5. The Geologic Time Scale
• the time line of the History of the Earth, is
based from the rock record.
• Geologic time is subdivided into hierarchal
intervals
1. Eon (the largest)
2. Era
3. Period
4. Epoch
6. • Subdivision of Geologic time is based from
significant events in the Earth’s History as
interpreted from the rock record.
7. Age of the Earth
• The Earth has a very long history — 4.5
billions of years of history.
• The age of the Earth is based from the
radioactive isotopic dating of meteorites.
8. Rocks Record Earth History
12.1 Discovering Earth’s History
Rocks record geological events and
changing life forms of the past.
We have learned that Earth is much older
than anyone had previously imagined and
that its surface and interior have been
changed by the same geological processes
that continue today.
9. A Brief History of Geology
12.1 Discovering Earth’s History
Uniformitarianism means that the forces
and processes that we observe today have
been at work for a very long time.
10. Fossils Indicate Evolutionary Pathways:
Precambrian
Little direct evidence of fossils, due to lack of hard body parts.
Fossil evidence include; algae, bacteria, and traces of soft body
organisms.
Paleozoic Era -- “Age of the Invertebrate”.
Invertebrates evolved into vertebrates;
First land plants evolved in the Silurian.
Abundance of fishes in the Devonian which is known as the “age of the
fishes”.
Lung fish evolved into amphibians throughout the the Mississippian
and Pennsylvanian.
Amphibians evolved into reptiles in the Permian and reptiles are known
as the first true land dwellers. Hard shelled eggs made this possible.
Mass extinctions of invertebrates including trilobites and numerous
other marine species occurred at the end of the Paleozoic Era.
11. Fossils Indicate Evolutionary Pathways:
Mesozoic Era -- “Age of the Reptiles”
Dinosaurs became dominant.
First birds are seen during this time
The end of the Mesozoic Era was marked by mass extinctions
of reptiles including dinosaurs and numerous other species.
Cenozoic Era -- “Age of the Mammals”.
Mammals evolve and dominate during this time.
Flowering plants are the dominant land plant.
Some mammals became extinct during the late Cenozoic
(11,000 years ago). These include the mastodon, mammoth,
saber-tooth cat, large ground sloth, giant bison and others.
12.
13. Nicholas Steno
• Steno’s principles – superposition, original
horizontality, and lateral continuity
• Stratigraphy – the study of layered rocks.
14. Relative Dating—Key Principles
12.1 Discovering Earth’s History
Relative dating tells us the sequence in
which events occurred, not how long ago
they occurred.
Law of Superposition
• The law of superposition states that in an
undeformed sequence of sedimentary rocks,
each bed is older than the one above it and
younger than the one below it.
16. Relative Dating—Key Principles
12.1 Discovering Earth’s History
Principle of Original Horizontality
• The principle of original horizontality means
that layers of sediment are generally deposited
in a horizontal position.
18. Relative Dating—Key Principles
12.1 Discovering Earth’s History
Principle of Cross-Cutting Relationships
• The principle of cross-cutting relationships
states that when a fault cuts through rock layers,
or when magma intrudes other rocks and
crystallizes, we can assume that the fault or
intrusion is younger than the rocks affected.
Inclusions
• Inclusions are rocks contained within other rocks.
• Rocks containing inclusions are younger than
the inclusions they contain.
22. Abraham Gottlob Werner
• a German geologist. Werner divided the
rock record into the following rock-time
units (from oldest to youngest):
– Primary
– Secondary
– Tertiary
– Quaternary.
• Used the Principle of Superposition
extensively to establish temporal
relationship among the rock units.
23. William Strata Smith
• used fossils primarily to identify rock layers
• observed that each layer or strata of
sedimentary rock contain a distinct
assemblage of fossils which can be used
to establish equivalence (correlation)
between rock units separated by long
distances. Moreover
• observed that these fossils succeed each
other vertically in a definite order.
24. Charles Lyell
• British Lawyer and Geologist, recognized
the utility of fossils in subdividing Geologic
Time on the basis of fossils. He was able
to subdivide the Tertiary by examining the
proportion of living vs.extinct fossils in the
rocks.
25. Relative Dating—Key Principles
12.1 Discovering Earth’s History
Unconformities
• An unconformity represents a long period
during which deposition stopped, erosion
removed previously formed rocks, and then
deposition resumed.
• An angular unconformity indicates that during the
pause in deposition, a period of deformation
(folding or tilting) and erosion occurred.
27. Relative Dating—Key Principles
12.1 Discovering Earth’s History
Unconformities
• A nonconformity is when the erosional surface
separates older metamorphic or intrusive
igneous rocks from younger sedimentary rocks.
• A disconformity is when two sedimentary rock
layers are separated by an erosional surface.
29. Correlation of Rock Layers
12.1 Discovering Earth’s History
Correlation is establishing the equivalence
of rocks of similar age in different areas.
31. EVOLUTION OF EARTH’S
HISTORY
Fossils are the remains or traces of prehistoric life.
They are important components of sediment and
sedimentary rocks.
Biostratigraphy - a sub-discipline of stratigraphy
which deals with the use of fossils in correlation and
establishing the relative ages of rocks.
Index fossils are widespread geographically, are
limited to a short span of geologic time, and occur in
large numbers.
32. Fossils and Correlation
12.2 Fossils: Evidence of Past Life
The principle of fossil succession states
that fossil organisms succeed one another
in a definite and determinable order.
Therefore, any time period can be
recognized by its fossil content.
Index fossils are widespread
geographically, are limited to a short span of
geologic time, and occur in large numbers.
33. Fossil Formation
12.2 Fossils: Evidence of Past Life
Interpreting Environments
• Fossils can also be used to interpret and
describe ancient environments.
