What is geological time scale?

Contributed by:
The geological time scale is based on the geological rock record, which includes erosion, mountain building, and other geological events. Over hundreds to thousands of millions of years, continents, oceans, and mountain ranges have moved vast distances both vertically and horizontally.
1. Geologic Time and Earth’s
Biological History
Designed to meet South
Department of Education
2005 Science Academic
2. Table of Contents
 What is the Geologic Time Scale? (slide 4) Standard 8-2.4
`Epochs, Eons, Eras, and Periods (slide 4)
 How was the time scale and it’s divisions created? (slide 5)
 A complete Geologic Time Scale with references to S.C. (slides 6-7)
 Principles of the Geologic Time Scale (slide 8)
 Principles of Superposition, Horizontality and Original lateral continuity
 Principle of biologic succession (slide 9)
 Charles Lyell, the Principle of cross-cutting relations & Inclusion principle (slide 10)
 Charles Darwin
 Relative Age Dating using unconformities (slide 10) Standard 8-2.6
 Relative Age dating using cross-cutting relations and index fossils (slide 11)
 Absolute Age dating (slide 13)
 Isotopes and radiometric dating
 Carbon dating (slide 14)
 How old is old? (slide 16)
 Breakdown of geologic time periods
 Are we now living in the Anthropocene Era?
(slide 17)
3. Table of Contents, cont.
(2 of 2)
The Earth through time (slide 19) Standard 8-2.5
 Ordovician and Silurian
 Devonian, Mississippian, Pennsylvanian (slide 21)
 Permian and Triassic (slide 22)
 Jurassic and Cretaceous (slide 23)
 Triassic and Quaternary (slide 24)
 Adaptation (slide 25) Standard 8-2.1
 Punctuated events through time (slide 26) Standard 8-2.3
 Comet impact
 Climate shift
 Volcanism
 Extinction (slide 29) Standard 8-2.7
 The study of fossils and fossil types (slide 30) Standard 8-2.2
 South Carolina Standards (slide 32)
4. What is the Geologic Time Scale?
Standard 8-2.4: Recognize the relationship among the units—era, epoch, and period—into which
the geologic time scale is divided.
What does the time scale represent?
 The geologic time scale divides up the history of the earth based on life-
forms that have existed during specific times since the creation of the planet.
These divisions are called geochronologic units (geo: rock, chronology:
 Most of these life-forms are found as fossils, which are the remains or
traces of an organism from the geologic past that has been preserved in
sediment or rock. Without fossils, scientists may not have concluded that the
earth has a history that long precedes mankind.
 The Geologic Time Scale is divided by the following divisions:
Eons: Longest subdivision; based on the abundance of certain fossils
 Eras: Next to longest subdivision; marked by major changes in the fossil
 Periods: Based on types of life existing at the time
 Epochs: Shortest subdivision; marked by differences in life
forms and can vary from continent to continent.
Table of Contents 4
5. What is the Geologic Time Scale,
 geologists had ?
Due to the fact that earlycontinued no way of knowing how the
discoveries of the Earth were going to develop, geologist over time have put
the time scale together piece by piece. Units were named as they were
discovered. Sometimes unit names were borrowed from local geography,
from a person, or from the type of rock that dominated the unit.
 Cambrian: From the Latin name for Wales. Named for
exposures of strata found in a type-section in Wales by British
geologist Adam Sedgwick.
 Devonian: Named after significant outcrops first discovered
near Devonshire, England
 Jurassic: Named for representative strata first seen in the Jura
Mountains by German geologist Humboldt in 1795)
 Cretaceous:
 The earliest From
time of the Earth the Latin
is called “creta”and
the Hadean meaning
refers tochalk by of
a period a Belgian
time for
which we have no rock record, and the Archean followed, which corresponds to the
ages of the oldest known rocks on earth. These, with the Proterozoic Eon are called the
Precambrian Eon. The remainder of geologic time, including present day, belongs to
the Phanerozoic Eon.
