Deep Time: How Old is Old?
Geological Time
Geologic Time
 Discovering the magnitude of the Earth’s past was a
momentous discovery in the history of humanity.
 This discovery forever altered our perception of
ourselves within nature and the universe.
Geochronology is the determination of the
absolute (numerical) age of a material.
The quest to determine the age of the Earth
and its rocks has tantalized geologists for centuries.
Geologic Time
 Deep time – the immense span of time that marks
geologic changes – is difficult for most humans to grasp.
 We think of time in human terms.
Our lives.
The lives of our parents and grandparents.
The lives of our children or grandchildren.
Recorded history.
 Our experiences are measured in decades.
 Human history is puny when compared with deep time.
 Understanding time permits assigning an age to…
 Rocks.
 Fossils.
 Geologic structures.
 Landscapes.
 Tectonic events.
Geologic Time
 Prior to the late 17th century, geologic time was thought
to be the same as historical time.
 Archbishop James Ussher, of Armagh, Ireland,1654,.
He added up generations from the Old Testament.
He determined that Earth formed on October 23, 4004 BCE.
 During the Renaissance, however, geologists began to
find evidence of a more ancient Earth.
 Nicolaus Steno – (1638–1686) Danish physician.
Observed marine fossils high in the Apennines.
Deduced that these were ancient animals in loose sediment.
Lithification and uplift suggested long periods of time.
 Geologic processes work on a much different time scale.
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Geologic Time
Lord Kelvin’s Absolute Age of Earth
 James Hutton (1726-1797) Scottish physician and farmer.
 Known as “the Father of Modern Geology” for his
“Principle of Uniformitarianism.”
“The present is the key to the past” - Processes observed
today are the same as those that operated in the past.
Geologic processes are slow; hence, large changes require
long periods of time. This idea requires a very old Earth.
 Of the abyss of time, Hutton wrote: “…we find no vestige of
a beginning; no prospect of an end.”
In the mid 19th century, Kelvin attempted to calculate the
amount of time the Earth has been a solid body by estimating
cooling rates by conduction.
Kelvin, an important physicist, reasoned that the Earth had to
obey the Law of Conservation of Energy: it could not have
some internal heating mechanism, and thus had to have
cooled over time from an initial molten state.
Kelvin’s Age of the Earth
Given off by radioactive decay. Kelvin’s best estimate
was ~25 million years.
This estimate was accepted by many scientists until the
20th century, and posed a serious threat to the concept of
uniformitarianism, which required a much older Earth.
This estimate neglected to consider that, although the
Geologic Time
 There are two ways of dating geological materials.
 Relative ages – Ages based upon sequence of formation.
Permit determination of which rock unit is older vs. younger.
Qualitative.
Developed 100s of years ago.
 Numerical ages – Ages specifying the actual number of
years that have passed since an event occurred.
Age is given a number.
Quantitative.
Recently developed.
Earth has cooled over time, it does have an internal heat
source: thermal energy
Relative vs. Absolute
 Example:
 Relative ages assign
proper order to events.
 Numerical ages assign
exact dates to events.
Defining Relative Age
 Several logical tools are useful for defining relative age.
 Principle of uniformitarianism.
 Principle of superposition.
 Principle of original horizontality.
 Principle of original continuity.
 Principle of cross-cutting relationships.
 Principle of inclusions.
 Principle of baked contacts.
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Defining Relative Age
 Uniformitarianism – The present is the key to the past.
 Physical processes that we observe today operated in the
same way in the geological past.
 Thus, modern processes help us to understand how past
geologic phenomenon were generated.
Defining Relative Age
 Horizontality and Continuity.
 Strata often form laterally extensive horizontal sheets.
 Subsequent erosion dissects once continuous layers.
 Flat-lying rock layers are unlikely to have been disturbed.
Defining Relative Age
 Inclusions – A rock fragment enclosed within another.
 Igneous xenoliths – Country rock that fell into magma.
 Weathering rubble – Debris from pre-existing rocks.