35. Assessment
1. the time line of the History of the Earth, is
based from the rock record.
2. He is the first to recognize the
correspondence of between rocks and
time
3. The study of layered rocks.
4. He observed that these fossils succeed
each other vertically in a definite order.
5. He observed that each layer or strata of
sedimentary rock contain a distinct
assemblage of fossils
36. 1. What are the different geologic times
Scale of the Earth?
2. Enumerate the important events on the
Geologic time of the Earth
38. Fossil Formation
12.2 Fossils: Evidence of Past Life
Fossils are the remains or traces of
prehistoric life. They are important
components of sediment and sedimentary
rocks.
• Some remains of organisms—such as teeth,
bones, and shells—may not have been altered,
or may have changed hardly at all over time.
The type of fossil that is formed is
determined by the conditions under which
an organism died and how it was buried.
Unaltered Remains
39. Fossil Formation
12.2 Fossils: Evidence of Past Life
Altered Remains
• The remains of an organism are likely to be
changed over time.
• Fossils often become petrified or turned to stone.
• Molds and casts are another common type of
fossil.
• Carbonization is particularly effective in
preserving leaves and delicate animals. It occurs
when an organism is buried under fine sediment.
40. Fossil Formation
12.2 Fossils: Evidence of Past Life
Indirect Evidence
• Trace fossils are indirect evidence of prehistoric
life.
Conditions Favoring Preservation
• Two conditions are important for preservation:
rapid burial and the possession of hard parts.
42. Structure of the Time Scale
12.4 The Geologic Time Scale
Based on their interpretations of the rock
record, geologists have divided Earth’s
4.56-billion-year history into units that
represent specific amounts of time. Taken
together, these time spans make up the
geologic time scale.
43. Structure of the Time Scale
12.4 The Geologic Time Scale
Eons represent the greatest expanses of
time. Eons are divided into eras. Each era
is subdivided into periods. Finally, periods
are divided into smaller units called epochs.
There are three eras within the
Phanerozoic eon: the Paleozoic, which
means “ancient life,” the Mesozoic, which
means “middle life,” and the Cenozoic,
which means “recent life.”
44. Structure of the Time Scale
12.4 The Geologic Time Scale
Each period within an era is characterized
by somewhat less profound changes in life
forms as compared with the changes that
occur during an era.
The periods of the Cenozoic era are divided
into still smaller units called epochs, during
which even less profound changes in life
forms occur.
45. Precambrian Time
12.4 The Geologic Time Scale
During Precambrian time, there were fewer
life forms. These life forms are more difficult
to identify and the rocks have been
disturbed often.
47. Difficulties With the Geologic Time Scale
12.4 The Geologic Time Scale
A sedimentary rock may contain particles
that contain radioactive isotopes, but these
particles are not the same age as the rock
in which they occur.
The age of a particular mineral in a
metamorphic rock does not necessarily
represent the time when the rock was first
formed. Instead, the date may indicate
when the rock was metamorphosed.
49. WHAT IS A GEOLOGIC
TIME SCALE?
System of chronologic
measurement relating
to stratigraphy to time
that is used by geologist
and other earth
scientists
50. INTRODUCTION TO
CONCEPT:
The period is the basic unit of geological
time in which a single type of rock system
is formed. Two or more periods comprise
a geological Era. Two or more Eras form
an Eon, the largest division of geologic
time. Some periods are divided into
epochs.
The major periods in the geologic history
of the Earth are (mya=million years ago):
52. PRECAMBRIAN
Hadean Eon
4.6 to 3.9 billion
years ago
“Rockless Eon" -
The solidifying of
the Earth's
continental and
oceanic crusts.
Archeozoic Eon
(Archean)
3.9 to 2.5 billion
years ago
"Ancient Life"
The first life forms
evolve
Proterozoic Eon
2.5 billion years
ago to 540 mya
*multi-celled, animals
appear, including
sponges.
*Single supercontinent
called Rodinia.
*First multicellular life:
colonial algae and soft-
bodied invertebrates
appear.
53. THE FIRST EON:
HADEAN
•Earth's formation by
accretion from the solar
nebula, 4.6 billion years
ago
•bombardment by
meteorites
•volcanism must have
been severe
•Earth changed from
liquid to solid
54. THE SECOND EON: ARCHEAN
•3.8 to 2.5 billion years ago
•Earth cooled significantly
•An atmosphere formed which lacked oxygen
and an ozone layer but likely contained
methane, ammonia, and other gases which
would be toxic to most life on our planet
today.
•primordial life began to evolve:
stromatolites, such as cyanobacteria
(blue-green algae) and archea (bacteria that
can survive in extreme conditions)
56. THE THIRD EON: PROTEROZOIC
•started 2.5 billion years ago and ended 543 million years ago
•stable land masses first appeared
•oxygen build-up in the atmosphere
•prokaryotic bacteria and the four eukaryotic kingdoms (plants,
animals, fungi, & protists)
•end of proterozoic:
first soft-bodied
animals
(fossil evidence)
59. PHANEROZOIC EON
Paleozoic Era:
The Paleozoic is bracketed by two of the
most important events in the history of
animal life.
1. Multicellular animals went dramatic
diversity explosion
2. Largest Mass extinction of mostly 90%
of marine organisms
68. PHANEROZOIC EON
Mesozoic Era:
Many of the Old life forms had just gone
extinct in the Permian Extinction, the
world’s largest mass extinction.
Depleted state was followed by an
explosion of new life forms such as
dinosaurs, mammals, birds and flowering
plants
78. PHANEROZOIC EON
Cenozoic Era:
The most recent Era of the three major
subdivisions of animal history.
Also called as the Age of Mammals
65.5 million years ago to present
