[dragnet2]

The last time this volcano went off, tens of thousands of years ago, was there activity leading up to the event? If so, how much activity, and what was the duration?

that was a little before my time.

Then how would they know if they were going to have any lead time in the next massive eruption?

[dirtboy]

Then how would they know if they were going to have any lead time in the next massive eruption?

Mostly by speculating as to changes in earthquake patterns and increases in elevation - given that scientists have never directly observed a supervolcano eruption (thank God for that), they can only speculate as to what would lead up to such.

[Godzilla]

Then how would they know if they were going to have any lead time in the next massive eruption?

vulcanologists and seismologists have studied hundreds of volcanos around the world of all types and eruption styles, many of which are expressed in the volcanic features at yellowstone. One of the common precursors is the detection of the movement of magma towards the surface.

If you want further details on the eruption history, start here -

Yellowstone Volcano Observatory

I'm sure your internet saavy enough to follow the link.