Geologic Time & Fossils
• People originally thought that fossils
were placed in rocks by supernatural
beings.
• Later (~450 B.C.) a Greek historian
suggested that fossils were evidence of
past life.
• Soon after (~300 B.C.) Aristotle
abandoned this idea and suggested that
life-like shapes grew within the rocks,
but were inorganic.
• During the Renaissance, the idea that
fossils represent remnants of past life
was once again believed.
• Nicholaus Steno started the modern
field of paleontology by suggesting that
fossil containing rocks started as loose
sediment with fragments of organisms
that later hardened into rock.
(Interlude D, & Chapter 12)
Early Fossil Sketches by Nicolaus Steno (1638–1686)
Types of Fossils
• Fossils mainly occur in sedimentary rocks and come in a variety of
fossil types
• (Body) Fossils – Whole bodies or pieces of bodies of past organisms
– E.g. bones, insects preserved in amber/tar, mammoths frozen in ice, etc…
• Trace Fossils – Features left behind that are evidence of past life.
– E.g. footprints, burrows, etc…
Shells in limestone
Dinosaur footprint in mudstone
Burrows in siltstone
Fossil Preservation
Not all organisms that die get preserved in rocks. In fact, only a small percentage
do, because it takes special circumstances for a fossil to be preserved.
• Death in an anoxic (oxygen-poor) environment
– Oxygen reacts with organisms and breaks them down, and scavengers may eat the
carcass.
• Rapid Burial
– If an organism dies in an environment that has a high sedimentation rate, it may get
buried before it rots or is scavenged.
• The Presence of Hard Parts
– Organisms with hard parts (bones, shells, etc…) take longer to break down. Soft shelled
organisms (e.g. jellyfish) decay very quickly.
• Lack of Diagenesis or Metamorphism
– Diagenetic and Metamorphic processes (e.g. recrystallization) can destroy fossils
• In a typical shallow marine environment, ~30% of organisms have a high
preservation potential (hard, sturdy shells), ~40% have a low preservation potential
(fragile shells), and ~30% have no sturdy parts and will only be preserved in very
rare circumstances. Of the 30% with a high preservation potential, only a few will
die in environments that will preserve them.
Extraordinary Fossils
• Although organisms with soft parts are rarely preserved, there are a number of
locations throughout the world that at one time had conditions that were just right
for preserving some delicate life forms.
• La Brea Tar Pits, Los Angeles – mammals that got trapped in tar seeps were
preserved, e.g. saber tooth tigers and mammoths
• Solenhofen Limestone, Germany – 150Ma, many rare and well exposed fossils
including the oldest bird fossil, Archaeopteryx.
• Burgess Shale, British Columbia, Canada – many soft shelled creatures, such as
sponges. Researchers have been able to determine the contents of the stomachs of
ancient creatures here.
A trilobite in the Burgess Shale
Archaeopteryx in the Solenhofen Ls.
Evolution and Fossils
• During a five year cruise (1831-1836) around the world, Charles Darwin conceived
the hypothesis that organisms had adapted to their surroundings with less fit
organisms dying out over long periods of time through the process of Natural
Selection. This hypothesis was novel, because it made many predictions that could
be tested.
• Since then it has been tested by nearly 200 years of scientific research and no peerreviewed evidence refutes it. It is now known as The Theory of Evolution.
• Study of fossil sequences has led to two main ideas about how evolution progresses
• (Phyletic) Gradualism – the idea that organisms gradually change over time. Highly
linked to Uniformitarianism.
• Punctuated Equilibrium – Evolution happens very slowly except during short
pulses of evolution, possibly caused by
– Climate changes, Mass extinctions, formation of new environments (faulting), the
isolation of a species (Galapagos)
• Most paleontology research suggests a combination of these two ideas.
James Hutton
The father of modern geology (1726-1797)
• Hutton made observations of geologic structures
around Scotland and came up with a the seminal
scientific principle used in geology today.
• The Principle of Uniformitarianism – Physical
processes acting today acted in the past at more or
less the same rates and are responsible for the
geologic features that we see today.
• Furthermore, if this is correct then the Earth must be
much older than human history allowed.
• Charles Lyell (1797-1875) took the ideas of Hutton
and Steno and published a textbook, Principles of
Geology, that derived several principles useful for
determining the relative ages of various geologic
structures.
