The Mineralogical Society of Southern California

Program Notes: Friday, February 14, 2025  at 7:30, join us for “Plutonic Orbicular Rocks of New Mexico”  Presented by Joan Karrie

My presentation, is a mostly non-technical discussion and comparison of the two occurrences of plutonic orbicular rocks in New Mexico attempting to answer the question “Do the New Mexico orbicular rocks fit the classical definition of an orbicular rock, and can we see this in hand sample?”

The three small outcrops in the Sandia Mountains of central New Mexico appear to conform to the classical definition of orbicular rocks. That is, they are coarse-grained, igneous rocks with concentrically layered, spheroidal structures composed mostly of feldspars and mafic minerals and are formed by rapid nucleation in a magma.  Classical plutonic orbicular rocks occur around the world, including in California; the most spectacular occurrence is at Mount Magma, Australia. 

The three small outcrops in the Zuni Mountains in west-central New Mexico are very different in appearance having none of the characteristics of the classic orbicular rock, except that they contain spheroidal structures.

A discussion of the formation of the Sandia Mountains precedes visual inspection of Sandia Mountain hand samples. The most widely accepted theory of the petrogenesis of classic orbicular rocks is presented in a non-technical format.  Specific features of the Sandia Mountains samples are used to verify that the Sandia samples show evidence of being classic plutonic orbicular rocks.

A discussion of the formation of the Zuni Mountains then precedes a visual description of the theory of formation of these orbicular rocks. Specific features of the Zuni Mountain samples are then shown and agreements between the samples and the theory of formation are discussed to propose that the Zuni samples show evidence of the theory proposed.

A comparison of the Sandia and Zuni specimens vs. the classical definition of plutonic orbicular rocks verifies that while the Sandia specimens fit that definition, the Zuni rocks are of very different origin.

Joan Karrie is a self-proclaimed geology enthusiast.  She has a bachelor’s degree in geology from Western Washington University in Washington state and has taken graduate-level courses in geochemistry, thermodynamics, and petrogenesis at the University of Arizona-Tucson.  Although employed as a database designer and project manager for most of her career, she never lost the initial fascination with the how and why of geological processes.  She especially enjoys translating Geology into English and bringing geological process to life for the non-geologist.  She has presented geological topics to audiences ranging from the New Mexico State Garden Club’s Annual Convention; the New Mexico Mineral Symposium in Socorro; her home club, the Albuquerque Gem and Mineral Society (AGMS); and at OASIS, a 50+ continuing education nonprofit.  She is currently the Secretary of AGMS

SPECIAL ZOOM MEETING NOTICE

MSSC members will be automatically on the invite list each month.

Non Members, you must request to attend the MSSC zoom meeting every month.  Email our Speakers chair, Carolyn Seitz at speakers@mineralsocal.org  no later than the Thursday before the scheduled meeting   Please include “ZOOM Meeting” in the subject line of your response. We will send you the meeting link.

Upcoming Events

all events, except field trips are via  ZOOM