IMES
Lecture 7
The Rock Record
and the Geologic Time Scale
Dr. Sharon K. Reamer
Department of Earthquake Geology
Material modified from:
Peter Copeland and William Dupré
University of Houston
The Rock Record
Where do we see rocks?
Where we don't see rocks :
• If we were to drill a hole into any
spot on the Earth, we would find
rocks that repesent the geologic
past
• In the top few kilometers of most
regions, we would probably find
sedimentary rock
• Drilling deeper (ca. 6-10 km), we would eventually find
older igneous and metamorphic rock
• In fact, thousands of relatively shallow holes have been
drilled on the continents in the search for mineral resources.
• These holes are major sources of information - mainly about
sedimentary rocks and their history.
Where do we see rocks?
• Much information from the seafloor sediments comes from
the hundreds of boreholes made by the Deep Sea Drilling
Project
• This ongoing project began in the 1960s with U.S. research
ships. Today the DSDP is a global research project
• Even with all the information from boreholes, geologists
mostly rely on places where rock is exposed at the earth's
surface.
• These exposed surfaces of "bedrock" are called outcrops.
Proportions of the Rock Types
• 5 % by volume of the upper crust
• 75% by area of continents
• Often the only record of geologic events:
e.g. Himalaya will someday be sandstone
Outcrop
Ground
surface
Geologists rely on outcrops,
places where bedrock - the
underlying rock beneath soil
- is laid bare.
Rocks are not found in
nature conveniently sorted
into igneous, metamorphic
or sedimentary rocks.
Instead, we find them
arranged in patterns
determined by the geologic
history of a region
Geologists map these
patterns to deduce the
geologic past
Where We See Rocks
• outcrops vary from region to region.
• the presence and types of outcrops depends
on the landscape.
• the landscape depends on:
– the geologic structure of the region
– its geologic history
–its present climate
Shawangunk Mountains, New York - part of the
Appalachian System - ancient mountains provide excellent
outcrops along steep slopes
Carr Clifton
Florida Keys – some of the best outcrops occur where
ancient coral reefs are exposed in the cores of the islands.
•There is no place on earth that is not moving both
vertically and horizontally - but very slowly
•At that time scale, continents, oceans and
mountains have moved great distances
•The geological processes that shape the Earth’s
surface and interior take place over millions of
years
•Geologists use the perspective of time to properly
sequence important events in Earth history:
•How many ice ages have occurred?
•What were the conditions in East Africa when
early humans were evolving?
•When were the Himalayas formed?
Amount of Time Required for Some
Geologic Processes and Events
The time scale is logarithmic: equal divisions are in
powers of 10
Times are given in orders of magnitude
Two ways to date geologic events
1) relative dating
– sedimentary structure
– fossils
– cross-cutting relationships
2) absolute dating
– isotopic - absolute age (radioactive decay)
– tree rings
The geologists who worked out the geologic time
scale started a (paradigm) revolution in the way we
think about time, our planet, even ourselves.
Sedimentary structure
1) sediments are deposited in horizontal layers and slowly
change into sedimentary rock
2) if left undisturbed by tectonic processes, the youngest
layers remain on the top and the oldest on the bottom
• Disturbances of horizontal layering also provide clues for
relative dating
• Events such as magma intrusions, faults, uplift and
deformation can "cut across" or are overlain by sediments
The Stratigraphic Record
Steno's Laws (Nicolaus Steno, 1669)
• Principle of Superposition :
– each layer of sedimentary rock in a tectonically
undisturbed sequence is younger than the one
beneath it and older than the one above it
– Layers are viewed as a kind of vertical timeline
(partial or complete record of deposition)
• Principle of Original Horizontality
• Laws apply to both sedimentary and igneous
rocks
Principle of Superposition
Youngest rocks
Oldest rocks
Steno's Laws
• Principle of Superposition
• Principle of Original Horizontality:
– sediments are deposited as essentially
horizontal beds.
– Although cross-bedding is inclined, the overall
orientation is horizontal
– If we find a sequence of sedimentary rock
layers that is folded or tilted, we know that the
rocks were deformed by tectonic stresses after
their sediments were deposited
Principles of original horizontality and
superposition
A vertical set of strata is called a stratigraphic succession
— a chronological record of the geologic history of a region
— the corresponding timeline based on this sequence is
called the geologic time of the sequence
— stratigraphic and sedimentary sequences are not the same!
— sequences are vertical changes in lithology of sediments of a
single environment
— stratigraphic sequences can contain layers from many
different environments – gaps among events occur
— the emphasis is on the chronological order of the
stratigraphic clock: BUT stratigraphy alone is inconclusive!
