This topic attempts to explain what “bench”
craters are and shows some examples of them.
These craters are also called "concentric" craters.
See Concentric Craters - Wiki
The topic is locked in order to keep it short and relevant.
Thanks go to Irene Ant
for help with this topic
The following diagram shows how craters are formed and the fourth figure, labelled Concentric Crater
, is actually a bench crater. (Melosh, 1989, Fig 5.17)
On the Moon it’s common to have a weak layer of regolith overlaying a strong layer of bedrock. If an object is large enough and travelling fast enough it will excavate all the regolith in its path and break through the bedrock. This will form a crater with part of the bedrock forming a shelf or bench in the wall of the crater.
The image below shows the Bench Crater which the Apollo 12 astronauts visited. The line with arrows shows the path they took around the crater.Bench Crater from: wikispaces
If you find a bench crater or what you think is a bench crater please post it here: Bench Craters
Information about bench craters is hard to find! Searching for bench craters through Google (or other search sites) gives a lot of hits on the bench crater meteorite, found by the Apollo 12 astronauts near the crater called Bench Crater
, but not much information about how bench craters are formed.
A quote from the Nasa site documenting the Apollo 12 landing helps a bit:Any impacting projectile bigger than sand-grain size will eject material down to a depth equal to about one quarter the diameter of the crater the projectile digs. At mare sites, bedrock is covered by about five meters of regolith and, in principle, will be exposed in any crater bigger than about 20 meters. Bench Crater is younger than either Surveyor Crater or Head Crater and, in addition to the obvious boulders, the bench itself is evidence that the impact reached bedrock. This shelf, part way down the wall, marks the depth at which the Bench impactor first encountered rock rather than soil.
The most useful information was found in a PDF document from UCL (University College London) concerning planetary geology.
The following description is based mainly on this document (from www.apl.ucl.ac.uk/lectures/3c11/
):Planetary Geology Paper
(See page 7 for useful figures and description).
This Modeling Impact Cratering
is a useful article suggested by Irene Ant. Image Examples
The following examples of bench craters are taken from the Crater Questions
Not all of them are Bench craters - I hope to add further examples when I find them so please keep posting in the Craters thread: Crater is at left centre and is a 'Mound' type crater
Entry #47 - ElisabethB
Entry#53 - steen
# ID: AMZ400198q
# Latitude: -2.9391°
# Longitude: 336.415°
Entry#119 - jules
Crater at top centre is a 'flat-floored' type crater
Entry #138 - Geoff
Entry #146 - DJ_59