Transcript How do we know…… - GSHS Mrs. Francomb

Geologic Time:
How do we know……?
Looking at the layers of the Earth and
interpreting the evidence.
CREATING A GEOLOGIC TIMELINE
Relative Dating
Radiometric Dating
REVIEW: RADIOMETRIC DATING
A procedure used to date materials
based on the amount of radioactive
elements present
This gives a specific age (within an
error margin)
RADIOMETRIC DATING:
SOME ELEMENTS DECAY
An element always has a specific number of
protons
They usually have the same number of
neutrons, but variations exist called “isotopes”
Radioactive Decay is when an unstable isotope
loses particles in its nucleus and becomes
another element
Decay is measured by its half-life
HALF LIFE DECAY
SLOWLY
DECAYING
ISOTOPES
CAN BE USED
TO DATE
ROCKS:
CARBON
DATING
• C14 Half-life:
5730 years
• Used to date
organic material
up to 60,000
years old
RELATIVE DATING
No.. Don’t ask your cousin out on a date
RELATIVE DATING
Before radiometric
dating…..
The process of
placing events in the
sequence in which
they occurred
without using exact
dates.
 Where are the
youngest rocks?
STRATA AND THE PRINCIPLE OF
SUPERPOSITION
Nicolaus Steno,
1636-1686
The oldest rock
layer will be at
the bottom and
the youngest at
the top.
THE PRINCIPLE OF ORIGINAL
HORIZONTALITY
Sediment is always
deposited flat
If the strata (layers) are
not flat, something
happened!
What might have
happened here?
A few other “dating”
guidelines:
BEWARE OF INTRUDERS!
 Magma can “intrude” into sedimentary rock that was already there!
CROSS-CUTTING
An igneous
intrusion is
always
younger than
the strata it
cuts through.
The layers had
to be there
first!
CROSS CUTTING
Inclusions
Embedded Fragments
Rocks that are
“included” in
another rock must
be older than the
rock in which they
are found.
Older
UNCONFORMITY
The layer or layers
of rock missing from a
strata sequence.
Where did they go?
Usually caused by
erosion
INDEX FOSSILS
ROCK LAYER CORRELATION
Matching of rock layers
from one area to
another.
Gives a more
comprehensive view of
a region’s geologic
history
D.U.D.E
Deposition
Uplift
Down-cutting
Erosion
200 MILLION YEARS AGO
PANGAEA
Our continents were part of one landmass:
Pangaea
Due to continental drift our continents are
where they are today, and continue to drift
apart.
EVIDENCE TO SUPPORT PANGEA
Matching reptile fossils found in South
America and Africa
Identical species of ferns found in all
southern continents.
Rocks in SW Africa and SE Brazil were
distinctively identical, and their age the same.
Plate tectonics and continental drift