GEOLOGIC
TIME
How old is the earth?
How old is the earth?
4.543 billion years!
(a lot of time = a lot of geologic history)
• Over the years
• Plants and
animals evolve
and often go
extinct
• Geologic periods
bounded by
significant
extinction events
3 Ways to Determine Age of a Rock
1.
Relative Dating
• Age based on relation to another object
2.
Correlative
• Age based on time-dependent geologic characteristic (use fossils,
volcanic ash, etc)
3.
Numeric/Absolute
• Measured age (#) using elements
1) Relative dating:
determining relative order of past events
Main principles:
• Law of original horizontality
• Law of lateral continuity
• Law of superposition
• Law of included fragments/inclusions
• Law of cross-cutting relationships
• Unconformities
1) Relative dating:
determining relative order of past events
Law of original
horizontality:
All sedimentary
rocks, lava,
pyroclastic flows,
and ash layers are
all deposited
horizontally.
1) Relative dating:
determining relative order of past events
Law of
Superposition:
The oldest rocks
are found at the
bottom of the
sequence, the
youngest at the top.
EXAMPLES:
Law of original
Horizontality:
Pancakes are made
horizontally / flat
Flat
Flat
Flat
EXAMPLES:
Law of
superposition:
The oldest pancake
is at the bottom of
the stack and the
youngest at the top.
Deposited last
(youngest)
Deposited 2nd
Deposited 1st
(oldest)
Practice) Determine relative ages
• Oldest 🡪 youngest
1) Relative dating:
determining relative order of past events
Law of lateral
continuity:
The layers of
sedimentary rocks
will continue
laterally in all
directions unless
separated due to
erosion/intrusion
1) Relative dating:
determining relative order of past events
Law of
cross-cutting:
Any geologic event
(faulting, folding,
volcanic dike) that
cuts across another
rock layer MUST be
younger than the
rock body that it
cuts across.
EXAMPLES:
Law of lateral
continuity:
Law of Cross-Cutting:
1) Relative dating:
determining relative order of past events
Law of Inclusions:
A piece of rock that
is included in a
sedimentary layer, is
older than the layer it
is included in.
EXAMPLE:
Law of Inclusions: Which is older? Chocolate chips
or the pancake? THINK: Which was made first?
Practice
●
Oldest?
●
Youngest?
●
Which is older: T
or G?
Unconformity: erosion of rock,
followed by deposition of more
sedimentary rocks.
Disconformity: Sedimentary layer deposited over eroded horizontal sedimentary layer
Angular unconformity: Sedimentary layer deposited over eroded angular (tilted) rock
Nonconformity: Sedimentary layer deposited over eroded igneous or metamorphic rock
Practice
● Is J older
or younger
than G?
2) Correlative dating:
determining age by correlation
Correlate rock
layers using
material with
known ages such
as fossils or key
beds (lava/ash
beds)
3) Absolute dating:
determining age in years by using radioactive decay
• Use radioactive isotopes
• U238, U235, K40, C14
• Half-life (decay rate is known)
• How long it takes for half the parent to decay
• Parent 🡪 Daughter Isotope
• # of half-lives passed
• Equation:
• Age (years) = Half-life (years) * Number of half-lives elapsed
How to calculate the age of a sample:
Equation: Half-lives elapsed X half-life = Age (in years)
Example: Calculate the age of a rock that
contains 25% U235 (parent) and 75% Pb207
(daughter) when we know half-life for U235/Pb207
is 713,000,000 yr.
To do this:
1. What is the parent half life?
25%
2. How many half-lives have elapsed if 25%
atoms are remaining?
100% parent->50 (1 half lives
elapsed)->25% (2 half lives elapsed)
3. What is the half-life for U235/Pb207 ?
713,000,000
4. Calculate:
2 half-lives X 713,000,000 yr = 1,426,000,000 yr
Sig figs & scientific notation: 1x10^9 yr
Complete Lab: 1-9,
13-17, 21-25
Use the equation
for absolute dating
from the slides NOT
the one given in the
lab.
Good luck!
Ask questions!