Erosion of Ejecta at Meteor Crater, Arizona

New methods for estimating erosion at Meteor Crater, Arizona, indicate that continuous ejecta deposits beyond 1/4-1/2 crater radii from the rim (0.25R-0.5R) have been lowered less than 1 m on the average. This conclusion is based on the results of two approaches: coarsening ofunweathered ejectainto surface lag deposits and calculation of the sediment budget within a drainage basin on the ejecta. Preserved ejecta morphologies beneath thin alluvium revealed by ground-penetrating radar provide qualitative support for the derived estimates. Although slightly greater erosion (2-3 m) of less resistant ejecta locally has occurred, such deposits were limited in extent, particularly beyond 0.25R-0.5R from the present rim. Subtle but preserved primary ejecta features (e.g., distal ejecta lobes and blocks) further support our estimate of minimal erosion of ejecta since the crater formed --50,000 years ago. Unconsolidated deposits formed during other sudden extreme events (e.g., landslides) exhibit similarly low erosion over the same time frame; the common factor is the presence of large fragments or large fragments in a matrix of finer debris. At Meteor Crater, fluvial and eolian processes remove surrounding fines leaving behind a surface lag of coarse-grained ejecta fragments that armor surfaces and slow vertical lowering.