The Rock and Fossil Record –
Geology and Time
Earth’s Story
• Catastrophism (#2) – belief that all geologic
change happens suddenly.
• Rare
• Mountains, canyons, seas, etc.
• Earth was only a few thousand years old
James Hutton & Earth’s Story
• Scottish farmer and scientist
• Uniformitarianism (#1) –
geologic processes that happen
today are same as those that
happened in the past.
• Weathering, erosion, deposition,
mountain building (uplift)
• Takes a very long time.
• Earth was millions of years old
• In 1788 he published Theory of
the Earth.
Charles Lyell’s Contribution
• British geologist
• Used new evidence to
reintroduce uniformitarianism.
• 1830-1833, he published 3
volumes called Principles of
Geology.
• This convinced most scientists
and people (including Charles
Darwin) that the Earth was
indeed very old.
A Happy Middle Ground
• Both catastrophism and uniformitarianism
work to shape the Earth
• Most change is gradual (uniformitarianism)
• Some change is sudden (catastrophism)
•asteroid or meteor impacts
•volcanic eruptions
•landslides
•tsunamis
•earthquakes
Relative Dating
• Relative dating (#3) – determining whether events or objects are
younger or older in comparison to other events or objects.
• Based on principle of superposition (#4) – younger rocks layers
are on top of older layers in undisturbed area
– sediments are laid down horizontally
– forces can change the position of the layers
Draw a picture of 4 rock layers with
a fossil in each layer. Label the
fossils from oldest to youngest:
Absolute Dating
• Absolute dating (#5) – any method of measuring the age of
an event or object in years
– measures a range of years from thousands up to millions
• One method is called radiometric dating
– Certain atoms have isotopes (#6) – an atom with same number of
protons but different number of neutrons
– Radioactive decay (#7) is when the unstable isotope atoms break
down to stable atoms
– A half-life (#8) is the time needed for half the unstable isotope atoms
to break down to stable atoms
• Radiometric dating (#9) – a method of measuring the percent
of remaining isotopes to give the age of the object
Types of Radiometric Dating
• Potassium-Argon (K→Ar)
– potassium-40 has a 1.3 billion year half life
– decays to argon and calcium
– date rocks older than 100,000 years
• Uranium-Lead (U→Pb)
– uranium-238 has a 4.5 billion year half life
– decays in steps to lead-206
– date rocks older than 10 million years
• Rubidium-Strontium (Rb→Sr)
– rubidium-87 has a 49 billion year half life
– decays to strontium-87
– date rocks older than 10 million years
• Carbon-14 (C14→N)
– carbon-14 has a 5,730 year half life
– decays to nitrogen
– date living things no older than 50,000 years
Tell your neighbor:
What is the difference between relative
dating and absolute dating?
relative dating compares ages of different objects while absolute
dating measures an actual age range of an object
Fossils and the Fossil Record
• Fossils found in the rock layers (aka fossil record)
provide a history of the Earth
– Only a rough sketch
– Some layers more complete than others
– Only a few organisms meet conditions
to become fossils
• Fossils can be found:
– bones and shells preserved in rocks
– amber: tree sap
– petrifaction: minerals replace organism’s
tissues, aka petrified wood
– asphalt: organisms trapped in tar pits
such at LaBrea tar pits in Los Angeles
– frozen: cold slows decay and preserves woolly mammoth
Geologic Time Scale
• Dividing Earth’s history into sections of time
Geologic Time Scale
• Eon (#10) – largest division of geologic time
– hundreds of millions of years of time
– 4 eons
•
•
•
•
Hadean – hell
Archean – ancient
Proterozoic – earlier life
Phanerozoic – visible life. Most of fossil record in this eon
• Era (#11) – 2nd largest division consisting of 2 or more periods
– Tens of millions of years
– Phanerozoic Eon divided into 3 eras
• Paleozoic – ancient life
• Mesozoic – middle life
• Cenozoic – recent life
• Period (#12) – 3rd largest division of geologic time
– Millions of years
• Epoch (#13) – 4th largest division of time
– Hundreds of thousands of years
Draw and label a geologic time
scale that has the 4 eons and 3
eras on it.
Phanerozoic Eon
• Paleozoic Era – Age of Fishes
–
–
–
–
–
543 to 248 million years ago (MYA)
Marine life
Modern land plants
Amphibians, reptiles, insects by end of era
Mass extinction killed 90% of species
• Mesozoic Era – Age of Reptiles
–
–
–
–
248 to 65 MYA
Reptiles dominate with small mammals
Birds evolved from dinosaurs
Mass extinction killed up to 20% of species
• Cenozoic Era – Age of Mammals
– 65 MYA to present
– Mammals dominate
Review Questions
1. Who proposed the idea of
uniformitarianism?
2. What is catastrophism?
3. What is the principle of superposition?
4. In your own words, write a definition for
relative dating.
5. T/F Absolute dating gives the exact, precise
age of something.
6. What type of radiometric dating is used to
date remains of living things?
7. What are the 4 divisions of geologic time
from largest to smallest called?
8. What do each of the following terms mean?
- Phanerozoic
- Paleozoic
- Mesozoic
- Cenozoic