7/30/2023 0 Comments Black obsidian crystal redditWe owe the unique occurrence of obsidian at Pu`u Wa`awa`a to the cone's unusual composition, which was the topic of this column 3 weeks ago. Hawaiʻian volcanoes haven't erupted more obsidian because most magma here is low in silica and, therefore, has a low viscosity. Most of the larger pieces have gray streaks that close examination shows to be layers of gas bubbles. The largest obsidian blocks are about 30 cm (1 foot) across. Luckily for our tires, the obsidian chunks make up only a small proportion of the deposit-perhaps 5 percent. The obsidian is exposed only in an old quarry where pumice was excavated from the cone for road fill. Pieces of this obsidian body were blasted out by subsequent phases of the eruption. The source of the obsidian was probably a dome that intruded into the base of the cone. The cone consists of layers of gray pumice, some containing blocks of black obsidian, that were deposited during explosive eruptions about 114,000 years ago. We refer to the former as "volcanic glass," reserving the term obsidian for the latter.Īccording to the International Association for Obsidian Studies, the sole source of obsidian in the state of Hawaiʻi is Pu`u Wa`awa`a, a broad, dome-shaped cone on the north flank of Hualālai Volcano. Most geologists and archeologists, however, prefer to make a distinction between the thin, glassy rind on an otherwise crystalline lava flow and the glass that erupts as thick flows or domes. Over eons, the silica molecules within the glass slowly rearrange into organized crystal structures.īecause "obsidian" is a generic term for dark-colored volcanic glass, people sometimes refer to the dense, glassy crust that forms on some Pāhoehoe flows here in Hawaiʻi as obsidian. Most obsidian is younger than 20 million years, because any obsidian older than that has devitrified, or changed from glass to crystalline rock. Obsidian is short-lived relative to most crystalline rocks. Obsidian is commonly banded or streaked, because the high viscosity of molten obsidian prevents impurities or bubbles from easily mixing with the surrounding magma. Different oxidation states of iron can tint the obsidian red, brown, or green. Volcanic glass is rich in iron and magnesium, and tiny crystals of iron oxide give the glass its dark color. Walking on an obsidian flow can be a nerve-wracking experience-wear gloves! Native Americans, of course, prized obsidian for arrowheads, and obsidian was traded hundreds of miles from its source. Without this structure, glass fractures in conchoidal, or smoothly curved, shapes, leaving edges that are sharper than the finest steel blades. When viscosity is high and heat loss is rapid, crystallization is inhibited.Ĭrystals impart a regular structure to materials. In nature, erupting a glass flow requires an unusually viscous magma, one that has both a high silicon dioxide (silica) content and a very low water content. Glass, be it volcanic or manmade, cools quickly from a molten liquid without forming crystals, the building blocks of the minerals that make up most rocks. So what is obsidian? Obsidian is dense volcanic glass, usually rhyolitic (the fine-grained equivalent of granite) in composition and black in color. No matter how many lava flows you've seen in Hawaiʻi, we guarantee you'll be astounded.
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