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

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

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.

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.

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.

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.

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

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

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.

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. ]

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 0.001818 0.000524 0.000179

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.

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

Atmosphere Pie Chart

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.

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

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.

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.

End of Chapter 2, part one Any Questions?