myers@hobiecat.Caltech.Edu (Bob Myers) (12/09/86)
In article <777@jc3b21.UUCP> larry@jc3b21.UUCP (Lawrence F. Strickland) writes: > >Now the show I saw contended that for North America, at least, the continent >was built up from deposition on an originally smaller continent based in >southern Canada. They called it the Canadian shelf or something like that. >They also noted that this rock PRE-DATED the formation of Pan-Gaea! That's Canadian Shield. It is the core of the present North American continent. Yes, it does predate Pangaea, by quite a bit. On the order of 1.5 billion years old, while Pangaea was only about 2-400 million years ago. Plate tectonics as a process is older than Pangaea. The Appalachians were formed by the collision of Proto-North America and Proto-Europe about 4-500 million years ago. Can't remember exact times, but that's about right. The Atlantic reopened about 180 million years ago, splitting North America and Europe along nearly the same boundry (but not quite). Continents have been drifting around the world for about the last 1.5 billion years, sometimes joining, sometimes breaking up. It is, however, real difficult to figure out the geography older than 200 million years ago. This is because we don't have any seafloor records older than that. All the oceanic crust is less than 200 million years old! There have been some attempts at reconstruction, despite the difficulties. Let's see, references.... Best I can do is geological journals for stuff that technical. Maybe you can find some of these journals in a University Geology library. Scotese, C. R., et al. 1979. Paleozoic base maps. Journal of Geology. 87: 217--277. Smith, A.G., et al. 1973. Phanerozoic world maps. Special Papers in Palaeontology. 12: 1-42. Whittington, H. B., et al. 1972. Ordovician geography and faunal provinces deduced from trilobite distribution. Philosophical Transactions of the Royal Society of London, Series B. 263: 235--278. These should provide a start. I don't know where to find much in the way of popular material dealing with this subject, though. New continental crust has been added, as well. Sometimes pieces of oceanic crust get jammed onto the continents. The volcanism near subduction zones will create island arcs that also often are accreted onto the continents. The Shield areas are the continental cores, generally the areas of low geologic activity older than 1 billion years. You should be able to find some relatively simple geology texts that can explain these aspects of plate tectonics. Example of one I like: Press, Frank, and Siever, Raymond. 1982. Earth. W.H. Freeman and Company, San Francisco. I just took a look at it and there is a Middle Ordovician (475-490 Million years ago) world map in there, p. 457. Have fun! -------------------------------------------------------------- Don't Should on yourself. Bob Myers myers@hobiecat.Caltech.Edu