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The San Francisco Volcanic Field, located in northeastern Arizona, is host to over 600 volcanoes. These volcanoes began erupting approximately 6 million years ago in the western portion of the field and through time, the locus of activity has migrated eastward. Eruptive products range from basalt to rhyolite, with basalt dominant. Pleistocene vents include Merriam Crater and two associated cinder cones as well as The Sproul, a spatter rampart. One, or several, of these vents produced the Grand Falls flow which spilled over into the Little Colorado River gorge and flowed both up and downstream. Lava filled the canyon producing a dam and continued to flow ~ 1 km beyond the eastern rim. This changed the course of the river creating the waterfall at Grand Falls. Quaternary volcanism began as a fissure eruption that culminated with the building of Sunset Crater cinder cone. The eruption, which produced a blanket of tephra and two lava flows, was most certainly witnessed by the ancestors of the Pueblo Indians and had a dramatic impact on their lives. The eruption may have caused a shift in population to places such as Wupatki, 30 km to the north, where farming in the arid climate may have been temporarily enhanced by a thin layer of ash that acted as a water-retaining mulch.

Melts that produced these dominantly basaltic cinder cones were derived by variable amounts of partial melting of an oceanic island basalt–like mantle source that underwent differing degrees of contamination from the lower crust. Subsequent fractional crystallization of olivine ° clinopyroxene further modified these melts. Discrete packets of these melts ascended rapidly to produce short-lived volcanic events in the eastern San Francisco Volcanic Field.

The purpose of this field trip is to examine these young cinder cones and their eruptive products in an effort to understand the origin of the eruptions as well as the effects they had on the physiography and native inhabitants of the area.

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