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Geologic Time Scale
Originally based on
relative age
- revised with development of
absolute age techniques
- refined as new information
becomes available.
The geologic time scale
is divided into:
The Pre-Cambrian Supereon
of 3 Eons
By end of Precambrian,
organisms developing
hard parts.
Life believed to
originate around
the beginning of
the Archean.
Stromatolite, Archean bacteria
The Pre-Cambrian Supereon
Geologic setting
- First rocks formed are igneous
- Crust formed through ‘differentiation’
- Earliest continental land masses form this way
- Erosion ‘erases’ all land older than 3.8 byo
- Oldest fossils are 3.5 byo (cyanobacteria microfossils)
- Produced oxygen, poisonous to other bacteria
- Atmospheric oxygen levels gradually increased; makes
Cambrian explosion possible
- At end of Pre-Cambrian, glaciation kills almost all
The Phanerozoic Eon (“visible life”)
Divided into 3 Eras
Cenozoic (“recent life”)
Mesozoic (“middle life”)
Paleozoic (“ancient life”)
The Paleozoic Era
Divided into 7
Beginning of
Cambrian period
Explosion of life
Life in the Paleozoic Era
First two periods (Cambrian & Ordovician) known as “age of
invertebrates” – first marine organisms with shells show up at
beginning of the Cambrian; commonly referred to as the “Cambrian
Middle of Paleozoic (Silurian & Devonian) known as “age of
fishes” – also during this time, first land plants and first insects
appear. Near end of Devonian, first amphibians
evolve from fishes. Life is poised to move on to
Late Paleozoic – amphibians are thriving, begin to
see first reptiles.
Geology of the Paleozoic Era
During the Paleozoic, there was much tectonic activity. In the
Ordovician, Gondwana formed (Antarctica, South America,
Africa, Madagascar, Australian, the Arabian Peninsula, and
Indian subcontinent). This would later be joined with Laurasia
(North American, Baltica, Siberia, Kazakhstania, & the North
China and East China cratons.) to form Pangaea.
Time of mountain building – Appalachians
formed, as well as mountains in what is
now western Europe.
This is thought to have caused significant
climate change, which may have
contributed to the extinction
event at the end of the Paleozoic.
The Great Permian Extinction
Occurred at the end of
Paleozoic Era (marks the end of
the Permian Period)
Estimates that 90% of all marine
species, 70% of all land species
went extinct
Possible causes:
- Impact event
- Volcanism
- Methane gas emissions leading
to climate change
- Sea level changes
The Mesozoic Era
Divided into 3 periods
Life in the Mesozoic Era
Mesozoic (“middle life”) is known as the “age of the dinosaurs.”
Dinosaurs ruled for over 100 million years. Pangaea continues its
breakup during the early part of the Mesozoic. The Rocky
Mountains and Himalayas form, the Atlantic Ocean basin widens,
and the Indian Ocean forms. More coastlines mean more habitats.
Also during this time, the first mammals, birds, and flowering plants
appear. We also see crocodiles, turtles, and primitive mammals
showing up in the fossil record of this era.
On land, lush forests and swamps thrive, eventually producing large
coal deposits along what is today the Gulf of Mexico, as well as
parts of western North America.
Towards the end of the Mesozoic, Pterosaurs become first
vertebrate to fly.
And then….
The Cretaceous Period
Death in the Cretaceous
A large asteroid or comet, approximately 10 km wide, slams
into what is now the Yucatan Peninsula in Mexico. Firestorms rage,
and the cloud of dust and debris that is ejected into the atmosphere
causes cooling of the planet and a significant drop in
photosynthesis. Estimates are that at least 60% of all life goes
extinct, including the dinosaurs.
But one group survives….
The Cenozoic Era
Divided into 2 periods
Further divided into 7 Epochs
Life in the Cenozoic Era
In the Cenozoic, oceans flourish, insects
thrive and diversify, mammal species increase
as they take advantage of the niches left
vacant by the death of the dinosaurs, large
grasslands support ancestors of modern
grazing land animals, Mastodons and Wooly Mammoths
survive until the last ice age, large herds of horses, camels,
and bison roam North America until about 12,000 years
ago, then go extinct.
Geology of the Cenozoic Era
In the Cenozoic, the Atlantic basin opens further,
continents move into position we see today,
Rockies and Appalachians grow and are worn
down, Alps and Pyrenees form, massive lava flows
produce Columbian Plateau, Cascade mountain
range forms, Colorado plateau raised several times,
Colorado river cuts out Grand Canyon.