 While the units making up the time scale are called geochronologic units, the actual
rocks formed during those specific time intervals are called chronostratigraphic units.
The actual rock record of a period is called a system, so rocks from Table
the of Contents 5
-Mastadons become extinct
-Human culture flourishes
0.01 Holocene Beaches and barrier islands form
-Accelerating extinction of many
Quaternary species
-Modern humans develop
1.8 Pleistocene Ice sheets form -Asians arrive and settle the
-Volcanic activity in North
5.3 Pliocene America and Africa Hominids develop
-Grand Canyon forms
Horses, mastadons, mammoths,
“Age of
23.8 Miocene Sandhills form in S.C. tigers, and camels live in South
Appalachians uplift; erosion
33.7 Oligocene Cats, dogs, and apes appear
Tertiary increases
-Grass spreads widely
Sea levels rise; deposits of marine
-Diverse array of animals develop,
54.8 Eocene sediments – limestone in S.C.;
including whales, rhinos, and
land bridges form
Earthquakes common; Georgia
-First horses appear (size of a cat)
65.0 Paleocene Embayment, Cape Fear Arch
-Tropical plants dominate
forms in Southeast
Mass extinction occurs at the end -T-Rex develops but number of
of the period caused by a dinosaur species decline
144 Cretaceous meteorite impact (Dinosaurs, -Snakes appear and first primates
ammonites and 25% of marine appear
life become extinct) -Angiosperms appear
“Age of Western US: orogeny of Rockies;
-First birds appear
206 Reptiles” Jurassic North America continues to rotate
-Golden age of dinosaurs
away from Africa
-Pangea begins to break apart First dinosaurs, mammals,
248 Triassic -Rocky Mountains and Sierra crinoids, and modern echinoids
Nevada form appear
-90% of Earth’s species become
extinct, including trilobites,
-Pangea forms
290 Permian blastoids, fish and amphibians
-Appalachians rise
because of heavy volcanism in
Siberia Table of Contents6
7. Paleozoic
“Age of
Modified after Carolina Rocks, contributed by J. Table of Contents 7
8. Principles Behind Geologic Time
 Nicholas Steno, a Danish physician (1638-1687), described how the position of a
rock layer could be used to show the relative age of the layer. He devised the three
main principles that underlie the interpretation of geologic time:
 The principle of superposition: The layer on the bottom was deposited first and
so is the oldest
 The principle of horizontality: All rock layers were originally deposited
 The principle of original lateral continuity: Originally deposited layers of rock
extend important
in principles have
all directions formed
until the
either framework
thinning forbeing
out or the geologic area
cut off by a of
rock layer. which is the study of layered rock (strata).
Geologist studying the
Younger stratigraphy in the Copper Basin,
Idaho. These rock layers were
deposited horizontally, and
uplifted later so they are now
tilted at an angle (along the red
arrow). (Photo contributed by K. McCarney-
 Decades later, other European scientists rediscovered
Castle) ‘Steno’s Laws’ and began
applying them. Abraham Gottlob Werner became famous for his proposal that all
rocks came from the ocean environment. He and his followers were called
“Neptunists.” An opposing view (by Voisins) argued that all rocks of the earth came
from volcanic environments. These scientist were called “plutonists.” Table of Contents 8
9. Principles Behind Geologic Time,
 James Hutton, a Scottish physician and geologist (1726-1797), thought the surface
of the earth was an ever-changing environment and “the past history of our globe must
be explained by what can be seen to be happening now.” This theory was called
“uniformitarianism,” which was later catch-phrased as “the present is the key to the
 William Smith was a surveyor who was in charge of mapping a large part of
England. He was the first to understand that certain rock units could be identified by
the particular assemblages of fossils they contained. Using this information, he was
able to correlate strata with the same fossils for many miles, giving rise to the principle
of biologic succession.
