North Crater Rafted Blocks at Craters of the Moon National Monument
North Crater cinder cone has of the more unique histories of the more than 25 cinder cones of the Craters of the Moon Lava Field. Five lava flows erupted from or near its base. Three of those flows tore apart North Crater and carried pieces of the cone more than 11 kilometers (nearly 7 miles) to the northeast.
Cinder cones form when loose rock fragments erupt from a central vent and pile up to create small, often circular, volcanoes. Some rock fragments are so hot they weld together when they land. Rock fragments called cinders make up most of North Crater, but ash, volcanic bombs, and taffy-like spatter are also common. Cinders and ash form when hot magma, highly charged with dissolved gas, rises to the surface and erupts explosively, shooting magma into the air. The sudden release in pressure allows gases in the magma to expand, creating tiny gas bubbles in the rock called vesicles. Not as highly charged with gas, spatter did not expand as it boiled from the vent. Flying through the air and crashing to the ground often gives large volcanic bombs weird and contorted shapes. The very light and loose cinders on North Crater contrast with the heavy and solid accumulations of taffy-like spatter and volcanic bombs also present.
In one explanation for the large hole in the northwest side of North Crater, the Great Rift cracked open the cone. Thick, pasty magma then easily pushed apart the weakened cone walls and floated away large pieces of the cone just like rafts floating down rivers. As these rafted blocks floated away, they shed loose, unconsolidated cinders and ash into the lava flow transporting them. Tremendous stresses caused the rafted blocks to break up into smaller pieces and mix into the upper portions of the transporting flow. Later, more lava flows buried many of the rafted blocks. Today, some of the large resistant blocks that did not break apart as they rafted, poke up through the younger flows. Enough unburied rafted blocks exist to suggest North Crater may have been 40 m (131 ft) taller and 200 m (656 ft) wider at the base before rafting events carried away portions of the cone. This North Crater would include the existing North Crater and occupy the area between the current North Crater and the highway, visitor center, and campground. Alternatively, several smaller North Craters, or cousins of North Crater, may have also existed throughout time and been partially rafted away and later rebuilt.
For further information on rafted blocks and cinder cones see:
http://volcanoes.usgs.gov/Products/Pglossary/pglossary.html (A photo glossary of volcanic terms compiled by the USGS.)
Foshag, W.F., and Gonazalez, J.R., 1956, Birth and Development of Parícutin Volcano, Mexico: U.S. Geological Survey Bulletin 965-D, p. 355-489.
Harwood, D.R., 1989, Cinder cone breaching events at Strawberry and O'Neill Craters, San Francisco Volcanic Field, Arizona: Masters Thesis, Northern Arizona University.
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