2. The most straightforward method of correlating
sedimentary rocks is to compare their fossils.
• Fossils = the remains or traces of organisms
preserved in rocks.
• Fossil = preserved from a past geologic age.
• Fossil = an animal or plant that lived many
thousands or millions of years ago that has been
preserved, or the shape of one of these
organisms, in rock.
• Fossil = any remains, trace, or imprint of a plant
or animal that has been preserved in the earth’s
crust since some past geologic or prehistoric
time.
3.
4. Fossils can be vertebrates, invertebrates,
plants or even footprints.
5. Becoming a Fossil Is Not Easy
• The process of making something into a fossil
is not an easy one.
• It takes fairly rapid burial in fine-grained
sediment to stop the three major agents of
destruction: decay, dissolution and
disarticulation.
• This is why hard skeletons in the fossil record
are more likely to have been preserved form
marine environments than from on land.
6. Similar rocks of the same age typically
contain similar fossils.
• Fossils hold the key to predicting the sequence of
layers in a given location and to matching outcrops of
similar rocks between different locations.
• Geologists can make more precise correlations using
index fossils--species that existed for relatively short
periods of time and are found over large geographic
areas.
• Index fossils are useful because their appearance in the
rock record represents a specific time interval that
scientists can use to identify and correlate rocks
chronologically between different regions around the
world.
7.
8. Lack of Continuity
of Sedimentary
Strata
When geologists try to
match rocks from
different areas of any
continent, they can
rarely follow the best
exposures of layers of
sedimentary rock for
more than a few tens
of kilometers before
they disappear
underground or are
removed by erosion.
(Note gaps in dates for
Grand Canyon strata)
9. Grand Canyon Fossils Match Fossils
Elsewhere in North America
• The Grand Canyon’s Redwall Limestone layer, about
350 million years old, contains an assemblage of fossils
that includes extinct corals, cephalopods and crinoids
(all sea bottom creatures).
• Similar fossil assemblages are found in limestone
quarries in Indiana and in deposits surrounding
Kentucky’s Mammoth Cave.
• The same fossils are even found in the Rundle
Limestone formation in the Rocky Mountains way up in
Banff, Canada.
• All of these strata were formed at the same time when
much of N. America was covered by a shallow sea.
10. Fossils similar to those in the Grand Canyon’s Redwall
Limestone strata are found in Indiana quarries and
around Kentucky’s Mammoth Cave.
11.
12. Examine the following illustration and predict which
rock strata in the Grand Canyon is most likely to have
formed in a depositional environment like the one
pictured.
13. Geologic Time: KEY POINTS
• Earth’s “history” is divided into 3 long spans of
time know as eons (Archean, Proterozoic, and
Phanerozoic).
• The Archean and Proterozoic were once
commonly called the Precambrian—a span of
over 4 billion years, from the time the Earth
first formed to when fossils become common
in rocks.
14. KEY POINTS (Cont’d)
• The most recent eon, the Phanerozoic, is
characterized by abundant fossils, especially
thoses with shells or hard skeleton parts.
• The Phanerozoic is divided into three eras
(Paleozoic, Mesozoic, and Cenozoic), which in
turn are divided into 13 periods (see Figure 8.14
and Table 8.1).
• The fossil record has been interpreted to record
the changes in the biosphere over time.
15.
16.
17.
18.
19.
20.
21. Early Earth at 4.3 Billion Year Ago
• Much hotter from:
1. Left over heat from planet’s formation period.
2. More volcanic activity than today.
• Regular bombardment of asteroids and comets.
• Little or no oxygen.
• Life arose in hot, acrid conditions like those in
the scorching, acidic, hot springs in Yellowstone
Park (some extremophile bacteria live in such
conditions today).
24. First Multicellular
Life and First
“Animals”
About 2 billion years
ago, oxygen began to
accumulate in the
atmosphere as the
result of O2 production
by blue-green algae in
the seas, and life
evolved beyond
primitive bacteria
around 1.7 b.y.a.
The earliest creatures
that might be labeled
“animals” appeared
some 580 million years
ago (R).
25. Cambrian
Period:
Explosion of fossils
with shells or hard
skeletons in the
seas.
Many of the major
animal groups can
be found in the
Cambrian period,
even though they
did not look
anything like their
descendents do
today.
26. The fossil record of marine animals demonstrates the
slow pace of evolution, with new families appearing at
a rate of roughly one per million years.
27. On average, the number of species represented by fossils
increases from the Cambrian onward. All major phyla had
appeared by the Cambrian.
28.
29.
30. “The Great Dying”
• The Permian-Triassic extinction (or P-T event) killed off an
estimated 96% of marine species and 70% of land species.
• The P-T extinctions may have taken a few million years.
• Two geologic events were ongoing at the time:
1. The supercontinent Pangaea was assembled during the
Period, crating a single worldwide ocean—reducing the
area of continental shelf, the shallow ocean floor around
continents that’s home to a majority of marine species.
2. And, thousands of eruptions, starting about 251 m.y.a.,
took place over 1 million years to form the Siberian Traps
(lava plateaus).