 The principle of biologic succession: Each age in the earth’s history is
unique such that fossil remains will be unique. This permits vertical and horizontal
correlation of the rock layers based on fossil species.
Even though these two outcrops are
Rock separated by a large distance, the
Outcrop 1 same rock layer can be correlated
with the other because of the
Correlati presence of the same shark teeth.
This lets scientists know that the
two layer were deposited at the
same time, even if the surrounding
Rock rocks look dissimilar from each
Outcrop 2
300 km Table of Contents 9
10. Principles Behind Geologic Time,
 During the early 1800’s, English Geologist, Charles Lyell published a book called
“Principles of Geology,” which became a very important volume in Great Britain. It
included all of Hutton’s ideas, and presented his own contemporary ideas such as:
 The principle of cross-cutting relationships: A rock feature that cuts across
another feature must be younger than the rock that it cuts.
 Inclusion principle: Small fragments of one type of rock but embedded in a
second type of rock must have formed first, and were included when the second rock
was forming.
 Charles Darwin (1809-1882) was an unpaid naturalist who signed up
for a 5-yr expedition around the world aboard the H.M.S. Beagle. On this
trip, he realized two major points. In spite of all species reproducing, no
one species overwhelmed the Earth, concluding that not all individuals
produced in a generation survive. He also found that individuals of the
same kind differ from one another and concluded that those with the most
favorable variations would have the best chance of surviving to create the
next generation.
 The theory of natural selection was credited to Darwin (along with Alfred Russel
Wallace) and he went on to write the famous “Origin of Species.” Darwin’s two goals
in that work were:
1. To convince the world that evolution had occurred and organisms had
changed over geologic time Table of Contents 10
11. Relative Age Dating
Standard 8-2.6: Infer the relative age of rocks and fossils from index fossils and the ordering
 the rock layers.
“Relative age” means the age of one object compared to the age of another, not
the exact age of an object. This method can only be used when the rock layers are in
their original sequence.
 All six of the original stratigraphic principles may be applied to determine the age of
a rock. This process is called age dating. Correlation of strata by rock unit type
(lithology) or fossil type (biology) using species, composition, or texture leads
scientists to extrapolate relationships over large areas of land. Because rock layers
can be “matched up,” we can guess that they were formed during the same period,
so they usually are the same age.
 Using the principles of original horizontality and superposition, we can conclude
that oldest rock is always on the bottom because is was deposited 1 st.
 Deciphering the sequence of a rock outcrop is sometimes complicated by a features
within the rock record called unconformities, which are specific contacts between rock
1. Angular:
layers. There Horizontal beds of
are three types areunconformities
uplifted and that help us determine relative
ages or eroded
of rock followed by new deposition of
horizontal beds. The figure to the right is an
angular unconconformity. Horizontal
bed of
2. Disconformity: Episodes of erosion or non- sedimentary
deposition between layers rock
Tilted bed of
3. Nonconformity: Sediment is deposited on rock
top of eroded volcanic or metamorphic rock
(indicates very long passage of time) Wikipedia (public domain)
Table of Contents 11
12.  Relative ages can also be determined using
Lyell’s principle of cross-cutting relationships. In
the figure to the right, both the gray and the
yellow horizontal strata needed to be in place for
the pink layer to cut them, therefore, the pink
layer is the youngest. (Image from Plummer/ McGeary, 7th
edition, 1996)
Relative Age dating with index fossils
 Biostratigraphy is the correlation of stratigraphic units based on fossil content.
Biostratigraphically useful species are known as index fossils (or guide fossils)
because they can be used as guides for recognition of chronostratigraphic units.
 Index fossils are widespread, have short temporal durations resulting from rapid life
spans, are abundant throughout their geographic and geologic ranges, and are easily
recognized (unique).
