File:Robinson Mountain & O'Leary Peak & Bonito Lava Flow (San Francisco Volcanic Field, Arizona, USA) (49126290441).jpg

Alkaline olivine basalt in the Holocene of Arizona, USA.

In the foreground is a Holocene basaltic lava flow erupted from the base of Sunset Crater, a cinder cone in Arizona's San Francisco Volcanic Field. Cinder cones are relatively small, steep-sided cones of loose igneous debris. They have few eruptions and a relatively large summit crater. Cinder cones are principally composed of scoria and vesicular basalt.

More than 600 eruptive centers - mostly cinder cones - are present in the volcanic field, and date from the Late Miocene to the late Holocene. The largest volcano in the area is San Francisco Mountain, an eroded stratovolcano. Cinder cones in the field have erupted a variety of igneous products, ranging from mafic to felsic to alkaline in composition - most are alkaline basalt.

Sunset Crater had an eruption in the late 11th century A.D., variously dated to 1064 or in the 1080s. The eruption involved lava flows and relatively widespread deposition of airfall scoria. This was the most recent activity in the entire San Francisco Volcanic Field.

This is the Bonito Lava Flow, which occurs to the west & northwest of Sunset Crater itself. The flow covers an area about 2 to 2.5 kilometers in diameter and ranges in thickness from ~1 to over 30 meters. Flow top morphology ranges from pahoehoe to aa, but is principally slabby pahoehoe to slabby aa.

The background hill at right is O'Leary Peak, a volcanic dome reportedly composed of Pleistocene-aged porphyritic rhyodacite. The lower hill in the left background is Robison Mountain, a Pleistocene cinder cone.