The Development of Textures and Structures on Lava Flow Surfaces

Structures on lava flow surfaces develop in response to stress during cooling and emplacement. Although flow features are common on the surfaces of basaltic lava flows, viscous silicic extrusions typically form structures through brittle fracture of the cooled crust. We have studied the development of a variety of surface features on active silicic lava flows and domes in order to delineate the factors responsible for their formation. By modeling the formation of these features, we can assess the emplacement histories of older flows not observed during emplacement, or those found on other planetary bodies.

One of my first volcanological research projects concentrated on the development and distribution of vesicular and nonvesicular lava textures on the surface of the Mount St. Helens lava dome (Anderson and Fink, 1990).

Left: Smooth toe at the foot of the predominantly vesicular September 1981 lobe (photo courtesy of the USGS).

By combining field mapping with detailed sampling and hydrogen isotopic analyses, we determined that highly vesicular lava formed when lava arrived at the surface with a relatively high water content. This water-rich lava would vesiculate and degas upon eruption, resulting in the bubble-rich surface texture. Smooth, relatively dense lava formed when the lava water content was low. Thus, lava arrived at the surface in a highly degassed state and would not experience any significant degassing and bubble growth during surface flow.

Perhaps the most interesting aspect of this work is that late Mount St. Helens dome eruptions (1984-86) contained very little vesicular lava, suggesting that these lava flows arrived at the surface in a more degassed state than those produced early in the dome's history (1980-83). Since the bulk chemistry remained unchanged during the 6-year history of the dome, we believe that the thoroughly degassed late-stage eruptions are a product of more intense gas loss during eruption, rather than from a change in the composition of the lava. We suggest that the larger, stronger dome existing from 1984-86 impeded the rise of additional lava to the surface, allowing for a longer stage of gas loss prior to arrival at the surface.

Recently, we have extended this work to include a study of blocks on lava flow surfaces (Anderson et al., 1998). We determined block size distributions on the surfaces of Holocene silicic lava flows at the Inyo Domes and the Medicine Lake volcano, and studied the development of blocks on the active Mount St. Helens and Mount Unzen lava domes to better understand the emplacement history of young viscous flows. We measured block chord lengths along perpendicular 25 m long transects within vent, jumbled and ridged morphologic units. Vent regions generally contain the largest average block sizes and largest range of average blocks, whereas ridged areas tend to have the smallest average blocks.

Left: Vent area of Obsidian Dome. There are two people at the base of the large fracture in the center of the photo! Note the high quality boot belonging to Dr. Mike Ramsey (Arizona State).

Observations at the active Mount St. Helens and Mount Unzen lava domes show that block size distributions reflect stress conditions during flow. High extrusion rates produce small primary blocks and lead to rapid fracturing of the flow surface, whereas low extrusion rates allow large slabs to form in the vent area and lead to less severe fragmentation. A dramatic increase in the size of blocks evident in active vent regions may indicate a significant decrease in eruption rate, and thus could signal the cessation of extrusion. On the other hand, if the extrusion rate is too high or the cooling rate too low, a rigid crust and accompanying blocks will not form on an eruptive time scale.

Below: Summit of the Mount Unzen dome (Japan) showing a decrease in block size outward from the vent area.

 

Blocks may fracture through mechanical and thermal processes as they move down slope. Most silicic lava flows reach a "steady-state" down slope where the average block size at the surface remains in the 20-30 cm size range with increasing distance from the vent. Fines (blocks <12 cm) do not accumulate on the flow surface because they slip towards the flow interior through void spaces between surface blocks. We therefore expect long silicic lava flows to have blocky surfaces throughout their lengths, an important consideration for evaluation of planetary lava flow emplacement. My collaborators on this project are Drs. Ellen Stofan (Proxemy Research), Jeffrey Plaut (JPL) and Dr. David Crown (University of Pittsburgh). This project benefited greatly from undergraduate involvement as 4 of my best students spent several weeks conducting field work in Oregon and California. We have now started to apply our findings to other volcanoes on the Earth and other planets.  We have done some additional work in Peru, and are looking at block sizes on planetary flows (Bulmer et al., 2005).

Left: Dave Crown and Jeff Plaut working in a jumbled area on obsidian dome.

 III. Resurfacing of Lava Flows at Mount Etna volcano, Sicily

We are now looking at processes that cause lava flows to widen and thicken at Mount Etna in Italy, where our recent field work has resulted in preparation of a manuscript that was recently accepted for publication (Duncan et al., 2004). My collaborators on this project include Dr. Ellen Stofan (Proxemy Research) Dr. Suzanne Smrekar (JPL), Dr. John Guest (University College, London), Dr. Angus Duncan (Luzon), Dr. Harry Pinkerton (Lancaster) and Dr. Sonia Calvari (Istituto Interazionale di Vulcanologia - Catania). We are assessing the magnitude of pressure gradients in lava tubes that feed these flows in order to determine the role of lava tube dimension and magmastatic head on processes that cause lava flows to thicken and widen. We have a couple of papers that discuss our findings (Duncan et al., 2004; Guest et al., 2004)

Resurfacing of pahoehoe lava over 'a'a at Mount Etna. The nimble and quick Ellen Stofan for scale.