Ejecta blocks around the Kings Bowl phreatomagmatic crater in Idaho: An indication of subsurface water amounts with implications for Mars

• Published in: Planetary and space science Vol. 222; p. 105564
• Main Authors: Sears, D.W.G., Sehlke, A., Hughes, S.S., Kobs-Nawotniak, S.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.11.2022
• Subjects: Ejecta blocks; Impact craters; Kings bowl; Phreatomagmatic craters; Kings bowl; MOLA; NASA; Phreatomagmatic craters; COTM; Ejecta blocks; DTM; NIH; Impact craters
• ISSN: 0032-0633; 1873-5088
• DOI: 10.1016/j.pss.2022.105564

The Kings Bowl lava field, associated with the Craters of the Moon National Monument and Preserve on the Great Rift in Idaho, U.S.A., contains many explosive pits, the largest of which has a substantial field of ejecta blocks. These features were formed when lava supply diminished, the fissure was evacuated, and ground water entered. We have documented the properties of the ejecta block field: crater shape, the ejecta field to the west and the tephra field to the east, the vesicularity and density of the blocks, the block size distribution and the number of blocks per unit area as a function of distance from the crater, and the aspect ratios of the blocks. To the degree possible we compare the data to those of ejecta blocks at other phreatomagmatic craters on rifts especially, Deception Island, South Shetland Islands, and White Island, New Zealand, and find them similar. Many of the Kings Bowl ejecta characteristics can be described by relationships that have previously been published to describe ejecta blocks around impact craters. We have also performed ballistic and energy calculations to derive ejection velocities for the blocks consistent with our data. The “velocity exponent” b in the relationship d ​∝ ​vi-b where d is ejecta size and vi is the initial ejecta velocity for our data is around −3.0 and in the range previously observed for impact ejecta (−3.7 to −0.32), although at the small end. Using the calculated ejection velocities (20–52 ​m/s) and the calculated total mass ejected (1.2 ​× ​108 ​kg), and correcting for the energy consumed in the fragmentation of the blocks, we calculate that the total energy of the explosion that produced the crater and the ejecta field is about 1012 ​J, roughly half used in fragmenting the blocks and half in their dispersal, and that about 4 ​× ​107 ​kg of water was involved in the explosion. Although subject to a number of assumptions, this method of calculating subsurface water from crater and ejecta block data could be used remotely to provide an indication of the amount of subsurface water in the vicinity of phreatomagmatic craters on Mars. •We document the ejecta block field at Kings Bowl phreatomagmatic crater Idaho.•The blocks obey many of the empirical equations for ejecta from impact craters.•We calculate ejection velocities (20–52 ​m/s) and mass ejected (1.2 ​× ​108 ​kg).•Thus the water/magma ratio was ∼0.30, which is near optimum for this process.•Subsurface water can be determined from ejecta blocks near such craters on Mars.