I have a couple of clarifications to Don's post from Lowell: the hotspot did not form at 40 Ma: it was already there-the Emporer (sp?) Sea Mounts extend NNW up to the subduction zone and are older than 40 Ma (back to 80 Ma?). The poster by W. Hamilton at AGU suggests (and I have heard elsewhere) that there was no major change in plate motion at 40 (42?) Ma: that instead the hotspot moved. I guess I'll have to look at Atwater's animations to see if she shows much of a change in Pacific plate motion at 40/42 Ma-I have them here somewhere. Following Lowell, I discussed with T. Atwater a decade ago whether the horizontal component of an oblique impact could be transferred into a plate. the horizontal momentum of a 60 deg impact (30 deg off of horizontal) for a 10 km object at 11 km/s (or whatever) is way more (orders of magnitude?) than the momentum of a plate. But, the problem is how to get that momentum just into the plate-at instantaneous events the plate would just be part of the mantle.....Chris