VOCABULARY


GEOLOGIC ERA: a subdivision of geologic time
that divides an eon into smaller units of time
The Phanerozoic Eon is divided into three such
time frames: the Paleozoic, Mesozoic, and
Cenozoic
PART I: GEOLOGIC TIME
AND THE
AGE OF THE EARTH
LET’S START WITH SOME BASICS…..
Scientists have always wondered how old the
earth was.
 But how were they to determine the Earth’s age?

1ST ATTEMPT: ‘BEGAT’ THEORY
 1640
 He
Archbishop James Ussher
calculated, using Hebrew
chronology (know as the ‘begat’
theory)
 CONCLUSION: Earth was
created in 4004 BC.
FATAL FLAW
This
is a calculation of
recorded history– not how
long the earth itself has been
around.
SALT IN THE OCEAN THEORY
1715
: English Astronomer Edmund
Halley (of Comet Halley fame)
Salinity of ocean
Measured in location and then
measured again 10 years later
FATAL FLAW
Although
the Earth’s first
waters were indeed fresh
water, the salinity of the
oceans is actually in
equilibrium with the earth’s
crust – and does not increase
over time.
THICKNESS OF THE
SEDIMENTARY LAYERS
THEORY
Late 1800’s Various geologists measured the total
thickness sedimentary rocks.
 They thought that this would lead to an
approximate calculation of the age of the Earth.

VARIOUS AGE CALCULATION
FATAL FLAW
They
assumed that each rock
type had an average rate of
deposition
This does not hold true
 Climate,
rock type, type of
deposition, angle of slope - all
affect the rate of sediments
being deposited
LOSS OF HEAT THEORY
 1859:
Lord Kelvin in 1859
 Argued that the Earth was originally
molten and continued to lose heat
from the center to the surface at a
uniform rate.
FATAL FLAW
Scientist
of his day did not
know about radioactive decay
This is keeping the earth’s
interior hot (not ‘cooling
down’)
RADIOACTIVE DECAY THEORY
1902
Ernest Rutherford
studying radioactive decay
Measured the ratio of
Uranium to its decay by
product Helium
FATAL FLAW
Over
time the helium gas
escaped from the rock sample
 This
gave an inaccurate ratio
RADIOACTIVE DECAY THEORY REFINED
1911,
Arthur Holmes radioactive
decay, but used a better set of
elements.
FATAL FLAW
Rocks
on the earth’s surface
are constantly being recycled
via processes associated with
erosion, weathering and plate
tectonics.
This destroys the original
ratios
METEORITE THEORY
 Using
radiometric age dating
techniques on meteorite samples
have revealed a still older date for
the age of the Earth
AGE OF THE UNIVERSE?
How does this age compare to the age of the
universe?
 February 2003, NASA’s Wilkinson Microwave
Anisotrophy Probe provided scientists with a
date

GEOLOGIC TIME SCALE
GEOLOGIC TIME SCALE
 Now
that scientists had a handle on
the age of the earth, they started to
think of how to divide it up into
sections
 Think
day
calendar – year, month, week,
NEPTUNIST THEORY
Neptunist
theories dictated that
all rocks had precipitated out of
a single enormous flood
PLUTONIST THEORY –
JAMES HUTTON (FATHER OF
GEOLOGY)
 The
interior of the Earth was hot,
and that this heat was the engine
which drove the creation of new rock:
 land was eroded by air and water
and deposited as layers in the sea;
 heat then consolidated the
sediment into stone, and
 uplifted it into new lands.
DEVELOPING THE IDEA
The
principles underlying
geologic (geological) time scales
were laid down by Nicholas
Steno in the late 17th century
 STENO’S
LAWS
 Over
the course of the 18th century geologists
realized that:
 Sequences of strata were often eroded,
distorted, tilted, or even inverted after
deposition;
 Strata laid down at the same time in
different areas could have entirely different
appearances;
 The strata of any given area represented
only part of the Earth's long history.
FOSSILS
 William
Smith - identification of
strata by the fossils they contained
 Enabled geologists to divide Earth
history based on
appearance/disappearance of
difference plants or animals
EARLY ATTEMPTS
Scientists divided the rocks of the
Earth's crust into four types
 Each type of rock, according to the
theory, formed during a specific
period in Earth history
 Primary,
 Secondary,
 Tertiary, and
 Quaternary