Climate in the Cenozoic Era
- Climate warm and humid at beginning
- By mid-era, temperatures cool
- Ice sheets cover Northern Europe, Siberia,
Greenland, and Antarctica
- At one point, one-fourth of all land covered by ice
- Last major ice age ends around 11,000 years ago
Peephole into the past…
fos•sil (\fä-səl\)
(From the Latin fossus, literally “having been dug up”)
1: a remnant, impression, or trace of an
organism of past geologic ages that has been
preserved in the earth's crust
Fossils provide clues to organisms and their
environments in Earth’s geologic past and are the basis
for the geologic time scale.
Scientists who study fossils are called paleontologists.
However, fossils are very rare and only offer a limited
view into the past.
Requirements for fossilization
In order for an organism to become fossilized, it needs:
Hard parts** – bones, shells, teeth, or wood
Rapid burial – organism must be buried quickly in anoxic
environment to avoid decomposition or decay. The longer an
organism is exposed to oxygen after death, the more it will
Conditions for fossilization are rare. Marine organisms
dominate the fossil record because as they die, they sink to
the bottom of the ocean and are buried by marine sediments.
Terrestrial environments usually do not have conditions
favorable for fossilization.
**Some organisms without hard parts have been preserved as fossils, but as a general
rule, the more hard parts an organism has, the greater the chance it will be preserved
as a fossil.
1. Sediment
An animal is buried by sediment,
such as volcanic ash or silt, shortly
after it dies. Its bones are protected
from rotting by the layer of
2. Layers
More sediment layers
accumulate above the
animal’s remains, and
minerals, such as silica
(a compound of silicon
and oxygen), slowly
replace the calcium
phosphate in
the bones.
3. Movement
Movement of tectonic
plates, or giant rock
slabs that make up
Earth’s surface, lifts
up the sediments and
pushes the fossil
closer to the surface.
4. Erosion
Erosion from rain,
rivers, and wind wears
away the remaining
rock layers. Eventually,
erosion or people
digging for fossils will
expose the preserved
Types of Preservation
Permineralization Empty spaces in
fossil are filled by minerals
precipitated out of groundwater.
Original structure of organism is
preserved. Also known as
“Petrification” (“turned to stone”)
Petrified wood
The Field Museum in
Chicago displays a
fossil of a
Tyrannosaurus rex.
Types of Preservation
Carbon Film
Original organic material is
preserved as a thin carbon
film. Soft tissue can be
preserved in this manner if
conditions are just right.
Types of Preservation
Molds and Casts
A mold is the cavity left by the
original shell after it dissolves.
mold of
A cast is produced when
that void is filled with
sediment or minerals.
Fern leaf
fossil cast
Types of Preservation
Original Remains
In rare cases, the original
tissue from an organism
will be preserved. This
can occur in various
ways, such as through
freezing, drying,
entrapment in amber or
tar, or burial in peat bogs.
Insects trapped
in amber
Skull of Sabertooth tiger
extracted from
La Brea tar pits
Types of Preservation
Original Remains
In rare cases, the original tissue from an organism will be preserved. This can
occur in various ways, such as through freezing, drying, entrapment in amber
or tar, or burial in peat bogs.
Types of Preservation
Trace Fossils
The marks left by an
organism in it’s
environment, such as
tracks, burrows, trails,
nests, and droppings
(known as coprolites).
Index Fossils
Also known as guide or zone
fossils – used to identify a
particular geologic period. Index
fossils come from species that
were short-lived during a
specific time period, very
abundant, and found over a
large geographic area.
Lophophyllidium proliferum
Pennsylvanian period
Asaphiscus wheeleri,
a trilobite from the
Cambrian shale of
Ever date a fossil?
Relative Age
(AKA – Biostratigraphy)
The geologic time scale was
originally developed by determining
the relative ages of rock layers in the
Earth. The relative age was
determined by comparing the fossils
found in that layer with those in
adjacent layers. This concept, known
as the principal of faunal succession,
states that fossils succeed each
other vertically in a specific, reliable
order over a wide horizontal area.
Ever date a fossil?
Absolute Age
(AKA – Geochronology)
When molten rock cools to form an
igneous rock, these radioactive
elements are trapped inside. Over
time, these radioactive elements
decay into into different, stable
elements. Scientists use these known
radioactive elements to determine the
absolute age of a rock or rock layers
by comparing the amount of
radioactive elements to the amount of
the related stable elements to
determine the rocks age.
