Relative Dating, Absolute Dating and Index Fossil Review

Layers of rock and sediment build up
overtime. Because of this the strata
creates a physical timeline.

Comparing the age of rocks and fossils to
other rocks and fossils bases upon its
position is called relative dating.

Relative Dating -
Rock Layers

OLDEST LAYER is at the BOTTOM
And the YOUNGEST at the TOP
unless there has been a
disturbance or the rocks have
been interrupted

LAW of Superposition

If a fault or another rock cuts
through another body of rock then
it is YOUNGER than the rocks it
cuts through and displaces

LAW of
Crosscutting

Sedimentary rocks are the only
type to contain fossils because
these rocks are formed on the
Earth surface at very low
temperatures and pressures.

Why are fossils only
found in Sedimentary
Rock ?

Are fossils used to define and
identify geologic periods.
1.

Easy to recognize

2.

Widespread (multiple columns)

3.

Lived for a short period of
time (shows up in one row)

Index Fossils

Radioactive dating determines the exact ages of rocks and fossils by looking at tiny particles called atoms.

Some atoms decay over time and by measuring the amount of decayed atoms in a sample we can measure how long it has been around for.

Radioactive Dating

Half-lives

When 1/2 of the element decays, this is known as a half life.

​All, radioactive, elements have a specific decay rate unique to that element.

How long in years ?

After you know how many half-lives have occurred you can multiply that number by the amount of time it takes for that element to decay.