers: 570 km thick
– Continental crust: Granitic
– Oceanic crust: Basaltic
EARTH’S CRUST
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The Earth’s crust is composed of almost all of the basic elements.
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Listed below (in order of abundance) are the eight (8) basic elements that
compose approximately 99% of the crust:
– Oxygen (O)
– Calcium (Ca)
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Silicon (Si)
Potassium (K)
Aluminum (Al)
Sodium (Na)
Continental Crust is composed mainly of a “granitic” rock type
– High silicon content
– Lower density (2.7 grams per cubic centimeter)
– Thicker (20-70 km thick)
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Iron (Fe)
Magnesium (Mg)
Oceanic Crust is composed mainly of a “basaltic” rock type
– Low silicon content
– Higher density (3.0 grams per cubic centimeter)
– Thinner (5-10 km thick)
TECTONIC PLATES
• Earth’s Crust is
broken into large
moving slabs:
Tectonic Plates
• Interaction between
plates drives
mountain building:
– Volcanic mountains
– Folded mountains
– Faulted mountains