THE HISTORY OF
THE EARTH
EARTH SCIENCE
CHAPTER 21
FOSSILS
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Paleontology is the study of life that existed in
prehistoric times
A fossil is any evidence of earlier life preserved in
rock.
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Shells
Bones
Petrified trees
Footprints
Impressions made by leaves
Burrows made by worms
FORMATION OF FOSSILS
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Fossils may form in several ways:
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From original remains
From replaced remains
Being preserved in molds and casts
Being preserved as trace fossils
Being preserved in carbonaceous film
ORIGINAL REMAINS
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In rare cases, organisms may be preserved in
their entirety.
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Frozen wooly mammoths in Siberia and Alaska
Prehistoric insects captured in resin
Usually, soft body parts decay and only the
hard parts are preserved.
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Bones
teeth
REPLACED REMAINS
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The soft parts of plants and animals have decayed
and the hard parts have been replaced by minerals
Circulating groundwater removes the original
organic material and replaces it with minerals such
as calcite, silica, and pyrite.
This results in an exact copy of the original plant or
animal.
Ex. Petrified wood
MOLDS AND CASTS
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Organisms can be buried in mud or other
sediments. As the sediments become rock,
the hard body parts become fossils.
If the fossil dissolves out of the rock, a hollow
depression in the rock called a mold results.
Minerals may then seep into the mold and fill
it, forming a cast, or copy, of the original
fossil.
TRACE FOSSILS
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Indirect evidence of life that is left behind
without being actual remains of the organism
or plant is called a trace fossil.
Trails, footprints, bite marks, burrows
CARBONACEOUS FILMS
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Decomposing organisms may be exposed to
intense heat and/or pressure.
Their tissues will undergo chemical reactions
that sometimes leave behind a very thin film
of carbon that resembles a silhouette of the
organism. This is called a carbonaceous
film.
RELATIVE TIME
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Most often, scientists have no way of
knowing the exact dates of geological events
on Earth.
Therefore, they often use a process called
relative dating.
Relative dating does not assign exact dates to
events but places events in a sequence, thus
saying when they occurred in relation to other
events.
THREE RULES OF
RELATIVE DATING
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The Principle of Superposition
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The Principle of Cross-Cutting Relationships
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In an undisturbed sequence of sedimentary strata, the
oldest rock layer will be at the bottom and the youngest
layer will be on the top.
An igneous intrusion is always younger than the rock it
has intruded or cut across.
Embedded Fragments
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Rocks that are embedded in another rock must be older
than the rocks in which they are embedded.
Example #1
Example #2
Example #3
Grand Canyon
Cross Section
1.
Oldest layer
2.
Youngest layer
3.
Oldest organism
4.
Place the following actions in the correct order of formation. (1-oldest, 5youngest)
-Granite dike intrusion, -folding of gray shale, -deposition of tan sandstone, -erosion
of tan sandstone, -deposition of red sandstone
GAPS IN RELATIVE TIME:
UNCONFORMITIES
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An unconformity indicates where layers of rock are missing
in the strata sequence—means evidence is missing
Angular unconformity
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Disconformity
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Younger, flat strata are deposited on top of older strata…older strata
could have been tilted during uplift
All layers are horizontal, but some are missing due to erosion before
the deposition of new layers
Nonconformity
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Occurs when sedimentary layers are deposited on top of igneous or
metamorphic rock
ROCK LAYER CORRELATION
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Correlation is the matching of rock layers from one area to
those in another area
Several methods are used to correlate rock layers:
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Walking the outcrop—outcrop is the part of the rock layer that can be
seen at Earth’s surface
Matching rock characteristics—see if the rocks have the same
appearance, color, and composition
Using index fossils--(next slide)
Fossils as Environmental indicators—the presence of certain fossils
can indicate climate and/or sea level changes
Matching key beds—a single rock layer that is unique, easily
recognizable, and widespread
Stratigraphic matching—when the sequence of three of more layers or
“strata” is repeated in two different areas
INDEX FOSSILS
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Index fossils are the remains of animals that lived and died
within a particular time segment of Earth’s history.
Remains are buried and animals become extinct, so those
rock layers can become distinctly tied to that time period.
Index fossils have four characteristics:
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The fossils are easily recognizable.
The fossils are abundant.
The fossils are widespread in occurrence.
The fossils occur only in a few rock layers because the organisms
only existed for a brief time period
ABSOLUTE TIME
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Absolute time identifies the actual dates of events.
Historical methods for measuring absolute time:
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Estimating rates of erosion and sedimentation
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Counting tree rings
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Not very accurate because rates are not constant
Still used today
One ring roughly equals one year
Have dated trees back to 2000 BC
Counting varves
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A varve is any sediment that is deposited on a yearly cylce
Used most accurately in glacial lakes
Have dated sediments back 15 000 years
RADIOACTIVITY
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Recall that isotopes are atoms of the same element
with different numbers of neutrons.
Many elements have radioactive isotopes.
Radioactive isotopes are those that exhibit
radioactive decay by emitting or capturing tiny
particles in order to try and achieve stability.
Whenever, particles are emitted or captured, it can
change the atomic mass and even the atomic number
(identity) of the atom.
THREE TYPES OF
RADIOACTIVE DECAY
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Alpha decay
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Beta decay
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Two protons and neutrons are emitted
So, atomic number of original isotope decreases by two and atomic
mass decreases by four
A neutron splits into a proton (+) and an electron(-). The electron,
called a beta particle, is emitted.
The atomic number of the original isotope increases by one and the
atomic mass does not change.
