Re: what will happen to areas formed by hotspots after a long period of time?

Plate tectonics carry the islands formed by the Hawaii hotspot away to 
the northwest. These islands also subside back into the ocean over time--
say hundreds of thousands of years--once they have been transported away 
from the uplifting influence of the hotspot. So for example the islands 
of Maui, Lanai, and Oahu were originally all part of the same island that 
was created over the hotspot. This larger island has sunk down leaving 
the individual islands we see today. 

The Big Island (Hawaii) is now "over" the hotspot but it will also move 
off to the Northwest and presumably sink down over time. The next one in 
the chain (Loihi) is still under construction to the Southeast of Hawaii 
and hasn't emerged from the ocean yet, so it is called a "seamount". We 
expect to see it above the ocean in the next 100,000 years or so.  

Visit this website to read more about the Hawaii chain that has been 
formed in this way, where many of the older islands are now below sea 
level: 
 http://pubs.usgs.gov/gip/dynamic/Hawaiian.html

From there you can also visit a more general page on hotspots also 
provided by the US Geological Survey (USGS): 
 http://pubs.usgs.gov/gip/dynamic/hotspots.html

Finally a book suggestion: Ask your librarian for help locating some 
books on this great topic.  A recent book published on this topic with a 
lot more technical detail is called "Oceanic Hotspots: Intraplate 
Submarine Magmatism and Tectonism". Books will give you a lot better 
picture than websites!

cheers
-Rob http://robfatland.net