I’m looking for a place on earth where plate subduction is taking place. I need a spot where there is a good balance between speed of subduction and a lack of associated volcanic activity on the ‘top’ (i forget what its called) plate.
For context: I’m working on a proposal for subductive disposal of radioactive waste and fissionable material. I need to make them go bye-bye.
Presumably more than once, for more than 20 minutes, after 1960… 
I guess it depends on what you mean by “lack” of volcanic activity on the overriding plate. Volcanoes are intrinsic to subduction zones, whether the collisions are ocean-ocean (one bit of ocean floor riding over another, just like the Marianas Trench) or ocean-continent (ocean floor is overriden by continental crust). As you can see on this map, trench locations coincide mostly with the Pacific Ring of Fire; even the two that don’t (the Java and Puerto Rico trenches), there are still volcanoes. Since there are a number of factors that contribute to the style (and possibly frequency) of volcanism, speed of subduction isn’t necessarily going to help you much in making a choice.
I can’t find anything at the moment that isn’t deeply buried (hah!) in a PDF document, but the general concept has been bandied about before. It has been rejected for a variety of reasons, the two main ones being 1) even the fastest subduction rates are only on the order of a few centimeters per year, so the waste sits around for a long time anyway, and 2) containers are likely to start leaking under high pressures and under the influence of “corrosive fluids” moving through the trench. If you’re that keen on the details, you can try this PDF file of a report issued by the Committee on Radioactive Waste Management in the UK as a starting point.
Can you be more specific as to the nature of your proposal?
On a related note, you might be interested in this patent: Collapsible waste disposal container and method of disposal of waste in subduction zone between tectonic plates
There’s subduction zones like youy see in your Geology 101 textbook and there’s subduction zones that occur in real life. They’re not the same thing. The biggest problem you’re going to have to overcome is that you don’t have two nice neat slbs of rock gently slipping over one another. There’s a whole big smoosh* of sediment getting in the way.
AFAIK (speaking as a geologist who used to work in nuclear seimic hazard assessment) what you need for disposal is 1) stability and 2 predictability. Even if you place your drum of radioactive goo at the deepest point of a tranch, how are you going to predict what is going to happen in the next hundred or thousand years? Will there be some plate movement? Yes. Will it be nice and steady, or might there be a big earthquake? Will your drum just sit in the sediment while the plate moves beneath it? No way to tell.
It might sound like a great idea on paper, but a piece of paper is a poor substitue for the real world.
There’s a massive subduction zone where the Indian subcontinent is ramming into the Asian continent, forming the Himalayas. Lots of sweiasmic activity, but not much volcanism. I worked on a geophysics study of this as an undergrad – they were trying to figure out if it was subduction, or crumpling, or something. It turned out deep subduction fit the dispersion graphs best.
On the other hand, it’s a rotten place to use for disposing of nuclear wastes – to get t the interface you’d have to drill through the Himalayas.
Why do you want to get rid of the wastes, anyway? Isolating them and safely storing is one thing, but you might want 'em. Read Heinlein’s “Expanded Universe”. Back in the 19th century they used to throw away the fractions that we now lump together as “gasoline” after distlling the stuff they wanted from petroleum – the stuff vaporized too easily to make a useful fuel for lamps, or for anything else they could do with it at the time. You might want your nuclear waste in the future, too.
I thought that was just a prehistoric subduction zone? Isn’t it just strictly collision of two continental plates now?
Well, I haven’t looked at this in many years, but it had the structure of a subduction zone, as verified by seismic waves paassing through it. I was under the impression that it was still moving, and the Himalayas still rising , albeit slowly. But I haven’t looked into this in ages.
From this site:
http://tlacaelel.igeofcu.unam.mx/~GeoD/colision/colision.html
That’s why it’s so much harder to climb Everest now than it was during Hillary’s day. It’s higher.
Although India is still moving northward into Asia and so is still properly called a convergent margin, it’s gone beyond being a subduction zone to being considered a continent-continent collision zone. The diagram in Cal’s link is dated and not the best representation of the tectonic processes going in the region as we now understand them.
Subduction begins when the denser of two colliding plates edges begins to slip underneath the less dense plate edge. Since oceanic crust (basalt) is denser than your average continental crust (generally granitic in composition), ocean crust gets overridden by the continent. Prior to about 55 million years ago, the northern edge of the Indian plate was a sea floor, so as the Indian plate moved northward, its leading edge was forced down under Asia.
After 55 million years ago, the continental crust of the Indian plate began to collide with the continental crust of Asia. Since both continental bits are approximately the same density, neither would sink readily beneath the other, and the deformation that’s produced the Himalayas began. That’s the point at which you’d say there ceased to be a subduction zone, and the collision zone was born.
There does appear to be some separation (delamination) of the lower crust from the upper crust in the region, owing to their different rheological properties, and that delaminated lower crust could allow continued motion down into the upper mantle (PDF file). The upper crust continues to deform in collision, though, so despite the movement of the lower crust it still doesn’t qualify now as a subduction zone.
sunfish, thanks for the links, they are a great help. I realize that volcanism is always associated with subduction, but I was hoping to find an area where it was minimized. To the other posters: I realize that this is not exactly the best method, storage being the easiest, but my goal in brainstorming this is to find a method of disposal of fissile material from disassembled weapons in order to reduce the proliferation risk, in addition to geting rid of waste.
