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September 2, 2004

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Obsidian, a scarce commodity in Hawai`i

People sometimes ask if there's any obsidian in Hawai`i. The short answer is yes, but if you want to see an obsidian flow, you'll have to go to the mainland. Instead of heading straight for Las Vegas, take a side trip to Obsidian Cliff in Yellowstone National Park, the Big Obsidian Flow in Newberry Crater, Oregon, or Glass Mountain in northern California. No matter how many lava flows you've seen in Hawaii, we guarantee you'll be astounded.

So what is obsidian? Obsidian is dense volcanic glass, usually rhyolitic (the fine-grained equivalent of granite) in composition and black in color. 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. 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. When viscosity is high and heat loss is rapid, crystallization is inhibited.

Crystals impart a regular structure to materials. Without this structure, glass fractures in conchoidal, or smoothly curved, shapes, leaving edges that are sharper than the finest steel blades. 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.

Volcanic glass is rich in iron and magnesium, and tiny crystals of iron oxide give the glass its dark color. Different oxidation states of iron can tint the obsidian red, brown, or green. Obsidian is commonly banded or streaked, because the high viscosity of molten obsidian prevents impurities or bubbles from easily mixing with the surrounding magma.

Obsidian is short-lived relative to most crystalline rocks. Most obsidian is younger than 20 million years, because any obsidian older than that has devitrified, or changed from glass to crystalline rock. Over eons, the silica molecules within the glass slowly rearrange into organized crystal structures.

Because "obsidian" is a generic term for dark-colored volcanic glass, people sometimes refer to the dense, glassy crust that forms on some pahoehoe flows here in Hawai`i as obsidian. 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. We refer to the former as "volcanic glass," reserving the term obsidian for the latter.

According to the International Association for Obsidian Studies, the sole source of obsidian in the state of Hawai`i is Pu`u Wa`awa`a, a broad, dome-shaped cone on the north flank of Hualalai Volcano. 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.

The source of the obsidian was probably a dome that intruded into the base of the cone. Pieces of this obsidian body were blasted out by subsequent phases of the eruption.

The obsidian is exposed only in an old quarry where pumice was excavated from the cone for road fill. Luckily for our tires, the obsidian chunks make up only a small proportion of the deposit-perhaps 5 percent. The largest obsidian blocks are about 30 cm (1 foot) across. Most of the larger pieces have gray streaks that close examination shows to be layers of gas bubbles.

Hawaiian volcanoes haven't erupted more obsidian because most magma here is low in silica and, therefore, has a low viscosity. We 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. The pumice, lava flow, and obsidian erupted from Pu`u Wa`awa`a are trachyte, the highest silica rocks found on the island.

Ancient Hawaiians used volcanic glass for small cutting tools, and numerous quarry sites have been found on flows with glassy rinds. Certainly they would also have used Pu`u Wa`awa`a obsidian if it were available. Geologists working at Pu`u Wa`awa`a in recent decades, however, have found no obsidian at the surface of the cone. As of this column's deadline, we have been unable to confirm if archeologists have traced any glass artifacts back to this unique source.

Activity update


One earthquake was reported felt on the island during the week ending September 1.Residents of Volcano Farm Lots noted a magnitude 2.8 earthquake at 2:57 p.m. August 29. The earthquake was located 8 km (5 miles) west-northwest of Kilauea's summit at a depth of 8 km (5 miles).

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity was notably high for the sixth week in a row, with 69 small earthquakes recorded in the summit area. The activity was higher than during the previous week, when only 31 earthquakes were recorded, but lower than the 80 earthquakes recorded the week before that. Most of the earthquakes are of long-period type and deep, 40 km (23 miles) or more.


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Updated: September 7, 2004 (pnf)