Melting Stones by Tamora Pierce

Residents on one of the Battle Islands south of Emelan have sent a call for help to Winding Circle temple: their plants and trees and the animals who live around them are dying off in patches, for no reason that they can work out.  From Winding Circle come two familiar figures: prickly green mage Rosethorn, whose plant magic will help her to decipher what's ailing the plant life on the island, and Briar's former student, the obstreperous stone mage Evumeimei, or Evvy, who is eager to travel once more.

With them comes the eager and nervous Dedicate Initiate Myrrhtide, who will help them to communicate with Winding Circle should they need to, and the companion Evvy met while she, Rosethorn, and her former teacher Briar were traveling in distant Yanjing, Luvo, a being who is best described as a walking, intelligent rock. Though supposedly Evvy and Luvo are simply along for the ride, they are pulled into the island's mystery when Evvy is caught up by the wild spirits loose in an immense bubble of molten rock quite close to the surface of the island.

What Evvy, Luvo, Rosethorn, and the people of the island can do in the face of a fast-approaching volcano will be anyone's guess. Can they evacuate the island?  Or worse--will they try to shift a volcano?