Raising an impact in Africa

By Phil Plait | December 22, 2010 9:38 am

When you look at the Moon, you see a surface covered in craters. Yet the Earth, which is bigger and has more gravity — and therefore, you’d think, be hit more often than the Moon — hardly has any craters!

The difference, of course, is that the Earth has weather and tectonic activity. Craters erode, and over time go away*. But not all of them do. Some are so big they take hundreds of millions of years to erode, while others are in dry climates where erosion is limited… like, say, in Algeria. Where the Tin Bider crater lies!

tinbider

This picture, from the Earth Observatory-1, shows the roughly 6 km (4 mile) wide crater, located in the high desert of northern Africa. It has a complicated terraced structure, indicating that the rock inside may have slumped after impact — a common feature in larger craters. It has undergone some erosion, too… not surprising, given its age of about 70 million years!

tinbider_flippedThere’s an interesting thing about this crater. North is up in the picture, so the sunlight is coming from the south, from the bottom of the picture. Regular readers of this blog know that this induces the well-known crater/dome illusion (another example can be found here). Our brains expect light to come from above, so when it comes from below the shadows send mixed signals to our brains, and we interpret craters as domes and vice-versa.

This illusion plays into this crater, because it’s actually raised above the surrounding plains! It’s still a crater, but it sits up higher than the desert around it. In the smaller picture above I flipped the picture over, and you may see that the crater now looks like it’s above the plains. I know some people have a hard time seeing this illusion, so for proof here is a topographical map of the region, from Earth Impact Database:

tinbider_topo

See? Blue represents low regions, red high. The crater is higher up! I’m thinking the area of the crater may have been uplifted over time, raising it above everything around it. Weird, huh? They also have an oblique 3D view that again shows it raised up. Amazing!

As always, it goes to show that you can’t always trust your eyes. It also shows me that there are interesting features on our own planet I hadn’t even heard of. But that, to me, is good news. There’s so much left to see and explore!

Image credits:EO-1 image created by Jesse Allen, using EO-1 ALI data provided courtesy of the NASA EO-1 team. Topographic image provided by Dr. Carlos Roberto de Souza Filho for the EID.



*
We also have an atmosphere which keeps anything smaller than a hundred meters across from hitting us, but those make craters on the order of a kilometer or two in diameter; it takes a decent telescope to even see such craters on the Moon.


Related posts:

- More Mars caves found
- Google Moon
- Radar love
- A lunar illusion you’ll flip over


Comments (37)

  1. Grizzly

    Uhhh. I have a reverse crater/dome effect, because it looks like a creter to me in the first picture and a dome in the second. I’m doomed, doomed I tell you.

  2. Chris

    It’s located at (27.601944,5.112222) GPS coordinates, for anyone wanting to take a trip out there or look on google maps.

  3. Grizzly:

    I have a reverse crater/dome effect, because it looks like a creter to me in the first picture and a dome in the second.

    Re-read the article. It is a dome, and the effect does make the first image look like a crater. Quoting BA:

    This illusion plays into this crater, because it’s actually raised above the surrounding plains.

  4. Oli

    @1. Grizzly: That’s supposed to happen…

  5. Martin

    Is it possible that the impact compacted the earth around it and prevented the area around the crater from eroding as fast as the rest of the region, causing the crater to be higher. Like the column in great deserts that are from another type of rocks or with a layer of waterproof rock above them, protecting them.

  6. dan

    @Martin

    That was my first thought as well. The heat from the impact could have fused the desert sand into rock. The sand blows away over time leaving the crater behind. If this is true, I wonder if there are other examples that have been covered by blowing sands over time, waiting to be discovered.

  7. Mike

    “We also have an atmosphere which keeps anything smaller than a hundred meters across from hitting us” <– if that's true, then why did you argue, in your review/debunking of movie myths, that blowing up a huge asteroid into millions of little pieces wouldn't do any good? Like they did in those blockbuster movies a few years back.

  8. Jason Dick

    I was thinking along the same lines as Martin, though it would still require some sort of uplift of the area in general for erosion to be a significant factor, or for the impact to have occured in an already higher-than-typical area of Mars.

  9. Mike (8): Good question! It’s because while something smaller than roughly 100 meters across (depending on what it’s made of and other factors) won’t hit the ground, it still explodes in the air, dumping all that vast energy (many megatons!) into the air. For a single object that’s bad but not global. But for an asteroid a kilometer across breaking up, you get 1000 such 100 meter rocks breaking up and exploding! That’s a HUGE amount of energy, so it’s best to not let it hit us at all.

    Mind you, for an asteroid 10 km across you get a million 100-meter pieces. Volume goes up as radius cubed, so things get evil pretty darn fast.

