Pictured below is a crack in the Southwest Rift Zone taken from a stop along Crater Rim Drive. This and other large cracks are the result of magma moving underground from the summit reservoir. Layers of ash in this rift are as large as 5 meters thick. The rifts formed in layers of ash from the 1790 eruption of Kilauea.
For more details: http://hvo.wr.usgs.gov/gallery/kilauea/caldera/srb980817016_caption.html
Karen and Steve in the rift zone.
Photo by Thom Wilch