The following information is from the National Park Service web site.
Ancient Indians undoubtedly witnessed the eruption of Sunset Crater Volcano in A.D.1064-1065 which blanketed the region with black cinder. Today the volcano's rim of red cinders and the lava flows near the cone seem to have cooled and hardened to a jagged surface only yesterday. Squeeze-ups and hornitos are just two of the fascinating volcanic features you'll encounter while exploring the park. To protect this fragile resource, Sunset Crater Volcano is closed to climbing and hiking. However, other cinder cones in the area may be climbed.
The cones and lava flows of the San Francisco volcanic field, which covers about 2,000 square miles of the southwestern Colorado Plateau, result from several million years of volcanic activity. These powerful underground forces changed the landscape dramatically beginning in the winter of AD 1064-65.
Sunset Crater appeared when molten rock sprayed out of a crack in the ground high into the air, solidified, then fell to earth as large bombs or smaller cinders. As periodic eruptions continued over the next 200 years, the heavier debris accumulated around the vent creating a 1,000-foot cone. The lightest, smallest particles blew the farthest, dusting 800 square miles of northern Arizona with ash. Perhaps as spectacular as the original pyrotechnics were two subsequent lava flows: the Kana-A flow in 1064 and the Bonito flow in 1180. They destroyed all living things in their paths.
The processes that created Sunset Crater also created a sculpture garden of extraordinary forms at its base. As new gas vents opened suddenly, spatter cones sprouted from the ground like miniatures of the cone itself. Moving lava developed a crust on the surface where it cooled; caves beneath drained away. Partially cooled lava, pushing through cracks like toothpaste from a tube, solidified into wedge-shaped squeeze-ups, grooved from scraping against the harder rock.
In a final burst of activity, around 1250, lava containing iron and sulfur shot out of the vent. The red and yellow oxidized particles fell back onto the rim as a permanent "sunset" so bright that the cone appears still to glow from intense volcanic heat.
To recieve credit for a find you need to take a picture of you and your GPS infront of the ice cave, and sign the log book at the visitor center