31. More genera were alive in the very recent past than
ever before (at least until the 6th and current, ongoing,
human-caused mass extinction).
32. Euthycarcinoids
and First Land
Plants
Tracks from the
primordial sea (p.
224): Euthycarcinoids
seem to be the first
sea creatures to have
invaded the land some
510 m.y.a. (in the
Cambrian)
The first plants to
invade the land from
the sea occurred
about 440 m.y.a. (in
the Silurian Period).
51. Numerical Time: KEY POINTS
• Radioactive decay occurs when a radioactive
parent isotope undergoes a change to its
nucleus and is converted to a daughter atom
and releases energy.
• Radioactive decay of unstable isotopes can be
used to determine the age of igneous and
metamorphic rocks.
• The half-life is the length of time it takes for
half of the radioactive material to decay.
52. What are isotopes?
• Isotopes are varieties of the same
element that have different mass
numbers (their nuclei contain the same
number of protons but a different
number of neutrons).
• In other words, isotopes are variants of
atoms of a particular element, which
have differing numbers of neutrons.
53. What are isotopes?
• Atoms of a particular element by definition
must contain the same number of protons but
may have a distinct number of neutrons,
which differs from atom to atom, without
changing the designation of the atom as a
particular element.
• The number of protons and neutrons in the
nucleus, known as the mass number, is not
the same for two isotopes of any element.
54. What are radioactive isotopes?
• Radioactive isotopes, also called
radioisotopes, are atoms with a different
number of neutrons than a usual atom
• It has an unstable nucleus that decays,
emitting alpha, beta and gamma rays
until the isotope reaches stability.
• Once it's stable, the isotope becomes
another element entirely.
55. What is radioactive decay?
• The process by which unstable
(radioactive) isotopes transform to new
elements by a change in the number of
protons (and neutrons) in the nucleus.
• In other words, radioactive decay is
when an unstable isotope changes to a
new element.
56. Radioactive decay is the process by which an unstable
atomic nucleus loses energy by emitting radiation in
the form of particles or electromagnetic waves, thereby
transitioning toward a more stable state.
57. PotassiumArgon Dating Technique
• Geologists have used this method to date
rocks as much as 4 billion years old.
• It is based on the fact that some of the
radioactive isotope of Potassium, Potassium-
40, decays to the gas Argon as Argon-40.
• By comparing the proportion of K-40 to Ar-40
in a sample of volcanic rock, and knowing the
decay rate of K-40, the date that the rock
formed can be determined.
58. PotassiumArgon Dating Technique
• As the K-40 in the rock decays into Ar-40,
the gas is trapped in the rock.
• When rocks are heated to the melting
point, any Ar-40 contained in them is
released into the atmosphere.
• The technique works well for almost any
igneous or volcanic rock, provided that
the rock gives no evidence of having
gone through a heating-recrystallization
process after its initial formation.
59.
60.
61. Carbon-14 Dating
Technique
Radiocarbon dating
(sometimes simply
known as carbon
dating) is a
radiometric dating
method that uses the
naturally occurring
radioisotope carbon-
14 (14C) to estimate
the age of once living
materials from today
up to about 58,000 to
62,000 years in the
past.
64. Radioisotopes provide numerical dating here. 1. Place fossils
in correct order according to relative ages, oldest to youngest.
2. How would you estimate the age ranges of C, G & K fossils?
65. Catastrophism, Unifromitarianism, and
now a Combination of Both
• Catastrophism is the idea that Earth’s features have remained
fairly static until dramatic changes were wrought by sudden,
short-lived, violent events (catastrophes) that were
occasionally worldwide in scope.
• By contrast, during most of the 1800s & 1900s, the dominant
paradigm of geology has been uniformitarianism.
• Uniformitarianism, also known as gradualism, according to
which Earth's features have been gradually but continually
changing, eroding and reforming at a roughly constant rate.
• Recently, however, the scientific consensus has been
changing toward a more inclusive and integrated view of
geologic events, reflecting acceptance of some catastrophic
events along with gradual changes
66. Georges Cuvier
(August 1769-May
1832
Cuvier was a major figure
in natural science research
in the early 1800s.
He was a proponent of
catastrophism --that many
of the geological features
of the earth and the past
history of life could be
explained by short-lived
catastrophic global events
that had caused the
extinction of many species
of animals.
Cuvier came to believe
that there had not been a
single catastrophe but
several.
67. Sir Charles Lyell
(November 1797 –
February 1875)
Lyell was the foremost
geologist of his day.
He is best known as
the author of
Principles of Geology,
which popularized
James Hutton's
concepts of
uniformitarianism
(slides 12-15) – the
idea that the earth
was shaped by slow-
moving forces still in
operation today.
68. Uniformitarianism
These (a) mud cracks
formed recently,
while (b) the mud
cracks preserved in
rocks are millions of
year old.
The concept of
uniformitarianism
holds that the
ancient mud cracks
formed under the
same conditions
necessary for the
formation of modern
mud cracks.
70. Now a Combo: Uniformitarianism is the main process,
intermittently interrupted by Catastrophism (e.g. asteroid
impacts or super-volcanoes or mega-earthquakes).
