Category: geomorph

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ask a geologist geology geomorph

[Ask a Geologist] Before there was terraforming, there were rocks

My friend Andrew Barton asked me, a bit out of the blue via twitter:

Given a now-earthlike planet terraformed 15 million years ago, which previously resembled an Earth-sized Iapetus or something, how obvious would the pre-terraforming rocks be, or what governs how deep they’d be now?

He also provided a bit more background as to the reason behind the question:

I’m still working out a lot of the details; this involves Esperanza, the terraformed habitable moon of HD 28185 b that was the setting of “The Paragon of Animals” in the March 2013 Analog, and hopefully additional stuff down the line. While tectonically active and geographically varied at present, it was entirely lifeless before the terraforming process began.

I should probably be ashamed to admit it, but I didn’t actually have an idea what Iapetus would be like. And didn’t look it up until this very moment (naughty, naughty) but I don’t think that would have changed the answer I sent him. Which is long and a bit rambling, but I was thinking it through as I went since the question was fairly general.

  1. How deep do the effects of the terraforming go? If it’s just a matter of soil modification/creation, I wouldn’t really expect most bare rock to be all that altered. If it’s a change to atmospheric chemistry that will completely redefine the way weathering works on the planet, that’s a whole different matter. Also, microbial life does have a profound effect on how any rock that’s exposed to air will degrade (we even see this deep in mines/hydrothermal vents) but did the terraforming, say, completely alter the habitats of the extremophiles?
  2. How tectonically active is your planet? If you’re getting regular tectonic activity like you see in modern Earth, there’s a good chance that you would get exposures of relatively pristine rock fairly regularly; if there’s a large earthquake that causes a major landscape drop, you’ll get a fault scarp where new rock will be exposed. These aren’t the most common events, but you’d probably get a bit of that happening during 15 million years. Of course, as soon as you expose the fresh rock face to the surface, it will start being effected by the terraforming.
  3. If you’ve got landscapes with a lot of relief (eg: mountains) then you’ll have an ongoing process of mass wasting (landslides, rockslides, etc) that can expose fresh surfaces.
  4. Weathering rates will determine a lot, but 15 million years isn’t that much time to redefine a landscape, particularly if you’re just changing the air and water chemistry and not effecting the tectonics at all. Erosion rates are generally less than 1mm/yr (but up to 10mm/yr in places like New Zealand and the Himalayas that have high relief, active mountain building, and plenty of moisture) and one thing you have to consider is that as material is being eroded from the surface, it’s not necessarily going to just expose something pristine beneath it… whatever is right beneath it will probably be in some way chemically weathered by the time you get to it, because water ruins everyone’s life.
  5. As far as “how deep” the pre-terraforming rocks would be, it’s basically just going to be anything below the zone where your new bacteria/weather can effect it. Which will vary wildly depending on the environment in question. In the classic case of a single non-stacked soil, you could potentially hit bedrock less than 1.5 meters down… but then that bedrock has been subjected to the presumably terraformed water regime. And how deep that water would go would be determined by things like the type of rock, its porosity, and how fractured it is.
  6. So basically your biggest problem, depending on what exactly the terraforming entailed, is trying to find rocks that have not been touched by air/microbes/water from the new surface.
  7. Probably your best bet if your people are digging would be to get below the water table if you want completely pristine rocks. In the majority of places, the freshwater table will stop at about 30-35 meters below the surface, but it can go as deep as 370 meters or so.
  8. Just as a note, for buried pre-terraforming rocks, what you’ll be looking for are sedimentary rocks. Those are the ones that will give you the clearest picture of surface and near surface conditions at the time of their formation (and early diagenesis). And that would presumably provide a very different environmental picture from what currently exists. (Like gosh there are no fossils of any kind in these older rocks…) The good thing is, those rocks have had the entire existence of the planet to form and be buried, so there ought to be plenty of them lurking just below the surface.

Tl;dr: That’s a really complicated question.

Obviously, I’m not the world’s greatest expert on this topic–any other geologists out there have thoughts? Did I get anything completely wrong? Just drop a note in the comments. I’m sure I didn’t think of everything.

Categories
geology geomorph

Slow Motion Landslide

This is just awesome:

The flow looks like it’s really cooking along… until people make an appearance in the video and you see just how much it’s been sped up. The flow is actually moving at around 50 cm per hour, which to us fast-living humans makes it practically solid ground.

More info over at the AGU Landslide Blog.

And I totally agree with the first commenter over at the post. This thing needs some Benny Hill music, starting right when the first person pops into the frame. WIN.

