SAN FRANCISCO MOUNTAIN—PAST AND PRESENT
It was during the last great era in geological history, and for the most part after the Colorado River had already started to cut the Grand Canyon, that molten masses broke through and flowed out on the northern Arizona plateau. Three great periods of such volcanic activity are represented by the rocks of this region and they range in age from probably at least a million to relatively recent years.
The first general period of eruption in the San Francisco Volcanic Field was characterized by a predominance of lava flows which formed a black rock known as basalt. These flows had an average thickness of about 50 feet and covered an area of 3,000 square miles. Accompanying them was the formation of about a hundred small cinder cones.
It was during the second period of volcanism that six isolated cones of large size and a somewhat greater number of small cones were formed by the eruption of lavas widely ranging in composition. San Francisco Peak, the largest of these cones and a dominant feature of the region reaches an elevation of 12,611 feet above sea level, or about 5,000 feet above the plateau surface. It is composed of five different types, of lava—mostly red or light colored—which represent a corresponding number of distinct stages in eruption. Since the termination of its building-up, the crest of this cone has been eroded and worn away to the extent of 3,000 feet as estimated from restored cross-sections. Other large volcanic craters of this period are Bill Williams (9,090 feet), Kendrick (10,418 feet), O’Leary (8,925 feet) and Sitgreaves (9,240 feet).
Two hundred small cones and about twenty cubic miles of lava were produced in this same general region during a third and relatively recent period of volcanic activity. Much of this material overlies that of the two preceding periods. Probably the most interesting feature of these cones and flows is their age, at least one cone being so recent that ash from it buried numerous pithouses built by Pueblo Indians during the eleventh century, A. D.
SAN FRANCISCO MOUNTAIN