Geologic Time & Fossils
Geologic Time & Fossils
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
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
-I am older than my sister.
Absolute age- A specific age
is given. (Uses radioactive
-Ex: That fossil is 65 million
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.
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
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
Ex: 1600g 800g400g 200g 100g 50g 25g 12.5g 6.25g
1st half life 2nd
Other ways of dating include
using tree rings, seasonal climate
changes, and distinctive sediment
Tree Rings- trees grow faster in
summer than winter. Rings =
Varves- glacial ice melts faster in
summer than winter. Bands =
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 Fossils
Fossils-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 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.
Observe how fossils can form.
Imprints are impressions made in soft
mud that hardens. Examples: leaves,
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-Flies or insects get trapped in tree sap and it hardens.
Ex. Jurassic Park.
Tar pits-Huge pits of tar. Usually near water-Animals would
go drink & fall in.
Ex. Bison, camels, wolves, vultures, saber-toothed catsSeen in movie-volcano and My Girl II.
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
Geologic Time Scale
This geologic time
scale is divided up
Eons, Eras, Periods
eOns, eRas, Period
Eons, Eras, and Periods
Eons are the longest units of time, measured in billions
Ex. Achaean Eon
Eras are the next unit, defined by differences in lifeforms found in rock, measured in hundreds of millions
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.
Dating of Rocks
Rocks are what define
our understanding of
time, so how do we
understand time from
Several principles are required if we
want to use rocks as a means for dating
happening today have been happening
since beginning of earth. Ex. Volcanic
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 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.
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 help us understand the different
organisms that have existed throughout
99% of all species that have ever lived
are now extinct
Fossils give us an understanding of