Fossils

Rock LayersThe 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.

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.

Index FossilsAn 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

Dating FossilsAbsolute 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.

Radioactive DecayExample elements are: Rubidium -87, half life

of 48.6 billion years. Decays to Strontium-87Uranium-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 800g400g 200g 100g 50g 25g 12.5g 6.25g

1st half life 2nd 3rd 4th 5th 6th 7th 8th

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

Earth’s 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 earth’s younger?

Earth’s History in FossilsFossils-clues to the pastA 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 doesn’t allow decay or other animals to destroy it.

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.

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.

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.

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

Tar PitsTar 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.

Interpreting Fossils

Many different life forms have existed at different times through earth’s history.

Scientists believe for every organism alive today at least 100 have gone extinct.

Fossils show evolution, how earth’s surface has changed, past climates and appearance and activities of extinct animals.

Geologic Time Scale

This geologic time scale is divided up into three descriptive units.

Eons, Eras, Periods and Epochs

eOns, eRas, Period ORP

Eons, Eras, and PeriodsEons 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

Dating of RocksRocks are what

define our understanding of time, so how do we understand time from rocks?

Geologic PrinciplesSeveral 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.

Youngest to Oldest

Answer:N, L, K, J, I, H, G, F, E, D, C, B, A

Relative & Absolute DatingWe 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.

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

Fossil Interpretation

Fossil help us understand the different organisms that have existed throughout Earth’s history.99% of all species that have ever lived are now extinctFossils give us an understanding of evolution