2. 8.1 Determining Relative Age
•Relative Age – age of an object in relation
to the ages of other objects.
• Sequences events in order, but does not give
exact age.
•Strata – layers of rock.
3. 8.1 Determining Relative Age
•Uniformitarianism – a principle that
geologic processes that occurred in the
past can be explained by current geologic
processes.
• Geologic forces happening now have been
happening for a most of Earth’s history.
• James Hutton 1700's
4. 8.1 Determining Relative Age
•Law of Superposition –
the law that a
sedimentary rock layer
is older than the layers
above it and younger
than the layers below it
if the layers are not
disturbed.
5. 8.1 Determining Relative Age
•Principle of Original
Horizontality –
sedimentary rock (if
left undisturbed) will
remain in horizontal
layers.
• If disturbed scientists
look for clues to
determine the original
order of layers.
6. 8.1 Determining Relative Age
•Graded Bedding – coarse or large
sediments are in bottom layers.
• If not, they were disturbed by tectonic forces.
7. 8.1 Determining Relative Age
•Cross-Beds – sandy sediments form
curved beds at an angle to the bedding
plane.
8. 8.1 Determining Relative Age
•Ripple Marks – small waves that form on
the surface of sand due to wind or waves.
• If undisturbed they point up.
9. 8.1 Determining Relative Age
•Unconformity – the break in the geologic
record created when rock layers are
eroded or when sediment is not deposited
for long periods of time.
• Nonconformity
• Angular Unconformity
• Disconformity
15. 8.1 Determining Relative Age
•Law of Crosscutting Relationships – the
principle that a fault or body of rock is
younger than any other body of rock that
it cuts through.
16. 8.2 Determining Absolute Age
•Absolute Age – the numeric age of an
object or event as established by an
absolute-dating process, such as
radiometric dating.
17. 8.2 Determining Absolute Age
•Absolute Dating Methods
•Rates of Erosion
•Rate of Deposition
•Varve Count
•Radiometric Dating
18. 8.2 Determining Absolute Age
•Rates of Erosion – by measuring the rate
that streams erode their beds.
• Only for streams between 10,000 - 20,000
years old.
19. 8.2 Determining Absolute Age
•Rate of Deposition – measuring the
deposition layers and comparing them to
known deposition rates.
• 30 cm per 1000 years.
• Not very accurate since deposition rates can
vary dramatically over time.
20. 8.2 Determining Absolute Age
•Varve Count – counting the banded layers
of sand and silt that are deposited
annually in a lake.
• Especially useful near ice sheets or glaciers.
• Similar to counting growth rings to
determine the age of trees.
21. 8.2 Determining Absolute Age
•Radiometric Dating – comparing the
relative percentages of a radioactive
(parent) isotope and a stable (daughter)
isotope.
22. 8.2 Determining Absolute Age
•Half-Life – the time required for half of a
sample of a radioactive isotope to break
down by radioactive decay into a daughter
form.
• Radioactive isotopes used are determined by
when rock layers formed.
23. 8.2 Determining Absolute Age
Parent Isotope Daughter Isotope Half-Life Effective Range
Carbon-14 Nitrogen-14 5730 years less than 70,000 years
Uranium-235 Lead-207 704 million years 10 million - 4.6 billion
Uranium-238 Lead-206 4.5 billion years 10 million - 4.6 billion
Potassium-40 Argon-40 1.25 billion years 50,000 - 4.6 billion
Thorium-232 Lead-208 14 billion years up to 200 million
Rubidium-87 Strontium-87 48.8 billion years 10 million - 4.6 billion
25. 8.3 The Fossil Record
•Fossils – the remains of animals or plants
that lived in a previous geologic time.
26. 8.3 The Fossil Record
•Paleontology – the study of fossils.
• Fossils are sought after in sedimentary rock
because sediment covers the organism and
prevents decay.
• Igneous and metamorphic rock forming
processes destroy the organic structures.
• Fossils in rock can form if the organism is
buried immediately after death and the hard
parts become preserved as rock forms
around them.
27. 8.3 The Fossil Record
• Interpreting the Fossil Record
• Shows us clues to the Earth's geologic history
• Climate changes are seen by marine fossils
being found in areas that are not under ocean
today showing the area once was under oceans.
• Michigan was for 300 million years laying down
our salt, gypsum, and dolomite, as well as
Petoskey Stones (our state stone) .
