Transcript ch-151-earths-history-fossils-hand

History is in the rocks
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Earth’s history is found in the rocks.
There is no evidence of Earth’s infancy.
Oldest rocks are 3.9 billion years old.
Fossils are the historical record of life.
Fossilization: Formation of fossils
• Fossils form when an organism is buried in
sediments (small particles of mud, sand, or
clay) soon after they die.
• Over time sediments build up over organism
preventing further decay.
• The type of fossil formed depends on
environmental factors.
Fossils form in sedimentary rock.
• Fossils are usually formed in sedimentary rocks
because the process of depositing sediments
prevents damage to the organism.
Fossil Classification: Molds
• Molds: when an organism is buried, it can decay,
leaving an empty space in the rock that is the
exact shape of the organism.
Fossil Classification: Casts
• Fossils are classified by
how they were formed.
• Casts: a mold of the
organism is created and
it is filled by minerals
from the surrounding
rock.
Fossil Classification: Amber
• Amber-preserved and
frozen fossils: entire,
intact organisms are
found in ice or
fossilized tree sap.
• Rare, but valuable to
science because the
delicate parts of the
organism are
preserved.
Fossil Classification: Petrified
• Petrified fossils: hard
parts of organisms are
replaced by minerals
(atom-for-atom)
• When the minerals
harden an exact stone
copy of the organism is
produced.
Fossil Classification: Imprints
• Imprints: Sometimes
fossils form before
sediments harden into
rock.
• Thin objects like leaves
or feathers fall into mud
and leave an imprint.
Fossil Classification: trace
• Trace fossils: are the
markings or evidence of
animal activities.
• They include footprints,
trails, and burrows.
Paleontology
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Paleontologist: a scientist who study ancient life.
What kinds of organism lived.
When the organisms lived, period of time.
Theorize organism’s behavior.
Ancient climate/environmental conditions.
Plant life, based on fossils (pollen/plants).
Condition, position, and location of fossils determine
geography and topography.
Dating methods for fossils
• Relative dating: determines whether one fossil is
older than another fossil because it is found deeper
in the Earth.
• Geological Law of Superposition: most sedimentary
rocks are laid down in horizontal layers with the
younger layers closer to the surface and the older
layers buried deeper.
Absolute age of fossils
• Radiometric dating techniques: involve the use of
radioactive isotopes.
1)Isotopes are atoms with unstable nuclei.
2)The nuclei break down over time .
3)The breakdown releases radiation.
4)The amount of radiation is measured.
5)The decay rate of each radioactive element occurs
at a steady rate.
6)Scientists use them as a type of clock.
Radioactive dating example
• If one looks at an
hourglass. One can see
how much sand needs to
go through and how
much more needs to go
through.
Calculate Age Using Half-lives
• A radioactive element has a half-life of 20
days.
• How much of a 16 g sample will be unchanged
after 80 days?
Carbon-14 Dating
• Fossils and archaeological artifacts that are
<50,000 years old.
• Carbon-14 breaks to half of its amount in
5,730 years.