Calling all geologists! Help us identify a mystery rock.

November 12, 2011 6:09 PM

Need help with a unique rock's identification / formation?

My nine year old son loves geology so we spend a lot of time exploring the DFW area for interesting rocks. Today, we came across this in a field in Tarrant County. From a distance it looked like petrified wood because of the organic-looking patterns. Based on the photo provided, can anybody help us identify this stone and explain its unusual features?

posted by punkfloyd to science & nature (13 answers total)

It could be sandstone.

According to Wikipedia, "Texas is approximately bisected by a series of faults that trend southwest to northeast . . . South and east of these faults, the surface exposures consist mostly of Cenozoic sandstone and shale strata . . . "

Here's some sandstone in Zion Nat'l Park, and some sandstone in Texas that looks pretty similar, I think, to the rock you saw.
posted by cp311 at 6:29 PM on November 12, 2011 [1 favorite]

It looks a bit red for sandstone to me. But if you can break chunks of it off fairly easily with your fingers, it probably is sandstone.
posted by zug at 7:44 PM on November 12, 2011 [1 favorite]

I suspect it might be hematite, an iron ore. That is one of its colors, and it is occasionally present in a flaky, "micaceous" habit.
posted by Nomyte at 7:56 PM on November 12, 2011 [1 favorite]

If it was sort of granular and fragile, it could be red sandstone. It could also be red chert. I think that there's some hematite in there, too.
posted by Ostara at 9:14 PM on November 12, 2011 [1 favorite]

What a beautiful piece of rock. I believe its sandstone with Liesegang banding. It looks rather like this cool Illinois sandstone.
posted by Long Way To Go at 9:22 PM on November 12, 2011 [1 favorite]

It's kinda hard to tell from just a picture. From just the picture alone, I suspect that it's petrified wood, just because the rock looks rather hollowed out and light, although that could just be a function of the picture itself. Plus the patterns look fairly complex and wiggly and have a lot of relief on the surface (surface is not smooth; it's rough). Metamorphism can cause these same complex patterns, but the surface wouldn't be so rough/ have so much relief. And so can other forms of precipitation of iron ore, or chert, but again, the surface would be more smooth.

But there are several very important things you can do to help identification of this rock.

1) How heavy it is? It's all very relative, of course, but petrified wood would generally be much, much lighter than any rock (granite, sandstone, shale, etc) of the same size. And iron ore would be much, much heavier than any rock of the same size.

2) Chip off a bit of rock with a hammer, so that you can see a piece of the "fresh" surface of the rock, and not the weathered surface. The weathered surface would of course be rained on and weathered away and can sometimes also be a totally different colour. What does the insides look like? Do they have any trace of lamination or layering? What are the layers made out of and what are their colours? If you look up real close, can you see what looks like individual sand grains (like what you find at the beach, but just all glued together). Careful - you might be mistaking the texture of the rock for individual sand grains. If you have a microscope or hand lens, now would be a good time to use it.

This is why they pay me the big bucks....
posted by moiraine at 7:54 AM on November 13, 2011 [1 favorite]

On a second examination, however, it does look like lightly-metamorphized sandstone which has been deformed by some kind of tectonicism in the area. Basically, geological layers in the earth have been squeezed and pulled apart in weird ways, so they end up deformed and looking like that. However, they haven't been subjected to great heat (<2>
The absence/presence of individual sand grains and weight should be a good differentiator.
posted by moiraine at 8:02 AM on November 13, 2011

Not sure why my last message was truncated, but here's the message in entirety:

On a second examination, however, it does look like lightly-metamorphized sandstone which has been deformed by some kind of tectonicism in the area. Basically, geological layers in the earth have been squeezed and pulled apart in weird ways, so they end up deformed and looking like that. However, they haven't been subjected to great heat (less than 200 C) or pressure, so they're still sandstones AND the "weaker" materials would still had been preserved (and not metamorphized out of the rock, so to speak). Therefore, when a rock of that nature is left outside to the rain, and snow, and sun, it will weather in that odd fashion, where the weaker laminations would be eroded out, leaving behind the stronger laminations and producing a surface with high relief (very rough surface). They would also be fissile/ brittle and foliated in the same direction as the main tectonic stresses.

The absence/presence of individual sand grains and weight should be a good differentiator.
posted by moiraine at 8:03 AM on November 13, 2011 [1 favorite]

I'm no geologist, but it looks like sandstone with some recent water erosion. The stuff was a playground favorite when I was a kid because it erodes easily, and erosion can have biological looking results. Given the proximity to Oklahoma, the redness is probably due to iron oxide (rust) in the soil. Though if the pattern is more than skin deep, moiraine's laminations theory is quite plausible to me.

In the future you might try applying scratch tests in the field or getting a sample.
posted by pwnguin at 12:53 PM on November 13, 2011

sandstone with some recent water erosion

If it is a sandstone, there would almost certainly be some weathering of some sort if it was exposed at the surface for any length of time. So, the question is not: "has weathering and erosion acted upon on it" (definitely YES), but rather, "why does weathering produce such a weird surface in the first place?" Weathering will only produce such a pattern in sandstones, if there was a pre-existing pattern in the first place, that is NOT caused by weathering. Weathering is merely the surficial expression of the heterogeneity found in sandstones (generally speaking, not true in 100% of cases but for the purpose of this question, assume to be true).

Sandstones are just layers of sand laid down on a flat surface. They would originally be flat, horizontal layers. IF this rock is a sandstone (and not petrified wood), then, this rock has been squashed/ deformed by tectonic activity (like mountain building).

So that explains the weird contorted shape. But why does some layers stick out more than others? In other words, why is the surface all rough and jaggeddy? Well, sometimes sandstones are interbedded or laminated with layers of shale. Shale is weaker and hence weathers away more easily. So the layering pattern that you see will be due to the alternation between sand (harder, less weathered) and shale (weaker, more weathered) layers.

Red sandstones are pretty common, especially in the desert, as penguin mentions. The redness comes from the presence of iron oxide in the sand grains (not necessarily iron oxide ore!). In more humid climates, the iron gets weathered/ leached from the sandstones, producing bauxite or laterite. That's why sandstones that originate from more humid climates are not red.
posted by moiraine at 2:11 PM on November 13, 2011

Oh and scratch tests are usually useful only when distinguishing between different minerals. I use scratch tests when I need to differentiate between fluorite and limestone.

Any rock (sandstone, basalt, metamorphic rocks) is amalgamation of many different minerals and compounds. A sandstone can be made up of a combination of limestone, iron oxide, pyrite, quartz grains. Therefore, a scratch test will only tell you that, yes, you have some hard minerals and soft minerals, but not useful when it comes to the depositional history, what type of rock it is, and any subsequent weathering or deformation.

You're much better off just looking at it very closely, with a hand lens if you have one. If this is a sandstone, you should be able to see the individual sand grains.
posted by moiraine at 2:19 PM on November 13, 2011

my Geologist father-in-law stated that this is an iron concretion. http://paleo.cc/paluxy/concret2.jpg
posted by pelican at 7:10 PM on November 18, 2011

sorry, I didn't link that correctly the first time.
posted by pelican at 7:11 PM on November 18, 2011