Ultima 8 inaccuracies [English | German] can be found at Hacki's Ultima Page [English | German].
Shawn Varga writes: "You need to summon an earth elemental and make it open the doors for you."
"Summoning the elemental is tricky... you typically require a patch of dirt to do so (and there's no such dirt in the cave). However, the game seems to distinguish dirt by pixel colour! If you target just the right pixel (I believe there is a proper pixel or two just above the doors themselves on the stone itself) you can actually bring in the elemental and make him open the door for you."
"Don't get too excited though. Even when opened, the doorway doesn't take you anywhere. It's just a dead-end empty space."