Dantes Peak is a good one if she liked Volcano. The Day After Tomorrow has several disaster scenarios in one movie. Twister - not so much a disaster movie but well worth a look if she likes that kind of stuff. There's the two asteroid impact films of Armageddon and Deep Impact, though the disasters only take place in short bursts.
category 6 day of disaster or the follow up category 7 the end of the world are good disaster movies u could also try 10.5 and its sequal 10.5 apocolypse. they were all tv mini series's that have been released on dvd.
or u could try super volcano which was also a mini series but has been released on dvd.
I absolutely love the Posoideon Adventure (the 70s version) although the latest version is quite good too. They've just released a special edition of the original at a very cheap price - see link (but only if you've got a multi-region DVD):
My personal favorite:
Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb
Or, one of the ultimate of the genre':
On the Beach (1959, starring gregory Peck and Anthony Perkins)... other good ones are The Sum of All Fears, WarGames (Matthew Broderick, "Shall we play a game?"), Armageddon, Backdraft ... and The China Syndrome, from 1979 (I was but a small child when I saw it!)