 Trilobites are a commonly used index fossil
because they are easy to recognize. We know
exactly when certain species became extinct, such
that we can compare rock layers that contain
trilobites with a second rock layer and, based on
position, determine if the second rock layer is
younger. The photo to the right is a trilobite from
the Mississippian period (photo courtesy of K. McCarney-
 Fossils found in many rock layers have lived for Table of Contents 12
13. Absolute Age Dating
 Absolute ages, or geochronometric ages, of rock can be assigned to the geologic
time scale on the basis of properties of atoms that make up the minerals of a rock.
Unlike relative dating, which relies on sequencing of rock layers (i.e. younger vs.
older), absolute dating can produce an actual age in years.
 The number of neutrons in a nucleus of an atom determines the isotope of the
element, just like the number of protons determines the identity of an element.
 Some isotopes are unstable and break down into other isotopes through a process
called radioactive decay. Radioactive decay is characterized by beta decay, where a
neutron changes into a proton by giving off an electron, and alpha decay, when
isotopes give off 2 protons and 2 neutrons in the form of an alpha particle and changes
into a new product. The original isotope is called the parent and the new isotope
What is called the daughter.
a Half-Life?
 Each radioactive parent isotope decays to its daughter product at a specific and
measurable rate. This measurement is reported in half-lives. The half-life of an
isotope is the time it takes for ½ of the parent atoms in the isotope to decay.
 If an isotope has a half-life of 4000 years, then after 4000 years ½ of the parent
isotope remains. After another 4000 years, ½ of ½ remains, or ¼ of the original
amount of parent isotope. In another 4000 years (12,000 years total), ½ more of
the remaining amount decays, so after 3 half-lives, there only remains 1/8 (½ of ½
of ½) of the original parent isotope.
 If a scientist knows the half-life of the parent and measures the proportion of
parent isotope to daughter isotope, he/she can calculate the absolute age of the
Note: This valuable
Radioactive method
isotopes can isincalled
be found the rockradiometric
record dating.
because radioactive isotopes are incorporated into the crystals of Table of Contents 13
14. Radioactive
Scientists used the proportion of parent
75% 87.5%
material remaining to the proportion of
daughter material produced in order to
50% predict the age of the rock. During
each half-life, only one-half of the
parent material decays to the daughter
DECAY product.
 Isotopes with very long half-lives are not suitable for dating rocks younger than ~1
million years because there are too few daughter atoms to be measured accurately.
 Experimental error limits measurements to those rocks younger than about 12 half-
lives of the isotope used.
Radiocarbon Dating
 Radiocarbon dating is a common method used to date anything that was once alive
(including plants) and up to 70,000 years old.
 All living things take in carbon from the environment in the form of carbon-12 and
carbon-14. When an organism dies, carbon intake stops and the carbon-14 begins to
decay at a known rate. Scientists can determine how much C-14 remains in an
organism by measuring radiation emitted by the C-14 isotopes.
 Carbon dating can be used on wood, plants, humans, and even old paper made out of
 The half-life of C-14 is 5,730 years. Because of this, it should not
Tablebe used with
of Contents 14
15. Commonly used radioactive isotopes
Parent Daughter half- Mineral or Material
Uranium238 Lead 206 4.56 BY Zircon, Uraninite, Pitchblende
Uranium Lead 207 704 MY Zircon, Uraninite, Pitchblende
Potassium Argon 40 1.251 Muscovite, biotite, hornblende, K-
40 BY feldspar, volcanic rock, glauconite,
Rubidium Sr 87 48.8 BY K-mica, K-feldspar, Biotite,
87 Metamorphics
Thorium Lead 206 75 KY Ocean sediments
Thorium Lead 208 1.39 BY Zircon, Uraninite, Pitchblende
KY- thousand years. MY- million years. BY- billion years
Uranium-Lead decay series (U-Pb series)
Carbon 14 Nitrogen 14 5730 yr Wood, bone, shell
 Unlike carbon-14 dating, uranium dating cannot be used to date formerly living
things; however, it is the most commonly used method in igneous rock dating
because of the abundance of zircon minerals.
 The subscripts of 235 and 238 are the atomic mass numbers of the element.
