Ramblings about what I encounter within the realm of the geosciences, as well as the occasional rant about nonsense.

08 June 2009

Accretionary Wedge: Time Warp

Lockwood at Outside the Interzone is hosting the June Accretionary Wedge. The rules are simple:
“Where and when would you most like to visit to witness and analyze an event in Earth’s history?” Suppose you have a space-time machine to (safely and comfortably) watch an event unfold; which event would you most like to see? Why? What do we already know or hypothesize about that event that appeals to you, or that you would like to test? What would be the result, the upshot, of knowing more about this event? You do not necessarily need to limit yourself to Earth, nor to the past. You do not need to limit yourself to a particular instant if peeking several times over a period of minutes or ages helps you envision the evolution of something. You do not need to limit yourself to environments that could support life as we know it... imagine being able to take a time-sampling of magmatic composition from 10 miles below the surface as a nascent mid-ocean ridge opens up, or examining the circumference of the vent during one of Yellowstone's mega-eruptions! I'll tell you, this technology is basically magic. (See the third law here.) Feel free to toss in a few "also-rans" of your favorite day-dreams, but please develop one.
So, just like the mainstay of the best Star Trek movies (excluding Wrath of Kahn of course), we will be time traveling. My first impulse is to say "I want to see the KPg extinction, because nobody really understands anything about it". There are lots of correlations, but there is no evidence, beyond circumstantial, for a causative agent. I could broaden this out to mass extinctions in general for the same reason. There are a good many geomyths propagating in the field of extinction studies, and it might be nice to get some answers to move the conversation forward. I would expand on this further, but volcanista beat me to the punch. Also, since this is tangential to what I am working on, I would like to expand my horizons.
I think I will take advantage of the non-limitation to Earth clause and couple that with the time-lapse clause. It might be fun to observe how Martian tectonism quieted down over time, and collecting data from that event would advance our current understanding of tectonics and planetary geology. We know that Mars has been, for all intents and purposes, tectonically quiet for quite a while. One line of evidence comes from the largest volcano in the solar system, Olympus Mons. This volcano is thought to have formed as the result of a hot spot. However, since there is no hot spot trackway, like we see in the Hawaiian islands, it is unlikely that the 'plate' that contains Olympus Mons has undergone any motion. Events of Olympus Mons have been dated at ~115 Maa, so Mars has been tectonically quiet for at least that long.

One idea as to why Mars is tectonically quiet is it no longer has a sufficient heat engine to drive tectonism. Tectonics happening on Earth is a side effect of several mechanisms, one of which is the Earth is cooling down (there are multiple inputs into the system, so it is not a simple linear heat decay as has been proposed in the past). However, if you were to fast forward the Earth cooling down, hypothetically the lithosphere will thicken. If this rigid zone becomes too thick, the plates will lock up and the Earth will become a tectonically quite body.

Similarly, it would be fun to observe Venus' runaway green-house effect from the beginning. Would Venus have initially experienced tectonism similar to the Earth, or did it always have a 'funky' form of tectonism? Venus is very similar to Earth in mass and, to my understanding, composition. However, it is a green house with no liquid water remaining. And with the surface temperatures capable of melting lead, we are currently restricted to observing Venus from orbit. Venus also experiences Global Resurfacing Events (The last one ended ~300 Maa). The Earth cools primarily via convection which manifests itself as Plate Tectonics. Venus, on the other hand, does not seem to have an analogous mechanism. So it is hypothesized that heat builds up in Venus' interior until it is sufficient to initiate global volcanism, which resurfaces the whole planet and the process of heat building up starts over.

These two transitions would be quite helpful in figuring out the dynamics behind tectonics and the evolution of terrestrial planetary bodies.

5 comments:

Lockwood said...

Noted and queued... nice one. Not one I had thought of for my own post for this AW, but definitely one about which I've spent too much time daydreaming.

Bryan said...

Thank you very much. This is an exciting topic for an Accretionary Wedge.

tim said...

Hello Bryan. Guess who?? I'm sorry I haven't been looking at your blog for sometime so I was just catching up. I know it's probably a bit stale for you by now and I hope you don't mind but of course this topic is of great interest to me. There are two points I would like to raise here and I would appreciate your thoughts, not least because I share some of your selections ofr this proposal but for slightly different reasons...

Firstly, you mention that tectonic activity has stopped on Mars which of course presupposes that it ever started. I would be very interested to know the evidence you have that it has been occuring in the past.

Secondly - this goes back to my old discussion with you guys before - in which circumstance could planet-wide volcanism and resurfacing NOT result in accretion over time and (you guessed it...) consequently, planetary expansion? I dont accept the crustal sag type explanation.

Bets regards, Tim

Bryan said...

Tim,

How goes the degree?

In answer to your first question. Martian tectonics is inferred from certain surficial features, such as the Valles Marineris. There is also evidence of "magnetic striping" on Mars. Similar to the magnetic striping found on Earth. This can be used to infer that analogous mechanisms are behind these features. Tectonics is the mechanism behind these processes on Earth, so it is probable that tectonics is the responsible mechanism on Mars.
__________________________________
In answer to your second question, it doesn't matter if you accept "crustal sag" or not, it happens. When material is extruded onto the surface, something has to fill in the void vacated by the extruded material. This void will be filled by a collapse higher in the gravitational gradient.

It's like a planet-sized game of Jenga. Material on the bottom is placed on top. The inevitable end of this scenario is collapse. Just like on Earth, mass will try and move as low as possible within the gravitational gradient.

tim said...

The degree is going great thanks and I'm still searching for the evidence which proves tectonics to me. The term starting in Jan should take it to the next level.

Re Mars, The analogous mechanism could also be used to corroborate expansion...

Your Jenga analogy is perfect. As one extrudes "planks" (there is probably a true jenga name for them?) the structure loses density. Sure there will eventually be some collapse but less dense material is constantly being deposited on top - Venus is a very good example. If you are suggesting that the Earth's gravitational force is sufficient to compact any accreted material down to its original density I would accept it. However, I have yet to read any evidence that this is the case.

Can you not imagine a topographic view of Earth, long after our surface water has boiled away? With resurfacing caused by random and sporadic planet-wide volcanic activity. Do you not think it would largely resemble the surface of Vennus?

Disclaimer

All the Latin on this page is from my vague recollections from High School. There are mistakes in the text. I just was trying to get the point across

Between Los Alamos,NM and White Rock, NM

Between Los Alamos,NM and White Rock, NM
The photo of the travertine spring was taken in the small opening in the center of the image.

Lectio Liber