Laying down the law!
Principle of
Original Horizontality
and the
Principle of Superposition
Sedimentary rocks are
composed of sediments
originally deposited in a
horizontal plane (Original
Horizontality), with the oldest
sediment on the bottom and
youngest on top
But alas, it’s not always this
Uniformitarianism is the assumption that
the same natural laws and processes that
operate in the universe now, have always
operated in the universe in the past and
apply everywhere in the universe.
“The present is the key to the past.”
Fossils as
record of life
The fossil record
shows life has
become more
complex and diverse
over time
Matching Up Rock Layers — Correlation
cor·re·la·tion [kawr-uh-ley-shuhn, kor-] noun
1. mutual relation of two or more things, parts, etc.
Correlation is the process of matching similar layers of
rock found in two or more different locations. Sedimentary
rocks that date from the same age can be correlated over
long distances with the help of fossils.
Principle of Fossil Correlation - Strata (i.e. layers of
rock) containing similar collections of fossils (called
fossil assemblages) are of similar age. Also, fossils at
the bottom of the strata are older than fossils closer to the
top of the strata.
Matching Up Rock Layers —
Evidence Used for Correlation
For example, if the same
Types of fossils were found
in a limestone layer in two
different places, it’s a good
Indication that the limestone
at each location is likely
the same age, and
therefore, one continuous
There are times when one or
more layers may be missing,
due to erosion or
“Finding Clues to Rock Layers”
1) Study the rock layers and fossils at Sites 1
and 2. On a separate sheet of paper, write
down the similarities and differences
between the layers at the two sites.
2) List the kinds of fossils that are found in
each rock layer of Sites 1 and 2.
Complete the “Analyze and Conclude” section
(questions 1-9). You do not have to do the
“More to Explore” activity
Make Observations about the fossils found in
each layer.
Column 1 Column 2
When you have completed the test, turn in your
“Fossils” vocabulary and your geologic time wheel.
Layers of rock are said to be
“conformable” when they are deposited
in a sequence that has not been
interrupted. This is rare, and most rock
layers show some form of interruption.
These interruptions, or gaps, in the
rock layers are called
When a rock or sediment
layer is eroded away, and
another rock layer is
deposited on top of it,
this becomes an
unconformity, meaning
there is a break in the
sedimentary record.
There are 3 basic types
of unconformities:
Angular unconformity
When there is layer
missing from a
geological sequence
due to non-deposition
or erosion, this is
known as a
disconformities are
found in parallel layers,
they are sometimes
difficult to spot.
Disconformity between Wingate and Chinle formations
The sequence of events is as follows:
1st - Subsidence and sediment deposition;
2nd - Uplift and erosion;
3rd - Renewed subsidence and deposition.
In the picture above, the disconformity is indicated by an irregular
black line between the 3rd and 4th rock unit from the bottom.
exists between
sedimentary rock
layers that have
been deposited on
top of pre-existing
and older igneous
or metamorphic
“The Great Unconformity”
An angular
unconformity exists
where horizontally
parallel layers, or strata,
of sedimentary rock are
deposited on tilted and
eroded layers that may
be either vertical or at an
angle to the overlying
horizontal layers.
Angular Unconformity, Grand Canyon, Arizona.
Angular unconformities - overlying and underlying rocks dip at different angles.
Underlying rocks were tilted and eroded before the younger rocks were deposited.
Matching Up Rock Layers —
Evidence Used for Correlation
Geologists often
can match up, or
correlate, layers
of rocks over
great distances.
Sometimes it’s
possible to walk
along the layers
for kilometers
and prove that
it’s continuous.
Matching Up Rock Layers —
Evidence Used for Correlation
In other cases, the rock
layers are exposed only
where rivers have cut
through overlying layers
of rock and sediment.
Reading the rocks…
“G” – deposited, then deformed, then eroded
“S, H, L” – deposited
“B” – lava intruded through the layers and formed
extrusive layer on top of “L”
Reading the rocks…
“S” layers deposited
“F” layers were cut by fault
Erosion, followed by deposition of “L”
Reading the rocks…
“A” through “L” – deposited
“H” – Igneous intrusion
“D” – Igneous intrusion
Erosion, “C”, followed by deposition of “S”
Somehow a "giant hotdog" also formed amongst the
Hamburger Rocks in southern Capitol Reef National Park!