Though each isotope has 92 protons in its nucleus, U-235 has 143 neutrons and U-238
has 146 neutrons.
 Igneous rocks, or the magma from which it was formed, often intrudes overlying
sedimentary rocks. By dating the magma, one can get at least a minimum age for
Table of Contents 15
16. How Old is Old?
 From the time of Hutton, scientists were convinced that the earth was much older
than the 6000 years predicted by the religious scholars.
 Charles Lyell tried to estimate the age of the earth through the amount of
evolution exhibited by marine mollusks in a specific time system.
 Another method was to estimate the rate of deposition for sedimentary rocks.
 Sir Edmund Halley proposed to estimate the age of the earth using salt content
of the oceans, assuming that the oceans were once non-saline and that salt
addition to the oceans corresponded in some linear fashion with time.
 Lord Kelvin estimated the age of the Earth at 24-40 million years. He proposed
that the Earth has been cooling since it formed, and he calculated the rate of
cooling using principles of heat conduction.
 It wasn’t until Henri Becquerel discovered radioactivity in 1896 and Madame
Curie isolated radium 2 years later that people realized that the Earth had it’s own
source of heat. Thus it became one of the most useful tools for future scientists.
 The oldest rocks found so far on Earth (based on zircon grains from
Australia) have been dated at 4.1-4.2 billion years.
 Meteorites have also been dated at 4.6 billion years. Meteorites are
considered to be remnants of a plant or asteroid that originally formed at the
same time as the Earth, so that the Earth’s age is currently estimated to be
4.6 billion years.
 The oldest fossils are preserved remains of stromatolites, which are layers
of lithified blue-green algae, dating to approximately 3.5 billion years before
Table of Contents
17. Precambrian: Earliest span of
Phanerozoic: Everything since
“Age of
Invertebrates” Carboniferous
(Missipp. & Pennsylvanian)
Mesozoic Eocene
“Age of Oligocene
Reptiles” Cretaceous
Cenozoic Miocene
“Age of Pliocene
Mammals” Quaternary
We are living in the Phanerozoic Eon, Cenozoic Era,
Quaternary Period, Holocene Epoch……..BUT
Table of Contents 17
18. A new concept has been gaining
momentum since it’s introduction by
Paul Crutzen in 2000. He proposed
that the Holocene Epoch is over and
a new geological epoch called the
Anthropocene has begun.
Mans’ impact on the Earth’s climate
and ecosystems since the Industrial
Revolution is quite evident. Support
for this theory comes from data
derived from glacial ice cores
showing the growth in greenhouse
gases starting from the 1800’s.
Does this justify a new Epoch on the
Geological Time Scale? Some
scientists question this, however,
there is no doubt that there has
been a shift in Earth’s atmosphere
and biosphere as we emerge from
the most recent ice age which ended
approximately 10,000 years ago.
This is strong indication that
geologic time is not a thing
Photo used withof the past!
permission from the cover of GSA
Today, Geological Society of America, Vol. 18, 2, Feb.
Table of Contents 18
19. The Earth Through Time
d 8-2.5: Illustrate the vast diversity of life that has been present on Earth over time by using the geolog
The Proterozoic:
 No life possible as the Earth initially
forms 4.6 billion years ago.
 Simple, single-celled forms of life appear
3.8 billion years ago, becoming more
complex and successful over the next 3
billion years: Prokaryotes then Eukaryotes
 Cyanobacteria begins producing free
oxygen (photosynthesis)
 Land masses gather to make up a
continent called “Rodinia”
 Explosion of life
 All existing phyla come into being at this
 Life forms in warm seas as oxygen levels
rise enough to support life
 Dominant animals: Marine invertebrates
(trilobites and brachiopods)
 Supercontinent Gondwana forms near the
South Pole (note position of present-day
PaleoMaps used with permission from Christopher Scotese and 19
are under copyright of C.R. Scotese, 2002
Table of Contents 19
20.  The 1st animals with bones appear,
though dominant animals are still trilobites,
brachiopods and corals
 The beginning of the construction of
South Carolina
 A very cold time in Earth’s history: there
was a great extinction due to ice caps in
present-day Africa
 Four main continents: Gondwana,
Baltica, Siberia and Laurentia
 First land plants appear and land animals
 Laurentia collides with Baltica and closes
Iapetus Sea.
