The Stone Sky by N.K. Jemisin

The third and final volume of the Broken Earth.   Essun awakes from a coma caused by her use of the obelisk to discover that the comm is on the road to where the attackers came from.  Her arm is partially stone.  The real problem is that the moon, which had been pushed out of it's orbit much farther away from Earth ages ago, is now on a collision course back to the planet it once circled.  Nassun is seeking a way to get to an island on the other side of the planet where she hopes to convert everyone left on Earth into stone people so they can survive the collision.  Essun eventually also realizes she must go to that same island to try to divert the course of the moon.