The Lava Lord sequence is just a movie or cut sequence (I wish they had put more of that in the game). When you get to the part where you enter a tunnel (that has a spa at the end), you will cause a slight earthquake as you walk through the tunnel.Right where you cause the tremor, you need to make another one, by grabbing one of the posts, in the right spot. The second earthquake you cause, allows you to enter the smaller tunnel to the side and continue on through the rift.
Tools for Wizardry(r) 7