Evolution of the Earth
Evolution of the Earth
and its Atmosphere
and its Atmosphere
4.7 billion years ago:
4.7 billion years ago:
♦
The proto-Earth was a loosely-packed ball of “dirt” (primarily
The proto-Earth was a loosely-packed ball of “dirt” (primarily
silicon, iron, nickel and their oxides), surrounded by a thin
silicon, iron, nickel and their oxides), surrounded by a thin
primordial atmosphere of hydrogen and helium (with traces of
primordial atmosphere of hydrogen and helium (with traces of
argon, neon and krypton).
argon, neon and krypton).
4.7 billion years ago – 4.3 billion years ago:
4.7 billion years ago – 4.3 billion years ago:♦
The planet settled over 400 million years into a densely packed
The planet settled over 400 million years into a densely packed
ball, with a molten core (due to pressure and high levels of
ball, with a molten core (due to pressure and high levels of
radioactivity.
radioactivity.
♦
Most of the hydrogen and helium either “escaped” by

2
2
),
),
water (H
water (H
2
2
O), carbon monoxide (CO), carbon dioxide (CO
O), carbon monoxide (CO), carbon dioxide (CO
2
2
),
),
ammonia (NH
ammonia (NH
3
3
) and methane (CH
) and methane (CH
4
4
) into the atmosphere.
) into the atmosphere.
♦
Most of the H
Most of the H
2
2
O vapor condensed to form the oceans.

.
.
♦
There was
There was
still no oxygen
still no oxygen
gas (O
gas (O
2
2
)!
)!
♦
Moreover, since CO
Moreover, since CO
2
2
and CH
and CH
4
4
are “greenhouse” gases, the
are “greenhouse” gases, the
climate grew much warmer, thereby preventing freezing and
climate grew much warmer, thereby preventing freezing and
accelerating surface “weathering”.
accelerating surface “weathering”.
Except for Oxygen, almost all of the Earth’s atmosphere today
Except for Oxygen, almost all of the Earth’s atmosphere today

→
N
N
2
2
+ 3H
+ 3H
2
2
Conversion of
Conversion of
some
some
H
H
2
2
O and
O and
most
most
CO
CO
H
H
2
2
O + CO +
O + CO +
sunlight


→
→
CaCO
CaCO
3
3
(limestone)
(limestone)
(CO
(CO
2
2
absorption)
absorption)
MgO + CO
MgO + CO
2
2→
→
MgCO
MgCO
3
3
(dolomite)
(dolomite)


and CH
4
4
accumulated (since it didn’t react with anything else
accumulated (since it didn’t react with anything else
at this point
at this point
).
).Evolution of the Earth
Evolution of the Earth and its Atmosphere
and its Atmosphere

4.0 billion years ago – 3.8 billion years ago:
4.0 billion years ago – 3.8 billion years ago:
♦
Volcanic outgassing slowed dramatically while carbonate rock
Volcanic outgassing slowed dramatically while carbonate rock
formation increased, thereby lowering CO
formation increased, thereby lowering CO
2
2
levels somewhat.
levels somewhat.
♦

☻
of primitive one-celled
of primitive one-celled
aquatic
aquatic
organisms which could survive in the oceans.
organisms which could survive in the oceans.
Life on Earth had begun, despite its inhospitable atmosphere!
Life on Earth had begun, despite its inhospitable atmosphere!Evolution of the Earth
Evolution of the Earth
and its Atmosphere
and its Atmosphere

3.8 billion years ago – 2.5 billion years ago:
3.8 billion years ago – 2.5 billion years ago:
♦
Some of the one-celled organisms in the oceans developed the
Some of the one-celled organisms in the oceans developed the
ability to conduct “photosynthesis”:
ability to conduct “photosynthesis”:Process
Process
“Equation”
“Equation”


6
+ 6O
+ 6O
2
2
(This provided food, in the form of glucose, to these primitive
(This provided food, in the form of glucose, to these primitive
aquatic plants, and very importantly,
aquatic plants, and very importantly,
began the release of
began the release of
oxygen into the atmosphere
oxygen into the atmosphere
!)
!)
♦
By 2.5 billion years ago, atmospheric oxygen accumulated to a
By 2.5 billion years ago, atmospheric oxygen accumulated to a
level of about 1% (by volume).
level of about 1% (by volume).Evolution of the Earth
Evolution of the Earth
and its Atmosphere
and its Atmosphere

2.5 billion years ago – 600 million years ago:
2.5 billion years ago – 600 million years ago:
♦

600 million years ago – 3 million years ago:
600 million years ago – 3 million years ago:
♦
Volcanic outgassing slowed to a relative trickle, but two more
Volcanic outgassing slowed to a relative trickle, but two more
key processes (outlined on the next slide) completely altered
key processes (outlined on the next slide) completely altered
the atmosphere, enabling plants and animals to leave the
the atmosphere, enabling plants and animals to leave the
oceans, and survive and evolve on land.
oceans, and survive and evolve on land.
♦
Between 450 million years ago and 350 million years ago,
Between 450 million years ago and 350 million years ago,
land plants and amphibian animals began to appear, and
land plants and amphibian animals began to appear, and
oxygen levels approached today’s value of nearly 21% by
oxygen levels approached today’s value of nearly 21% by
volume.
volume.
♦
By the end of this period, the atmosphere contained much
By the end of this period, the atmosphere contained much
less CO
less CO
2
2
than before (280 ppm) and very little CH
than before (280 ppm) and very little CH
4