 Coral reefs expand and land plants begin
to colonize barren land.
 First millipede fossils and sea scorpions
(Euryptides) found in this period
PaleoMaps used with permission from 20
Christopher Scotese and are under copyright Table of Contents 20
21. Devonian (Age of the
 Pre-Pangea forms. Dominant
animal: fish
 Oceans still freshwater and fish
migrate from southern hemisphere
to North America.
 Present-day Arctic Canada was at
the equator and hardwoods began
to grow.
 Amphibians, evergreens and
ferns appear
 The Acadian Orogeny, leading to
S.C. metamorphism
PaleoMaps used with permission from Christopher Scotese and
Mississippian: are under copyright of C.R. Scotese, 2002
 First seed plants appear
 Much of North America is
covered by shallow seas and
sea life flourishes (bryoza,
brachipods, blastoids)
 Modern North America
begins to form
 Ice covers the southern
hemisphere and coal swamps
formed along equator.
 Lizards and winged insects 21
first appear. Table of Contents 21
22.  Last period of the Paleozoic
 Pangea forms. Reptiles
spread across continents.
 The Appalachians rise
 90% of Earth’s species
become extinct due to
volcanism in Siberia. This
marks the end of trilobites,
ammonoids, blastoids, and
most fish.
 First dinosaurs appear
 First mammals- small rodents
 Life and fauna re-diversify
 Rocky Mountains form.
 First turtle fossil from this period
 Pangea breaks apart
PaleoMaps used with permission from Christopher Scotese and
are under copyright of C.R. Scotese, 2002
Table of Contents 22
23.  Pangea still breaking apart
 Dinosaurs flourish “Golden age of
 First birds appear
 North America continues to rotate
away from Africa
 T-Rex develops
 First snakes and primates
 Deciduous trees and grasses
 First flowering plants
 Mass extinction marks the
end of the Mesozoic Era, with
the demise of dinoaurs and
25% of all marine life.
PaleoMaps used with permission from Christopher Scotese and are
under copyright of C.R. Scotese, 2002 Table of Contents 23
24.  First horses appear and
tropical plants dominate
 Grasses spread and whales,
rhinos, elephants and other
large mammals develop. Sea
level rises and limestone
deposits form in S.C. (Eocene)
 Dogs, cats, and apes appear
 Horses, mastadons, camels,
and tigers roam free in S.C.
 Hominids develop and the
Grand Canyon forms (Pliocene)
 Modern humans develop and
ice sheets are predominant- Ice
age (Pleistocene)
 Holocene Humans flourish
PaleoMaps used with permission from Christopher Scotese and
are under copyright of C.R. Scotese, 2002
Table of Contents 24
25. Adaptation and ‘Survival of the
Standard 8-2.1: Explain how biological adaptations of populations enhance their
survival in a particular environment.
 Some populations, whether mammals, amphibians, or reptiles are better adapted to
living conditions than others, even within the same species, so they are better at
surviving than others. Because their chances of surviving are increased, their chances
of reproducing offspring are better, and their offspring will possess the same strong
traits. This is the basis for natural selection over long periods of time.
 Natural selection refers to the process where over long periods of time, helpful
variations can appear in a species while “unfavorable” one disappear. For example, a
group of frogs living on the rocky side of an island may, over time, adapt a gray skin
color to help blend in with their rocky environment in while a group of frogs living on
the more lush, vegetated side of the island may develop a green skin color to blend in
with their particular environment. Even though the frogs are of the same species, they
are able to incorporate different traits to help them survive in their environments.
