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GWU / Geology / GEOL 1005 / Why are volcanoes in subduction zones so dangerous?

Why are volcanoes in subduction zones so dangerous?

Why are volcanoes in subduction zones so dangerous?


School: George Washington University
Department: Geology
Course: Historical Geology
Professor: Catherine forster
Term: Winter 2016
Cost: 25
Name: Historical Geology Week 5 Notes
Description: These notes cover class on February 11, 2016. February 9th was no lecture due to the exam.
Uploaded: 02/11/2016
5 Pages 26 Views 3 Unlocks

Cathy K. (Rating: )

its useful but not always

February 11, 2016

Why are volcanoes in subduction zones so dangerous?

∙ When we think of volcanoes  subduction zones  

∙ Taiwan  

o Two subduction zones  

o Two faults parallel the subduction zone of the Eurasian Plate o Philippine plate sub ducting under the Eurasian Plate

o Earthquake wasn’t on a subduction zone but instead on a fault  ∙ History of the Earth and history of life  

o Hadean

 Started 4.54 billion years ago  

o Archean

 Started 4 billion years ago  

 Ended 2.5 billion years ago  

o People trying to figure out how old the earth was  

 Looked at the thickness of sedimentary rocks  

∙ 1861  Earth calculated to be 3 million years old  

When did the archean era begin and end?

∙ 1910  discovery of much more sedimentary rocks,  

found to be 1.6 billion years old  

∙ Problems with doing it this way  

o Rates of sedimentation changes  

 1897  Lord Kelvin  interested in heat  

∙ Calculated that it was 24-40 million years old  

∙ Didn’t realize that the Earth has an internal heat  


 John Joly  

∙ Looked at the salt in ocean and the salt that was  

feeding into the rivers

∙ 90 – 100 million years old  

 1905  radioactive decay first used  Rutherford and  


∙ Dated the oldest rocks that they knew of  

How did we figure out how old the earth is?

∙ Found that it was 500 million years old  Don't forget about the age old question of What is the difference between dna & rna?

 1907  Boltwood  uranium turned into lead  

∙ Dated some uranium containing rocks  

∙ Came up with the date 1.64 billion years old  

∙ Two things happen to the Earth  

o Gravitational compression

 Applied pressure that creates heat  

 Heating up the Earth  leads to heat within the Earth  

 A lot of material starts to melt or partially melt  

o Radioactive decay  

 This leads to an internal heat source  

 A lot of material starts to melt or partially melt

o This allows the earth to start to differentiate  

 This is what gives the Earth its different layers  

 The material that moves toward the center are the denser  and heavier materials  

∙ Like nickel and iron We also discuss several other topics like What are the antioxidant enzymes?

 Lighter materials came out toward the mantle and crust  ∙ Silicate rich  

 Before this differentiation, the surface of the Earth was  probably molten  

∙ Liquid Outer Core  

o The liquid circulates  doesn’t just sit there  

 This sets up an electric field  the geodynamo

 Electric field sets up a dipolar magnetic field  

∙ Magnetic field  

o Polarized, we have a north and a south  

o Magnetite and other magnetized minerals  

 Igneous  

 When it crystalizes, it crystalizes with that magnetic field  ∙ Aligns itself with the magnetic field (points north)  

 If it weathers out and becomes a sedimentary grain

∙ Also orients itself north-south with the magnetic field  of the Earth  

 One way we can go back in time and figure out the  

orientation of the plate  

∙ If it isn’t pointing north, we know how much it has  


 Amount of inclination tells you the latitude that the mineral was formed in  

∙ Magnetic inclination  latitude of formation

 Every 4-5 million years the poles reverse themselves  ∙ Not precise, there is some variation  

∙ Reverse magnetism  

∙ Oldest whole rock  Nuvvuagittug Greenstone Belt  If you want to learn more check out What wealth can do is bring leisure, audience, peers, train experts, readers, who place social value?
If you want to learn more check out What is the purpose of offering an argument?

o Dates to 4.28 billion years  

o In Quebec  

o Remnants of some of the oldest continental crust on Earth   North American craton  

