LRP writes:
I spent much time in thinking about your last post. I even popped in to the University to talk to someone I work with who has a Masters degree in Geology. But he like me could not see how palaeomagnetism and radiometric dating help in finding the answer to my original question which is how do we know WHEN the continents drifted apart.
I can't explain your geologist colleague's inability to help you with this. He'll have to explain that himself. I'm merely repeating to you modern geological theory, something you can learn in any introductory geology book. Here's a rather long excerpt from a layman's geology book,
Building Planet Earth by Peter Cattermole, published in 2000, pages 120-121:
Wandering poles and continental drift
One of the reasons why geologists initially were so sceptical about Wegener's ideas was that there was no obvious reason why the continents should shift around, nor was there any plausible mechanism to achieve movement. An answer to the problem was first suggested by the great British geologist, Sir Arthur Holmes, who proposed that because the solid mantle was at very high temperature and under substantial pressure, it could actually flow over long periods. Slow motions of this kind would be more than ample to drag along rafts of the less dense lithosphere; although slow, the 'currents' would be extremely powerful. The situation is actually more complex than this, but Holmes certainly was on the right track.
Modern work has established that the lithosphere of the Earth is divided into a number of semi-rigid plates, seven of which are large, and a further five of which are of reasonable size. The boundaries between them are marked by zones of active seismicity and volcanicity. To be precise the term, 'continental drift' really refers to drift of the plates, rather than the continents alone; however, the result is much the same. Drift occurs because the plates are moving relative to one another, and the continents are carried along as part of lithospheric adjustments driven by mantle motions. Once this basic principle became accepted, it was relatively quickly that supporting evidence came along to dot the is and cross the ts, so to speak.
The most simple line of evidence comes from the almost perfect fit which can be achieved for some continents which once were joined; consider, for example, the structure of the Saharan Shield, which is around 2000 million years old. The structural grain of the rocks runs north-south towards the interior but then swings west-east towards the Atlantic margin. There is a well-defined boundary between these ancient rocks and younger ones which run into the ocean off the coast of Ghana. If drift is a fact, and Africa and South America once connected, there should be similar rocks with similar trends on the complementary side of the latter continent. Indeed, this is so; the boundary and a similar structural grain are found in the Brazilian Shield.
Further support for the theory comes from fossil remains. Fossils retrieved from ancient strata in Africa and Greenland, for instance, show that during the Silurian, Greenland was in tropical latitudes, while Africa was in the grip of glaciation! Then again, comparison of index fossils from the Phanerozoic rocks of both Gondwanaland and Laurasia shows that while the two were at one stage widely separated by the Tethys Ocean, at other times they were very close together, if not actually joined. The latter certainly was true between 350 and 220 million years ago.
The most convincing evidence, however, comes from palaeomagnetism, and it was this that finally clinched matters once and for all (although not immediately winning over all geologists). As I have previously mentioned, we can define the positions of the continents with respect to latitude by locating their past positions as shown by their palaeomagnetic imprint. During the late 1950s some curious facts had begun to emerge: as more and more palaeomagnetic measurements were made from different continents, it was found that, for any individual continent, if the magnetic pole positions were traced for different periods in time, data would not cluster around a single point but would trace out a path across which the pole appears to have passed. This could only be interpreted in one of two ways: either the magnetic pole had moved, or the continents had; initially it seems easier to accept that the magnetic pole position had changed. These paths were called polar wondering curves.
Once similar paths had been measured from different continents, it immediately became clear that this interpretation could not be the correct one. For instance, when polar paths for North America and Europe were plotted and compared on a map, while the pole position converges at the present time, 500million years ago, in the Early Palaeozoic, the poles were far apart. The same was found to be true for other continents too. There could be only one interpretation: the continents had moved. When the ancient pole positions for the different continents were 'put together', they were found to match those positions suggested by Wegener.
Now, you were aware that geologists think they know where each continent was over time, but because you didn't know how they knew you thought - what? That they were making it up?
Anyway, you now have the information you need to assess for yourself whether the evidence is convincing. Let me know if you have any questions about more detailed parts of the process. Something you said hints at the possibility that you believe the motion of the continents is derived from radiometric and paleomagnetic data from a single ancient volcanic eruption on each continent, but that's not the case at all. Geologists had to gather this data from basaltic material from many geologic ages. Something else you said hints that you believe basaltic rocks, once cooled, can change their magnetism to reflect the prevailing magnetic field, but this also isn't true. The magnetic orientation prevailing when the rock cools is frozen there for as long as the rock remains cool.
--Percy
™ Version 4.2