—global correlations need supporting evidence: fossils
Relative dating - Paleontology
• The study of life in the past based on the
fossils of plants and animals.
Fossil: evidence of past life
• Fossils that are preserved in sedimentary
rocks (fossil record) are used to determine:
1) relative age
2) the environment of deposition
Ammonite fossils – large group of Petrified Forest, AZ – ancient logs
(nearly) extinct invertebrates
m.y. old – wood replaced by silica
The Fossil Clock: Paleontology
• To most of us, it seems obvious that fossils are the remains of
ancient organisms.
• Some fossils look much like relatives of animals that are still
living today
• Others, like the trilobites shown earlier, are extinct and do
not resemble any animals living presently
• The most common fossils in rocks of the past 1/2 billion
years are the shells of invertebrate animals (clams, oysters,
ammonites)
• Much less common are the bones of vertebrates (mammals,
reptiles, dinosaurs)
• Plant fossils are abundant in some rocks (coal beds)
Trilobites preserved as fossils in rocks from
365 mya (Ontario, Canada)
•The ancient Greeks were the first to hypothesize about
fossils.
•In the 17th century, Nicolaus Steno compared what he
called “tongue-stones” in the Mediterranean region with
the teeth of modern sharks and concluded that the stones
were the remains of ancient animals.
•Paleontology is the study of the history of ancient life
from the fossil record.
•In 1859, Darwin proposed the theory of evolution and
revolutionized scientific thinking about the origins of
millions of species of animal and plant life
Using Fossils to Correlate Rocks
1. Fossils found in some rock layers in
outcrop A are the same as those
found in some rock layers at
2. Layers with the
outcrop B, some distance away
same fossils are
the same age
Using Fossils to Correlate Rocks
A composite of the two outcrops would show formations I
and II both overlying formation III and therefore, younger
than III.
Cross-Cutting Relationships
• Formation: is a series of rock layers in a region
that has about the same physical properties and
may contain the same assemblage of fossils. A
formation represents (geologic) time
• A cross-cutting relationship is a particular
geometry of rocks that allows geologists to place
a rock unit in relative chronological order.
TIME 1 :
beneath the sea,
sediments
accumulated in
flat layers called
sedimentary
beds
TIME 2 : Later,
tectonic forces
caused uplift,
folding and
deformation of the
sedimentary beds
during mountain
building
TIME 3 : a dike from molten
magma intrudes the folded
layers
the dike cuts
across the folded
layers: intrusion
followed
sedimentation and
folding
TIME 4 :
faulting
displaced the
layers including
the dike.
faulting displaced the dike and
sedimentary beds – faulting came last
Cross-cutting Relationships
Cross-cutting relationships allow us to place geological
events within the relative time frames given by the
stratigraphic sequences.
Cross-cutting Relationships
1. Tectonic forces caused uplift, folding and deformation of
sedimentary layers during mountain building
Cross-cutting Relationships
2. A dike from molten magma intruded the folded layers.
Because the dike can be seen to cut across the folded layers,
it is clear that it came later.
Cross-cutting Relationships
3. Faulting displaced the layers and the intruding dike.
Because both the sedimentary beds and the dike are
displaced, the faulting had to have taken place after.
Unconformity : Markers of missing time
• Formation: is a series of rock layers in a region
that has about the same physical properties and
may contain the same assemblage of fossils. A
formation represents (geologic) time
• A cross-cutting relationship is a particular
geometry of rocks that allows geologists to place
a rock unit in relative chronological order.
• Unconformity: is a surface between two layers that
were not laid down in an unbroken sequence. An
unconformity also represents (geologic) time
• An unconformity also represents tectonic forces
that raised the land above sea level, where it was
eroded, for example.
TIME 1 : beneath the sea, sediments
accumulated in beds A then B then C then D
TIME 2 : Later, tectonic forces caused uplift of the
beds, above sea level, exposing them to erosion
TIME 3 : erosion stripped away bed D and part of C,
leaving an irregular surface of hills and valleys
The irregular
surface of C is
preserved as an
unconformity
(disconformity)
TIME 4 : subsidence below the sea allowed a
new layer, E, to be deposited over C.
Types of unconformity
• Geologists classify different types of unconformities by
the relationships between upper and lower sets of layers:
– Disconformity : the upper set of layers overlies an
erosional surface developed on an undeformed, stillhorizontal lower set of beds
– Nonconformity : upper beds overlie metamorphic or
igneous rock
– Angular Unconformity : upper layers overlie lower
beds that have been folded by tectonic processes and
then eroded to a more or less even plane. The two sets
of layers have bedding planes that are not parallel.
