Friday, April 10, 2015

Will The Plate We Live On Sink? More Tectonics Questions.

Q: Dear Geologist, we are students from the Schiller Gynasium in Berlin and we would like to ask you some questions about platetectonics.
- Emma and Lili K

A: Hi, Emma and Lili,
My grandmother and her family came from Bavaria (Goggingen/Augsburg), so I have a soft place in my heart for Germany. I'll try to answer your questions in order below: 

Q: Is it possible that the plates we live on are going to sink?

A: The plates we live on will not sink. They have a higher silica content than the Earth as a whole and are thus less dense (average density of 2.67 grams/cc), so they float on the denser Mantle material. The plates that DO sink during subduction tectonics are ocean floor segments. These plates are made of  injected Mantle material that comes in at ocean floor spreading centers like the Mid-Atlantic Ridge. This material has more iron, magnesium, and calcium, so it is usually more dense (average density about 3.2 grams/cc or even higher). When these ocean floor segments meet continental crust in normal plate tectonic collisions, the ocean floor segment is usually - but not always – over-ridden. There are rare instances where segments of oceanic crust are rafted onto continental margins by this messy collision process, and these are called ophiolites. Examples are in Cyprus (the Troodos ophiolite), Oman (the Semail Ophiolite) and northern California, USA (the Josephine Complex). These ophioloites are fascinating to study, and often host pods of dense chromite (more than 7 grams/cc) in them. A chromite pod the size of a large room may be worth over a million dollars/euros.

Q: Can the mantle and the outer core mix (as they are both liquid)?

A: The Mantle and outer Core are indeed liquid according to seismic refraction studies, but they have already been largely segregated by gravity over the last 4 billion years or so. If there is mixing, it is local in nature. Some laboratory studies and modeling suggest that the outer core is actually growing, as the gravity segregation apparently is still continuing. 

Q: Would it be possible that the pangea forms again?

A: Yes, it is possible to have another super-continent like Pangaea, as the Pacific continues to narrow on almost all margins. However, it will take hundreds of millions of years to accomplish this. It will likely not be a clean recombination, but more likely a complex amalgamation of crustal segments and fragments. The Atlantic opened up once before, then closed again, and then opened again a second time. It is now widening at the Mid-Atlantic Ridge, so the Atlantic grows while the Pacific narrows. As the history of the Atlantic Ocean shows, this may change at any time in the future.

Q: Thank you for taking time to read our e-mail!

A: I'm glad to help. You will perhaps be the next generation of Earth scientists, and discover things that we don't know about today. 

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