 The inclusion is older than the material enclosing it.
Defining Relative Age
 Superposition.
 In an undeformed sequence of layered rocks…
Each bed is older than the one above, and…
younger than the one below.
 Younger strata are on top; successively older strata below.
Defining Relative Age
 Cross-Cutting Relations.
 Younger features cut across older features.
 Faults, dikes, erosion, etc., must be younger than the
material that is faulted, intruded or eroded.
Defining Relative Age
 Baked contacts.
 Thermal metamorphism occurs when country rock is
invaded by a plutonic igneous intrusion.
 The rock that is thermally altered (baked) must have been
there first (it is older).
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Relative Dating
 Determining relative ages empowers geologists to easily
unravel complicated geologic histories.
Geologic History
 Tectonic compression and folding.
 Beds had to be present to be folded.
Geologic History
 Deposition of horizontal strata below sea level in the
order 1, 2, 3, 4, 5, 6, 7 and 8 (oldest to youngest).
Geologic History
 Intrusion of a granitic igneous pluton.
 Inclusions of older sedimentary rocks lie within the granite.
 Uplift above sea-level.
 Erosion.
Geologic History
 Extensional normal faulting.
 Faulting cross-cuts the granitic pluton.
Geologic History
 Intrusion of a dike.
 Dike cross-cuts the normal fault.
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Geologic History
Geologic History
 Erosion to present landscape configuration.
 Erosion removed the volcano and cross-cuts the dike.
 Relative ages help to unravel a complicated history.
 Do the previous 6 slides completely explain this picture?
Fossil Succession
 Fossils, remnants or traces of once living organisms, are
often preserved in sedimentary rocks.
 Fossil are useful for relative age determination.
 Often, several types of fossils will occur as an assemblage.
 Fossils are time markers.
Fossil Succession
 Species evolve, exist for a time and then become extinct.
 Fossil appearance, range and extinction can date rocks.
 Fossil organisms succeed one another in a known order.
 A time period is readily recognized by its fossil content.
Fossil Succession
 The first and last appearance of a
Unconformities
 Each fossil has a unique range.
 An unconformity is a gap in the rock record via…
 Non-deposition or…
 Erosion.
 Overlapping ranges provide
 Unconformities are significant; they are gaps in time.
distinctive time markers.
 Permit stratigraphic correlation.
 There are three types of unconformity.
specific fossil type is its range.
 Locally.
 Regionally.
 Globally.
Unconformity Animation
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Unconformities
 Three unconformity types:
 Disconformity – Parallel strata bracketing non-deposition.
Due to an interruption in sedimentation.
May be difficult to recognize.
Unconformities
 Three unconformity types:
 Angular unconformity – Represents a huge gulf in time
Horizontal marine sediments deformed by orogenesis
Mountains eroded totally away.
Mountain remnants flooded by the sea.
Sediment deposited horizontally on the deformed sediments.
Unconformities
 Earth history is recorded
in strata.
 Missing strata represent
a part of the historical
record that is lost in that
location.
 The Grand Canyon…
Unconformities
 Three unconformity types:
 Nonconformity – Metamorphic or igneous rocks overlain
by sedimentary strata.
Crystalline ig/met rocks were exposed by deep erosion.
This eroded surface became the substrate for sediment.
Angular Unconformity
 James Hutton was the first to recognize the enormous
time-significance of angular unconformities.
 “Hutton’s Unconformity”, Siccar Point, Scotland, is a
common destination for geologists.
Stratigraphic Correlation
 Stratigraphic columns depict rock layers in a region.
 Drawn to scale to accurately portray relative thicknesses.
 Rock types are depicted by graphical fill patterns.
 Divided into formations – distinct and mapable rock units.
 Formations are separated by contacts.
 Thick layers of strata.
 Numerous gaps.
 A partial record of
geological events there.
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Stratigraphic Correlation
 In 1793, William “Strata” Smith was the first to note that
strata in widely separated regions could be matched.
 The rocks looked similar and were in a similar order.