• Relative Age – the age of features relative to each
other.
• Absolute Age / Numeric Age – A numerical measure
of the age of an object (e.g. 13Ma, 500 yrs.)
Relative Age vs. Absolute Age
• There are two ways of
dating geological materials.
– Relative ages – Based upon
order of formation.
• Qualitative method
developed 100s of years
ago.
• Permit determination of
older vs. younger
relationships.
– Numerical ages – Actual
number of years since an
event.
• Quantitative method
developed recently.
• Age is given as a number.
Physical Principles For Determining Relative Ages
•
Although these principles were laid out in 1857, they are still key to the modern
study of geologic structures.
We will discuss each in detail, but here’s the list.
1. The Principle of Uniformitarianism
2. The Principle of Superposition
3. The Principle of Original Horizontality
4. The Principle of Original Continuity
5. The Principle of Cross-Cutting Relations
6. The Principle of Inclusions
7. The Principle of Baked Contacts
The Principle of Uniformitarianism
• “The Present is the Key to the Past!”
• Physical processes that we see operating today, also operated in the past at
approximately the same rates.
• Geologists do recognize that the rates of some processes have changed through time
because of factors such as climate change, and occasional catastrophic events
(meteor impacts, floods, etc…).
• So, in these rocks shown below, these Paleozoic mud cracks in tidal flat deposits
(bottom right) and modern mud cracks observed in tidal flats near the Persian Gulf
(bottom left) must have formed in a similar manner and in similar amounts of time.
Modern mud cracks
Paleozoic mud cracks
The Principle of Superposition
• In a sequence of non-overturned sedimentary rocks, each layer must be younger than
the one below because a layer of sediment cannot accumulate if there is no substrate
on which it can collect.
• Sedimentary rocks that are intensely folded to the point that they are overturned,
violate this.
The Principle of Original Horizontality
• Sediments settle out of a fluid and are subjected to gravity, so they are deposited on
relatively horizontal surfaces. If deposited on a slope, the sediment would gradually
be transported downhill until a horizontal substrate is reached.
• So, when we see folds, we know that rocks have undergone deformation.
A sequence of horizontal strata in WI
Sediments are deposited on this horizontal tidal flat
surface in France
The Principle of Original Continuity
• Sediments generally accumulate in continuous sheets. If you find a layer that is cut
by a canyon or river, then you can assume that the layer once spanned the canyon
and was later eroded away.
The Principle of Cross-Cutting Relations
• If one geologic feature cuts another, the
feature that has been cut must be older.
The Principle of Inclusions
• If an igneous intrusion contains fragments of
another rock, the fragments must be older than the
intrusion
• If a layer of sediment deposited on top of an
igneous layer includes pebbles of the igneous
rock, then the sedimentary layer is younger.
• The xenoliths in an igneous body or the pebbles
or sand grains in a sedimentary rock are all older
than the rock that they are in.
The Principle of Baked Contacts
• Igneous intrusions bake (contact metamorphism) surrounding rocks. The rock that
has been baked must be older than the intrusion.
Geologic Time Animation
Fossil Succession
• Fossil Assemblage – A group of fossils that is found in a rock.
• William Smith (1769-1839), an English engineer in the 19th
century, was hired to survey an excavation for a network of
canals to transport coal and iron around England.
• Smith realized that certain rock layers had
distinctive fossil assemblages and that a
particular fossil species can be found only
in a limited interval of strata. This is called
the Principle of Fossil Succession.
• Index Fossils - only lived during a
very specific time interval.
Unconformity – A Gap in Time
• Hutton became very interested in the rock outcrop shown below. He soon realized
that according to uniformitarianism, the lower rocks must have been deposited and
tilted, and then eroded to a flat surface and then the red sandstone was deposited on
top of the older tilted beds. Therefore, there must be some period of time that is not
represented between the formation of these two units.
• Hiatus – A break in the rock record (missing time)
• Unconformity – A period in which rocks are not being deposited, possibly involving
erosion of existing rocks.
– Unconformities come in three flavors…
Siccar Point, Scotland
Disconformity
•
•
•
Disconformity –
An erosional
surface within a
sedimentary
sequence.
Marks missing time
(break in
sedimentation)
within a
sedimentary
sequence.
