Abstract

An unusual structure in a basalt flow near Wagon Mound, New Mexico, consists of large, polyhedral blocks, each having a fresh rim or shell and a spheroidally weathered interior. These blocks have features attributable to both columnar jointing and spheroidal weathering and are termed columnar-spheroidal structures.

It is concluded that meteoric water has interacted with the basalt to produce the rims, as evidenced by (1) small clastic dikes emanating from water-laid ash and surrounding each block, (2) a decrease in the ferric-ferrous ratio of the basalt from the rim to the core of each block, and (3) a change in the alteration of olivine from iddingsite in the rim to serpentine in the core. The influence of meteoric water appears to have been to increase the oxygen fugacity in the rock adjacent to fractures through which superheated steam was escaping to the surface from below. These areas of the rock later became the rims of each block. In contrast, volatiles within the remainder of the rock, away from the fractures, were buffered by the rock and were at a much lower oxygen fugacity. Varying susceptibility of the secondary minerals formed during this stage of cooling to later chemical weathering resulted in the present structure.

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