  10. BargeArse

    For another spectacular crater, look up ‘Gosses Bluff Range, Australia’ on Google Maps. This is rather cool as it is quite old and has had about an estimated 3km thickness of overlying sediment eroded since impact (a very long time for that to happen for continental shield). You can also see the magnificent folding of the West McDonnell ranges surrounding the crater (which is now an aboriginal reserve), which is on the drive out to Uluru (or Ayers Rock) from Alice Springs.

  11. mike burkhart

    One mor reasion you don’t see craters on Earth is because you are standing on the Earths surface and the craters are miles long . If you went to the Moon and stood on its surface you would not see the craters on the Moon. Also if a Asteroid were going to collide with the Earth it might be possable to use nukes to alter its orbit pushing it away form Earth. And Merry Chirstmas everone.

  12. Joseph G

    @#8: IIRC, a KT-sized asteroid asplodes with enough force to launch many pieces of it back into space, which then rain down around the globe, and all that kinetic energy is radiated away as heat they burn up. Basically, the whole darn planet got broiled. Phil’s kickass book goes into much greater detail, of course (no, I’m not a shameless suck-up. Really. Oh, hush.)
    So if enough large-but-not-large-enough-to-hit-the-ground meteors hit us at once, we’d probably wish it was just one big rock.

    One of my fondest childhood memories was visiting Meteor Crater in Arizona. As a kid, it really captured my imagination. Pictures really don’t do it justice. Incidentally, I have an image of the crater today on one of my credit cards. I guess I thought that “giant hole in the ground” was apropos of my financial situation :D

  13. Joseph G

    I’m remembering… On that same trip, we also visited the Kitt Peak Observatory. I think I was 8 or 9 years old. If I was somewhat interested in space before, that trip sealed the deal!
    I can still remember us walking up the steep hill toward the telescope building. The wind was so cold my fingers felt frozen solid, but I was really excited. Below us, Arizona stretched out to the horizon – it was hard to believe that we were in the desert. At the telescope… it’s hard to describe the feelings without sounding melodramatic. I felt like I was in a holy place. That building sequestered on a remote mountain, like a monastery, vaulting majestically into the heavens like a cathedral. Here, people were gazing into the depths of the universe, trying to determine the ultimate answers to the ultimate questions. It was awe-inspiring!

  14. Tom K.

    Fascinating! How did they get the Federation of Planets Emblem in the middle of it? Maybe a giant communicator?

  15. Tribeca Mike

    Erosion? Typical liberal pro-science flapdoodle! There ain’t no way in Heck that immense craters could have been swept away by the wind in a mere 6,000 years, unless you take into account the biblical flood, of course.

    (Oh, and thanks for the fascinating info and photos.)

  16. @6 Martin: Yep, that’s what I was thinking. I often see this in my research on martian craters, and this is the working hypothesis for why you get “pedestal” craters on Mars where you have the crater and its ejecta bound by a cliff that can fall over 1 km to the surrounding surface.

  17. MGary

    I think you meant a *topographical* map, not a topological map, which would be a creature of a different nature entirely.

  18. MGary (19): Ha! You can take the scientist out of math class… Thanks, I fixed it. :)

  19. Naomi

    Could it be something similar to what we see on Mars with inverted channels? The processes of forming the crater changes the composition of the rock in the heat and pressure of the impact (whereas on Mars, it’s sedimentary rock having a different composition to the surrounding stuff), and the surroundings get eroded preferentially.

    In this case, the surrounding sand is blown away, but things like the breccia from the impact remains.

    Edit: Oops, and that’s pretty much what comment six says. Late to the party again!

  20. XPT

    Very nice! In fact I was sure it was a depression in the first photo but couldn’t retain this perception in the second. I’m still having trouble seeing it raised in the larger one!

  21. Dunc

    The difference, of course, is that the Earth has weather and tectonic activity.

    And oceans. Don’t forget the oceans!

  22. Donovan

    Is it just me, or does that upwelling, together with the crater, look like a dragon eating a hot pocket?

    As usual, Phil, it’s awesome to schooled!

  23. Dr. Hovind

    “I’m thinking the area of the crater may have been uplifted over time, raising it above everything around it. Weird, huh?”

    Oh. give up Phil. There has never been any clearer proof than this phenomenon of the global flood.

  24. Jon Hanford

    @7

    “I wonder if there are other examples that have been covered by blowing sands over time, waiting to be discovered.”

    In 2003, a large double crater (Arkenu 1 & 2) was discovered partially buried in the Libyan desert. Radar imaging from a Japanese satellite was used in the discovery: http://www.thelivingmoon.com/43ancients/02files/Earth_Images_08.html#Arkenu

    (The craters are situated in a hazardous region of the Libyan desert close to WWII minefields)

  25. WJM

    Uhhh. I have a reverse crater/dome effect, because it looks like a creter to me in the first picture and a dome in the second. I’m doomed, doomed I tell you.

    Or are you domed, domed?

