Because “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 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 Hawaiʻi is Pu`u Wa‘awa‘a, a broad, dome-shaped cone on the north flank of Hualālai 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. (USGS)
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. (USGS)
Since the raw material is quite small, there is no tradition of formal tool manufacture as in other parts of the world where obsidian is a preferred resource for manufacturing flake blades and a range of bifaces (e.g., arrow and spear points).
Based on retouch, microscopic edge damage, step flake scars, and crushing, most researchers have assumed that small sharp flakes were used for light or fine cutting and scraping tasks. Some have speculated that volcanic glass flakes may have been used for circumcision. (Weisler in Liritzis and Stevenson)
Volcanic glass was used by Hawaiians to make cutting tools like knives for butchering birds and other animals as well as for doing fine wood work. (NPS) They appear to have been used for short periods of time then discarded. (McCoy et al)
Volcanic glass was limited to a few small, informal tools types, typically scrapers or multi-purpose cutting tools. Basaltic volcanic glass, sometimes referred to as tachylite, forms by the rapid cooling of lava, and has few, if any, crystals and no corresponding internal crystal structure. Rarer in Hawai‘i is high silica volcanic glass from trachyte cones. (Lundblad, et al)
Another form of volcanic glass are thin strands drawn out from molten lava that have long been called Pele’s hair, named for Pele, the Hawaiian goddess of volcanoes.
Scientists say Pele’s hair is “volcanic glass that has been stretched into thin strands by the physical pulling apart of molten material during eruptions. Most commonly it forms during fire fountain activity.” (Batiza)
Missionary Titus Coan describes Pele’s Hair: “All at once the scene changes, the central portion begins to swell and rise into a grayish dome, until it bursts like a gigantic bubble, and out rushes a sea of crimson fusion …”
“… which pours down to the surrounding wall with an awful seething and roaring, striking this mural barrier with fury, and with such force that its sanguinary jets are thrown back like a repulsed charge upon a battle-field, or tossed into the air fifty to a hundred feet high, to fall upon the upper rim of the pit in a hail-storm of fire.”
“This makes the filamentous vitrification called ‘Pele’s hair.’”
“The sudden sundering of the fusion into thousands of particles, by the force that thus ejects the igneous masses upward, and their separation when in this fused state, spins out vitreous threads like spun glass.”
“These threads are light, and when taken up by brisk winds, are often kept floating and gyrating in the atmosphere, until they come into a calmer stratum of air …”
“… when they fall over the surrounding regions, sometimes in masses in quiet and sheltered places. They are sometimes carried a hundred miles, as is proved by their dropping on ships at sea.”
“This ‘hair’ takes the color of the lava of which it is formed. Some of it is a dark gray, some auburn, or it may be yellow, or red, or of a brick color.” (Titus Coan)
A single strand, with a diameter of less than 0.5 mm (~0.02 inch), may be as long as 2 m (~6.5 feet). The strands are formed by the stretching or blowing-out of molten basaltic glass from lava, usually from lava fountains, lava cascades, and vigorous lava flows (for example, as pāhoehoe lava plunges over a small cliff and at the front of an ‘a’a flow.)
Pele’s hair is often carried high into the air during fountaining, and wind can blow the glass threads several tens of kilometers from a vent. (USGS)
