A Stratigraphic Study of the 610 A.D. Eruption of the Mono Craters

A SURF Proposal by Aron Meltzner

Collaborating with Matthew Dawson

Mentor: Kerry Sieh

Submitted: 2 March 1998

Although much attention has been focused recently on the Long Valley caldera resurgent dome in eastern-central California and on associated volcanic hazards at Mammoth Mountain, little is known about the details of the most recent (latest Pleistocene to Holocene) eruptions in the greater Long Valley caldera complex, specifically in the Mono and Inyo Craters chain. In general, activity within the resurgent dome has not been linked with the formation and later eruptions of the Mono and Inyo Craters, however, there may be good reason to connect the two. Despite recent activity in the resurgent dome east of Mammoth Mountain, the resurgent dome has experienced eruption only once every 100,000 to 200,000 years since the catastrophic caldera-forming event 760,000 years ago, and it last erupted roughly 50,000 years B.P.; meanwhile, an eruption has broken out in the nearby Mono-Inyo chain roughly every 250 to 750 years during the past 5,000 years, and an event such as the latter is statistically more probable in the near future than an eruption of the resurgent dome. Dr. Kerry Sieh has hypothesized that the Holocene eruptions of the Mono and Inyo Craters may have immediately followed magmatic intrusions into the resurgent dome and subsequent lateral sub-surface evacuation of that magma into adjacent magma chambers under the Mono-Inyo chain (Sieh, personal communication); such a phenomenon has been observed elsewhere, particularly in association with events at Kilauea and during the 1912 eruption at Katmai, Alaska, and it could explain the otherwise unlikely observation that the Long Valley caldera resurgent dome has been expanding over the past two decades. The validity of such a hypothesis would provide a direct link between the Long Valley caldera resurgent dome and the Mono-Inyo Craters chain, and in light of recent seismic activity, this could significantly increase the potential volcanic hazard in the Mono-Inyo Craters chain in the near future.

The Mono and Inyo Craters comprise a young volcanic chain with a violent and exciting history, and there is strong evidence that another eruption in the region is very likely in the near geologic future. It is imperative that we study the records of what has occurred in the region, to help us understand the nature of past eruptions -- i.e., volume and explosiveness of the eruptions; extent of pyroclastic falls and flows, lava flows, and ash falls; and the timing of the events (e.g., if three or four separate vents erupted in the same eruption, did they erupt contemporaneously, or were the individual vent eruptions separated by a week, a few months, or more?) -- so that we can gain insight into what may happen in the future. Sieh and Bursik have already done a thorough field investigation of the most recent (14th century A.D.) eruption of the Mono Craters, but that is only one eruption; others still need to be studied. Dr. Sieh has continued the study by beginning an investigation of the 610 A.D. eruption of the Mono Craters, although much data has yet to be collected, and much analysis remains to be done. As a SURF project this summer, I propose a study in which I¹ continue the research of Dr. Sieh, by collecting more data in the field, and by processing, mapping, and analyzing the new data subsequently in the GIS (Geographical Information Systems) computer lab.

There is too much data to be collected and too many questions to be answered, to even attempt to accomplish everything in one summer, but with adequate preparation, scrupulous (but systematic) note-taking, and good judgment in the field, significant progress can be made toward answering some of the most important questions. We² will spend approximately five weeks in the field,³ digging pits at strategically-planned sites, and studying the stratigraphy of the rocks in each pit. For each volcanic layer, we will note its color, grain or clast size, texture, mineralogy, and any bedding structures such as cross-bedding, all of which will help us correlate volcanic layers from one site to the next. We will interpret each layer's depositional nature (i.e., pyroclastic flow, lava flow, ash fall, etc.) from its geographical distribution and the size distribution of its rock fragments. The sorting of a layer's ash and clasts will allow us to infer the original water content of each layer, and hence to infer the viscosity and explosiveness of the primordial lava as well. We will note how the thickness of each layer varies with proximity to a potential source vent, and if we are able to obtain enough data, we will interpolate to get an estimate of the volume of material in that particular layer. We will pay close attention to the boundaries of each layer, provided we get said data -- if a layer "pinches out" at a particular point, we know the depositional extent of that particular layer; but if a layer does not "pinch out" and instead merges with another layer of different composition, we will have gained the valuable information that two vents, with separate magmatic sources, were erupting and depositing ash and debris contemporaneously. Otherwise, we will be able to interpret the sequence of eruptive events simply by noting the order in which the beds were deposited.

In addition to taking notes (and photos) in the field, we will be using a GPS hand-held receiver to determine the latitude and longitude of each site (+/- 50 m), and we will record on site on a portable laptop computer, the thickness of each layer at each pit, and other relevant information. The ArcView GIS application we will be using will allow us to store and analyze the information we input as data points, along with the surveyed coordinates for each site. Either in the field or in the lab when we return, we will analyze the data, using spreadsheets and graphical methods (in particular, compiling contour maps) as appropriate to map out the extent and varying thickness of each bed, and to map out various cross-sections that will reveal the "big picture": the spatial and temporal depositional distribution of the eruptive material as a whole. We will spend the final weeks of the SURF interpreting the data, attempting to explain our findings (and attempting to pinpoint the source vents or craters of the 610 A.D. eruption), possibly comparing our data to the data already collected from the 1350 A.D. eruption, or perhaps we will discover some intriguing pattern or anomaly, which we will want to focus our attentions on during the last few weeks of the SURF. It is hard to predict exactly what we will find, but most certainly, the knowledge we gain will be invaluable in improving the science community's general knowledge of the Holocene history of the Long Valley caldera complex, and it will shape the general understanding of volcanic hazards in the region now, and into the future.

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