 The rocks contained the same fossils.
 After years of work, he made the 1st geologic map.
 A geologic map shows the spatial distribution of rocks.
 Today, these maps provide fundamental resource data.
Stratigraphic Correlation
Stratigraphic Correlation
 Lithologic correlation – Based on…
 Lithology (rock type).
 Sequence – The relative order in which the rocks occur.
 Limited to correlation between nearby regions.
 Fossil correlation – Based on fossils contained in rocks.
 Applicable to much broader areas.
The Geologic Column
 A composite stratigraphic column encompassing all
Earth history has been constructed by correlating
incomplete sections across the globe.
Correlation Example:
 Parks of Arizona and Utah.
 Formations can be traced
long distances.
 Overlap is seen in the
sequence of rock types.
 Overlap between columns
is used to create composite
stratigraphic columns.
Geologic Time
 The geologic column can be divided on the basis of time.
 These time divisions yield the geologic time scale.
 The “calendar” of Earth’s history.
 The structure of the geologic time scale.
Geologic Time
 Subdivisions are named time intervals typifying…
 The nature of life (“zoic” means life).
Example: Proterozoic.
 A characteristic of the time period. Example: Carboniferous.
 A specific locality.
Example: Devonian.
Eons – The largest subdivision of time (100s to 1000s Ma).
Eras – Subdivisions of an eon (65 to 100s Ma).
Periods – Subdivisions of an era (2 to 70 Ma).
Epochs – Subdivisions of a period (0.011 to 22 Ma).
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Geologic Time and Life
 Life first appears on Earth ~ 3.8 Ba.
 Early life consisted of anaerobic single-celled organisms.
 Oxygen from cyanobacteria built up by 2 Ba.
 ~ 700 Ma, multicellular life evolved.
 ~ 542 Ma marks the 1st
appearance of hard shells.
 Fossil preservation increased
dramatically thereafter.
 Life diversified rapidly, a
development known as the
“Cambrian Explosion.”
The Geologic Time
Scale
The Geologic Time Scale
 Names of the Eons.
 Phanerozoic “Visible life” (542 Ma to the present).
Started 542 Ma at the Pre-Cambrian – Cambrian boundary.
Marks the 1st appearance of hard shells.
Life diversified rapidly afterwards.
 Proterozoic – “Before life” (2.5 to 0.542 Ga).
Development of tectonic plates like those of today.
Buildup of atmospheric O2 ; multicellular life appears.
 Archean – “Ancient” (3.8 to 2.5 Ga).
Birth of continents.
Appearance of the earliest life forms.
The Geologic Time Scale
 Names of the Eras.
 Cenozoic – “Recent life.”
65.5 Ma to present.
The “Age of Mammals.”
 Mesozoic – “Middle Life.”
251 to 65.5 Ma.
The “Age of Dinosaurs.”
 Paleozoic – “Ancient Life.”
542 to 251 Ma.
Life diversified rapidly.
 Hadean – “Hell” (4.57 to 3.8 Ga).
Internal differentiation.
Formation of the oceans and secondary atmosphere.
Numerical Age
 The relative age of geologic events is well-established.
 We can now assign an actual numerical age to events.
 Based on radioactive decay of elements in minerals.
 Radioactive decay of an element proceeds at a fixed rate.
 Decay rates are known for all elements from lab work.
 Radioactive elements act as internal clocks.
Radioactive Decay
 All atoms of an element have the same atomic number.
 An element’s identifying number.
 Equal to the number of protons in the atom’s nucleus.
 Atomic mass – Sum of the protons (p) and neutrons (n).
 Numerical dating is called…
 Radiometric dating, or…
 Geochronology.
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Radioactive Decay
 Isotopes – Variations of an element that…
 Have the same # of p but a different # of n.
 Isotopes have similar but different mass numbers.
 Stable – Isotopes that never change (i.e. 13C).
 Radioactive – Isotopes that spontaneously decay.
 Hydrogen has 2 isotopes: stable deuterium and
radioactive tritium.
