G. Lang Farmer
Ph.D. University of California, Los Angeles, 1983
Professor, Geological Sciences
Chair of Geological Sciences Department
E-mail: farmer@cires.colorado.edu
Office: ESCI (Benson) 422A
Phone: 303-492-6534
Web: Prof. Farmer (Dept. of Geological Sciences)
Research Interests
Application of radiogenic isotope systematics to earth sciences.
Current Research: Mining the North American Volcanic Rock Database
The Farmer laboratory has been involved for the past eight years in the development and population of an online database for the age and chemical compositions of western North America volcanic rocks. This North America Volcanic and Intrusive Rock Database (NAVDAT; http://www.navdat. org/) now contains information for nearly 65,000 separate rocks samples, allowing variations in the compositions of these rocks through time and geographic position to be interrogated at an unprecedented level of detail.
Figure 1: Sodium contents for mid-Cenozoic volcanic rocks from the southwestern portions of North America, using data extracted from the North American Volcanic and Intrusive Rock database (NAVDAT).
Mid-Cenozoic volcanic rocks in the Never Summer Mountains, northcentral Colorado.
Our efforts in 2009 to mine the database concentrated on intermediate- to silicic-composition magmatism that occurred during the mid-Cenozoic “ignimbrite flare-up” in western North America. This immense volcanic event is generally attributed to a melting event in the upper mantle, related in some fashion to shallowing and resteepening of the subduction angle of oceanic lithosphere underthrust beneath the continent. To address what exactly produced the flare-up, we reexamined space-time-composition patterns in mid-Cenozoic magmatism in the Rocky Mountain region, using more than 5,500 individual rock chemical analyses now compiled in NAVDAT for rocks of this age. We divided the Rocky Mountain and northern Mexico regions into 15, 5° x 5° grid elements and interrogated volcanic rock ages and compositions from each. At this scale, the ignimbrite flare-up clearly occurs in two pulses: from 40-60 million years ago (Ma) north of about 45°N latitude and from 20-40 Ma to the south—in Colorado, New Mexico, west Texas, and northern Mexico. The chemical compositions of the mid-Cenozoic volcanic rocks, in contrast, vary little with latitude, but instead show longitudinal variations (Figure 1), from largely calc-alkaline in the west to alkaline (basically high sodium contents) in the east. These observations are at least consistent with the volcanic episodes being ultimately related to subduction of oceanic lithosphere, but with major west-east changes occurring in the mantle sources of the magmas parental to the volcanic rocks, potentially as a function of the dip angle of the underthrust oceanic lithosphere. We plan to continue to investigate these regional variations in volcanic rock compositions in the upcoming year, expanding our studies to comparing space-time-composition patterns in volcanism in the western United States and northern Mexico.
