Hundreds of millions of years... Pacific plate sliding under and down underneath the North American plate. Sliding down, mashing minerals together, heating them with tremendous pressure and heat from far below. All that pressure and very slow movement ...over millions of years... caused granite to form below, then to rise slowly, creating the Sierra Nevada range.

That granite was formed some miles from where it is now... straight down below its current spot. The Lone Pine earthquake of 1872 saw the eastern face of the Sierra rupture as the rocks to the west rose and the Owens valley to the east fell by a few feet.

Measurements show the Sierra range continues to rise -- by millimeters each year. (Not the reason they changed the elevation numbers for Whitney.)

Here's a geologist giving a talk in the Sierra:
Ron Wolf - Sierra Nevada Geology: http://youtu.be/2Ym7cf44cCg

The huge granite blocks on the summits have likely sat there relatively unchanged for millions of years. And they will likely be there in yet a million more.

Here's another short and sweet explanation from a young woman backpacker/science teacher: http://youtu.be/rnaA84zepLk