•Geological Time
• The Painted Desert, Arizona. This landscape has a
story to tell, and each individual rock and even the
colors mean something about the past.
• Fossils
• Early Ideas about Fossils
– Herodotus
• realized that fossil shells found far from oceans were
remnants of an ancient sea.
– Aristotle
• Believed in Spontaneous Generation
• Thought that fossils grew in place in rocks.
– Leonardo da Vinci
• Argued that fossils were remnants of living organisms
from the Earths past history.
– William “Strata” Smith
• Discovered a relationship among the strata from
different areas.
• Found that each strata has a distinct group of fossils
associated with it and that these fossils were different
from the fossils found in other strata.
• Types of Fossilization
– Preservation or alteration of hard parts
• This is the actual preservation of the hard parts themselves
• Could include shells, bones, exoskeletons, etc…
– Preservation of the shape
• Mineralization
– The filling of pores with calcium carbonate, silica, or
pyrite
• Replacement
– Dissolving of the original material and depositing new
material one ion at a time (ion exchange)
– Preservation of signs of activity
• Footprints
• Bones of prey
• The fossil record of the hard parts is beautifully preserved,
along with a carbon film, showing a detailed outline of the
fish and some of its internal structure.
• Origin of
molds and
casts. (A)
Formation
of a mold.
(B)
Formation
of a cast.
• These logs of petrified wood are in the Petrified
Forest National Park, Arizona.
• Reading Rocks
• Arranging Events in Order
– Principle of uniformity
• Stated that the present is the key to the past.
• Look at geological features of today’s Earth as being
shaped by its past.
• States that the processes that are going on today to
shape the Earth are the same processes that have
always been going on.
– Principal of original horizontality
• Based on the premise that sedimentary rock is
deposited in flat-lying layers.
• This means that if a sedimentary layer is not
horizontal, it has been subjected to forces that have
moved or changed it.
• If it is horizontal, we can use this to study organisms
that lived on the Earth during the same periods of
time.
• The principle of
original
horizontality. (A)
Sediments tend
to be deposited in
horizontal layers.
(B) Even where
the sediments are
draped over an
irregular surface,
they tend toward
the horizontal.
– Principal of superposition
• As sedimentary layers are laid down they are
compacted and cemented together, which keeps the
organisms in that layer together.
• The principle of superposition. In an undisturbed
sedimentary sequence, the rocks on the bottom were
deposited first, and the depositional ages decrease as you
progress to the top of the pile.
• The Grand Canyon, Arizona, provides a majestic cross
section of horizontal sedimentary rocks. According to the
principle of superposition, traveling deeper and deeper into
the Grand Canyon means that you are moving into older and
older rocks.
– Principal of crosscutting relationships
• Any igneous or metamorphic rock that cuts across a
sedimentary strata must be younger than the layer it
crosscuts.
• This is helpful in tracking events in the Earth’s history.
– Shifting sites of erosion and deposition
• As sedimentary rock is eroded and moved to another
place, there will always be a time lapse between the
shifting and the compacting and cementing.
• A granite intrusion cutting across older rocks.
– Unconformity
• States that as there is a time lapse between
sedimentation and compacting and cementing there
will be a surface break within the sedimentary
deposition.
– Angular unconformity
• This is where the bedding planes above and below the
area of unconformity are not parallel.
• This is due to tilting or folding followed by a period of
erosion and a period of deposition.
• Angular unconformity.
Development involved
some deformation and
erosion before
sedimentation is
resumed.
• A time break in the rock record in the Grand Canyon,
Arizona. The horizontal sedimentary rock layers overlie
almost vertically foliated metamorphic rocks. Metamorphic
rocks form deep in the earth, so they must have been
uplifted, and the overlying
• Correlation
– Principal of faunal succession
• Recognizes that life has changed throughout time.
• As one group of organisms disappears from the fossil
record it is replaced with new ones.
• The same form is never exactly duplicated
• This allows fossils to be put in chronological order
based on their appearance in the strata and the fossil
record.
– Index Fossils
• A fossil that occurred widely, but for a very brief time
period.
– Correlation
• The use of index fossils and the other methods of
reading fossils to make it possible to compare rocks
that have been exposed in two locations
– Relative dating
• Fossil dating using these methods is called relative
dating.
• It is a way of dating fossils and geological events
relative to other fossils and geological events.
• Deciphering a complex rock
sequence. The limestones must be
oldest (law of superposition),
followed by the shales. The
granite and basalt must both be
younger than the limestone they
crosscut (note the
metamorphosed zone around the
granite). It is not possible to tell
whether the igneous rocks predate
or postdate the shales or to
determine whether the
sedimentary rocks were tilted
before or after the igneous rocks
were emplaced. After the
limestones and shales were tilted,
they were eroded, and then the
sandstones were deposited on top.
Finally, the lava flow covered the
entire sequence.
• Similarity of fossils suggests similarity of ages, even
in different rocks widely separated in space.
• This dinosaur footprint is in shale near Tuba City, Arizona.
It tells you something about the relative age of the shale,
since it must have been soft mud when the dinosaur stepped
here.
• (A)Fossil
trilobites from
Cambrian
rocks. (B)
Fossil
ammonites
from Permian
rocks.
• Geological Time
• Early Attempts at Earth Dating
– Introduction
• Archbishop Ussher of Ireland counted up the
generations of people mentioned in the Bible and
decided that the Earth was created at 9:00 AM on
Tuesday, October 26 4004 BC
• At the end of the 18th century James Hutton reasoned
that the Earth must be much older
• People began to look for a way of dating the Earth
using some sort of Geological Clock.
– Geological Clocks
• Process must have been present since the Earth was
first created
• The process must occur at a uniform rate
• The process must be able to be measured
– Clocks considered
• Rate of salt addition to the ocean
– Resulted in an age for the Earth of 100 million
years
• Rate of sediment deposition
– Resulted in an age for the Earth of 20 to 1,500
million Years
• Rate of cooling of the Earth
– Resulted in an age for the Earth of 20 to 40 million
• Modern Techniques
– Radioactive decay
• Measure the rate of radioactive decay of unstable
elements within the crystals of certain elements.
• Since decay rates are constant the ratio of the decay
product to the remaining amount of the original
element can be used as a measure of the age of the
material in which it is found.
– Geomagnetic time scale
• Looks at the duration and number of magnetic field
reversals occurring during the past 6 million years.
• Used in combination with Radiometric dating.
• The Geological Time Scale
– Eras
• Cenezoic
– Time of recent life
– Fossils resemble modern day living organisms
• Mesazoic
– Time of middle life.
– Some fossils resemble modern day living
organisms
– Some are different from any living organism on the
Earth today.
• Paleozoic
– Time of ancient life.
– Fossils are different from anything found on the
Earth today.
• Precambrian
– Time before the time of ancient life.
– Very few fossils are found in this time period
– Periods
• Eras were further divided into Periods
– Epochs
• Periods are further divided into Epochs
• Geologic history is divided into four main eras. The
Precambrian era was first, lasting the first 4 billion years, or
about 85 percent of the total 4.6 billion years of geologic
time. The Paleozoic lasted about 10 percent of geologic
time, the Mesozoic about 4 percent, and the Cenozoic only
about 1.5 percent of all geologic time.
• The divisions of
the geologic
time scale.
• The periods of the
Paleozoic era, which
refers to the time of
ancient life. Ancient
life means that the
fossils for this time
period are very
different form
anything living on the
earth today. Each
period represents a
dominant life form of
a different species of
animal. This pie chart
compares the relative
time that each period
lasted.