ERS111: Lecture 6 – The Age of Rocks

In this lecture we will cover how we know the age of the

Earth:

• How we are able to determine the age of a rock or

fossil – both in an absolute sense (i.e. writing the age
as a number) and in a relative sense (i.e. this rock is
older / younger than that rock).
• The Geologic timescale, and considering time on a
geological scale

At the end of the lecture you will be able to:

• Explain how old Earth is, and how we know it
• Describe the different methods we can use to
determine the age of a rock
How do we know how old the Earth is? How do
calculate the age of rocks?

How do we know the Earth looked liked this 250

million years ago?
How do we know how old things are?
Relative vs. absolute dating

Relative Absolute

• The principles of • Radiometric dating

stratigraphy
• Unconformities
• Index fossils
The Principles of Stratigraphy

The Principle of Original

Horizontality:
Layers of sediment are
originally deposited
horizontally under the
action of gravity
The Principles of Stratigraphy

The Principle of
Superposition:
Sedimentary layers
are deposited in a
time sequence, with
the oldest on the
bottom and the
youngest on the top.
The Principles of Stratigraphy

The Principle of Lateral Continuity:

Layers of sediment initially extend
laterally in all directions; in other
words, they are laterally continuous
 Rock relationships - Unconformities
An unconformity is a rock interface which represents a GAP
in the geologic record, like pages missing from a book
Types of Unconformities
Represents an unknown period of time
Index Fossils

• Small size
• Abundant / large sample sizes
• Worldwide distribution
• Easy to identify
• Good preservation
• Narrow stratigraphic range
Microfossils
Through the work of thousands of geologists, we have
built up a picture of when, and where, these
organisms lived
Radiometric dating – absolute dating
Uses the natural radioactive decay of certain elements as
a stopwatch

Only certain elements (e.g. U or

K) in certain minerals (e.g.
zircons) can be used in this way
Atoms and Isotopes
An atom is the basic building
blocks of all matter
Nucleus:

Protons + Neutrons

Electron shells
surround nucleus

# of protons defines
element
# of neutrons can
change = isotopes
 Carbon has 15 isotopes – 12 and
Atoms and Isotopes 13 are stable, all others decay –
some quick, some slow
98.9% ~1% of >0.1%
of all C all C is of all C
is C12 C13 is C14

X X
Radioactive decay

The time it takes for half of original isotope to decay

to stable isotope
 After ~10 half lives, original isotope
Half lives
is pretty much undetectable
Common isotope systems
 Requirements for absolute dating

• The half-life of the parent isotope must be appropriate

for the length of the period to be dated
• The initial concentration of the isotope must be known
(or extrapolated)
• There must be a connection between the event being
dated and the start of the radioactive decay clock
(crystallization or capture of carbon)
How do we know how old the Earth is? How do
calculate the age of rocks?

How do we know the Earth looked liked this 250

million years ago?
How old is the Earth?

Plate tectonics destroyed all evidence of the first rocks – so we must

use something else

These also formed in the swirling disc around the young Sun at the
same time that Earth was forming
How do we know the Earth looked liked this 250
million years ago?

Measure magnetic inclination and

declination of rocks on every
continent - magnetostratigraphy
 How do we know the Earth looked liked this 250
million years ago?

Hot spot trails – markers of

moving tectonic plates over Get samples
a static hot spot from seamounts
– work out how
Progressively older

old they are

Big Island, Hawaii –

location of hot
spot
 How do we know the Earth looked liked this 250
million years ago?

Put the data collected by hundreds / thousands of

geologists together….
 How do we know the Earth looked liked this 250 million
years ago?
Sedimentary rocks preserve structures which indicate
certain environments
 How do we know the Earth looked liked this 250 million
years ago?

The fossils of
species that exist
in narrow
ecological niches

Crocodiles – large
reptiles; require
warm conditions;
lakes, rivers,
inland seas
Geologists classify huge
portions of geological time
into Eons, Eras and Periods

Life started in the Archean

Eon; the Ediacaran biota
were in the Ediacaran
Period, which is part of the
Neoproterozoic Era, which
is part of the Proterozoic
Eon.

