Copy these questions into your
science notebook. Leave space to
write the answers.
1. What was earth’s atmosphere like during the
Hadean? the Archaean? the Proterozoic?
2. When did the earth’s crust become solid?
3. What caused oxygen to accumulate in the water?
4. Why are stromatolites important?
5. What important information does the banded iron
formation give us?
6. Why did many species die out during the
Proterozoic?
Earth’s Atmosphere
•What was earth’s early
atmosphere like?
•How did it change
over time?
•When did it become
what it is today?
Geological Time
(There is a 250 million year gap here - a lot
happened during this time, but we’ll have to
talk about it in a different class.)
...is measured in
MILLIONS and
BILLIONS of years!
The earth’s
atmosphere
developed during the
earliest part of earth’s
history, which is
known as the
Precambrian.
Hadean Eon
...started 4.6 billion years ago (bya) when the
earth was formed, and ended when the oldest
existing rock on the earth’s surface was formed,
around 3.8 bya.
“Hadean” refers to “Hades,” the Greek word for
hell. The earth’s surface was molten rock.
Hadean Eon
The earth was being
bombarded with cosmic
material that was still
abundant in the solar
system. This added
heat to the earth’s
surface.
Eventually the earth
cooled and a crust of
rock began to form.
Hadean Eon
Some scientists think the earth’s first atmosphere
had lots of nitrogen. Others believe that it had
carbon dioxide and other volcanic gases. There
was almost certainly lots of water vapor.
We have no direct evidence about the atmosphere
at that time, so we can only make inferences based
on other information.
Archean Eon
...from 3.8 bya to 2.5 bya. “Archean” comes from a
Greek word meaning “ancient.” The first oceans
had formed.
The atmosphere was composed of gases emitted
during volcanic eruptions, which were much more
frequent then. This is called “outgassing.” The
atmosphere contained nitrogen, carbon dioxide, and
possibly low levels of oxygen, plus water vapor.
Archean Eon
...is when life first
began on earth.
Then, organisms
developed photosynthesis. This let
them make their own
food using energy
from the sun.
These ancient
organisms are called
cyanobacteria.
Cyanobactera
...were plentiful during Precambrian times.
We know this because there are plenty of
cyanobacteria fossils, called stromatolites. The two
scientists are standing in a field of stromatolites.
Cyanobacteria
...still exist today, although only in a very
few places, such as these waters off of
Australia.
Cyanobacteria give the sea its blue-green
color in this photo.
Cyanobacteria
...fossils are important
because they tell us
how our atmosphere
came to have oxygen.
Cyanobacteria used
photosynthesis, and
they thrived.
A waste product of photosynthesis is oxygen. The
oceans became rich in oxygen...
...but the atmosphere still had very little oxygen.
What happened to all that the oxygen?
Banded Iron (Fe) Formations
Fe ions abundant in water.
Oxygen produced by cyanobacteria is pervasive in
the ocean.
Fe rusts (joins with oxygen) and precipitates out.
Oxygen depleted, no rust in the precipitate.
Oxygen is pervasive, more rust.
....etc.
Proterozoic Eon
“Protero” is from the Greek word for “former, and
“zoic” is from the Greek word for “life.”
Scientist once believed that the earliest life began
during this eon (2.5 bya to 542 mya).
We now know
that early life e.g.
Cyanobacteria)
already existed
during the
Archean Eon.
Proterozoic life
(artist’s conception)
Proterozoic Eon
...was when oxygen finally began to accumulate
in the atmosphere.
This resulted in the death of life forms that were
not adapted to living with oxygen.
But it made
possible an
explosion of
eukaryotic
life forms.
Proterozoic life
(artist’s conception)