The Great Unconformity of the Grand Canyon
An angular unconformity
between the horizontal
Tapeats sandstones above
and the steeply folded
Precambrian Wapatai
shales below
The Great Unconformity of the Grand Canyon
TIME 1 : beneath
the sea, sediments
accumulated in beds
TIME 3 : erosion stripped away the tops
of the folded layers, leaving an uneven
plain with exposed portions of several
folded layers
Where the folded
layers and the new
sediments meet is
preserved as an
angular
unconformity
TIME 2 : later, tectonic forces
caused uplift, folding and
deformation of the sedimentary
layers during mountain building
TIME 4 : subsidence below the sea
allowed new sediments to be deposited
on the former erosion surfaces.
Angular unconformity,
Shinumo Creek, Grand Canyon
Paleozoic
sediments ~ 550 my
Hakatai Shale ~ 1 b.y.
South rim of the Grand Canyon
South rim of the Grand Canyon
250 million years old
Paleozoic Strata (sediments)
550 million years old
1.7 billion years old
Precambrian
(metamorphic)
Great Unconformity
Blacktail Canyon, River Mile 120, Grand Canyon
Tapeats Sandstone
(~550 million years old)
Vishnu Schist
(~1700 million years old)
Angular unconformity,
Siccar Point, Berwichshire, Scotland
Hutton was here
Devonian (old red sandstone)
(~409-354 million years old)
Silurian rocks
(~439-409 million years old)
Sequence stratigraphy
(seismic stratigraphy)
• Stratigraphic analysis in which the major
geologic units are unconformities
• Subtle geometric features are often
revealed in seismic profiles
• Used widely with seismic data
• The basic unit used for sequence
stratigraphy is the sequence, a series of
sedimentary beds, bounded above and
below by an uncomformity
1. Seismic measurements along profiles can be used to
create seismic maps to reveal sequence stratigraphy
2. Geologists use these maps to "see" individual beds
in a stratrigraphic sequence
3. The seismic sequence
reveals changes in
sedimentation such as
those that occur at a
river delta
4. A sequence of delta
sediments B,
accumulates over
previous sediments A
5. The sea level rises, and
the shoreline recedes
inland
In this way, successive global
climate changes can be mapped
according to rising or falling sea
level and depositional sequences
6. Another sedimentary
sequence C accumulates
over sequence B
How old is the Earth?
Principle of Uniformitarianism
The present is the key to the past.
— James Hutton (Principles of Geology, 1829)
• The earth is a product of ordinary
(geological/evolutionary) processes
operating over very long time intervals
• Natural laws do not change — however,
rates and intensity of processes may.
How old is the Earth?
1) Bible: In 1664, Archbishop Usher of Dublin used
chronology of the Book of Genesis to calculate that
the world began on Oct. 26, 4004 B.C.
2) Salt in the Ocean: (ca. 1899) Assuming the oceans
began as fresh water, the rate at which rivers are
transporting salts to the oceans would lead to
present salinity in ~100 m.y.
How old is the Earth?
3) Sediment Thickness: Assuming the rate of deposition
is the same today as in the past, the thickest
sedimentary sequences (e.g., Grand Canyon) would
have been deposited in ~ 100 m.y.
4) Kelvin’s Calculation: (1870): Lord Kelvin calculated
that the present geothermal gradient of ~30°C/km
would result in an initially molten earth cooled for 30
– 100 m.y.
Flawed Assumptions
• The Bible is not a science text
• Salt is precipitated in sedimentary formations
• Both erosion and non-deposition are major parts of the
sedimentary record
• Radioactivity provides another heat source
The Geologic time scale
• We have now several ways of correlating rock strata with a
time sequence of geologic events:
– relative ages of sedimentary rocks by the laws of
superposition and by the local and global fossil record
– we can use deformation and unconformities to date tectonic
episodes in relation to a stratigraphic sequence
– we can use cross-cutting relationships to establish the
relative ages of igneous bodies or faults cutting through
layered rocks
The Geologic time scale
• Combining these methods, geologists in the 19th and 20th
centuries used these relative dating principles to work out
the entire geologic time scale, a relative-age calendar for the
Earth’s geologic history.
• In addition to relative dating, absolute (isotopic) ages have
also been used to derive the exact ages of rocks
The Geologic Time Scale
The geologic time scale is divided into four major time units,
in order of decreasing length:
(1) Eons
(2) Era
(3) Period
(4) Epoch
The heat inside the Earth
• The discovery of radioactivity at the turn of the century by
Henri Becquerel (1896) (uranium) and Marie Curie (1897)
(radium) provided the source of the heat to override
Kelvin’s calculations
• In 1905, Ernest Rutherford deduced that radioactivity could
be used to give an accurate age for a rock.
