Hawaiian Volcano Observatory: Kīlauea – Current Maps

Longer answer: the matter was decided in State by Kobayashi v. Zimring in 1977 (Supreme Court of Hawaii). They found that there was no applicable common law precedent, since land created by eruptions that impacts private title is very rare. The only precedent they found in Hawaii dates to 1877 (before Hawaii was US territory!) and it's not clear how to generalize it (note that Hawaiian land was the property of the King at the time). The closest thing is accretion by riverbanks and the like, which everyone in the case agreed wasn't particularly relevant. The court basically said "we're making completely new law here, so we can do whatever we want" and chose granting the land to the public interest as the proper solution.