Unfortunately, I can’t submit a working paper that advocates having Superman hurl it all into the sun…
If you’re interested in that way of disposing of nuclear waste, perhaps you ought to consider simply dumping the stuff into fluid lava, such as they have in Hawaii.
That wouldn’t get rid of it. The purpose of subducting it would be to remove it completely. Someone could always dig it out of a lava flow. Plus, you;d have a highly readioactive lava flow.
What if we drilled into the mantle, and simply poured the stuff in? Would that work? I think we’re pretty close to being able to drill into the mantle now, aren’t we? Why not skip the slow subduction process, and simply bore down and push our radioactive material straight in?
How stable are the antarctic ice sheets? What would happen if we buried nuclear canisters say, 4 km down inside an ice sheet? Any chance that that material could leach back to the surface within, say, 50,000 years?
There are lakes (liquid!) deep beneath the surface ice in Antarctica. See Lake Vostok as an example. They are of great interest to astrobiologists and others studying life in extreme environments and psychrophiles. That may complicate things a bit.
It’s wonderful that everyone has (apparently) so much confidence in our ability to safely deliver catastrophic elements into the Marianas, ultra-deep wells or whatever (I’m not including the lava flow as that was blindingly stupid) but certainly this is just an exercise in freewheeling discussion and not something you’d bring up anywhere outside of a bar.
As Tapioca asserts, stability and predictability are key, two things inherently absent in subduction zones. You can’t possibly deliver them safely to said point, you don’t know what the result would be and you’re burdening future generations with the prolific litany of your result.
There are better options. In fact, I can think of few worse.
Yeah, I was aware of that. There are many of them. In fact, Lake Vostok is what got me thinking about it. They believe that if they find life there, it will be a good indication that it could evolve on Europa, since they don’t believe there has been a substantial exchange of organic material between Lake Vostok and the surface for perhaps millions of years.
So what would happen if we dropped nuclear waste down there? Not in the lake, but say 3 or 4 kilometers down in ice shafts? How long would it take before the stuff could reach anywhere that could affect people?
Not so - the lava would melt and mix it, and you’d end up with rock only very slightly more radioactive than normal.
IANAD, But I suspect that there would be just a tad bit more effect than you are suggesting. Especially since we are talking about a massive amount of HEU, PU, and reactor fuel waste.
Well… not really, and certainly not on an industrial scale. Super-deep drilling efforts to date have not reached the mantle, only making it into the lower crust before various problems forced a stoppage.
The deepest hole ever drilled into continental crust reached a maximum depth of about 12.2 km, in Russia. Drilling eventually stopped after 24 years when the Russians found that borehole temperatures were higher than anticipated, and the drill bits would not have operated at the original target depth of 15 km. A similar effort in Germany has stopped at about 9.1 km. Those scientists found that, by the time they were able to pull up the drill string to put on fresh drill bits and then go back down into the hole, the bottom of the hole had begun to pinch shut; they were essentially drilling the same section of the hole over and over. The problem here was that the lower crust doesn’t behave like a brittle material; under high enough temperatures and pressures, rock deforms plastically (i.e., it flows without being molten).
One of the objectives of the Integrated Ocean Drilling Program, in cooperation with the Japan Agency for Marine-Earth Science and Technology, is to drill, for the first time, into the mantle through oceanic crust, but it hasn’t happened yet. (For some additional info on the remarkable ship that would be used for this project, and the drilling process involved, look here.)
As both Tapioca Dextrin and lieu have pointed out, stability is key. Ice sheets flow; the ice cracks and/or deforms plastically, and the bases of the ice sheets are often conduits for water flow. (In fact, part of the West Antarctic ice sheet is actually grounded 1300 m below sea level, so basal glacier water would be free to mix with sea water.) So apart from any concerns about Antarctic ice remaining stable in a global warming scenario (see, e.g., this discussion, they just aren’t good places for radioactive waste disposal.
If you were here in the U.S., I’d say you were feeling inspired by last night’s airing of The Fellowship of the Ring when you said this, 
but we’re not talking about getting rid of the One Ring here.
Natural concentrations of radioactive materials are pretty low. An economically viable uranium ore has just 1 to 4 pounds of uranium oxide per ton of ore, or a concentration of 0.05 to 0.2 percent. Industrial processing eventually produces yellowcake (concentrated uranium) that is 70 to 90% uranium oxide by weight, which may then be enriched even further for use as fuel in reactors. It’s the high level and transuranic waste products of this highly refined radioactive material that CynicalGabe is presumably looking to unload, in the thousands of pounds quantity. The exact level of radioactivity is of course going to depend on what’s in the waste, but I’d say it’s not trivial, and the material not easily downblended for other uses, if it can be at all.
In any case, since lava often solidifies very quickly after being erupted, I suspect you can’t quite depend on melting and mixing to reduce radiation levels to acceptable levels. And even if you did find a lava tube with a fast-moving flow - how comfortable would you be, knowing that you could have no idea where the waste ended up on the landscape, and at what concentration?