• He was able to date a rock with uranium in his laboratory
• Over the next few years many more radioactive minerals
were discovered and many more were dated.
• This was the start of isotopic dating, the use of naturally
occurring radioactive elements to determine rock ages
Age of the Earth
• In 1956 Clare Paterson measured decay of Uranium in
meteorites
• These measurements showed that the age of solar
system, and therefore the Earth, was 4.56 billion years
old
• This age has been modified by less than 10 million years
since then
• Paterson completed the discovery of geologic time.
Absolute geochronology:
How does it work?
• Radioactive elements have nuclei that
disintegrate spontaneously
• Isotopes are different forms of the same
element (atomic mass) containing the
same number of protons, but varying
numbers of neutrons.
235U, 238U, 87Sr, 86Sr, 14C, 12C
• Energy is emitted during decay – this is
called radiation. The original atom is the
parent, the decayed atom is the daughter
Radioactive Decay of
Rubidium to Strontium
• A neutron in a Rubidium-87
atom decays or
decomposes, ejecting an
electron
• and producing a proton,
which changes the atom to
strontium-87
• Radioactive decay is an
inherently random process
which behaves in a
statistically predictable
pattern
• (unstable) radioactive elements (parents) decay to stable,
non-radioactive elements (daughters)
• The rate at which this decay occurs is constant and known
• If we also know the amount present of parent and
(original) daughter we can calculate the reaction time
Proportion of Parent Atoms Remaining as a
Function of Time
The half-life of a radioactive
isotope is defined as the
time required for half of it to
decay.
Poisson distribution
Isotopic dating
• Radioactive decay is a random process and can be
statistically calculated using the Poisson (or other)
probability distribution.
• Decay rates are unaffected by environment.
• Each radioactive element has its own decay rate.
– Slow-decaying elements can be used to date old
rocks (rubidium-87).
– Fast-decaying elements are useful for recent
dating (carbon-14).
• Geologists use a number of naturally occurring
radioactive elements to determine the ages of rocks
Isotopic dating
• If the decay rate is known and the daughter atoms
can be counted, we can calculate the length of the
radioactive clock
• Minute quantities of isotopes (parent and daughter)
atoms are measured with a mass spectrometer
• Crystallization resets the parent-daughter ratio–with
some uncertainty – atomic clock is reset to zero –
igneous rocks can be dated to time of crystallization.
• Metamorphism may also reset the clock
• Erosion may cause errors in determining parentdaughter ratios
Major Radioactive Elements Used in Isotopic Dating
Isotopic dating is only possible if a measurable number of
parent and daughter atoms remain in the rock.
Geologically Useful Decay Schemes
Parent
Daughter
Half-life (years)
235U
207Pb
4.5 x 109
238U
206Pb
0.71 x 109
40K
40Ar
1.25 x 109
87Rb
87Sr
47 x 109
14C
14N
5730
Oldest rocks on Earth
• Slave Province, Northern Canada
– Zircons in a metamorphosed granite dated at 3.96
Ga by the U-Pb method
• Yilgarn block, Western Australia
– Detrital zircons in a sandstone dated at 4.10 Ga
by U-Pb method.
• Jack Hills, Western Australia
– new improved methods - dated at 4.4 Ga!!!
• Several other regions dated at 3.8 Ga by various
methods including Minnesota, Wyoming, Greenland,
South Africa, and Antarctica.
Age of the Earth
• Although the oldest rocks found on Earth are 3.96 Ga
(crystals from 4.4 Ga), we believe that the age of the
Earth is approximately 4.6 Ga. All rocks of the age 4.6
to 4.4 Ga have been destroyed (the rock cycle) or are
presently covered by younger rocks.
• The current age of the earth is based on rocks brought
back from the Moon (4.4 Ga), and meteorites (4.6 Ga),
that are thought to be good representatives of the early
solar system as well as more complicated geochemical
modeling (oxygen isotope ratios).
• This data suggests that the present chemical
composition of the crust must have evolved for more
than 4.5 Ga.
The geologic timescale
and absolute ages
• Isotopic dating of intebedded volcanic rocks
allows assignment of an absolute age for
fossil transitions
• Sometimes daughter elements become lost or
disappear during recrystallization or
outgassing
• Precambrian and Paleozoic rocks have error
margins on the order of a few 100,000 years
The big assumption
The half-lives of radioactive isotopes are the
same as they were billions of years ago.
Test of the assumption
Meteorites and Moon rocks (that are thought
to have had a very simple history since they
formed), have been dated by up to 10
independent isotopic systems all of which have
given the same answer. However, scientists
continue to critically evaluate this data.
