Fountains of lava, pyroclastic flows, and ash clouds climbing high in the atmosphere—all wrapped in an element of danger— make for charismatic science. The appeal of volcanoes is so widespread that they have even found their way into our homes: look in the rock collection of any child, and nestled between the rose quartz and fool’s gold, you’ll probably find obsidian.
This glassy, volcanic rock has fascinated humans for millennia as much for its beauty as for its usefulness in arrowheads. At first glance, the origin of obsidian appears simple, as lava cools quickly enough that crystals can’t form. However, the lustrous, black depths of these enigmatic rocks hold mysteries about their formation that continue to puzzle earth scientists today.
The appearance of obsidian depends on the absence of water. It may seem natural that molten rock ten times hotter than water’s boiling temperature would be free of water. At the depths and pressures from which magma comes though, water is perfectly happy to coexist with and remain dissolved in the molten rock. Amazingly, a gallon of magma deep underground can contain up to two cups of water. However, the lavas from obsidian flows that form dome-shaped hills lose almost all of their water before cooling to glass. This happens as water naturally separates from the magma during its rise from deep chambers to the surface, but removing the bubbles that form while creating obsidian’s signature smooth texture proves to be a troublesome process.
When magma ascends to the surface and depressurizes, bubbles begin to form as water boils out of the molten rock. These bubbles then increase magma buoyancy, which speeds the magma’s rise to the surface. In cases of runaway bubble formation, the accelerating magma will explosively become a foam and eventually be ejected at high speeds out of the volcano as ash and pumice, a highly bubbly rock. Though seemingly opposites in appearance, pumice and obsidian have identical sources; however, the obsidian somehow was able to lose the majority of its bubbles before freezing while the molten rock was still liquid while the pumice was not.
Waiting for the foam to settle on a hastily poured beverage can sometimes feel like an eternity, but foamy magmas require even more time for the bubbles to leave a lava, taking months or longer. The stickiness of molten rock keeps the bubbles from rising out easily. For reference, the average obsidian lava is about a million times more viscous than honey—and up to one-hundred billion times depending on the temperature, water, and crystal content. This is a similar viscosity to the famous “pitch drop” experiments—a funnel of liquid tar that drips only about once a decade. Complicating matters more, the lava must remain hot enough to stay molten and not freeze into glass for the period that bubbles are slowly escaping the foam.
How then can the obsidian stay molten long enough for the foam to collapse? At volcanoes with massive obsidian flows, slow moving rivers of fire flows make hill-sized domes on the surface during oozing, relatively peaceful eruptions. Hot and insulating lava erupted before and after the future-obsidian can then sandwich the bubbly layer acting as a blanket, holding the heat in and keeping the obsidian liquid.
Obsidian flow with massive volcanic glass at Obsidian Dome, California.
Obsidian flow with large pieces of volcanic glass, Obsidian Dome, California. Adding to the puzzlement of volcanologists, chips of volcanic glass in ash beds, far from these thick, layered flows can be found by visitors to many volcanic systems like Mono-Inyo Craters in California, Newberry Caldera in Oregon, and Yellowstone Caldera in Wyoming . These chips, or obsidian pyroclasts—Greek for “shards of fire”—can’t have been kept hot enough for months in their ash beds. Instead, they must have formed underground, in the heart of the volcano, and been ejected with the pumice and ash during an explosive eruption.
Observations of obsidian pyroclasts reveal several mysterious differences from obsidian flows. First, these quarter sized bits of pyroclastic glass manage to retain much of their water, up to 25 times as much water as in flows, and their water content can vary greatly from one side of a chip to the other. Second, the pyroclasts preserve complicated starburst-shaped bubbles that cannot be explained through the foam settling process called upon to explain obsidian flows.
To address these complications and improve understanding of the fiery inner workings of volcanoes, scientists have developed a new mechanism to explain the formation of obsidian during explosive eruptions. Rather than having the entire magma foam up as a whole and then lose its bubbles during and after an effusive eruption, scientists suggest that pyroclast forming magmas do fragment and explode, temporarily becoming pumice and ash rather than a cohesive lava flow.
Before these fragmental bits of molten rock can be rocketed to the surface though, some may stick to the inner walls of the volcano. This accumulation of hot ash and pumice can then stay stuck long enough to heal back into a cohesive mass like squishing together bits of hot wax. The gas trapped between these sharp fragments of ash then forms the previously unexplainable starburst bubble shapes, solving the first part of the mystery.
Volcanic ash under an electron microscope. Gas becomes trapped between sharp points and cusps of these small bits of glass. Image window measures 1.5 mm across. Credit: Emma Nicholson
Having the obsidian form underground rather than at the surface also explains how it retains water: the higher pressures keep water in the magma right up until the shards get ripped off the inner wall of the volcano and flash freeze during eruption. The water variability in each pyroclast would also be expected to smooth out over time, so scientists cleverly use the diverse water contents across chips to estimate the time between ash sticking to the wall and pyroclasts being erupted. Before this new mechanism was developed, thousands of hours would have been required to explain the formation of these pyroclasts, but this new method can make smooth glass out of hydrated magma in less than a day.
We’ve taken a brief look into the origins of this beautiful rock, but many mysteries remain from the small—how do snowflake, mahogany, and rainbow obsidian form? — to the fundamental—what triggers volcanic eruptions, why do they stop, and can we predict them? Volcanoes are complex and chaotic systems, and scientific studies of obsidian tell more than just interesting stories about an attractive rock. The investigation of these pyroclasts has given us a rare, direct look into the processes happening inside of the volcano before, during, and after eruptions.