 The theory of natural selection, sometimes referred to as ‘Survival of the fittest,’
started with Charles Darwin’s 5-year trip around the world on the HMS Beagle. During
this time, he noticed variations within the same species, especially in the Galapagos
Turtles, and noted that some of the variations were favorable and some were not. He
concluded that not all members of a species survive, which is why the world is not
overpopulated by any one species. The practicality of their adaptation must be a
determining factor for who survives and who does not. He published his findings on his
return to England and wrote the classic work “The Origin of Species.”
Table of Contents 25
26. Punctuated Events Through Geologic
Standard 8-2.3: Explain how Earth’s history has been influenced by catastrophes (including the
impact of an asteroid or comet, climatic changes, and volcanic activity) that have affected the
conditions on Earth and the diversity of its life-forms.
 Environmental changes on earth are usually an indicator of a species
extinction (or a species addition). These changes can be brought about by
an asteroid or comet impact, volcanic activity, or climatic changes like the
onset of ice ages.
1. Impact:
 The most well-known extinction is the extinction of the dinosaurs.
Scientists think that this mass extinction was caused by a large comet that
impacted the earth in present-day Mexico, causing a massive quantity of dust
to rise up into the atmosphere, possibly blocking out the sun and affecting
the oxygen levels Earth. Many plants died, and the animals that depended
on those plant for life died as well. In addition, it may have become very cold
in a short period of time.
 It took millions of years for the earth to recover, and when it did, the large
dinosaurs were gone forever.
 Certain species of birds, however, did survive and began Table
to flourish.
of Contents Birds
are thought to be direct descendants of dinosaurs.
27. 2. Climate Changes
 Climate has always been a constantly changing phenomenon. The earliest
atmosphere was devoid of free oxygen, and it wasn’t until the earliest life forms
evolved that the present-day atmosphere began to form approximately 600 million
years ago.
 During the Paleozoic, warm shallow seas and tropical climates were common. Life
forms that could not adapt to these conditions disappeared.
 Throughout the Mesozoic era, plate movement shifted the continents and only the
animals and plants with the greatest ability to adapt could survive the extreme
changes in temperatures that occurred as a consequence. Plants with seed coverings
and animals with constant internal temperatures (warm-blooded) lived during this era.
 Climate continued to change during the Cenozoic and continues to change to this
day, as issues of “Global Warming” have been on the fore-front for over a decade. It
was only ~12,000 years ago that the world was in an “ice age” mode. Also, many
mountain ranges formed during this era, causing climate differences due to elevation
 Ice ages have occurred many times in Earth’s history. Climate shifts like these may
be caused by magnetic polar reversals or variation in the tilt of the earth (called
Milankovitch cycles). Obviously, not all life can adapt to the extreme cold.
Table ofAlso, not all
Contents 27
28. 3. Volcanic Activity
 Significant volcanic activity, which produced ash clouds in the air and lava flows on
the Earth’s surface, was common during the Precambrian. It was extremely hot, and
most life forms could not exist in these conditions.
 Volcanism is a common byproduct of tectonic plate collision. If one plate collides
with another and is pulled underneath it, a subduction zone is formed underneath the
plates and a volcanic arc forms on the Earth’s surface. During the Paleozoic and
Mesozoic, continents were regularly colliding with each other and volcanism was
common. Plate boundaries are still the most common sites of volcanoes today.
 If volcanism is significant enough to produce
mass quantities of ash and volatile gases, wind
can carry these into the upper atmosphere all
around the world, potentially enveloping the
earth in semi-darkness and reducing insulation
on earth. Obviously, this would have an effect
on all living things on Earth.
 A cause and effect phenomenon, catastrophic
events impact life on Earth, whether through an
extinction or creation of new traits for
adaptation to already existing plants and
animals. Table of Contents 28
29. Why Extinction?
Standard 8-2.7: Summarize the factors, both natural and man-made, that can contribute to the
extinction of a species.
