File:Fumarole-oxidized scoria (upper Holocene; west of Sunset Crater, San Francisco Volcanic Field, Arizona, USA) (49135613312).jpg

Oxidized scoria in the Holocene of Arizona, USA.

This is a small mound of airfall scoria that erupted from Sunset Crater, a cinder cone volcano 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.

The surface scoria deposit seen here partially covers the Bonito Lava Flow, which occurs on the western & northwestern sides 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 reddish coloration is from oxidation by post-eruption fumarole activity. Fumaroles are hot steam vents.