Transcript Geo-time scale measurements notes

Geologic Time Scale
Methods of Measurement
Background
• So far we have discussed
Earth’s formation
• We have also discussed
how the Earth was
organized during its
formation.
What Next?
• We know how the Earth was formed and how it
changed during formation. But there are two
lingering questions.
▫ What has happened since then?
▫ How do we know?
Dating Techniques (Review)
• Dating Earth is HARD!
• Nowhere on Earth will you find a complete rock
record from day Earth’s formation up to today.
IT DOESN’T EXIST.
• We have only had the ability to accurately date
rocks for about 60 years.
• We are tiny.
▫ Time units for humans are seconds, hours, days,
years.
▫ Time units for Earth is millions, hundreds of
millions to billions of years.
Dating Techniques (Review)
• Earth’s history is recorded in rocks of the crust…
• Geology seeks to unravel this history!
• Two basic time keeping methods:
• Relative dating - Place events in their proper
order.
• Absolute dating - Determine, in years before the
present, when event actually occurred.
Relative Age Dating
• Most of the work was done in the 1800’s by
geologists who…
▫ Gathered information from numerous rock
exposures.
▫ Constructed a sequential chronology based on
Earth’s life through time.
Relative Age Dating
• Puts rock layers in age order from oldest to
youngest.
• Operates using the principle of
Uniformitarianism- all present day processes
have operated throughout geologic time.
• Processes like…
▫
▫
▫
▫
Weathering and Erosion
Deposition
Volcanism
Plate Tectonics
Relative Age Dating
• There are several principles of relative age
dating that will ensure that rocks are always
placed in chronological order.
Principles of Relative Age Dating
• Superposition: In a sedimentary sequence
that has not been overturned, the oldest rock is
always at the bottom.
Principles of Relative Age Dating
• Original Horizontality: Sedimentary layers
are deposited flat. Any disruption must have
come after layers were deposited.
Principles of Relative Age Dating
• Cross-cutting relationships: If a rock unit
cuts across another, it is always younger!
Principles of Relative Age Dating
• Inclusions: if a unit contains inclusions
(fragments) of another rock unit, the inclusion is
older.
Relative Age Dating
• Look at slide 8 and put the layers in order from oldest to
youngest using relative age dating.
1___
2___
3___
4___
5___
6___
7___
8___
9___
10___
11___
12___
Correlation
• When rocks are deposited it usually happens
over large areas.
• Many of the continents may share many of the
same rock layers.
• Correlation: the matching of rock layers from
one area to another. This can be done by…
▫ Using similarities in the rock
▫ Examining key beds
▫ Examining index fossils
correlated sequence
1
Kansas
2
Indiana
3
Ohio
4
Pennsylvania
Mostly
nonmarine
limestone
Where did the
geologic time
scale come from?
Built up
over time by
correlation and
relative dating
of rocks
from around
the world!
Absolute or Radioactive Dating
• Relative age dating and correlation put the
ENTIRE rock record together for us but did not
tell what the real ages were.
• Unstable or, radioactive, elements are
everywhere.
▫ If you know what the radioactive element will turn
into, and how long it takes to decay you can
calculate the age of anything.
Radioactive Dating
• Half-life: The amount of time it takes for half of
the parent (radioactive) element to decay into
the daughter (stable) element.
Radioactive Dating
• Half-lives can be anywhere from billions of years
to fractions of a second.
• If you had a sample that was 25% Thorium-232
and 75% Lead-208, how old is that sample?
Summary
• Relative age dating has allowed us to put
together all of Earth’s history into a story.
• That story has chapters preserved all over the
planet, we just had to put them in the right
order.
• The geologic time scale was put together using
relative age dating.
• We have had more exact dates for the rock layers
for only about 60 years.