 Extinction of a species occurs when no more members of a particular species
remains. Extinction through time is very common, and, in fact, nearly 90 percent of all
species that ever lived on Earth are now extinct.
 Organisms that cannot survive a catastrophic or significant change in earth’s climate
usually become extinct. Extinctions are a way of clearing the path for new kinds of life
that is potentially more advanced. This is a natural part of life’s process.
 Natural phenomena that can contribute to the extinction of a species include global
climate changes, volcanic explosions, and celestial impacts.
 The influence of humans on the environment
do not include comet impacts or volcanism;
however, man has caused extinctions all the
same. Over the past few hundred years, man
has cut rainforests and woodland forests,
destroying natural habitats. Pollution from
industrial plants and vehicles has also affected
the air we breath and contributed to greenhouse
gases, which drive global warming. We are
looking at the potential extinction of many
species due to this warming trend.
 In addition to threatening less-adaptive
creatures than ourselves, man is negatively 29
impacting biological resources that our own Table of Contents
30. Standard 8-2.2 The study of Fossils
Summarize how scientists study Earth’s past environment and diverse life-forms by examining
different types of fossils (including molds, casts, petrified fossils, preserved and carbonized remains
of plants and animals and trace fossils)
 A fossil is the preserved remains of an organism that has died. Fossils tell
scientists, called paleontologists, about living things such as their biology
and environmental conditions over earth’s history through the rock record.
In addition, they give clues to the conditions of the earth (i.e. climate) at the
time that the fossil was preserved and possibly relate changes of an
organism over time.
 Definitions of fossil types:
Mold fossils: when sediments bury an organism and the sediment hardens into
rock. The organism decays slowly inside the rock, leaving an cavity in the shape of
the organism.
 Cast fossil: The cavity or mold mentioned above can filled in with mud. When
the mud hardens, it takes on the shape of the organism.
 Petrified fossil or permineralized fossil: Minerals like calcium can soak into the
buried remains of an organism. The mineral replaces the remaining bone and
changes it into rock.
 Carbonized fossil: When organism parts are pressed between layers of mud or
clay that hardens over time, squeezing the decaying organism away and leaving a
carbon imprint in the rock, since all living things contain carbon. Table of 30
31.  The fossil record, like the rock record, is an important record for
understanding life on earth before the dawn of man.
 Extinctions and new life forms are also found within the fossil record.
 Fossils can also show structural similarities and differences in organisms
over time revealing the diversity of life forms on earth. Nearly 90 percent of
organisms that have lived on the earth are now extinct.
A trilobite cast from the
Mississippian Period. Brachiopods in a sandstone
Carbon imprint of Extinct. matrix and an individual
fish remains, age brachiopod cast. Extinct.
fossil (cast),
cut and
Belemnite fossil (cast),
cut and polished.
Related to
Related to present-day
squid. Extinct.
snail. Extinct.
Table of Contents 31
32. South Carolina Science Academic
Standards: Grade 8
Standard 8-2: The student will demonstrate an understanding of Earth’s biological
diversity over time. (Life Science, Earth Science)
8-2.1 Explain how biological adaptations of populations enhance their
survival in a particular environment.
8-2.2 Summarize how scientists study Earth’s past
environment and diverse life-forms by examining different types of fossils
(including molds, casts, petrified fossils, preserved and carbonized remains
of plants and animals, and trace fossils).
8-2.3 Explain how Earth’s history has been influenced by
catastrophes (including the impact of an asteroid or comet, climatic
changes, and volcanic activity) that have affected the conditions on Earth
and the diversity of its life-forms.
8-2.4 Recognize the relationship among the units—era, epoch,
and period—into which the geologic time scale is divided.
8-2.5 Illustrate the vast diversity of life that has been present
on Earth over time by using the geologic time scale.
8-2.6 Infer the relative age of rocks and fossils from index
fossils and the ordering of the rock layers.
8-2.7 Summarize the factors, both natural and man-made,
Table of Contents that