more research results
Some general info for anyone interested...
Most of the ocean floor is Basalt (volcanic rock), but you'll get granite off a continental shelf (a steep cliff near the continents underwater). Anywhere near land there is a layer of silt ("sand" bottom type) with a thickness varying according to how far from land you go. Closer to land, particles are bigger and there are lots of them, and smaller stuff gets transported farther out. I'm not sure yet what the distances are, but they are large enough. I mean, we're not talking beaches and shorelines, but miles and miles. More on that as I find info.
Where does the floor turn into pure basalt? For one thing, anywhere you get a *ridge* of the plates you're going to have mostly basalt, i.e. "rock" bottom. The silt stays in the basin and the plates force their way up, for a double-whammy as far as creating pure rock bottom's go.
As for DW, how much of this matters? Generally any time the bottom is within a reasonable distance of the layer, it's going to be silt, as far as I can gather. I'm not 100% sure on that, but indications so far are that this is the case. (Except for ridges.) The deeper regions will have basalt bottoms, mostly because they are farther away from the continents, but then the sound channel there is so deep I'm not sure it effects anything in DW. Essentially the floor becomces "bottomless" at these depths, as sound is concerned.
Just some thoughts, correct me where I'm wrong.
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