It’s much easier to say

“during the Cambrian
Period” than to say
“between 480 and 550
million years ago”.
• Four Eons
• Hadean: 4.5 Ga –
4.0 Ga: Formation
of the Earth
• Archean: 4.0 Ga –
2.4 Ga: Oldest
rocks, oldest life
• Proterozoic: 2.4 Ga
– 541 Ma: Rise of
oxygen and
complex life
• Phanerozoic: 541
Ma – Present: Rise
of complex
animals
Hadean Eon Archean Eon

Proterozoic Eon Phanerozoic Eon

 Geologic Timescale
• Four Eons, Ten Eras (4 of which we’ll focus on)
• Hadean: 4.5 Ga – 4.0 Ga: Formation of the Earth
• Archean: 4.0 Ga – 2.4 Ga: Oldest rocks, oldest life
• Proterozoic: 2.4 Ga – 541 Ma: Rise of oxygen and
complex life
• Neoproterozoic 850-541 Ma: Snowball Earth
and Ediacara biota
• Phanerozoic: 541 Ma – Present: Rise of complex
animals
• Paleozoic: Age of Fish, 541-252 Ma
• Mesozoic: Age of Dinosaurs, 252-65 Ma
• Cenozoic: Age of Mammals, 65 Ma - Present
Neoproterozoic Era Paleozoic Era

Mesozoic Era Cenozoic Era

Geologic Timescale
• Four Eons, Ten Eras, Twelve Periods
• Neoproterozoic:
• Ediacaran: Oldest multicellular animals
• Paleozoic:
• Cambrian: Cambrian explosion
• Ordovician: Major diversification of marine
invertebrates
• Silurian: First vascular plants
• Devonian: First trees and insects,
diversification of fish
• Carboniferous: Vast coal-forming swamps, first
reptiles
• Permian: Supercontinent Pangea, largest
mass-extinction event
 Ordovician Period Devonian Period

Carboniferous Period Permian Period

Geologic Timescale

• Four Eons, Ten Eras, Twelve Periods

• Mesozoic
• Triassic: First dinosaurs
• Jurassic: First birds
• Cretaceous: First flowering plants, extinction
of the dinosaurs
• Cenozoic
• Paleogene: Modern mammals and first
grasses
• Neogene: First humans
 Triassic Period Cretaceous Period

Neogene Period

Paleogene Period
Let’s wrap our heads around Geological Time

100 years ago

The Great War is becoming a distant memory. The H1N1
flu pandemic is starting to ease off. Your Great
Grandparents were growing up. 3 generations to be born
before you.

1,000 years ago

Vikings raid and pillage Northern Europe; travelled as far
as Newfoundland. Beowulf is written. New Zealand is
discovered by Māori explorers.
40 generations have still to be born before you. Lots of
common ancestors.
Let’s wrap our heads around Geological Time

10,000 years ago

Cattle have just been domesticated. Megafauna roam

Europe, and go extinct at this time. Beer is invented. The
Ice Age has just finished. 500 generations to be born
before you. We are probably all related at this stage.

100,000 years ago

Homo Sapiens still in Africa. Clothes are a recent
invention. Earliest stone structures. Perhaps 2 or 3 other
Hominid species on Earth.
5,000 generations to be born before you.
Let’s wrap our heads around Geological Time

1,000,000 years ago

Homo Sapiens is ~700,000 years away from evolving;
Homo Erectus is the most widespread Hominid species.
Multiple glaciation events covered the Northern
Hemisphere in ice.

10,000,000 years ago

Hominids have yet to evolve; our evolution has yet to split
from Chimpanzees or Gorillas. Climate is becoming much
colder due to ocean circulation changes; leads to increase
in grasslands. North and South America are separate
continents.
Let’s wrap our heads around Geological Time

50,000,000 years ago

Earth’s average temperature was 12oC higher than today.
Mammals dominated the land; proto-whales and
elephants first appear. Himalayas start to form.

100,000,000 years ago

Dinosaurs have ruled the land for ~100,000,000 years.
Flowering plants evolve and spread. Birds have started to
diversify; early egg laying mammals remain small. Atlantic
Ocean starts to form; continents still very close to each
other.
Let’s wrap our heads around Geological Time

500,000,000 years ago

Complex life has only just evolved. Life teems in the
ocean; most of it small and simple. No land organisms at
all; no land plants.

1,000,000,000 years ago

Life consists of bacterial mats and algal blooms – mostly
single celled organisms.

There is still 3,500,000,000 years of Earth’s

history to go….
Key points to know:

• How we are able to determine the age of rocks:

• The Principles of Stratigraphy
• Index Fossils
• Rock relationships
• Radiometric Dating
• How we use these methods to track the history of the Earth and
the continents
• The geological timescale – what events happened in what order

Chapter Reading:
Essentials of Geology – 10.1 to 10.8, 11.1 to 11.6 (we’ll cover the
detail in this chapter in further lectures)
Portrait of a Planet – 12.1 to 12.8, 13.1 to 13.8 (same as above)