Editor's note:Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Drew White, graduate student, and Lauren Harrison, assistant professor, both in the Warner College of Natural Resources at Colorado State University.

The Yellowstone Plateau Volcanic Field has some of the largest rhyolite lava flows on Earth. Have you ever wondered what these flows look like in their interior, or how the unique textures that can be observed in rocks across Yellowstone National Park formed?

The Yellowstone Plateau Volcanic Field is one of the largest rhyolite fields on the planet and well known for its voluminous high-silica lava flows and ignimbrites (ash flows caused by massive explosive eruptions that result in thick volcanic deposits). The most recent eruptive phase of large rhyolite lava flows are the Central Plateau Member rhyolites, the youngest of which is 70 thousand years old. The high-silica composition results in a very viscous (sticky and resistant to flow, like toothpaste) lava with characteristic textures that record how the lava flow developed. Let’s take a closer look at these textures as they would be observed in a cross-sectional outcrop of a rhyolite flow, starting from the bottom and moving upward.

Schematic cartoon of an idealized rhyolite lava flow with structures identified. Figure modified from Sweetkind et al. (2015)

As the lava erupts, it is pushed through a volcanic conduit that connects the subsurface magma reservoir to the surface. Once it emerges at the surface, the exterior quickly quenches into a glassy carapace as the hot (~700–800 C, or ~1300–1500 F) magma contacts the cool air. This carapace starts to fragment under the stress of the still-molten flowing interior, which results in blocks that form a “flow breccia” that makes up both the bottom and top parts of the flow. The part of the carapace that is not fragmented is the glassy zone known as the “obsidian zone.” Obsidian is a volcanic glass that cools so rapidly crystals do not have time to form. There is also a very distinct core to a rhyolite flow that consists of dense, “stony” rhyolite that exhibits flow banding—swirls in the rock that form during both the movement and cooling of the flow. A temperature gradient forms through the flow as it cools from the outside inward, varying properties of the lava such as viscosity, which is lower at higher temperatures (the hot molten center of the flow will have a lower viscosity and move slightly faster). This variation causes deformation and orientation of crystals and glass in the lava that can result in the banding pattern. The flow bands are mostly parallel to the base of the flow and become more vertical towards the center of the flow, reflecting how the hotter, less viscous interior was able to deform more readily than the cooler margins during emplacement.

Photos of Central Plateau Member rhyolite flow structures from the Yellowstone Plateau Volcanic Field. A) An ogive from a road cut along Firehole Lake Drive. Ogives are pressure ridges that form perpendicular to the direction of flow from the compressive stresses that deform the highly viscous lava as it moves. B) A roadside outcrop exhibits rhyolitic flow banding, including an obsidian-dominated band (the glassy black rock) near the bottom of the outcrop. Photos by Lauren Harrison, Colorado State University, taken in May 2024.

As the flow cools, exsolution of gases such as water vapor plays a major role in the formation of textures. During the cooling process, the solubility of different gases decreases, causing them to escape from solution and form bubbles. If these bubbles are trapped in the lava, the final solidified rock will have small holes, called vesicles, that are a good indication of the amount of gas exsolution that occurred in that part of the lava flow. In fact, the occurrence of vesicles decreases from many at the top of the lava flow to almost none in the center! If these bubbles were present while the lava was still flowing, they may be stretched in the direction of flow.

If the bubbles coalesce enough to form a permeable network, the volatiles will move upward to escape the flow into the atmosphere, causing alteration of the surrounding lava and turning it red from oxidation. If a lot of vapors are exsolved upon further cooling of the lava, spherulites can develop. Spherulites are circular features of radially growing crystals that form when volcanic glass loses gases during cooling (which is a process called devitrification). They often develop in the glassy margins of a rhyolite flow. Other textures caused by gas escape are most noticeable towards the top of the flow where there is a such a high concentration of vesicles that it forms a frothy glass—pumice—where there are more vesicles than actual lava.

Rhyolite lava flow textures from Long Valley and Yellowstone calderas. A) Photograph of well-developed spherulites in a lava flow from Long Valley Caldera in Eastern California. This high-silica rhyolite flow is very similar to the Central Plateau Member rhyolites of the Yellowstone Plateau Volcanic Field and exhibits many of the same textures. Here, spherulites form from volcanic glass losing gases and causing the very fast crystallization of quartz and feldspar needles that radiate concentrically from a central nucleation point. The largest spherulites in the photo are the size of a hand. B) A flow banded high-silica rhyolite from Yellowstone National Park. Hand lens for scale. Photographs by Lauren Harrison, Colorado State University, taken in May 2024.

As the flow finally cools and begins to solidify, the upper part of the carapace begins to fracture, creating an upper flow breccia similar to the basal flow breccia. While the top and bottom breccias are both the result of fragmentation of a glassy carapace, they record different parts of the emplacement process. The base records early cooling and stress caused by motion of the lava flow, while the top records brittle cooling, contraction, and degassing processes.

Together, all these observed textures record a history of emplacement processes that can be observed in rhyolites globally. Many of these features are hard to see in Yellowstone National Park due to the huge scale of the rhyolite flows, and because many of the flows have been eroded by glaciers since they erupted, but there are some great exposures in outcrops and road cuts—for example, along Firehole Canyon Drive, a few miles south of Madison Junction. Take a look at the outcrops alongside the road to see if you can tell whether you are looking at the bottom, center, or top of an old rhyolite flow!