Chapter 13
CLUES TO
EARTH’S PAST
Section 1: FOSSILS
• Types of fossils
– Mineral replacement
– Carbon films
– Coal
– Molds and casts
– Original remains
– Trace fossils
– Trails and burrows
Mineral Replacement
• Permineralized
remains result
from minerals in
water or moist
sediments
replacing the
cells of dead
organisms,
leaving a copy
of the organism.
Permineralized wood
Carbon Film
• Living organisms
contain carbon.
• When they die
and get covered
by sediments, the
pressure and heat
squeeze out
everything but a
carbon “picture”
of the organism.
Coal
• Coal is a FOSSIL fuel.
• Animation of coal
formation:
http://www.sceyencestudi
os.com/movies/coalformati
on.swf
Molds and Casts
• An organism falls into a
soft sediment, like mud.
• More sediment covers
the organism.
• The organism decays or
becomes permineralized.
• The sediment hardens,
and if the rock is cracked
apart, a mold and cast
are formed
Mold: the
impression—or
indentation-- of
the organism.
Cast: the
raised copy of
the organism.
Original Remains
• The whole organism is preserved.
– In amber (tree resin)
– In ice
– In tar
In Ice
37,000 year
old frozen
remains of a
baby wooly
mammoth
found in
Siberia,
Russia.
Iceman
5,000 year old
remains of man
found frozen in a
glacier in the
Alps.
Tar
•Mammoth
•Mastodon
•Saber-tooth
cat
•Giant sloth
Radiometric dating of preserved wood and bones
found in the tar pits of La Brea, California has
given an age of 38,000 years for the oldest known
materials found there.
Giant sloth
Saber-tooth cat
Trace Fossils
• Trace fossils represent activities of the animal
while alive, rather than part of the dead
creature.
Dinosaur
footprints
Trails and
Burrows
• Trails and burrows
are evidence of
where animals
have walked or
lived.
Trails and
Burrows
• Modern ant burrows
More Fossil Evidence . . .
• Fossilized
nests
• Coprolite
(fossilized
feces)
Index Fossils
• Index fossils are fossils of organisms that lived
for a short period of time that are found in
many places.
• They are used to help date layers of Earth
because only certain fossils will be found in
certain layers.
More on Index Fossils
• Index fossils can help geologists correlate layers
of Earth and determine the relative age of the
rock they in which they are found.
Ancient Environments
• Fossils can help scientist determine what the
environment was like in an area long ago.
• Antarctica has fossils of ferns, so it must have
been warm and tropical at one time.
• Kansas has fossils of huge fish and other sea
creatures. It is very likely it was once covered
by a shallow sea.
Kansas Under Water
Here is
where
Kansas is
believed to
have been
located
when
covered by a
shallow sea
between 82
and 87
million years
ago.
Section 2: RELATIVE AGES OF ROCKS
• The principle of superposition states that the
oldest rocks are on the bottom.
Relative Ages
• Rock layers can be compared to each other in
terms of older, younger, between, etc.
• They are compared, or RELATED, to each other
by age.
• There is not an exact number of years used in
relative dating.
• If an older index fossil is found in a top layer,
and a younger fossil is found in a lower layer, a
geologist would conclude that the layers may
have been flipped over during compression.
UNCONFORMITIES
• Unconformities in rock layers can be
difficult to see in reality.
• Unconformities are gaps in the rock
sequence where some layers are missing.
• Three main types of unconformities
– Nonconformities
– Disconformities
– Angular unconformities
A Modern Unconformity
Comparing layers in two trash pits at two universities.
How an Angular Unconformity Forms
2
3
1
2: The layers are
uplifted by forces 3: New layers are then
within Earth, then deposited over the
1: Horizontal rock
angled layers, creating
eroded.
layers are deposited
an angular
over time. A is the
unconformity where
oldest layer.
the new layers meet the
angled layers.
Disconformities
1
2
2: Upper layers are
eroded away.
1: Horizontal rock
layers are deposited
over time. 1 is the
oldest layer.
3
New layers are
deposited on top of
old layers, leaving a
gap in the history
where the other
layers should be.