Categories
geomorph

Fluvial response to tectonic activity

Lunch today was quite fun; we had a “lunch and learn” that involved a lot of really tasty brownies and an hour-long talk by John M Holbrook. It was for the most part a general overview of the surface process quirks of meandering rivers and how those affect their usefulness as reservoirs. Most basically, oil companies like drilling in point bars since that’s where the best sand packages are to be found. Understanding the complex ways that rivers meander and stack up point bars over time (and that coarse channel fill can act as a fluid transmitter between sets of point bars at times) is a way to try to maximize drilling effectiveness.

Not that mapping out historical meanders is an easy task even for a river that’s still active and not buried hundreds of feet down and only visible via cores or seismic. Dr. Holbrook used a particular section of the Mississippi River (where it crosses the New Madrid seismic zone) to illustrate these concepts, and said that when they were trying to map out the old meanders by taking sediment cores, they were wrong about 30% of the time.

I thought the most interesting part of his talk was a brief look at fluvial response to tectonic activity, particularly how a river reacts to displacement on a fault that it crosses, since that normally means a change in grade on both sides of the fault. Looking at the Mississippi’s reaction to the displacement on the faults it crossed, the basic response was for the river to straighten out (cutting off meanders) on the downdropped side of the fault (where the gradient decreases) and resume meandering on the uplifted side. Which makes a lot of sense, really, though the other interesting thing was how quickly this response occurs. (Answer: very quickly.)

He’s got a paper in the pipe1 about using the Mississippi to examine tectonic activity on the New Madrid fault, which is currently in review for Tectonophysics. Apparently it’s a bit controversial since what the river seems to show is that earthquakes along the fault system are temporally clustered, which doesn’t necessarily fit with the current consensus on the seismic zone. So I hope that it does get published and I can find a way to get my hands on the paper, since it sounds like an interesting read.

1 – If it gets published, here’s the title: Restored river courses reveal millennial-scale temporal clustering on a midplate fault

Categories
geomorph planetary geology

Mars Geomorph Porn

There’s a lovely blog post over at The Planetary Society explaining a couple of images from IAG’s Planetary Geomorphology Working Group’s May 2010 featured images.

This is some cool stuff, since it’s very much connected to the ongoing “water on Mars” debate, and the geomorphological argument has to do with water leaching minerals over a fairly long period of time. Another of the images that the blog post doesn’t cover looks at:

However, with the addition of infrared color, two distinct units of altered minerals can be discerned, and using spectroscopic information, these have been identified. Here at NE Syrtis, there is a unique stratigraphy of iron sulfate overlying carbonate, which is being exposed by the erosion of overlying lavas (Mustard and Ehlmann, 2010). This suggests a transition in the aqueous alteration environment from neutral-to-alkaline to acidic that is preserved in the rock record.

Aqueous alteration environment… squee! With of course the added fun of wondering what might have caused the pH of that environment to go from neutral-ish to acidic. Interesting stuff, to be sure.

I didn’t know about the images of the month, but I’m going to start checking them out for sure! Geomorphology was one of my favorite undergrad classes, and there’s some very neat stuff on that site. For example, comparison of catastrophic flood bed forms on Earth and Mars that was April’s set of images. Looking at land features via aerial/satellite imagery isn’t perfect, but I think it’s great to see our knowledge of our own planet being applied to the images we’re getting from Mars.

Categories
geomorph rivers

Modeling Meanders

Alfalfa Sprouts Key To Discovering How Meandering Rivers Form

Some very cool stuff from the world of Geomorphology. Now that we’re realizing that channelizing rivers sometimes isn’t the best idea (well, as far as the flood plains and nearby shores are concerned, it’s never a good idea) and trying to get them back to their natural state, we’ve never managed to copy nature. We can put a man on the moon, but we can’t make a meandering river, to paraphrase. So this is some very cool modeling on how the process works, which means some day we might be able to get the meanders right.

*Quick terminology: Meandering rivers are those wandering, looping rivers we’re so familiar with. Such as The Amazon or the Mississippi or the Nile. You’re probably not familiar with braided rivers unless you live near the mountains or other sources of extremely coarse sediment, but here are a couple examples: Waimakariri River, drainage near the Yukon River. Basically, braided rivers have a lot of in-channel sediment deposits that the river cuts through in a multitude of small channels.

I definitely want to see if I can get my hot little hands on a copy of their results. It sounds extremely interesting. (Though I’m sure all the really technical stuff will make my head spin.) Also, the researcher does bring up some good questions about Mars and Titan. We can be pretty sure that neither place has or ever had the verdant banks that would help build meanders. So the real question is, how would meanders form in an environment without vegetation? What would provide the bank stability that lets the point bars grow? Maybe that’ll be the next experiment, after they’re done with their alfalfa jungle.

By the way? Best use for Alfalfa sprouts outside of a turkey sandwich. Truly.