Why is does this rock look so unusual?

October 16, 2008 2:11 PM

Why does this rock have so many stress lines and pits on it?

I found this very unusual rock in New Hampshire. What could have caused it to look like this? Also note, it is non-magnetic and about two feet across.
Image

posted by Java_Man to science & nature (14 answers total)

It's sandstone from when the area was a seabed. Sandstone is a sedimentary rock and the holes could have been caused by organic matter that disintegrated or air pockets before the pressure created the sandstone.
posted by JJ86 at 2:21 PM on October 16, 2008

Some of those "stress lines" are actually intrusions (dikes) of igneous rock.
posted by Cool Papa Bell at 2:24 PM on October 16, 2008

It looks like the sandstone was deposited eiher on top or below something more resistant... I can't tell in the photo, but probably a more silicious (more quartz) sandstone, or one with a finer grain size. The holes are just weathering features that are exploiting weaknesses that were already in the rock.

The stress lines could be quartz veins, which began as small cracks in the rock underground which then had several thousands of litres of fluid forced through it. This fluid would have precipitated quartz out onto the edges of these cracks. They look too fine and regular to be dikes to me, but it's entirely possible.
posted by twirlypen at 2:31 PM on October 16, 2008

Yeah, those aren't "stress lines." it looks like quartz, so i don't think it's igneous intrusions, probably hydrothermal. (from what i can see in the picture) when a hot fluid (water with stuff dissolved in it) flows through cracks in a rock, eventually the 'stuff' (in this case, silicates, which come out of solution easily) precipitates out and forms crystals that fill the crack. The holes are from weathering (something was there, and now it's not. either organic matter, or a larger pebble that has been worn away or popped out). although my Sedimentology and Stratigraphy class was about 5 years ago, so it's been a while.
posted by Green Eyed Monster at 2:35 PM on October 16, 2008

Interesting, I didn't know dikes could form that way ...
posted by Cool Papa Bell at 3:20 PM on October 16, 2008

Ms. Zamboni (a planetary geologist) says:

The fact that there are three straight parallel lines with two (also straight) parallel lines crossing them at a 30-60 degree angle indicates to me that these are systematic joints (possibly what you're referring to as stress fractures). Such joints are often filled in later by precipitated minerals. The precipitated mineral(s) in this case could conceivably be quartz, but silica only dissolves in very hot water, and we're looking at sandstone, which forms under low temperature and low pressure conditions, and this sample doesn't show evidence of metamorphism. I wouldn't be surprised if the veins are something like calcite. There is no good way to tell what they're made of from these images.

I agree that the holes are from weathering. They look to me like they could either be from removed pebbles or from water dripping onto those areas for a (very) long time. The fact that there is what looks like a layer that does not have holes points to the removed pebbles hypothesis. You'd get alternating layers of large sediments (sand + pebbles) and small sediments (no pebbles) as the shoreline moved back and forth.

Do you have a higher-res image?
posted by zamboni at 3:30 PM on October 16, 2008

Here are some more pictures of the rock. Thanks, everyone, for all of the help!
posted by Java_Man at 3:43 PM on October 16, 2008

I agree that the pits you are seeing are just surface weathering. I don't think there had to be pebbles or voids from organic matter. It doesn't seem to me to be such an odd weathering pattern. I also agree that the fine lines are veins of more resistant material that formed from the precipitation of minerals from a fluid that was following fractures in the rock. I cannot tell from the picture much else. Geology is best done with hand samples and not pictures. I am a geology grad student and honestly I cannot tell whether I am looking at an igneous or a sedimentary rock there, although the people above who say it is a sandstone are likely correct.
posted by DanielDManiel at 3:45 PM on October 16, 2008

Damn, i didn't preview. Unfortunately the new pictures don't help too much more, but zamboni's observation that there does seem to be a band through the middle that is hole free certainly does help the pebble hypothesis that I was dismissive of at first. It bothers me that you don't see any pebbles still un-plucked but they could have been of a different composition and just dissolved out. Still, I want to take a rock hammer to the damn thing and get a fresh surface to look at the grains and mineral composition. Now I know how frustrating it must be for geologist working on Mars exploration stuff who have to work largely from just pictures.
posted by DanielDManiel at 3:57 PM on October 16, 2008

Looking at the new photos...

it looks like the near-vertical line leaning to the right that separates the pockmarked side from the relatively smooth side is not a vein but a depositional feature. Is is made of coarse grains that you can see? As opposed to the other lines, which I imagine are very fine grained material that just appears to be a smooth surface? I'd say this line is due to a period of deposition of larger grains, in between one period of deposition which gave the smooth portion, and a later (or earlier, depending on which way is up) period which deposited the weaker layer that has now been weathered.

The other sets of lines, which are at 60 and 30 degrees to each other, are your veins.
posted by twirlypen at 4:45 PM on October 16, 2008

Looks like limestone or dolostone with dissolution dimpling.

Google "karst dissolution " in the image section and you should find examples like yours.I see that feature in many karst areas and caves.
posted by plumberonkarst at 6:45 PM on October 16, 2008

More from Ms. Zamboni:

Now I know how frustrating it must be for geologist working on Mars exploration stuff who have to work largely from just pictures.

We work with way more than just pictures! For in-situ studies (rovers and landers), we have, depending on the spacecraft, instruments that give us elemental chemistry, gas chromatography, mineralogy, iron mineralogy, grain and dust magnetization, 3D images with 20/20 vision, microscopic images, wet and dry chemistry labs...Spirit and Opportunity even have drills that are their equivalent of rockhammers, and the upcoming Mars Science Laboratory is going to be able to do X-ray diffraction. Some people, myself included, work with a combination of these, while others focus on specific instruments (generally spectrometers). The rovers' main cameras, which take most of the pictures that get publicized, actually have 13 visual and near-IR filters that were chosen to make this instrument particularly useful for using spectroscopy to investigate how the mineralogy varies across entire scenes. Orbiters carry full instrument suites as well -- visual, near-IR, thermal IR, gamma ray spectroscopy, altimeters, magnetometers, mapping the planet's gravitational field, etc.

As for this sample, it looks to me like the grain size is probably too large for it to be limestone, but the image isn't as crisp as I'd like to identify the type of rock. Now if we had some thermal infrared spectra...

I'd also say that it's conceivable that the holes are a result of water dripping onto the stone for an extended period of time. It doesn't seem terribly likely though, especially because of that unpitted region.
posted by zamboni at 9:53 PM on October 16, 2008 [1 favorite]

I can take some macro shots of it this weekend or try to chip at it, if that can help. Thanks again, everybody. Let me know if there is anything in particular that I should get in the shots. Unfortunately, I don't have a thermal infrared spectrometer handy. ;)
posted by Java_Man at 10:18 AM on October 17, 2008

I should also mention that this rock was found about 40 miles from the ocean, if that means anything.
posted by Java_Man at 11:26 AM on October 17, 2008