Nonconformity
• A nonconformity can
occur when igneous rock
invades rock layers that
already exist.
Another Look at Nonconformity
AND . . .Another Nonconformity
• Layers A – D
were uplifted
from pressure.
• The top of C
was eroded.
• E – G were
then deposited
on top.
• E is next to C
in one place.
A look at all three . . .
http://www.indiana.edu/~geol105b/images/gaia_chapter_6/unconformities.htm
Correlating Rock Layers
• Correlation means “matching up.”
• Rock layers in many places can match up.
Layers
exposed in
Zion National
Park, Utah
Layers exposed
in Grand Canyon,
Arizona
Layers exposed
in Bryce
Canyon, Utah
Stratigraphic Matching
• Stratigraphic
matching uses
correlation to
match patterns
in layers in
different
regions.
Correlation lines
Section 3: ABSOLUTE AGES OF ROCKS
• Absolute age of rocks is the age IN YEARS of
the rock.
• The age is determined by using radiometric
dating, which looks at how radioactive
material decays within rock layers.
• Two types of radioactive decay:
– Alpha decay
– Beta decay
Alpha Decay
• In alpha decay, two
neutrons and two
protons (particles in
the nucleus of an
atom) break away
from the parent atom
as an alpha particle.
• The resulting daughter
atom becomes a new
type of element.
Beta Decay
• In beta decay, a
neutron from an
parent atom’s nucleus
changes into a
neutrino and a proton.
• The resulting daughter
atom becomes a new
type of element.
Half-Life in Radioactive Decay
• A radioactive element decays in a known amount
of time.
• Half of all of the element will decay in a time
period known as a “half-life.”
• Carbon-14 takes 5,730 years to break down
through beta decay into nitrogen, but half of it
will remain.
• After another 5,730 years, half of the remaining
half will decay.
• Scientists can determine the age of the fossil by
comparing the amount of carbon-14 to the
amount of daughter nitrogen in the fossil sample.
How Does
Carbon-14 Get
Into
Living Things?
More on Carbon-14
• Since carbon is found in all living things,
carbon-14 is a good element to use for dating
fossils.
• The bad thing is, it can only date fossils up to
75,000 years old because it will decay
completely into nitrogen by then.
• Other radioactive elements with longer halflives are used to radiometrically date older
rock.
Using Radiometric Dating to Find Ages
of Rocks
• Aside from using Carbon-14 to date fossils,
radiometric dating only works on igneous and
metamorphic rock.
– Igneous rock is “new” rock, and when it cools, all the
particles in a sample are the same age.
– Metamorphic rock is changed under heat and
pressure. All particles in a sample will have changed
at the same time.
• Sedimentary rock contains particles of rock from
many different samples, so the age of the
particles can be different within the sample.
Relative vs.
Absolute Age
• Relative dating
determines if
one layer is
older or younger
than another.
• Absolute dating
uses radioactive
materials in
igneous and
metamorphic
rock to
determine their
ages in years.
Oldest Rocks on Earth
• The oldest rocks on Earth are
about 3.96 billion years old.
• Earth is about 4.5 billion years old.
• The rock older than 3.96 billion
years probably eroded away or
was subducted.
• Scientists use meteors that have landed on Earth to
determine the age of Earth.
– Meteors that hit Earth had to have come from nearby in our
galaxy, which is a little bit older than our Earth, since the
galaxy began at the same time.
– Particles in the galaxy formed together into stars and planets
due to gravitational pull. Meteors were a part of the debris.
A Galaxy
Uniformitarianism
• In the mid-1700s James Hutton estimated that
Earth is much older than previously thought.
• He came up with the principle of
uniformitarianism.
• It states that the Earth processes happening
now are similar to those in the past—and
those yet to come.
• The principle is often stated as “the present is
the key to the past.”
Earth processes happen
one of two ways . . .
• Earth processes can happen slowly over
thousands or even millions of years.
– Plate movement
– Changes in environments are usually slow
• Earth processes can happen violently and
quickly.
– Volcanic eruptions can build mountains quickly.
– Comets, asteroids, and meteorites can collide with
Earth, changing things abruptly.
• Dinosaur extinction
• Weather changes due to sun being blocked by debris