Hard to recognize,
may have a wavy
surface and/or a
thin soil present
Show unconformity
animations
Disconformities
Angular
Unconformity
• Angular Unconformity rocks below this type of
unconformity were tilted or
folded before the
unconformity developed.
• The rocks above and below
this type of unconformity
have different orientations
Siccar Point, Scotland
Nonconformity
• Nonconformity –
Sedimentary rocks
overlie intrusive igneous
and/or metamorphic
rocks.
• Thus the igneous or
metamorphic rocks have
to have cooled and been
eroded to some base
level
Nonconformity
Granite
Stratigraphic Columns
• Geologists summarize information about
the sequences of rocks in what are called
Stratigraphic Columns, (or Strat
Columns).
• Show information about formation
name, grain size (horizontal size), and
unit thickness (vertical size).
• Boundaries between formations are
called contacts.
Lithologic Correlation
• Geologists find the same formations present in locations that are sometimes many
kilometers apart. Because of this, we can more easily define the age relations of rock
units in different locations.
• Correlation – the process by which the age relationships between rock units
exposed at different localities are determined.
• Units that are present in one location but missing in another are said to “pinch out”.
• Formations may also have differing thicknesses in different locations.
• This is lithologic correlation and is
mainly useful over small-med distances
Fossil Correlation
• Over large distances geologists
can use the presence of certain
index fossils, fossils that only
are found within rocks of a very
limited age, or fossil
assemblages to correlate
between units that are far apart.
• In this case, the Monte Cristo Limestone, found
in the mountains north of Las Vegas, contains
similar fossils to those observed in the Grand
Canyon’s Redwall Limestone.
• Therefore, they
correlate to each
other and must
have been formed
at the same time.
Geologic Maps
• Because stratigraphic formations can be correlated, geologists can plot out the
locations of geologic formations (and other info, faults, strike&dip) on what are
called geologic maps.
• Geologic maps show the spatial distribution of rock formations and are an essential
tool to geologists. The astute reader can use them to find many geologic structures,
such as folds, unconformities, and faults.
• Typically geologic maps are overlain upon topographic maps, so that the reader can
get a feel for the 3D aspects of the region.
Stratigraphic Correlation
• National parks of Arizona and Utah.
– Formations can be traced long distances.
– Overlap is seen in the sequences of rock types.
– Overlapping rock columns are used to build a composite.
Correlation among rock strata in 3 national parks.
The Geologic Column
• A composite stratigraphic column exists.
– Constructed from incomplete sections across the globe.
– It brackets almost the entirety of Earth history.
Absolute Dating
Absolute Dating – Radioactive Decay
• Although since the days of Hutton, geologists have been able to determine the
relative ages of geologic structures, it wasn’t until the 1950’s that we became able to
determine the absolute age of rocks.
• Some elements have unstable isotopes (atoms with the same # of protons, different #
of neutrons), which decay and “radiate” energy.
• Radioactive elements decay at a constant rate, which can be measured in a lab and
are specified in years.
• Geologists can now look at radioactive elements within rocks to determine their age.
This field is called radiometric dating, or geochronology.
Absolute Dating – Radioactive Decay
• After a given time, unstable isotopes undergo radioactive decay, which converts
them into different elements and releases heat.
• These reactions change the atomic number (# protons) of an element and include:
– Alpha decay – Ejection of two protons and two neutrons (Helium nucleus) from the
nucleus.
– Beta decay – Transformation of a neutron into a proton by ejecting an electron from one
of the neutrons in the nucleus.
– Electron capture – Joining of a proton with an electron to form a new neutron.
• The original isotope that undergoes the change is the Parent Isotope
• The isotope that is created is called the Daughter Isotope
Half-Life
• Physicists cannot specify how long and individual isotope will
survive, but they can determine how long it will take for half of
a group of isotopes to decay.
• This time is called the half-life.
• So, lets say that some sample contains 16 units of the parent
isotope to begin with…what happens as time passes?
Useful Isotopes For Dating Rocks
• Not all radioactive isotopes are useful for geologists, they must be somewhat
abundant in rocks, and have half-lives long enough to record geologic time.
• Carbon dating (half-life 5730 yrs.) is only useful for dating very young soils.
