1. Geologic Time Geologists have their own system of defining and
naming periods in the earth’s history, based on the
ages of rocks..
How can you tell how old a rock is?

2. Geologic Time Dating rocks – The age of a rock refers to
when it solidified
or when, as a solid, it was changed by heat
and pressure into a different form.

3. Geologic Time The earth is 4.54 billion years old.
The oldest rocks that are still around are about
4 billion years old.
On the Big Island, there are rocks that are just
a few minutes old – they just cooled from lava
into solid rock.

4. Geologic Time How do we know how old a rock is?
(we’ll get to this in CH 8 of your book)
Radiometric dating: an absolute measurement
based on the steady decay of radioactive atoms.
Stratigraphic dating (strata = layers): The relative
positions of rocks can tell you their relative ages

5. Radiometric Dating For any sample of radioactive atoms, every n years
half of them will change into something else.
Potassium becomes Argon
Uranium becomes Lead
Carbon-14 becomes Carbon-12…..
Each of these has a characteristic n – it’s “half life”

6. Radiometric Dating
This is a KEY slide!

7. Radiometric Dating
For Potassium -> Argon, the half life is 1250 million
years.
So a rock that has no argon in it is very new.
One with about the same amount of potassium as
argon is about 1250 million years old – half of the
original potassium has changed.
More argon – older.

8. Radiometric Dating How do you measure Potassium and Argon?
- Ionize a sample – heat it until it vaporizes and glows
Run the vapor past a magnet; this bends different
elements by different amounts
count the ionized
particles with an
array of detectors

9. Stratigraphic Dating
Rocks are often found in layers, oldest on the bottom.
You can often trace layers through breaks and distortions.

10. Stratigraphic Dating
Layers in the Grand Canyon

11. Back to Geologic Time…
For historical reasons, geologists don’t just refer to
however many million or billion years old the rock is.
(For a long time they knew a lot about relative ages
but not the actual total age of the earth.)
They have a system of names that reflect
Important patterns or stages in earth’s history
Where rocks of certain ages were first identified.
(Devonian rocks fron Devonshire, England
Jurassic rocks from the Jura mountains)

12. Back to Geologic Time…
EONS
ERAS
PERIODS
EPOCHS.

13. Back to Geologic Time…

14. Eons - The largest division of time.
Hadean – first 560 my. Named after Hades (hell).
Meteor impacts, volcanoes, earthquakes.
Archaen – next 1.5 by. Still hot, small proto-continents,
no free oxygen, only life was bacteria.
Proterozoic – next 2 billion years. Surface stabilizing,
oxygen building in atmosphere, major ice age,
single-celled and very simple life.
Phanerozoic – the last 540 my. Largely like now.

15. Archaen 4.75 BYA – formation of Earth
4.03 BYA – oldest known rocks. Earth still being bombarded by meteorites.
3.8 BYA – Earliest probable microfossils
3.6 BYA – Earliest oxygen-forming bacteria
3.2 BYA – First colonial algae; large chunks of land become stable

16. Proterozoic/pre-Cambrian
2.5 BYA – the Oxygen crisis; banded iron formations show earliest oxidation of rocks (rust)
2 BYA – significant oxygen in atmosphere
1.8 BYA – complex single-celled life (eukaryotes – cells have nuclei)
6.3 MYA – oldest good fossils of multicellular life (sponges, worms)

17. ERAS - Subdivisions based on life
The Phanerozoic eon is divided into 3 Eras based
on major changes in fossil life forms. These also
show changes in climate and movements of the
continents.
The boundaries of Eras are based on major
extinctions – many many forms disappearing suddenly
from the fossil record.

18. ERAS - Subdivisions based on life
Paleozoic (“old life”) – first complex life forms to the
Permian extinction, at the same time as
continents came together into Pangea
(540 – 225 mya)
Mesozoic (“middle life”) – rise to the fall of the
dinosaurs (225 – 65 mya)
Cenozoic – rise of the mammals, still going on

19. PERIODS Smaller subdivisions of the Eras, also based on
changes in the fossil record.
So there were dinosaurs around throughout the
Triassic, Jurassic, and Cretaceous periods – but
Different ones in each period.
(Not all of the dinos in Jurassic Park existed in the
Jurassic Period!)

20. EPOCHS Are, yes, shorter divisions in the most recent periods.
Mainly important for mammal and human fossils and
some recent rocks.

21. Geologic Time
One view of the relative lengths of the geologic time divisions (does not separate the Hadean Eon)

22. Another way of looking at the relative durations of the major divisions.

23. What to Know?
So now you’re thinking, how much of this do we have to know, anyway?
Geologists have their own time scale
Earth is 4.55 billion years old
How we date rocks
Why radiometric dating works
What is stratigraphy
Eons, Eras, Periods, Epochs
…..yes there’s more

24. What to Know? Names, order, and dates for Eons and Eras.
Names of Periods, in order
Be ready to understand and use these in descriptions of
rock formations, fossils, etc.