RICHARD DANIEL HARWOOD
A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in Geology, Northern Arizona University
The primary tool in reconstructing cinder cone breaching events is the location of the rafted mounds of cone material on the surface of the breaching lava flow. This indicates: 1) the relative timing of the breach in relation to the lava extrusion event, 2) the initial stratigraphic position of the breach, and 3) the number of lava flow units.
Detailed mapping of the volcanic rocks at Strawberry Crater and O'Neill Crater in the San Francisco volcanic field, north-central Arizona, has revealed unique eruptive and breaching histories for the two cinder cones. The sequence of events at Strawberry Crater are: 1) building of the basaltic andesite cinder cone by strombolian style eruption, 2) possible intrusion of a magma body into the cone structure, resulting in over-steepening of agglutinate rim deposits and initial breaching of the cone on the east side at a stratigraphic position just below the rim, 3) extrusion of a basaltic andesite block lava flow, which rafted mounds of cinders and agglutinate away from the cone, enlarging the breach, and 4) extrusion of a dacite vitrophyre plug from a vent located in the floor of the breach. Timing of the slump gap in the southern rim of the cone is uncertain, and could have occurred at any time after the cone was built. Post-eruption events have been limited to deposition of Sunset Crater airfall pyroclasts and erosion.
The sequence of events at O'Neill Crater are: 1) extrusion of the first basaltic andesite block lava flow either prior to the cone or as a non-breaching unit after the cone was built, 2) building of a basaltic andesite cinder cone, 3) extrusion of the second block lava flow, which breached the cinder cone to the east, and 4) extrusion of dacite vitrophyre as a small dome slightly off center of the cone, and as two small plugs west of the cone. A large slump fan of cone material was deposited to the west at some time after the building of the cone, but prior to the extrusion of the dacite plugs. Post-eruption events have been limited to deposition of Sunset Crater airfall pyroclasts and erosion.
Statistical analysis of breach azimuths for 36 cinder cones in the San Francisco volcanic field results in a bimodal distribution with mean vectors of S57.7E (122.3°) and N67.9W (292.1°). These directions are roughly perpendicular to the inferred least principal horizontal stress direction of approximately N50E (50°) for the volcanic field. Five local controls on breaching mechanisms appear to operate in determining the direction of breach for cinder cones: 1) local topographic stress regimes, 2) local fault/joint system control, 3) wind direction/cone strength, 4) vent location of the breaching lava, and 5) substrate buttressing of the cone structure. The connection among the proposed local controls and the inferred regional stress regime is not fully understood. The breaching control at Strawberry Crater appears to be a combination of substrate buttressing (5) and cone strength (3), while at O'Neill Crater it appears to be the local fault system (2) and the vent location of the breaching lava (4).
This work is dedicated to Dr. Arthur B. Metzger.
He has been, and shall be, a mentor and tutor.
He is someone I call friend.
Unlike so many of my colleagues, I cannot thank anyone for funding this project. Not that I didn't try to get support, it's just that no one wanted to give me their money.
I did, however, receive a multitude of moral and intellectual support from a number of special people. Dr. Richard Holm was instrumental in the choice of this thesis topic in two capacities. The first was by planting the seed of an idea while I was an undergrad by stating that Strawberry Crater had not been studied in detail. Secondly, when I went to him seeking a thesis topic he refused to give me any suggestions, thus forcing me to develop a topic myself, and thus giving that seed of an idea fertile soil in which to grow.
Dr. Charles Barnes caused a major headache to develop when he asked the seemingly simple question, "why do your two cinder cones breach to the east?" It is a question that will probably keep me busy trying to answer all the implications for years to come.
Many thanks must go to the rest of the faculty for putting up with me both as an undergrad and as a grad student. They taught me what I needed to know to research this topic, and much, much more.
My fellow grad students helped me keep my sanity and keep in shape physically during the many hacky sack sessions both during the day and under the lights at night. It's a wonder I managed to get any work done! Thanks to my roommate, Mike Buren, and also to Mike Lindholm for the many discussions while I was doing the field work and analysis of the data. Thanks also to Mike Doe, Mike Kelly, Matt Kaplinski, Ed Basham, Chuck Collum, Phil Anderson, Mike Sanders, Hilary Mayes, Mark Labrenz, Mark Capps, Myron Cook, Dave Rothstein, Tom Ring, Matt "why are you dressed so funny" Nation, Scott Johnson, and anyone else who I've forgotten in this list.
Lastly there are a few people that were not directly involved with the project but who lent their support; Barb and Bob Zoellner for having confidence in me, and when that failed, had a barbecue anyway; Bob Baron, friend and mentor who is still convinced that part of my brain must be dead because I used an IBM instead of a Mac; Darrell Richards for being my brother and best friend and listening to all my gripes, grievances, and frustrations; Mark Burton for being my brother and helping me relax in L.A. when I needed a vacation; and Dr. Arthur Metzger to whom this work is dedicated.
The greatest thanks and love is for my parents, Cliff and Cathy Harwood, for raising me, allowing me to make my own decisions, seeing me through school, and supporting whatever I chose to do, no matter how crazy. Not everything has worked out, but you were always there to help me get back on my feet.
Lava Flow - Breaching of the Cone
Lava lake model
Dike intrusion model
Magma body intrusion model
The breaching event
Lava Flow - Breaching of the Cone
1. Initial breach model
2. Two flow unit model
The breaching event
Breaching of Cinder Cones in the San Francisco Volcanic Field
Controls on Breach Direction
1. Regional stress regime
2. Fault/joint system control
3. Breaching - lava vent location
4. Substrate buttress
5. Wind direction/cone strength
6. Topographic stress regime
Breach Direction of Strawberry and O'Neill Craters
The Significance of Rafted Material