Volcanos generate in the Nether with Biomes O' Plenty installed (a lot of them, since the Nether is full of lava), and sometimes it causes server freeze for a relatively significant amount of time (enough to time out all connected players), and if I get Java thread dump, I see:

"Server thread" prio=6 tid=0x0000000013058800 nid=0x1ac8 runnable [0x000000001ac6e000]
java.lang.Thread.State: RUNNABLE
    at mrtjp.projectred.exploration.GeneratorVolcano.func_76484_a(GeneratorVolcano.java:31)

World gen can't run in the nether. Unless you have another mod messing with nether worldgen, its not on my end. The very first thing the generator does is checks if its in the nether and returns.

Well, I do indeed have Biomes O' Plenty which replaces chunk provider. The check you have will probably generate volcanos in Twilight Forest and all Mystcraft ages too. I think checking dimension ID would not be a terrible idea.

Edited the issue title.

My thought are:

• Generate ores only in dimension with ID = 0 or > 1
• Generate volcanos and marble caves only in dimension with ID = 0
• Maybe sometime later: Add Mystcraft symbols for volcanos and marble caves - that would be nice. I looked at Mystcraft API and it doesn't look hard to do.

Volcanoes are now restricted to ID 0 and >1, and non hell biomes.
Mystcraft api is not open to the public.

This issue also occurs with Natura when it overrides the nether worldgen. Disabling volcanoes or nethergen fixes it.