We are looking at upgrading our countertops to granite. We have a floating laminate floor under a movable island that we don't wish to anchor. The island would have a 40"x50" top surface. How does the weight of the granite affect the floor?
A granite slab of the size you describe and 1 1/4 inches thick would weigh about 300 pounds. A 3/4 inch slab, about 200 pounds.
This is on top of an island that may weigh 100 to 300 pounds, depending on how it is constructed. if you add some contents, the total weight with the thicker granite will probably be between 500 and 700 pounds. If people lean on it, this could add another 50 to 100 pounds of pressure.
The maximum weight of the island, top contents and dynamic pressure is probably about 800 pounds. This is divided over four (or more) support points. Each support is carrying about 200 pounds of weight. This is comparable to a good sized person standing on something the size of the area of the support point. If this were a tiny, sharp metal leg, it might well damage the surface of the floor. If it were a wide 3x6 inch board, the risk would be very small.
Tiny metal glides might do damage. Broad teflon slides, probably not. If you need to use wheels, large diameter rubber ones would probably be fine. Using the thinner granite (if it is well supported and doesn't have a significant overhang) might be better.
The expansion of laminate is pretty small especially in a conditioned space. Also the forces of the flooring when expanding far outweigh the forces of any island that you will make. You would probably be looking at a few thousand pounds before it even comes close to cause buckling.
I will say though that you should think about the quality of the laminate when deciding on the legs. A high quality with a good locking system will not be affected by a weighty island at all. But if you have some cheap stuff or it has a bad locking mechanism I would be afraid to "move" the island if it had small leg bottoms. I could definitely see it scratching or moving the laminate in some cases.
Lots of ways around this issue though and probably better to plan before heavy granite is on it. that is a different question though - but with the granite top weighing it down I would personally put it on industrial quality rollers.