They take half damage from it. That would be base 90 damage from Fire Blast. Also, if I recall the only thing HP Ground really hits harder than HP rock that would be a threat is Heatran, who still takes neutral damage from Rock. Dragons would take Base 140 from Rock.
If you fear it, you'll run either flying, poison, or rock attacks on at least one Pokemon. You can't just hope your team remains unchanged, otherwise CAP isn't doing a very good job of changing the metagame.The chance that you don't carry a Flying attack on a given team is high, and most users of Rock attacks don't like switching or staying in on this.