  26. psuedonymous

    The ‘illusion’ is reversed for me too. The first image appears to be a crater on top of a plateau, whereas the ‘flipped’ image looks like a dome in a basin.

  27. Buzz Parsec

    In the 3-D view, it looks like half the rim has completely eroded away, and left a channel instead. Is the rim softer than the crater floor, being a pile of ejecta? Then during a time when the local climate was much wetter, a river may have eroded the rim but left the crater floor. (There does appear to be what might be a dry river bed passing by the southwestern edge of the crater in the main picture.)

  28. Joseph G

    It does look quite Martian…

  29. Gary Ansorge

    Hmm, four miles wide, so about 16 times wider than Meteor Crater in Arizona.
    70 million years ago that area was probably green and well occupied by critters. Just off the top of my head, I’ll bet for a hundred mile radius around the impact zone, there was a lot of well cooked meat.

    The asteroid that made a really bad day for the dinosaurs was half again as wide as this crater. So many big impacts in such a short time frame. Hopefully, clusters such as these won’t occur as often today as they did then(or be as big). (Have we “cleared” our local orbit yet?).

    It would be way cool if earth captured a large asteroid in geosynchronous orbit. I wonder how large it would have to be to be broken up by tidal stresses? Rings, RINGS I say, for lovers and near earth (asteroid) miners.

    Merry Xmas, Y’All.

    Peace.

    Gary 7

  30. Joseph G

    @Gary Ansorge: Merry Christmas/Spacemas to y’all, too :)
    That’s an interesting question (re tidal forces). I’m sure that someone sufficiently skilled in math could ferret out the forces involved.

    IIRC, the anime “Cowboy Bebop” took place in a universe where the mother of all industrial accidents had essentially blown up an enormous chunk of the moon and showered the earth with fragments to the point that it became pretty much uninhabitable*. I believe the earth had rings of moon debris, too.

    *This actually resolves an important realism issue with a lot of sci-fi that involves space colonization, being “Why the heck did people spend all that money to colonize the rest of the solar system, and in a relatively short time (a hundred years or so)?”
    Let’s face it, we could colonize mars or the moon NOW if we really wanted to. But it’s an unfathomably huge chunk of change with very little payoff other then the coolness factor, as things stand now.

  31. Gary Ansorge

    32. Joseph G

    “But it’s an unfathomably huge chunk of change with very little payoff other then the coolness factor, as things stand now”

    “Now”,,,ummm, yeah, but, it won’t always be like “now”. When Gerard O’Neille first postulated Power Sats in the ’70s, he used the figure $100 billion to build the first (10 GigaWatt) sat and $ 10 billion for each succeeding one. That was predicated on the figures NASA predicted for space shuttle launch costs. (and we know how well THAT went). These days, I’d probably be inclined to wait on either nuc powered primary launchers(the Nuc Light Bulb), Space tethers or mag launchers(like those from Launch Point Technologies). That reduces the launch costs to more reasonable levels(around$ 100 to $200/lb). The big thing about Power Sats is the 24 hr solar availability and higher intensities(1350 watts/m^2 vs 400 on earths equator at noon).

    ,,,but the power is just an economic justification that provides a rationale for other kinds of space development. Once you have the installed infrastructure and a primary economic driver, the rest follows.

    Gary 7

    PS I too, really enjoyed Cowboy Bebop. Reminded me a little of the Firefly universe.

  32. Joseph G

    @Gary Ansorge: Thanks, I now have a list of stuff to Google today ;)
    Cowboy Bebop did indeed rule. In some ways it was much “harder” sci-fi then a lot of anime (I’m thinking of giant humanoid robots here). That and Firefly both – why oh why do the good shows get cancelled??

    And Merry Belated Christmas to anyone who celebrates it!

  33. Keith Bowden

    I think it’s time to blow this scene, get everybody and the stuff together.
    Okay 3, 2, 1 Let’s Jam

    Okay, time to watch Bebop again. Crank up the surrounds! (Now if I can only remember between which episodes the movie fall in…)

    I’m another one with a general reverse-perception on the crater/dome illusion; it always looks like a dome when the light comes from above rather than below.

  34. Joseph G

    @35: Ah yes, the music – yet another reason that show kicked quite a lot of posterior.

  35. Daniel W

    Well, it’s just a bit after post but it’s linked to day’s entry…

    Pretty neat illusion! @ Keith 35: Bebop FTW! Music is indeed epic!

    About the fact that it’s domed rather than a crater, I suspect it may actually be an erosional feature (maybe with regional uplift however) – it’s quite possible the heat/shock of impact could fuse the rock, making it more resistant to erosion. Thats similar to why you see tall, thin, ancient volcanic necks occasionally poking out of the plains, the basalt is much more resistant to erosion than relatively loose ash that made up the mountain.

    Thanks for the awesome blog!

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