Got my curiosity up, went and looked it up.
The “crack” is on the southern edge of Kilauea Volcano, on Hawaii, the Big Island. About 10% of the island is slipping into the ocean along what they call the Hilina Slump. Apparently it is about 5000 cubic miles of “stuff” slipping about 10 cm/year. In 1975, it slid 11 feet, making the “crack” 26 FEET wider along a 37 mile long stretch!
One source (http://www.science-frontiers.com/sf115/sf115p09.htm) says, “If the entire 4,760-cubic-mile block decided to break off, it would probably create a magnitude-9 quake and a tsunami 1,000-feet high. All the coast-hugging cities of the Hawaiian Islands would be swept away. And LOOK OUT Australia, Japan, and California.”
See the map a Comment #8. It shows a lot of the slides have occurred on the southern edge of the Big Island. A few years back I think it was CSI Miami had a show which involved just such an event. A chunk of land slid off one of the islands—Canaries or Azores and sent a 20 foot high wall of water into Miami. Their murder case was not caused by the tsunami, but it complicated matters a lot.