View of the Hamburger Rocks in the Halls Creek valley in Southern Capitol Reef
National Park. A red layer of silty sandstone in the upper Navajo
Sandstone(probably stream floodplain deposits) stand out in contrast to white
sandstone (dune sand deposits) that makes up most of the formation.
Read pages 150–158 in “Cells and Heredity”
Read pages 70-77 in your Glencoe textbook
“Clues to Earth’s Past” handout – Section 1
Define the following terms:
- Fossil
- Sedimentary rocks
- Petrified fossils
- Permineralization
- Mold
- Cast
- Carbon film
- Original remains
- Trace fossils
- Index fossil
- Relative dating
- Absolute dating
- Half-life
- Radioactive elements
- Fossil record
- Extinct
- Gradualism
- Punctuated Equilibrium
- Principle of Superposition
- Principle of Original Horizontality
- Uniformitarianism
- Unconformities
- Disconformity
- Nonconformity
- Angular Unconformity
- Eon
- Era
- Period
- Epoch
The Cambrian Period
Geologic setting
- Oceans, shallow seas cover Earth
- Six major continental land masses in southern hemisphere
- Marine invertebrates, like trilobites, flourished
- Preserved in fossil record because of their hard parts
- Mild, probably warmer than today
The Ordovician Period
Geologic setting
- Supercontinent Gondwana formed
- Significant volcanic activity
- Shallow seas uplifted
- Marine blastoids, corals,
snails, and clams first appear
- Early land animals and plants
- By late Ordovician, oxygen levels decreased
- More than 100 marine invertebrate families go extinct
The Silurian Period
Geologic setting
- Large glacial formations melt
- Sea levels rise
- Major mountain building events in NW Europe and
eastern North America produce mountain chains
- Tropical seas, coral reefs first appeared
- Many changes in populations of fish
- Wide rapid spread of jawless fish species
- Stabilized during Silurian
The Devonian Period
Geologic setting
- Three major continental land masses near equator
- North America and Europe collide, produce Appalachian
- Much volcanic and earthquake activity
- Amphibians among first land vertebrates
- First wingless insects, first trees and forests appear in
fossil record
- Brachiopods, corals, and
ammonites flourished
- Much like today
- By end of Devonian, glaciation
The Pennsylvanian &
Mississippian Periods
Geologic setting
- Shallow, warm marine waters flooded through continents
and receded several times
- Land masses move closer together
- Uplifting of land along with formation of lowlands, deltas
- Tropical swamp forests and terrestrial habitats widespread
- Lizard-like amniot appeared – laid eggs on land
- Led to evolution of birds, mammals, and reptiles on land
- More tropical and humid than today
- Large ice sheets developed over South Pole
The Permian Period
Geologic setting
- Pangea forms
- Intense mountain building occurs
- Interior portions of Pangea were dry, with miles of dunes
- Mass extinction occurs at end of Permian
- 90% of marine, 70% terrestrial species extinct
- Glaciers recede, climate becomes arid
- Great seasonal changes and extreme temperatures
- Along Pangea’s east coast, rain produces near-tropical
Life in the Mesozoic Era
Geologic setting
- Pangea starts to fracture by mid-Triassic
- Mountains form along west coast of North and South
- Continents much higher than sea level, no shallow seas
- Volcanoes spewed out massive lava flows
- First dinosaurs appear
- First crocodiles, turtles, and small, primitive mammals appear
- Influenced by large tectonic events
- Most of Earth was arid and dry
The Jurassic Period
Geologic setting
- Pangea rotates, breaks apart
- Central Atlantic and Gulf of Mexico water basins form
- Mountain building leads to uplift of Nevadan range
- Abundant life forms, including great plant-eating dinosaurs
- Stalked by vicious flesh-eating carnivores
- Lush fern and cycad growths
- Oceans full of fish, squid, and coiled ammonites
- Pterosaurs, first vertebrates to adapt to life of flight
- Warm and dry, perfect for the animals of this period
The Cretaceous Period
Geologic setting
- Several smaller continents formed with extensive coasts
- More coastlines = more near-shore habitats
- Rocky Mountains and Alpine-Himalayan mountains form
- Atlantic basin widens; Indian water basin forms
- Hadrosaurs, or duck-billed dinosaurs common
- Forests start to look more like modern forests
- Ferns dominate dry lands, conifers dominate swamp lands
- About 60% of species go extinct at end of Cretaceous
- Caused by large meteor impact near Yucatan Peninsula
- Seasons more pronounced, climate becomes cooler