Phil Plait over at Bad Astronomy has a nice post about how plate tectonics proves to be a major problem for creationists. I suggest you go read it since he can explain it alot better than I can. Here's the rundown, though, for those of you who like clicking as few links as possible:
Plate techtonics is a well known process (although, to be honest, it's really only been widely accepted since the 1960s). We know about it because we can directly observe it happening. We can also determine the actual rate of continental drift: it's a very very slow one (an inch or so a year). Based on geologic data that suggests the orientation of the plates in the past, we know that it has taken the plates millions of years to get where they are today. The problem for creationists lies with heat. As with most (all?) geologic processes, the movement of tectonic plates around the Earth releases heat. Since the plates move slowly, not alot of heat is given off by this process. However, Creationists claim that the Earth is a mere 6000 years old; what's more is that they explain away plate tectonics by claiming God made the plates move really really fast after the flood to get to their current positions. However, such an incredibly rapid motion would, accoring to the National Centre for Science Education, release enough heat to boil away the oceans and completely melt the Earth's crust!
Of course, I can already anticipate some wacky creationist counters to this. They would probably point out that God would just raise the boiling/melting points of the Earth and its oceans. The heat, then, wouldnt cause a problem. Well, except that raising the boiling/melting point means raising the environmental pressure (Pressue and temperature are both intimately entwined. 'Boiling point', for instance, is defined as the "temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid"). In order to prevent catostrophic environmental changes from happening from the release of that much heat, the environmental pressure would have to such incredible levels that living creatures would find it very difficult to continue living. The only way to get around this would be for God to change the very laws of thermodynamics. And all for only a period of 40 days so the world could appear to be older than it actually is. That God guy - not a fan of thermodynamics.