Radioactive Decay
 Alpha decay.
 Ejection of 2 p and 2 n.
 Decrease in atomic number by 2;
atomic weight by 4.
 Beta decay.
 Ejection of an electron from a
neutron.
 Increases atomic number by 1;
atomic weight unchanged.
 Electron capture.
 An electron is absorbed by a p.
 Decreases atomic number by 1;
atomic weight unchanged.
Radioactive Decay
 Radioactive decay steps along a decay chain.
 Decay creates new unstable elements.
 These new elements also decay.
 Decay proceeds until it reaches a stable element endpoint.
Radioactive Decay
 Parent isotope – The isotope that undergoes decay.
 Half-life (t½) – Time for ½ of an unstable nuclei to decay.
 t½ is a characteristic of each isotope.
 After one t½ one-half of the original parent remains.
 After two t½ one-quarter of the original parent remains.
 After three t½ one eighth of the original parent remains.
 Daughter isotope – The product of this decay.
 As the parent disappears, the daughter “grows in”.
Radiometric Dating
 The age of a mineral can be determined by…
 Measuring the ratio of parent to daughter isotopes.
 Calculating the amount of time by using the known t½ .
 Must pick the right mineral and the right isotope.
 The isotope must have a t½ large enough.
 The mineral must preserved all parent and daughter atoms.
 Great care is necessary for geochronological analyses.
What is a Radiometric Date?
 Radiometric dates give the time that a mineral began to
preserve all atoms of parent and daughter isotopes.
 Requires cooling below a “blocking temperature.”
 If rock is reheated, the radiometric clock can be reset.
 Ig / Met rocks are best for geochronologic work.
 Sedimentary rocks cannot be directly dated.
 They do not cool through a blocking T.
 Grains in a sediment will reflect a variety of ages.
 Other means are used to establish sediment ages.
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Radioactivity and Half Life
In 1895 Bequerel discovered radioactivity. After this, a
series of scientists mapped out what would become known
The Radioactive
Clock
as radioisotope geochronology:
the determination of absolute ages of geological
materials by understanding the decay of radioactive
atoms.
Recall that radioactive atoms decay into stable atoms at a
constant rate. This rate is related to the half life:
the amount of time it takes for 1/2 of a number of
radioactive atoms to decay.
Long-Lived Radioactive Isotopes
Radioactive atoms, to be geologically useful, must have (geologically)
long half lives. These are a few of the principal isotope systems used in
geochronology of ancient materials. All have half lives of >700 million
years.
From this it is clear that after a number of half-lives have
passed (~10), there is little parent material left to decay.
U-Pb Zircon Geochronology
The mineral zircon (ZrSiO4 ) is the most
widely used high-precision geochronometer.
Uranium is similar in ionic size and charge to zirconium, so all
zircon crystals have abundant parent (U) atoms, yet, when they
form, zircons do not contain much lead (Pb--the daughter).
no, you don’t need to memorize these elements
Zircon is resistant to both chemical and physical attack, so it
survives the rigors of geological life very well.
Also, since uranium decays on two tracks (U-238 to Pb-206,
Th
and U-235 to Pb-207), it has two simultaneous ‘clocks’ at work.
U
Zircon
Break out the Calculators
Zircon is widespread in intermediate to
The relationship between the half-life and decay
constant is given by the relationship:
felsic igneous rocks, but it is not too
abundant: it is called an accessory
λ = 0.693/half-life
mineral, in that it makes up <0.01% of
most rocks.
•Such that the decay constant for 14C is 1.2 x 10-4 yr-1
(0.693/5730 years)
So, in order to determine the age of a
rock, you have to process a fairly large
and for 235U is 9.9 x 10-10 yr-1 (0.693/700 million years)
amount of rock.
It also that comes in very small crystals.
