112lect19 - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

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GY 112 Lecture Notes D. Haywick (2006) 1 GY 112 Lecture Notes Evolution of the Earth's Atmosphere and Hydrosphere Lecture Goals : A) The Early Atmosphere B) The Oceans and Hydrosphere C) The Change Textbook reference: Levin 7 th edition (2003), Chapter 6; Levin 8 th edition (2006), Chapter 8 A) The Early Atmosphere We have spent considerable time mentioning that the early Earth was very different from today. The same can be said about the early atmosphere. Today, our atmosphere contains only a few major gases: O 2 : 21%; N 2 ; 78%; Ar: 1%; CO 2 ; 0.03% (but rising!) When the Earth was first forming (refer to an earlier lecture) it is likely that there was a primordial atmosphere of some sort. During the Early Hadean Eon, the atmosphere might have consisted of water vapor, nitrogen, argon, carbon dioxide etc. There might also have been some hydrogen and helium (left over from the initial formation of the solar system), but most of these gases would have been blown away when the sun ignited and the solar wind began. In addition, gases like hydrogen and helium can largely escape the Earth's gravity well anyway, so they would not have hung around for any great length of time. Well all this begs the question, "Where did our "modern" atmosphere come from?” The most likely source is volcanoes. During the Hadean, volcanic activity would have been extreme. Volcanoes were literally everywhere and volcanic eruptions must have been occurring non-stop. Volcanic eruptions produce a lot more than lava and ask. They produce tremendous amount of gas and it is quite likely that the early Earth's atmosphere was primarily derived from these volcanic eruptions. Today volcanic eruptions produce H 2 O, CO 2 , H 2 S, SO 2 , NO 2 , N 2 O (generally written as N x O y ), HCl and several other gases (most of which smell bad). To the best of my knowledge, volcanic eruptions generally do not produce much CH 4 or N 2 , but we suspect that these gases were also in the Earth's early atmosphere. Perhaps they managed to hang around from the primordial solar nebula or they leaked into the atmosphere from the interior of the Earth through other mechanisms. One thing is clear however, the water vapor that was produced from the volcanic eruptions eventually condensed in the atmosphere to form water droplets (clouds) and rainfall. The oceans almost certainly formed from this rainfall. Incidentally, rain for the first several million years would have been extremely acidic. Water combines with CO 2 , SO 2 , HCl and N x O y to form many strong acids:
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GY 112 Lecture Notes D. Haywick (2006) 2 H 2 O + CO 2 = H 2 CO 3 ( carbonic acid ; weak) H 2 O + SO 2 = H 2 SO 3 (sulfurous acid; weak) H 2 O + SO 3 = H 2 SO 4 ( sulfuric acid ; strong) H 2 O + NO x = HNO 3 ( nitric acid ; strong) H 2 O + HCl = HCl ( hydrochloric acid ; strong) Given the acidity of the rain that fell on the early Earth, it is reasonable to assume that chemical weathering of the rocks was widespread. Many minerals would have dissolved,
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This note was uploaded on 02/04/2012 for the course GLY 112 taught by Professor Haywick during the Spring '12 term at S. Alabama.

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112lect19 - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

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