Fossils

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Fossils
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Rock Layers

• The Law of Superposition states that the oldest
  rock layers are normally on the bottom.
• Cross cutting relationships says that an
  intrusion or fault is younger than the layers it
  cuts through.
• An unconformity is when erosion results in a gap
  in the rock record.

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Relative Age and Absolute Age

• Relative dating of rocks means that we define the
  age of rocks by surrounding layers of rock
• This does not give us specific time dates.
• -I am older than my sister.
• Absolute age- A specific age is given. (Uses
  radioactive decay)
• -Ex That fossil is 65 million years old.

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Index Fossils

• An index fossil lived only during a short period
  of time so we can use it to date common fossils
  found in the same area.
• Brachiopods Trilobites

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Dating Fossils

• Absolute dating is using the breakdown of
  radioactive elements to date an object.
• Radioactive decay is when one element decays
  (breaks down) into another element and releases
  radioactive particles.
• Radioactive elements have an unstable nucleus
  that breaks down over time. We can use this
  information to date things.
• This decay rate always occurs at the same rate
  for the same element BUT the decay rate varies
  from element to element.
• The decay rate can be a few years, thousands or
  billions of years.
• Parent element decays to the daughter element.

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Radioactive Decay

• Example elements are Rubidium -87, half life of
  48.6 billion years. Decays to Strontium-87
• Uranium-238, half life of 4.5 billion years.
  Decays to Lead-206. Carbon-14 decays to
  Nitrogen-14 in 5730 years.
• A half life is the time it takes for half of a
  radioactive substance to decay.
• Ex 1600g? 800g?400g? 200g? 100g? 50g? 25g?
  12.5g? 6.25g
• 1st half life 2nd 3rd
  4th 5th 6th 7th
  8th

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Other methods

• Other ways of dating include using tree rings,
  seasonal climate changes, and distinctive
  sediment layers.
• Tree Rings- trees grow faster in summer than
  winter. Rings years
• Varves- glacial ice melts faster in summer than
  winter. Bands years

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Earths Age and Fossils

• The oldest rocks have been found in South
  America. They are over 4 Billion years old!
• The oldest rocks on the moon are 4-4.6 billion
  years old. We think that the earth and the moon
  formed at about the same time. Why are earths
  younger?

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Earths History in FossilsFossils-clues
to the past

• A fossil is the remains or evidence of a living
  thing.
• Most common are bones, shells, pollen grains and
  seeds. Most are not complete organisms. The soft
  parts undergo decay.
• Decay is the breakdown of dead organisms into the
  substances from which they were made.
• Most fossils found in sedimentary rock. Being
  quickly covered by sediment doesnt allow decay
  or other animals to destroy it.

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• Petrification-Once living material is replaced by
  minerals, turning it to stone.
• Replacement may occur if water dissolves
  material but minerals in water replace the
  material.
• Ex. Petrified Forest in Arizona.

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Fossil types continued
Mold- The is empty space left after decay that
has same shape as organism. A cast is a mold
that has been filled. Sea shells Observe how
fossils can form. Imprints are impressions made
in soft mud that hardens. Examples leaves,
feathers Trace fossils- Fossils that mark
evidence of an animal. Examples footprints,
tracks, burrows etc.
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Preservation of entire organisms

• Very rare. Three ways.
• Original Preservation- Soft and hard parts of
  animals remain

Freezing Sudden weather changes preserve
organisms before decay takes place. Ex. Wooly
mammoths and very early people.
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Amber

• Amber-Flies or insects get trapped in tree sap
  and it hardens.
• Ex. Jurassic Park.

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Tar Pits

• Tar pits-Huge pits of tar. Usually near
  water-Animals would go drink fall in.
• Ex. Bison, camels, wolves, vultures,
  saber-toothed cats- Seen in movie-volcano and
  My Girl II.

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Interpreting Fossils

• Many different life forms have existed at
  different times through earths history.
• Scientists believe for every organism alive today
  at least 100 have gone extinct.
• Fossils show evolution, how earths surface has
  changed, past climates and appearance and
  activities of extinct animals.

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Geologic Time Scale

• This geologic time scale is divided up into three
  descriptive units.
• Eons, Eras, Periods and Epochs
• eOns, eRas, Period
• ORP

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Eons, Eras, and Periods

• Eons are the longest units of time, measured in
  billions of years.
• Ex. Achaean Eon
• Eras are the next unit, defined by differences in
  life-forms found in rock, measured in hundreds of
  millions of years.
• Ex. Paleozoic Era
• Periods are the next unit down, they are defined
  by the life-forms that are abundant or that
  become extinct, measured in tens of millions of
  years.
• Ex. Cambrian

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Dating of Rocks

• Rocks are what define our understanding of time,
  so how do we understand time from rocks?

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Geologic Principles

• Several principles are required if we want to use
  rocks as a means for dating time
• Uniformitarianism- Processes happening today have
  been happening since beginning of earth. Ex.
  Volcanic Activity
• Original Horizontality- Sedimentary rocks are
  deposited in flat layers
• Superposition- Undisturbed older rock is below
  younger rock
• Cross-Cutting- an intrusion or fault is younger
  than rock it cuts across.

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Youngest to Oldest
AnswerN, L, K, J, I, H, G, F, E, D, C, B, A
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Relative Absolute Dating
We are able to date objects based on Relative
Dating- Based on where we find fossils in the
rock record Example- Ms. Jeffreys is older than
her students Absolute Dating- This type of
dating uses the known breakdown rate of
radioactive elements to give an exact age.
Examples of Radioactive Elements are Carbon 14 -
Nitrogen 14 Example- Maria is 16 years old.
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What are fossils?

• A fossil is considered to be any trace of
  evidence left behind by an organism in the past.
• Ex. Bones, footprints, etc.
• Most commonly found in sedimentary rocks

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Fossil Interpretation
Fossil help us understand the different organisms
that have existed throughout Earths history. 99
of all species that have ever lived are now
extinct Fossils give us an understanding of
evolution