June 01, 2010
Pumice is a glassy volcanic rock that's so full of bubbles that most examples will float on water. Reticulite is an extreme form of pumice in which the bubbles have coalesced, leaving only a tenuous reticular network of glassy lava behind in the interstitial spaces between the bubbles. While pumice is more characteristic of the ejecta from the silica-rich magmas of stratovolcanoes such as Mount St. Helens, it may also be formed from the more silica-poor basaltic magmas of shield volcanoes such as those of Iceland and the Hawaiian Islands. Reticulite is formed only by very high fountains of basaltic lava that contain dissolved gasses such as water vapor and carbon dioxide. To form much reticulite, a lava fountain must be at least 1000 feet (300 m) high. Molten lava is shot into the air at such high velocity that gasses dissolved within it suddenly exsolve as the surrounding atmospheric pressure abruptly drops. The specimen of reticulite shown here was photographed near the Pu'u Loa petroglyphs in Hawaii Volcanoes National Park on April 3, 2007. It was probably formed by lava fountaining at Kilauea Volcano's Pu'u O'o vent sometime during the early to mid-1980s. Reticulite is so light that it's frequently transported many miles downwind of an eruption site. The Pu'u O'o vent lies roughly 7 miles (11 km) to the north of the Pu'u Loa site. Embedded in the far right-hand side of this specimen are several strands of "Pele's Hair", a fibrous form of basaltic glass that's also formed in lava fountains. Pele is the Hawaiian Goddess of Volcanoes.
Camera Maker: SONY; Camera Model: CYBERSHOT; Image Date: 2005:04:01 18:37:58; Focal Length: 7.0mm; Aperture: f/7.1; Exposure Time: 0.0020 s (1/500); ISO equiv: 100; Exposure Bias: none; Metering Mode: Center Weight; Exposure: program (Auto); Light Source: Daylight; Flash Fired: No; Color Space: sRGB