The only known source of obsidian in the Hawaiian Islands is Pu`u Wa`awa`a. This ʻopihi-shaped cinder cone on the north flank of Hualalai holds xenolithic blocks of black obsidian, deposited during explosive eruptions about 114,000 years ago, in its grey pumice. The obsidian chunks probably are pieces of a dome that intruded into the base of the cone, and were blasted out by subsequent eruptive phases.
What, exactly, is obsidian, and why doesn’t Hawaiʻi have much?
Obsidian is often used as a generic term for any volcanic glass. But if we want to be more specific, obsidian is a dense volcanic glass formed from exceptionally viscous magma with both a high silicon dioxide (silica) content (over 70%) and a very low water content. It is usually black in color because of iron and magnesium in the magma from which it formed. However, different oxidation states of the iron can tint it red, brown, or green. Because the magma is so viscous, it does not mix evenly, and so is often banded or streaked.
To make glass, the magma must not only have a lot of silica, but must cool without forming crystals. High viscosity and rapid cooling inhibit crystal formation. Without crystals, the obsidian is able to fracture in conchoidal (smoothly curved) shapes that follow the flow lines of the magma. Breaking the obsidian leaves edges that are sharper than a razor!
As rocks go, obsidian is very young, less than 20 million years old. As time goes by, silica molecules within the obsidian slowly move into organized crystal structures devitrifying the glass, changing it to crystalline rock.
Because “obsidian” is often used as a generic term for dark-colored volcanic glass, people sometimes refer to the “skin” or glassy crust that forms on some pahoehoe flows here in Hawai`i as obsidian. However, we prefer to make a distinction between the ʻili ā Pele, the thin, glassy rind on an otherwise crystalline lava flow, and the glass that erupts as thick flows or domes. In English, we prefer to call it “volcanic glass.”
According to the International Association for Obsidian Studies, the only source of obsidian in Hawai`i is Pu`u Wa`awa`a
The obsidian can be found in an old pumice quarry on the puʻu. The obsidian chunks make up only about 5% of the deposit, with the largest pieces about 30 cm (1 foot) across. Gas bubbles, from when the obsidian formed, show as gray streaks in the larger pieces.
The pumice, lava flow, and obsidian erupted from Pu`u Wa`awa`a are trachyte, the highest silica rocks found on the island. Magma from all of the other volcanoes in Hawaiʻi has much less silica than Hualalai, so they cannot form obsidian.
Before contact with European people, the Kanaka Maoli (Hawaiian people) used volcanic glass to make small cutting tools. Old quarry sites have been found on flows with ʻili ā pele. If Pu`u Wa`awa`a obsidian were accessible, the people of ka wā kahiko (ancient times) would have used it, but geologists have not found obsidian at the surface of the cone. If you find a tool made from Hualalai obsidian, we would love to know about it!