• Shows how living things have evolved over
time
28. 8.3 The Fossil Record
• Types of fossils
• Mummification
• Amber
• Tar seeps/asphalt
• Freezing
• Petrification
• Imprints
• Mold
• Coprolites
• Gastroliths
• Trace fossil
29. 8.3 The Fossil Record
•Mummification – form in dry places
where decomposing bacteria can not live.
30. 8.3 The Fossil Record
•Amber – fossils form when insects land on
a tree and get covered in sap which
hardens.
• Sometimes DNA can be extracted
• Jurassic Park
31. 8.3 The Fossil Record
•Tar Seeps/Asphalt – fossils form when an
organism fell into a asphalt/tar pit while
trying to drink he water above and was
preserved.
• Le Brea asphalt/tar pit in LA, California
32. 8.3 The Fossil Record
•Freezing – frozen fossils form when
organisms from the last ice age died and
froze slowing down the decay process.
33. 8.3 The Fossil Record
•Petrification – petrified fossils form when
pores in wood, bones, or shells fill with
dissolved minerals (silica, calcite, pyrite)
and crystallize over time.
34. 8.3 The Fossil Record
•Imprints – when original, organic material
partially decays, leaving behind a carbon
film.
35. 8.3 The Fossil Record
•Mold – a mark or cavity
made in a sedimentary
surface by a shell or other
body part.
•Cast – a fossil that forms
when sediments fill in the
cavity left by a
decomposing organism.
36. 8.3 The Fossil Record
•Coprolites – fossilized dung.
• Can preserve what the animal ate.
37. 8.3 The Fossil Record
•Gastroliths – stones that were used in the
digestive tract of some dinosaurs.
38. 8.3 The Fossil Record
•Trace Fossil – a fossilized mark that is
formed in soft sediment by the movement
of an animal.
• foot prints, burrows, boring, etc.
39. 8.3 The Fossil Record
•Index Fossil – a fossil that lived during a
relatively short geological time that can be
used to absolute date the rock layer in
which they are found.
Ammonite Trilobite Mesosaurus
40. 9.1 Geologic Time
•Geologic Time Scale — the standard
method used to divide earth's long natural
history into manageable parts.
•Based on significant events.
• Extinctions
• Appearances of new organisms
• Formation of mountain chains or oceans
• Ice age
• Widespread volcanic eruptions
43. 9.1 Geologic Time
•Era – major time division.
1. Precambrian
2. Paleozoic
3. Mesozoic
4. Cenozoic
44. 9.1 Geologic Time
• Precambrian – 4600 mya – 543 mya
• 4600 mya Earth forms.
• 3800 mya crust hardens.
• 3500 mya first prokaryotic (no nucleus) life forms.
• Single-celled, photosynthetic (make food from the sun),
cyanobacteria which released oxygen into atmosphere
which created ozone layer (03) which blocked UV rays.
• 2500 mya first eukaryotic (nucleus and other
organelles)life forms and animal like (ate their
food) amoeba.
• Ended with an explosion of multicellular ocean life.
• Paleozoic explosion of life
46. 9.1 Geologic Time
• Paleozoic – ("ancient life") 543 mya – 248 mya
• Sponges, corals, snails, clams, squid, trilobites,
fish, sharks.
• Ferns, club moss, horsetails, and conifers appear
on land toward the end of era.
• Wingless insects, salamander-like amphibians
during middle, followed by winged insects and
reptiles by the end of the era
• Ends in largest mass extinction in history.
• 90% of all species went extinct because of… ice age?
52. 9.1 Geologic Time
• Primate – group of mammals that include
humans, apes, monkeys, lemurs who have
opposable thumbs (can grab things) and 3-d
binocular vision.
• First primate (45 mya)
• Chimps and humans separate (30 mya – 5 mya)
• Hominid – primate characterized by bipedalism
(walk on 2 legs), longer back limbs (legs), and
lack of a tail.
54. 9.1 Geologic Time
•Hominids Through Time
• Earliest, 6 mya – 7 mya in Africa
• Australopithecines
• Used stone tools
• Walked on 2 legs (no opposable toe)
• Flatter forehead (larger brain)
• Humanlike jaws, teeth and feet
• "Lucy" 3.2 mya (Australopithecus afarensis)
