It wouldn't have stayed up there very long, either; 30 miles up is a pretty good vacuum. So I doubt it 'led' Moses and the Hebrews anywhere... or if it did, God must have been pissed off at them for not heading for Thera.
I read a theory a few years back that asserted the crossing of the Red Sea by the Hebrews was actually nearer the mouth of the Nile in an area called the "Reed Sea" which was more of a marsh and only a couple feet deep in some parts, but also much wider. It is based on a theory that the original Hebrew was misinterpreted and mistranslated (Of course, the similarities of "Red" and "Reed" are apparent in ENGLISH...what about Hebrew?).
This of course would all jibe with the Thera theory. If the plume did reach at least 30 miles (or higher, who knows?) it could have been seen by the Hebrews near the Nile Delta and might have appeared as a smokey staff by day and a torch by nite. If it continued to erupt and spew for several days or weeks, the light could have been reflected off the smoke particles forming the plume to a significant height.
One other issue that supports this theory is the Exodus story relating to how the Pharoahs men and chariots were caught in the rising sea. The popular locations of this at the Red Sea are fairly narrow and would have allowed them time to retreat safely to the shore, even if the were forced to abandon their chariots. If it happened near the "Reed Sea" at the Delta, it could have been tens of miles wide. As the Hebrews were exiting the east side the Pharoahs men could have been nearer the middle and would not have time to exit prior to being swamped.
One other note. The actual falling and rising of the Red Sea is a matter of contention. However, the Thera theory might explain this. As the initial tsunami hit the coast and then retreated, it would have drawn the water out of the Delta. There is an effect called "the Washbasin" where the water advances, retreats and advances again. With the "Reed Sea" so shallow , it would not have been unreasonable to see it drop to a level navigable on foot. As the washbasin effect takes place (or a secondary explosion at Thera that produced a second tsunami), the water would fill up again to a greater depth than normal, trapping the Pharoahs forces and drowning them.
Sorry if this is a known theory, I'm just an interested amatuer.
Keep in mind, from the Nile delta (which was probably farther south than it is now), only the part of the plume which was 30 miles high or higher would have been visible above the horizon. It was probably more 'blip' than plume.
Just roughly, and doing the math in my head, a plume sixty miles high would only have stuck up 3 or 4 degrees.
(Of course, the similarities of "Red" and "Reed" are apparent in ENGLISH...what about Hebrew?)Yeah, exactly. That "theory" is nonsense. If the Hebrew term "jam suf" meant "red sea" and some other term (made up for the purposes of this comparison) "jam suuf" meant "reed sea", the "theory" would make sense. But it doesn't. :')