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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 at that time.
– 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.
AGE OF THE EARTH
• 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 book,
rocks 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
– (1) RELATIVE DATING: A ‘relative time’
measuring system
• based on the sequence of layering of the rocks
and the evolution of life as ‘recorded’ in the
rocks
– (2) ABSOLUTE DATING: A ‘radiometric
time’ measuring system
• based on the natural radioactivity of chemical
elements in some of the rocks
RELATIVE DATING: NICOLA STENO’S LAWS
and STRATIGRAPHY (BEDROCK LAYERS)
Principle of Original Horizontality
- Bedrock layers formed from sedimentary
material are deposited in a horizontal
position under gravity
- Any deviations from this position are due
to outside forces disturbing the rocks later.
• Law of Superposition
– Wherever uncontorted (no faults, no
folds) layers are exposed at the surface,
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
• In the late 18th century,
James Hutton, a Scottish
geologist, proposed a
fundamental principle in
Geology:
• Uniformitarianism
– A theory that natural agents
(wind, rain, etc.) at work on and
within the Earth have operated
with general uniformity through
immensely long periods of time
– Uniformitarianism went directly
against “Catastrophism”:
• Prevailing thought that all
Earth’s features were formed
by catastrophic events
Picture at right may
have been Hutton’s first
clue as to age of
material with later
disruption, assuming
that materials on bottom
were originally
deposited horizontally
and then uplifted much
later
Charles Lyell – “Principles of
Geology”
• Charles Lyell, in the 1830s, followed up on
James Hutton’s work.
• Lyell viewed the history of the Earth as
vast and directionless.
• 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.
– Changes to the Earth’s surface were gradual
and over time, great changes could be effected:
the Earth was vastly old.
• “The present is the key to the past’
• Darwin used Lyell’s “Principles of
Geology” to decipher the volcanic rocks
on the Canary Islands
WILLIAM ‘STRATA’ SMITH – Fossil Record
and Relative Time Scale
• William "Strata" Smith, a civil engineer and surveyor
in the early 19th century, 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
• Age-dating the fossils also provides an agedate for the rock layer in which they were
found.
PUTTING IT ALL TOGETHER
• DEVELOPING A 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 deposited sediment form as horizontal or nearly
horizontal layers.
(2) Principle of Superposition
Within a sequence of undisturbed sedimentary or
volcanic rocks, the layers become 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 HORIZONTALITY
and SUPERPOSITION
Principles of Dating
RELATIVE DATING - How it works
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Correlation by Physical Continuity
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Correlation by 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
• Each column represents the
sequence of sedimentary
beds at a specific locality
• The same beds are
bracketed within the lines
connecting the three
columns.
• Adjoining beds that possess
similar or related features
(including fossils) are
grouped into a single, more
conspicuous unit called a
formation
FAUNAL SUCCESSION
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 (‘what’s there”) - geologists also look for an
Unconformity (“what’s missing”):
– An Unconformity is 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
Discnformity
Angular
unconformity
Angular Unconformity
Examples of angular
unconformities
DISCONFORMITY
Disconformity, Death Valley, California. Disconformities are
unconformities in which the younger material is roughly parallel to
the contact.
This photograph shows the rocks being parallel on the left side.
However, on the right it shows the gravel cutting down into
the marble to indicate erosion.
Photo is approximately 1 meter across.
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
NON CONFORMITY
Stratified rocks upon
unstratified rocks
(sedimentary rocks
overlying metamorphic
or plutonic rocks).
ABSOLUTE DATING – RADIOMETRIC
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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 in
‘nucleus’’
• Most isotopes are radio-active and unstable
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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
Radioactive 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
• Unstable radioactive (“Parent”) isotope → stable, non-radioactive
(“daughter” ) 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 ‘parent-to-daughter’ 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 Parent
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
(C14), 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
remaining 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 – non-Radiometric
- 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
THREE MAJOR ERAS IN
GEOLOGIC TIME SCALE
• (1) Paleozoic Era – appearance of
complex life
– Approximately 600 million years ago to 250
million years ago)
• (2) Mesozoic Era – Age of Dinosaurs
– Approximately 250 million years ago to 65
million years ago)
• (3) Cenozoic Era – Age of Mammals
– Approximately 65 million years ago to
present
TERTIARY
Red Arrows point to mass extinction
dates
The Great Permian Extinction
• At the end of many large
time units on the Geologic
Time Scale, mass
extinctions took place.
– Index Fossils used for
dating, remember
• 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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(1) 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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(2) 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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(3) 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
• 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
The disappearance of
‘dinosaurs’ from the fossil
record ~ 65 million years ago
– A very rare mineral in
terrestrial rocks
– 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
ma
Shocked Quartz
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To visualize this type of deformation,
imagine a perfectly vertically stacked
deck of playing cards.
Now slant the stack by pushing the
upper part of the deck a little to the
side. This is a rough analogy of what
happens when quartz goes through a
lattice offset.
