1. Geology The Atmosphere and its Development
Moray Coast U3A
Geology
The Atmosphere and its Development

2. A complete introduction By David A Rothery ISBN 978-473-60155-0
Geology
A complete introduction By
David A Rothery
ISBN

3. Our text-book for today’s Geology session
We will now be looking at the first part of Chapter 2, The Atmosphere and its Development.

4. The Atmosphere
Compared to the size of the planet our atmosphere is very thin.
It’s so thin as to be virtually unbreathable at the top of very high mountains. (8 or 9 km above sea level.)
Clouds rarely occur higher than 12 km.
At 200 km the atmosphere is so thin that it effectively offers no resistance to satellites.

5. Gases The word “gas” was devised quite recently in human history
The origin is Dutch from about the 1650’s
The modern scientific use is from about 1779
A gas is a substance that has no fixed shape and no fixed volume
Man only really grasped the concept of gas after the discovery of the air pump and a vacuum.

6. Our atmosphere and life
As far as living things are concerned the importance of the atmosphere is out of all proportion to its size.
It contains the oxygen we need to breathe
It moderates the temperature of the planet
It shields the planet from radiation that would be harmful to life.
Our present atmosphere is very different from the one that existed soon after the Earth formed.

7. Our early atmosphere
The Earth’s primordial atmosphere is thought to have comprised mainly of gases that escaped from the interior by means of venting from volcanoes
It was probably, carbon dioxide, nitrogen, sulphur dioxide and water vapour
Simple life forms began to slowly change this atmosphere

8. Simple Life Simple life forms started to change the atmosphere
The first life on Earth may have originated near underwater hydrothermal vents (black smokers)
These life forms are known as extremophiles and can survive in extremes of e.g. temperature, pressure, acidity, alkalinity or radiation
Carbon dioxide and sulphur dioxide gases were broken down
This allowed higher, more complex, organisms to develop
The changes were slow

9. Green Plants
The first, very simple, green plants evolved in the sea at about 2 billion years ago. Then only about 0.2% of the atmosphere was oxygen
With the emergence of the first land plants, about 500 million years ago the oxygen level was about 7%
It had risen to its present level by about 370 million years ago. This is the time animals first spread onto the land.
At one time the oxygen level was higher than it is today at 21% but higher levels also present dangers.

10. Changes not due to life
Some carbon dioxide dissolves in water and ends up as carbonate rocks
Much of the sulphur dioxide formed ended up in rocks also.
Some of the inert or noble gases found in the atmosphere are the daughter products of radio-active decay.
Some water vapour and ammonia may have come from comets that bombarded the primordial Earth.
[ The current I.U.P.A.C convention is that the spelling of the word sulphur is now sulfur and sulphate is now sulfate. ]

11. The composition of the Earth’s Atmosphere excluding water vapour
Gas
Nitrogen
Oxygen
Argon
Carbon Dioxide
Neon
Helium
Methane
Volume p.p.m.
780,840
290,460
9,340 40
18.18
5.24
1.79
Volume %
78.084
20.946
0.9340
0.04

12. There are small quantities of other gases
Water vapour can vary from 0.1% to about 5.0% but this also depends on temperature.
Most ozone (about 90%) forms in the ozone layer which is 15 to 30 km above the Earth’s surface. The ozone layer contains between 2.0 p.p.m. and 8.0 p.p.m. of ozone.
Industrial pollution produces other, dangerous, gases such as nitrogen dioxide.

13. Chemistry Insert
Normal atmospheric oxygen and ozone are both made up of only oxygen atoms. The formula for oxygen is O2 and the formula for ozone is O3.
Oxygen O2
Ozone O3

14. Atmosphere Pie Chart

15. More Cycles Nitrogen circulates in a “Nitrogen Cycle”
Carbon circulates in a “Carbon Cycle”
Human activity can disrupt these long established and stable natural cycles.

16. A Final Thought Our atmosphere is not an infinite volume of air
Our atmosphere is thin, delicate and fragile
Almost all life on Earth depends on this thin layer of gas to sustain life
Damage to the atmosphere may take a long time to recover

17. Our Responsibility
With awareness and understanding comes responsibility
Personally I think that the atmosphere, and all the Earth’s resources are not ours to exploit selfishly. We are custodians. We have an obligation to look after and maintain the planet for future generations.

18. End of the first part of Chapter 2, Our Atmosphere
Key topics include:- Our Atmosphere is thin but essential to life.
Our atmosphere now is very different from that of the early Earth.
Where our atmosphere came from and the importance of green plants
The approximate composition of the atmosphere today.

19. End of Chapter 2, part one
Any Questions?