• To figure out the age geochronologists must then: collect rocks, separate and
pulverize the appropriate minerals, extract the parent/daughter isotopes (laser, or
with acid), and then analyze the parent/daughter ratio (mass spectrometer).
Radiometric Dates:
What Do They Mean and What Rocks Can We Date?
• At high temps, isotopes in a crystal vibrate so rapidly that chemical bonds can break
easily. So, parent/daughter isotopes are meaningless.
• The “radiometric clock” starts when crystals become cool enough for both parent
and daughter isotopes to become locked in. This temperature is called the blocking
temperature.
• The blocking temperature is typically significantly cooler than the melting
temperature and not all minerals have the same blocking temperature.
• In the case of igneous rocks, a radiometric date tells you when the lava/magma
cooled to a low temperature.
– In a lava flow, different isotopes should yield very similar ages, because flows cool
quickly.
– In plutons, different minerals may yield different ages since plutons may take thousands
to millions of years to cool
• In the case of metamorphic rocks, a radiometric date tells you the time when the
rock cooled from high grade metamorphism to a low temperature (unroofing).
• Most sedimentary rocks cannot be radiometrically dated.
– But the ones that can give dates of the clasts and not the sedimentary rock
Other Dating Techniques
• Geologists also use various other
techniques to determine the absolute
ages of various geologic units.
• Most of these techniques are based on
processes that have drastic seasonal
variations producing rhythmic layering
– Layers in glaciers
– Seasonal sediment fluctuations in lakes
– Etc…
Rhythmic layering in a glacial ice
Magnetostratigraphy
• Geologists can also look at the magnetostratigraphy, the magnetic polarity preserved
in rock layers, to determine ages.
• If you encounter a lava flow, you can get a radiometric and magnetostratigraphic
age. This is how we know the ages of magnetic reversals of the Earth’s magnetic
field.
Fission Tracks
• In certain minerals, the ejection of an atomic particle during
radioactive decay damages the nearby crystal creating a line called a
fission track.
• As time passes, more isotopes decay
and more fission tracks are formed.
• Geologists can measure the rate at
which fission tracks are produced in
the lab.
• The number of fission tracks per
volume of a crystal represents the
age of the crystal.
Dating Sedimentary Rocks
• Since we can’t directly
calculate the numerical
age of most sedimentary
layers (cosmogenic dating
is an exception),
geologists rely on
bracketing ages of strata
with datable igneous
rocks.
Age of the Earth
• Before radioactivity was discovered
in 1896, scientists made many
estimates of the Earth’s age that
were all incorrect and too young.
– Sedimentation ~1mm/yr thickest known
sediment 20km = 20 Ma Earth.
• What about unconformities and
metamorphic rocks?
• What about the Hadeon?
– Oceans started as freshwater now salty
= 90 Ma Earth
• What about evaporites?
• Were oceans always on Earth?
– Time to cool something as hot as the
Sun into Earth = 20 Ma
• What about radioactive heat production,
and mantle convection?
grrr!
Age of the Earth
• The oldest rocks that geologists have found are quartzites
(metasandstones) from Australia = 4.1-4.2 Ga.
• If we assume that most of the objects formed at about the same time
from the interstellar cloud (i.e. nebular theory)
– Meteorites are 4.57 Ga. So, Earth must be ~4.57 Ga.
• We probably won’t find rocks much older than 4.1-4.2 Ga (Hadeon
Eon), because the Earth was so hot that most rocks didn’t solidify for
the first nearly half billion or so years after Earth’s formation.
Geologic Time Scale
Geologists have scoured the Earth for the last few hundred years
by using the techniques you just heard about have come up with
the story of the history of the Earth.
Geologic Time Scaled to One Earth Year
• Just for fun, lets say that scale down geologic time to one Earth year
and note the days of certain events.
• Earth Forms - Jan 1st
• First Rocks - Feb 21st
• First Shelled Invertebrates - Oct 25th
• First Amphibians - Nov 20th
• Pangaea Formed - Dec7th
• First Mammals, Birds, and Dinosaurs - Dec 15th
• Dinosaurs Go Extinct - Dec 25th
• First Homo Sapiens (Human Ancestors) - Dec 31st
• The Last Ice Age Ends - Dec 31st 11:59 P.M.
• All Recorded Human History – Last 30 seconds before midnight.
– Human history is 0.000001% (1/100 billion) of Earth’s history.