Frequently used decay schemes
have half-lives which vary by
a factor of > 100
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
147Sm
144Nd
106 x 109
Timeline: earth
absolute ages
evolution
geologic time
scale
The ribbon of geologic time
HADEAN
ARCHEAN
PROTOZEROIC
PHANEROZOIC
Hadean is the oldest eon: earliest rocks date to 4.4 by
HADEAN
ARCHEAN
During the Archean eon (4-2.5 by), the geodynamo, plate
tectonics, and the climate system were established.
PROTOZEROIC
In the Archean, some fossils of bacteria exist
The next set of continents formed during the Protozeroic (2.5
by-540 my) eon.
Interactions between plate tectonics and climate geosystems
were close to the state achieved in later times (except O2)
HADEAN
ARCHEAN
Multicellular life forms emerged during this era.
PROTOZEROIC
PHANEROZOIC
The best understood eon is the Phanerozoic (~540 my).
Beginning of evolutionary "Big Bang".
Rock formations of this age contain lots of shells and fossils.
HADEAN
ARCHEAN
Most of the world’s oil and gas reserves were formed during
this eon
PHANEROZOIC
Three domains in the tree of life
Came into existence in the
Archeon: 3.5 Ga
Cambrian "Big Bang"
• Precambrian-Cambrian Eon (Paleozoic-Phanerozoic Era) at
542 Ma
• Extremely fast radiation of life forms.
• Every animal group that exists on the earth today, and
many that have become extinct: all animal phyla from the
tree of life (~30) originated in the Cambrian
• Theory of Evolution: shows us how but not why
• Driven by natural selection: adaptation of populations
of organisms to new environments and is responsible for
the development of new species
• Species will change over many generations as a result of
preferential survival of organisms
Cambrian
radiation
Evolutionary barrier overcome?
Development of skeletons?
Environmental change?
= genetic possibility +
environmental opportunity
Three significant eras make up the Phanerozoic:
Paleozoic
– 543-251 mya
– Age of fishes
– Photosynthesis
Mesozoic
– 251-65 mya
– dinosaur extinctions
Cenozoic –
– 65 mya until now
Diversity of organisms
Paleocene-Eocene
(55 Ma)
800 Global warming:
methane production in
the oceans
End-Permian
(250 Ma)
of
Devonian event 95%
all species
364 Ma
600
75% of
all species
400
End-Cretaceous
(65 Ma)
Cambrian
radiation
200
0
600
Silurian event
429 Ma
End-Triassic
208 Ma
400
200
Age (Ma)
0
Knife lies on layer marking End-Cretaceous mass
extinction
Extinct Organisms
from the Fossil Record
The ribbon of geologic time
HADEAN
ARCHEAN
PROTOZEROIC
PHANEROZOIC
Astrobiology: the search for ET
Places to look for extraterrestrial life
● Habitable zones around stars
● Environments in our solar system
● Mars
● other places
Rarely, extremely recent
geologic processes can be
documented using
historical records (old
maps, land surveys)
(brown is new land including
sand, silt and mud brought in
by waves and tides from 18871988)
Reconstructing
relative sequence
of events
Generalized Stratigraphic Section of Rocks Exposed in the
Grand Canyon
Some of the Geologic Units Exposed in the
Grand Canyon
Michael Collier
Poisson distribution
P(r ) 
 r e
r!
Characteristics of the radioactive counting experiment are:
1. An interval of time is chosen, and the number of events
occurring in that time interval is counted
2. the probability of an event occurring in an interval does not
change
3. the mean number of events in an interval is a constant
4. event occurrence can be counted, but non-occurrence
cannot.
the detection of a particle (event) = r
mean number of detections in a given interval = 
P(r ) 
 r e
r!
Double it and add 1
number of
half-lives
0
1
2
3
4
5
number of
parents
64
32
16
8
4
2
number of
daughters
0
32
48
56
60
62
D/P
0
1
3
7
15
31
What if the rates have varied?
What we think happened:
rate
of 
decay
 time
What if the rates have varied?
What we know didn’t happen:
rate
of 
decay
 time
Best initial D = 0
Two ways around this problem:
1) Choose minerals with no initial
daughter.
2) Use a method that tells you the initial
concentration of D and P.
Minerals with no initial daughter
•
40K
decays to 40Ar (a gas)
• Zircon: ZrSiO4
ion
radius (Å)
Zr4+
0.92
U4+
1.08
Pb2+
1.37
Calculating Relative
Plate Motion
geologic formation
offset
satellite
measurements
seafloor spreading
1871
1968