Nargeta2
that does make sense...but now i am at the same place i started...
Well the White Materia(WM) that fell from Aeris' hair wasnt a Revive or Revival Materia. It was the Holy Materia, hich would activate Holy from within the planet, just as the Black Materia(BM) was the Meteor Materia and activated Meteor from orbit. The WM needed a successful prayer and plea to the planet, which Aeris provided. She was killed after the prayer though, and she died there. That was that. The pale glow of the WM as it touches the sand under the water was the signal that the prayer went through.
The reason Aeris is needed to stay dead from that point on is that after Sephiroth is broken from his frozen mako chamber and summoning of Meteor via the BM, he fell into the planet along with Cloud. Cloud was ejected from the core but Sephiroth remained since his physical being wasnt complete. Sephiroth stayed near the center of the planet, immersed in the lifestream, and also guarding Holy which he realised was being activated at this point though stayed dormant due to no immediate threat to the planet. His guard is what was broken in the final battle of the game and Holy made a quick jump to the surface. Its effect backfired though and began to cause more harm when combined with Meteor. This is where Aeris being dead comes in, since she is part of the reason the Lifestream flowed from the Planet to the site of Meteor and Holy's congregation and saved the planet.
Does that make sense to you?