NAMES OF THE GEOLOGIC TIME
SCALE
The
process was dominated by
British geologists, and the names of
the periods reflect that dominance.
DIVISION OF THE TIME SCALE
 LARGEST

to SMALLEST ….
EON

Divided into ERAS
 Divided into PERIODS


Divided into EPOCHS
Divided into AGES
TODAY’S ACTIVITY

CREATE A SCALE MODEL OF GEOLOGIC
TIME
1 mm = 1 million years
 Your list will allow your team to plot major
biologic and geologic events


HOMEWORK – SGQ #1

Discuss what was happening to the planet during the
Precambrian Supereon. Feel free to include drawings or
illustrations. Be sure to include by what processes the crust and
oceans formed, when and over how much time.
TICKET OUT THE DOOR
 When
did the geologic time scale start?
TODAY’S VOCABULARY


GEOLOGIC ERA: a subdivision
of geologic time that divides an eon into smaller
units of time.
NOTE: The Phanerozoic Eon is divided into
three such time frames: the Paleozoic, Mesozoic,
and Cenozoic represent the major stages in the
macroscopic fossil record.
EONS ……
Largest division of geologic time
 The Hadean, Archean and Proterozoic Eons (were
as a whole formerly called the Precambrian)



This covered the four billion years of Earth history
prior to the appearance of hard-shelled animals
PHANEROZOIC EON – life started to appear in
the geologic record

Takes us to the present day
EONS

DIVIDED INTO
ERAS
PHANEROZOIC EON

PALEOZOIC ERA


MESOZOIC ERA


Early life, mostly in the water
Middle life, age of dinosaur
CENOZOIC ERA

Present life, age of mammals
ERAS DIVIDED INTO PERIODS
The "Cambrian," (the Roman name for Wales)
 The "Ordovician," and "Silurian", named after
ancient Welsh tribes, were periods defined using
stratigraphic sequences from Wales.
 The "Devonian" was named for the English
county of Devon
 The "Carboniferous" was simply an adaptation
of "the Coal Measures," the old British geologists'
term for the same set of strata. The
 The "Permian" was named after Perm, Russia,
because it was defined using strata in that region
by a Scottish geologist Roderick Murchison.

However, some periods were defined by geologists from
other countries.
 "Triassic" was named in 1834 by a German geologist
Friedrich Von Alberti from the three distinct layers
(Latin trias meaning triad) —red beds, capped by
chalk, followed by black shales— that are found
throughout Germany and Northwest Europe, called
the 'Trias'.
 The "Jurassic" was named by a French geologist
Alexandre Brogniart for the extensive marine
limestone exposures of the Jura Mountains.
 The "Cretaceous" (from Latin creta meaning 'chalk')
as a separate period was first defined by a Belgian
geologist Jean d'Omalius d'Halloy in 1822, using strata
in the Paris basin and named for the extensive beds of
chalk (calcium carbonate deposited by the shells of
marine invertebrates).

IMPORTANT TO NOTE ……..
 The
time scale is depicted in its
traditional form with oldest at the
bottom and youngest at the top -- the
present day is at the zero mark.
HOW DO WE DECIDE WHERE TO
PUT THE DIVISIONS?
 Numerous
high-resolution radiometric
dates have been generated that has led to
improved age assignments of key geologic
stage boundaries,
 Also used:
 Occurrence/disappearance of fossils
 Global geochemical variations,
 Milankovitch climate cycles, and
 Magnetic reversals
FOR TODAY
FINISH YOUR TIME LINE
 COLOR CODE ALONG THE TIME LINE TO
SHOW THE DIFFERENT ERAS

RED = PRECAMBRIAN (pre-life)
 BLUE = PALEOZOIC (ancient life, mostly
in the oceans)
 GREEN = MESOZOIC (age of reptiles –
dinosaurs)
 YELLOW = CENOZOIC (age of mammals –
the Era we are now living in)

AFTER TIMELINE….
Questions that go with the activity
 WORKSHEET:
 HOMEWORK: Worksheet and SGQ #2


The boundaries for the geologic eras were established
because of mass extinctions. What are mass
extinctions? When did the five major mass
extinctions of the Earth past occur? For each discuss
what scientists feel caused them and what species
and what % were wiped out.
TICKET OUT THE DOOR
WHICH
ERA DO YOU LIVE
IN?
 Precambrian
 Paleozoic
 Mesozoic
 Cenozoic
GEOLOGIC HISTORY OF WESTERN US
http://jan.ucc.nau.edu/~rcb7/Text_WUS.html
 http://cpgeosystems.com/wnamtectonic.html

RESOURCE WEBSITES

http://pubs.usgs.gov/gip/geotime/contents.html