Agathla Peak is a remnant of an ancient volcanic structure emplaced during the Oligocene about 25 million years ago. All that remains is a severely eroded volcanic plug or diatreme comprising brecciated volcanic rock. The composition of Agathla Peak is minette, a rare igneous rock rich in biotite and orthoclase (potassium feldspar).
Semken, S.Black Rocks Protruding Up: The Navajo Volcanic Field. New Mexico Geological Society Guidebook, 54th Field Conference,
Geology of the Zuni Plateau , 2003, p. 133-138. http://semken.asu.edu/pubs/semken03_nvf.pdf