Does plate tectonics mean oil and coal fields get burned during the subduction process? Cause of volcanism?

Answer 1

I am sure some oil fields get subducted, and probably some minor coal deposits. However, the presence of oil or coal in the subduction zone plays no significant role in volcanism.

Oil often forms in the sort of sedimentary basins that are formed by subduction. However, the bulk of the sediment ends up in a wedge of sediment that is likely to be scraped off of the subducting plate. In many cases this would cause the oil to be released, creating paths for it to migrate up and out of the sediment. This sort of natural oil seepage is not uncommon, especially in areas where there is active tectonic activity and structural deformation. An example (which is not a subduction zone) are the natural oil seeps off the coast of southern California that leak so much oil they are visible from the surface of the ocean.

Coal does form in small quantities in the same depositional environments where oil is common (delta sediments) but the coal fields that are large enough to be mined typically form over extensive shallow swamps that were often on continental plates, which rarely get subducted. So it would be less likely that a coal field would have been subducted.

As for a relationship to volcanism, there is almost none. Volcanism is related to melting of the subducting plate quite deep in the crust near the mantle, and to hot spots where the crust is either thinner or the mantle hotter, or both. The oil or coal would convert to graphite at temperatures over about 400 C. What does play some role in the volcanism is the water that is carried down with this subducting plate, and it is likely that water would be released from oil or coal as it converts to graphite. Quite a few other rocks will release water from their chemical makeup under the extreme pressure. This water is believed to be the cause of melting in the subduction zone. The presence of the water lowers the melting point of the rock it is mixed with.
http://www.stmarys.ca/conted/webcourses/GEO/GEO99/pubigneous/subduction.html

The water content of the magma that forms above subduction zones also plays a role in causing more explosive eruptions. Magma with high water content, high dissolved gas content, and what is called a felsic magma tend to produce very explosive volcanism of the type that is more common in subduction zone volcanism.
http://www.geology.sdsu.edu/how_volcanoes_work/Controls.html

This is a technical journal article that explains how carbon is stable and plays little if any role in magmatism in subduction zones:
http://www.cosis.net/abstracts/EGU05/09219/EGU05-J-09219.pdf?PHPSESSID=687e5b36eb3953bf4f05142902659035

This link is a technical journal article on this subject that does have some excellent diagrams that demonstrate how water causes magmatism:
http://www.perplex.ethz.ch/papers/Ruepke_EPSL_04.pdf

Answer 2

Most oil and coal fields are found in continental or margional lithosphere. Not many are on the oceanic, so not many get subducted. But if they did, they would burn up (volitalize) and help cause volcanism.

Mostly though, volcanism in subduction is caused by more common volitals, like H2O, CO2. These are caried down by the plate, mainly in the form of weak minerals. These minerals break down with pressure and temperature into stronger minerals that don't contain water and carbon dioxide. The leftover gases and liquids travel into the mantle (the mantle wedge) and hydrate the mantle, causing melting. The melt rises and then... volcano!

Answer 3

Subducting coal and oil do NOT support volcanic activity...at all. Usually, it's an oceanic plate being subducted and they don't contain any oil, etc. Phase changes that occur with pressure and temperature drops allow the mantle rocks to partially melt, they in turn begin to rise and melt adjacent rocks...this is the origin of magma. (If any vapor, especially water, is present, then the process gets going really fast!)

Still...an interesting question!

Answer 4

not the cause of volcanos is this burning, but in simplr terms both burning and reforming do occur. there are deep process oils, and carbon is formed , sometimes as graphite and sometimes as diamond,a balance to the oils that require hydrogen to form