As the compression wave from the
blast passes through the sand grains,
planes of atoms in the quartz get
"shifted" slightly to the side relative to
adjacent planes of atoms.
These latice offsets create zones of
optical interference in the sand grain
which, under a microscope, show up
as two or more groups of dark lines
that intersect each other
Microtektites
How do they form?
1) A comet or asteroid impacts the Earth,
probably at an oblique angle.
2) Terrestrial (Earth) rock is melted and
ejected into the upper atmosphere at
hyper-velocities.
3) Tektites rain down.
Iridium – Boundary Layer Clay
• The asteroid hit a geologically
unique, sulfur-rich region of the
Yucatan Peninsula and kicked
up billions of tons of sulfur and
other materials into the
atmosphere.
• Darkness prevailed for about
half a year after the collision.
• This caused global
temperatures to plunge near
freezing
Chicxulub Crater, Yucatan Peninsula
ALTERNATE THEORY FOR
CRETACEOUS-TERTIARY EXTINCTION
• VOLCANISM:
• CAMP – Central Atlantic Magmatic Province
– Massive basaltic eruptions that broke up the supercontinent Pangaea and opened up the Atlantic Ocean Basin.
• The eruption of the Deccan Traps approximately 6564 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
Central Atlantic Magmatic Province
• Volcanic events flooded the center of the
former supercontinent of Pangaea with
molten rock
• The area—which today stretches around
the Atlantic, across parts of Canada, the
eastern US, Europe, South America and
Africa—is referred to as the CAMP, or
Central Atlantic Magmatic Province.
CENTRAL ATLANTIC
MAGMATIC PROVINCE
Deccan Volcanic Province in
India
• The Deccan volcanic
province in India today
covers an area the size of
France or Texas.
• The original size is
estimated twice this size,
but was reduced by
erosion.
• Arrows show the direction
of the largest lava flows
1500 km across India and
into the Gulf of Bengal.
EARTH STRUCTURES
• The Earth is composed of four major
layers:
– Inner Core
– Outer Core
– Mantle
– Crust
EVIDENCE OF EARTH’S 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: a sequence of rocks that appears to
represent a section through oceanic crust
• 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
DIRECT EVIDENCE:VOLCANOES
Magma ejected by a
volcano may have its origin
in the Mantle layer of the
Earth
Mantle minerals include
pyroxenes, amphiboles,
biotites and plagioclase
KIMBERLITE PIPE DIAGRAM
The complex volcanic magmas
that solidify into kimberlite and
lamproite are not the source of
diamonds, only the ‘elevators’ that
bring them with other minerals and
mantle rocks to Earth's surface
DIRECT EVIDENCE:
OPHIOLITE SUITE (SEA FLOOR)
The sequence of ‘layers’ of rock found
on all ocean floors.
An idealized ophiolite sequence
shows an upper layer consisting of
deep sea sediments (limestones,
cherts, and shales), overlying a layer
of pillow basalts.
Pillow basalts overly the sheeted
dikes of basalt material.
Beneath the sheeted dike complex
are gabbros that likely represent the
magma chambers for the basalts.
The marine sediments are typically
from animal and terrestrial material
settling at the bottom of the ocean
The Pillow lavas, dykes, gabbros and
peridotite are typical mantle materials
INDIRECT EVIDENCE: SEISMIC
WAVES
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CORE:
INNER
AND
OUTER
INNER AND OUTER CORE –
represent approximately 31% of
Earth’s mass
The intense heat of the core is
derived from decay of radio-active
isotopes
– Inner core
• The inner core is under such
extreme pressure that it
remains solid
– Composed mostly of iron
(Fe) and some Nickel (Ni)
– Temperatures over 7,0000
F (4,3000 C)
– ~2200 Km across
– Outer core
• The outer core is under less
pressure and is molten
– Composed mostly of iron
(Fe), Sulphur (S), and
Nickel (Ni)
– Temperatures of 6,7007,7000F (3,700 – 4,3000C)
– ~3000 Km across
MANTLE AND CRUST
– MANTLE– represents 68% of Earth’s
mass -~2900 km thick
• 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) and a number of
other minerals
– 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:
5-70 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)
Silicon (Si)
Aluminum (Al)
Iron (Fe)
– Calcium (Ca)
Potassium (K)
Sodium (Na)
Magnesium (Mg)
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Continental Crust is composed mainly of a “granitic” rock type
– High silica content (a combination of Oxygen and Silicon)
– Lower density (2.7 grams per cubic centimeter)
– Thicker (20-70 km thick)
– Underlies most continents
•
Oceanic Crust is composed mainly of a “basaltic” rock type
– Low silica content
– Higher density (3.0 grams per cubic centimeter)
– Thinner (5-10 km thick)
– Underlies most ocean basins
TECTONIC PLATES
• Earth’s Lithosphere
(crust and upper
mantle) is broken
into large moving
slabs: Tectonic (or
Lithospheric)
Plates
• Interaction between
plates drives
mountain building:
– Volcanic mountains
– Folded mountains
– Faulted mountains