File:Basaltic spatter (Bonito Lava Flow, upper Holocene; near Sunset Crater, San Francisco Volcanic Field, Arizona, USA) (49130381812).jpg

Basaltic spatter in the Holocene of Arizona, USA.

This is an accumulation of alkaline olivine basalt spatter erupted from a spatter cone near Sunset Crater, which is itself 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.

The material seen in photo was erupted from a spatter cone atop 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. In places, lava was squeezed upward through fissures in the solidified flow top. At other sites, lava erupted from spatter cones. Also known as "hornitos", spatter cones are small to very small, steep-sided volcanic cones formed by the accumulation and solidification of blobs of lava that sputtered from a vent. They are usually basaltic in composition.