∙ Oldest minerals  Found in the Naryyer Gneiss

o From Australia  

o Old parts of this Australian craton  

o Inside are some really tough minerals  zircon  

 Taken the zircon out and dated at 4.4 billion years  

 Layers in the zircons show the way it grew

∙ Added layer after layer until it reached its final size

∙ Oceanic crust  

o Begins to differentiate to its more mafic parts and its more felsic  material  

o Felsic material starts to rise and accumulate  

 Becomes continental crust  

∙ Water begins to form  

o Early in Earth history, we have a lot of volcanism going on   Lots of lava but also a lot of gas

 Huge reservoir of gases  

∙ One of these is water vapor  

o Water vapor begins to condense and form water on the surface  ∙ Everything coming onto the surface is coming out of these volcanoes  o Greenstone Belts  

 Oldest rocks  

 Chlorite  causes it to be green  

 Pillow basalts  very mafic We also discuss several other topics like What is an absolute space?

∙ Extruded underwater  

∙ Underwater volcanism that extrudes up and up  

∙ Oldest evidence of water that we know of is 3.8 billion years ago  o Found in Greenland  

o Water and air leave their mark  

o These rocks show evidence that they were deposited by water  ∙ 3.5 billion year old  Australia  

o Shows that the continental crust built up enough  

o Shows that there is now land above sea level  

∙ Atmosphere of our early Earth  

o Chemically aggressive atmosphere  

 Contains water vapor  

 Contains a lot of methane, ammonia, formaldehyde, carbon dioxide and hydrogen cyanide  

 A lot of greenhouse gases coming out of volcanoes  

o Essentially no free oxygen  

 No ozone layer  

 High layer of UV radiation in the air  

o Water and metal oxides were in the oceans  

 This forms acids  

∙ Hydrochloric, sulfuric and others  

∙ pH of these oceans were 2 – 4  

o These are the conditions that early life formed under  ∙ Earliest Life  Don't forget about the age old question of What do conceptual scheme relativists believe?

o Build complex carbon-based molecules  

 Basis for life

 1953  Miller and Urey  

∙ Took some flasks and filled them with water  

∙ Added the gases that they thought should be there  o Ammonia, water vapor, methane, etc.

∙ Added some electrical charge for lightning  

∙ Finally uncorked the flask and found what had  

formed in there  

∙ What they found was long chains of amino acids  o This is what builds proteins  

∙ Found that they could generate these building blocks of life  

 Very easy to generate amino acids  

∙ Amino acids won’t form under free oxygen

 Found that they can make nucleic acids  

∙ Building blocks for DNA and RNA  

o First life

 Originated in the ocean  

 Originated near spreading centers (warm)

 Were chemosynthesizers  

 Possibly happened as early as 3.7 billion years ago  o Greenland Isua Formation  3.7 billion years  

 Found Carbon isotopes  

∙12C : 13C = 100 : 1  

∙ Carbon 12 is preferred by organisms (lighter  


∙ Biomarker  

 Isoprene chain  

∙ Organic molecules  

∙ Isoprene is used as the cell wall of the most primitive organisms that we know of today  Archea  

o Archeans can live in extreme environments  

 Can live without oxygen  

 Are methanogens  

∙ Can tolerate really hot temperatures  

 These conditions sort of replicate the conditions on early  Earth  

o 3.5 billion years ago  first body fossils in Australia   Fossilized cells that look like a second kind of organisms  called cyanobacteria

 Structures called stromatolites

∙ Structure that are formed by cyanobacteria  

∙ Bacteria lives in shallow water

o Sediment settles on them, bacteria move up,  

sediment settles, bacteria move up, etc.  

o Cyanobacteria  

 Global by 3.5-3.4 billion years  

 Use photosynthesis  

∙ Consumes carbon dioxide in the atmosphere  

∙ By product is free oxygen  

 Over time, this free oxygen builds up and the amount of  free oxygen in the atmosphere starts to go up  

∙ This changes many things  

 Iron  comes in two forms  

∙ Ferrous Iron  soluble in water  

o Stable in reducing conditions (no oxygen)

o Gray or black in color  

∙ Ferric Iron  

o This is what happens in oxidizing environments

o Red, essentially rust  

o Insoluble in water  

 Precipitates out of the water  

∙ Free oxygen in the air from cyanobacteria  

o Causes banded iron formations  

o 2 – 2.4 billion years ago

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