Electron capture
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A proton captures an electron and becomes a neutron.
The atomic number of the original isotope decreases by one and the
atomic mass does not change.
CONTINUAL DECAY
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The original element in the decay process is
called the parent isotope.
After each decay process, a daughter isotope
is formed.
If the daughter isotope is also radioactive,
decay continues until a stable, nonradioactive
isotope is formed as the daughter.
HALF-LIFE
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Radioactive isotopes decay at a constant rate. This
rate is not affected by changes in temperature or
pressure, or by the passage of time
The amount of time it takes for exactly one-half of
the radioactive atoms in a sample to decay to a stable
product is called the half-life.
After two half-lives, 25% of the original sample will
remain. ( ½ x ½ = ¼).
After three half-lives, only 12.5 % of the original
sample will remain. ( ½ x ½ x ½ = 1/8
RADIOMETRIC DATING
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Scientists measure the amounts of a parent
and a daughter isotope within a rock or
mineral and use the ratio to find the age of the
rock
Carbon-14 is used to date organic material.
Uranium-lead, Rubidium-strontium, and
potassium-argon are used to date rocks and
minerals.
RADIOCARBON DATING
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Radioactive carbon-14 and stable carbon-12 both
exist in the tissues of living organisms.
After death, the carbon-14 begins to decay.
Scientists use the ratio of carbon-12 to carbon-14 to
determine how long ago the plant or animal died.
Two serious limitations:
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Can only be used to date things that were once alive
Carbon-14’s half-live is 5730 years (short amount of
time), so it can only be accurately used to date items that
are 70 000 years old or younger.
URANIUM-LEAD DATING
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Radioactive Uranium-238 decays to form
stable lead-206.
Uranium-238 is 4.5 billion years, so it is
possible to date the oldest rocks of Earth’s
crust using this isotope.
Is naturally found in zircon, which is in some
igneous rocks—very rarely found in
sedimentary or metamorphic rocks
RUBIDIUM-STRONTIUM DATING
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Rubidium-87 decays to form strontium-87.
The half-life of Rubidium-87 is 47 billion
years (over 10x the age of Earth)
Is the best method for dating the oldest of
Earth’s rocks
Is found in common minerals like feldspars
and micas.
POTASSIUM-ARGON DATING
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Potassium-40 decays to form Argon-40.
The half-life of potassium-40 is 1.3 billion
years.
Are found in micas, feldspars, and
amphiboles.
Can be used to date sedimentary and
metamorphic rocks as well as igneous ones.
Geologic Time
Dinosaurs
Mesozoic Era
Title: Geologic Timeline
• The Earth is about 4.6 byo based on the absolute dating of
rock samples. The Earth’s history is broken down into
Eons, eras, periods and epochs. Each time frame does not
last an even amount of time and the division of time is
based on geologic and biologic events or evolutions.
Precambrian Eon
Today, Cenozoic Era
Geologic Time
Defined by changes in species or major geologic events. Time
frames are NOT uniform (NOT evenly divided!)
Earth is 4.6
BYO
Fossil evidence
shows life at 3.5
BYA
Life evolved
between 3.9-3.5
BYA
Precambrian ERA
4,600-544mya
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87% of the Earth’s History
Seas formed
Mountains grew
Oxygen built up in the
atmosphere
– Cyanobacteria
• Life : bacteria, jellyfish,
algae (Prokaryotes and
Eukaryotes)
• No life on land yet, only in
seas!
Precambrian Life
Ediacara
Stromatalites!
Paleozoic ERA 544-248 mya
• Age of Fishes
• Appalachian
Mountains form
• Warm, shallow seas
cover the majority of
North America
Paleozoic Era
• Periods
• 1. Cambrian Period
– “Explosion” of life
– Trilobites (skeletons, hard
parts evolved)
• 2. Ordovician Period
– corals, sponges, first fish
– Mass extinction- 57% of
marine species (glaciation)
• 3. Silurian Period
– first land plants
Paleozoic ERA
• Periods
• 4. Devonian
– age of fishes (armorplated fish and
amphibians) lungfish,
coelacanths
– Ferns (seeds!)
– mass extinction! 50%
of marine species(glaciation)
• 5. Mississippian
– Crinoids
(Invertebrates)
– Many amphibians
Paleozoic Era
• 6. Pennsylvanian Period
– Insects (cockroach,
centipede, dragonfly)
– first reptiles
– Coal deposits
• 7. Permian
– abundant reptiles
– Mass extinction- 95% of
land and marine species
(regression)
Mesozoic Era 225-65my
• Age of Reptiles
• Mass extinctions at
the beginning and end
of this era
• Mississippi River
System forms
• Rocky Mountains
• Atlantic Ocean
Mesozoic Era 225-65my
• Periods
• 1.Triassic
– Pangaea breaks up
• 2. Jurassic
– First birds
• 3. Cretaceous
– first flowering plants
• Life Forms of Mesozoic Era
– Dinosaurs, First Mammals,
and Modern Fish
• Mass extinction
– Meteorite, volcanism
– Wipes out dinos
Cenozoic ERA 65mya- today
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Age of Mammals
Himalayan Mountains
Alps
Grand Canyon
Great Lakes
Chesapeake Bay
Cenozoic Era
• Periods
• 1. Tertiary
– Mastodons, Mammoths,
Primates
– Early humans
• Homonids (walk upright)
• Neanderthals
– extinction (45% of species)
• 2. Quaternary
– Civilization begins10,000 years
ago with end of most recent
ice age and invention of
agriculture