Individual zircon grains are 10-100 microns long
(that’s 0.01 to 0.1 millimeters).
half-lives:
U-238: 4.5 Byr
U-235: 0.7 Byr
*It can be thought of as the percent chance that any given
radioactive atom will decay in any given year
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•Where D/P is the daughter to parent ratio and ln is
the natural log function. In order to determine the
age of a mineral or rock (containing a radioactive
isotope), we need to measure the daughter/parent
ratio and know the decay constant for that particular
isotopic system.
Consider a lava flow that covers a landscape. As the lava
solidifies individual mineral grains grow as atoms are
transferred from the liquid melt to the solid crystal. Some of
the elements incorporated into the mineral grains are
radioactive. Thus…
Example:
87Rb:Decays
to 87Sr at constant rate
238U: Decays to 206Pb at constant rate
235U: Decays to 207Pb at constant rate
40K: Decays to 40Ar at constant rate
14C: Decays to 14N at constant rate
(λ) = 1.42 x 10-11 per year
(λ) = 1.55 x 10-10 per year
(λ) = 9.85 x 10-10 per year
(λ) = 5.543 x 10-10 per year
(λ) = 1.21 x 10-4 yr-1
Given an extrusive igneous rocks that contains the
mineral sphene. This mineral has loads of uranium
(U) in it at birth and little lead (Pb).
Now let’s say that the lead to uranium ratio is 0.07.
•How old is this rock?????
•What do we need to answer the question?????
Age of the Earth:
Modern Perspective
Our understanding of the age of the Earth is based mainly on
the ages of meteorites.
Multiple different isotope systems (U-Pb, Rb-Sr, Re-Os) give
consistent ages in the range of 4.57 - 4.55 billion years.
ratio is 0.07
Given our current model for how the solar system formed,
it is likely that all the inner planets,
including the meteorite parent bodies,
formed around the same time.
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The Earth’s Oldest Crustal Rocks
The Acasta gneiss in Canada’s NWT was formed 4.0 Byr ago.
Along with similar metamorphic rocks in southern Greenland,
these are the most ancient pieces of crust remaining on Earth.
If the Earth is 4.5+ billion years old,
why don’t we have older crustal rocks?
Dendrochronology
Tree rings give both absolute age constraints and
paleoclimate/ paleoenvironment information.
The
Earth’s
Oldest
Mineral
This zircon crystal comes from a sandstone from Western
Australia. Although the rock it came in was significantly younger,
it testifies that the Earth had a crust by 4.4 billion years ago:
within 100 million years of the planet’s formation!
Other Geochronometers
Many layered materials can be
used to count time: from lakes
that deposit organic-rich
sediment annually (varves) to
the seasonal layers in
continental glacial ice.
Each come in useful in
particular studies.
Some individual bristlecone pines carry 4000 yr of rings, and allow
chronologies that date back ~9000 yr.
This Alaskan tree recorded the
massive earthquake of 1964.
Other Numerical Ages
 Magnetostratigraphy – Magnetic signatures in stratified
rocks can be dated against the global reference column.
 Fission-track analysis – Radioactive decay releases
particles which leave tracks in crystals. The number of
decay tracks is proportional to age.
Deep drill cores in ice from Greenland and Antarctica have
sampled ice layers dating back over 180,000 years.
Other Numerical Ages
 Numerical ages possible without radioactive isotopes.
 Growth rings – Annual layers build up in trees, shells and
spring deposits.
 Rhythmic layering – Annual layers accumulate in lakebottom sediments and in glacial ice.
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Dating the Geologic Column
Geologic Time
 Geochronology can’t date sediment deposits.
 The immensity of time is beyond human comprehension.
 It can, however, constrain these deposits.
 Metaphors can be useful for illustrating the scale of time.
 The age of Earth (4.57 Ga) can be compared to pennies.
 Lining up 4.57 billion pennies…
 Cross-cutting relations help to bracket the age of
sediments with datable igneous and metamorphic rocks.
 Yields age ranges.
 Dates improve
with better data.
 This technique has
defined the age of
major boundaries in
the geologic column.
87,400 km long.
More than twice around Earth.
Geologic Time
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