The argument of the contribution of life to sedimentation interests me: How much faster would you suspect sedimentation to proceed due to the existence of life? In other words, how fast would sedimentation proceed on a lifeless earth if there was "just" the effects of freezing water that cracks rocks, or the transportation of rocks by water and the resulting collision of rocks resulting in breaking down of more material etc. Plus, even without life there is wind that moves sand that reduces rocks to more sand. As I formulate the question, of course I realize how very impossible the idea is of Earth without life but with the same amount of water and the same atmosphere and with the same weather. Still: is there any way to tell the relative contribution of life to sedimentation versus that of the other forces causing sedimentation?
"Anyway: did you know that most of that whole tectonic movement is being suspected as having been *caused* by Life in the first place?" I can see some close connection between sedimentation and tectonics but I feel you may be overemphasising the role of *life* (My emphasis). For example, what about the sedimentation from volcanic eruptions and water erosion? I am a little unhappy about the time scale here too. The earliest tectonic activity I have found so far is the amalgamation of the super continent of Rodinia around 1,200 to1,000 million years ago. Sea life I have traced back to 2,000 mya based on presence of oil and gas hydrocarbons. Also I believe the contribution from volcanic eruptions long ago was much greater and from *life* much smaller as *life* presumably took a long time to spread and enlarge. I am not dismissing that *life* played some part, but I fail to see the major part ("most"). However, I find the subject interesting. Please Register or Log in to view the hidden image!
rewt2004 I had not seen your post when I replied to mercurio, having been away from the comp for some time and not hitting refresh. I can see some substantial contribution, but not major. This especially 1,200 mya.
Here is some background (from "Earth" DK 2003): Ocean sediments Most of the ocean floor is covered in layers of sediment. In some places these are over 5km (3 miles) thick and have taken more than 100 million years to accumulate. On the continental shelf, most sediments are gravels, sands, and muds that are derived from the erosion of rocks on land and transported by rivers to the sea. Deep-sea sediments come from many different sources. Some are fine silts that have migrated down from the continental shelf through mechanisms such as turbidity currents. Others derive from sand and dust that have been blown by winds off the continents, or from icebergs that carry rock particles away from glaciers in polar regions and then drop the particles as they melt. ... Another important type of sediment is biogenic ooze, the skeletal remains of microscopic organisms that once lived in the ocean."
Ok, I understand your skepticism here, it does not seem to make a lot of sense to think that life would be the sole originator of tectonic movement, and at a certain point in time it cannot have been, since it wasn't there yet. Erosion, volcanic activity, meteor and comet impacts are not to be discounted, and precede life anyway. Point is, I think life actually benefits from planetary tectonic movement. It creates all kinds of energy fluxes that it can benefit from, and however disastrous it may seem locally, in the long run a volcanic eruption is a blessing. Life explodes on such abundance. So Nature preferring the kind of life that adds greatly to sedimentation in order to keep things going, tectonically, is not out of the question. It could well be as simple as that. As to the exact numbers in the division of organic/anorganic sedimentation, I have not found any specific documentation on it yet. It's a pretty fresh insight. One thing you guys clearly overlooked in my previous post is the addendum about Australias and Europes output of sediment per square mile. The difference is quite striking. Green Europe puts out 20 times as much sediment as the dusty Outback. Coincidence?
Mercurio, your ideas are intriguing, but wrong. Just to address a couple of points. No.Two factors are at work. 1.Rainfall. Water plays a major role in weathering, erosion and sediment transport. Annual precipitation is much higher in Europe, in general, than Australia, in general. 2. Interior drainage. I believe the figures you have quoted are for sediments delivered to the ocean. A substantial amount of drainage in the outback is internal. You are proposing four factors as originators of tectonic movement.. Life - please give even one piece of evidence.. Erosion - its longevity is a consequence of plate tectonics. Through isostatic adjustment it can induce uplift - not at all the same as plate tectonics. Volcanic activity - again a consequence not a cause. Bolide impact - A large one will certainly screw up the local stress environment. It may even induce large scale vulcanism of the flood basalt type. But it wont induce plate tectonics. We know what causes plate tectonics - mantle convection currents. So, by what mechanism are you proposing that life promotes/sustains/amplifies plate tectonics?
The more I think about it, the less I feel my earlier question of how much erosion and sedimentation is caused by life on the planet is answerable in simple terms. After all, even the specific development of the atmosphere is now generally understood to be closely connected to the development of life. I feel though that currently there is a trend to overemphasize the influence of humans on nature (we influence the weather, yes, but plate tectonics, bloody unlikely!) Sure, I also would imagine that life's (life in general) contribution to geology is substantial but still smaller than other modes of erosion and sedimentation which have existed long before and without life, and which are still the major cause of shaping the surface of the planet. This is in talking about life in general, not just humans. Even humans could have some small influence on sedimentation, ice shield formation or melting thereof, and hence ultimately on seismic activity. However, I believe that this influence is so small that the idea put forward in a recent commentary in the NYT that the earth "fought back" in Indonesia, is wild over-speculation of the New Age type. (http://www.nytimes.com/2004/12/29/opinion/29winchester.html?oref=login) - I'm referring to an article in the NYTimes from Dec 29, 2004, titled "The year the Earth Fought back", by the author of the book "Krakatoa, the day the earth exploded.", in which he puts together some speculations about connections between big seismic events and the suspicion that they often appear in clusters. It's an interesting article, except it has a too wide open metaphysical edge).
Ophioliolite, undoubtedly the other factors you mention like internal drainage twist the figures. On the other hand, high levels of precipitation and life also seem to go hand in hand, so the one does not rule out the other. On rereading I see I gave the impression there are four factors, I did not mean to do that, only to indicate those things precede life or are not influenced by life and therefore life could never be their sole cause. The mechanism I mentioned earlier: pure and simple weight of sediment (wedging): http://www.odp.usyd.edu.au/odp_CD/slope/slindex2.html If mantle convection was the only cause of plate tectonics, why does it not stop, like on other planets: http://science.nasa.gov/newhome/headlines/ast29apr99_1.htm or on Venus, our 'sister', which has a radically different kind of plate tectonics: https://www3.imperial.ac.uk/portal/page?_pageid=46,73409&_dad=portallive&_schema=PORTALLIVE Rewt20004: Nature does not prefer anything, but evolution decides what stays or not. If there is a feedback between prolongued plate tectonics and greater opportunity for the abundance of life, it will be exploited, I think.
But I'd never have guessed sediments getting sucked under at subduction zones to take an active, rather explosive part in it all, with Mount St. Helens as an example: http://www.ig.utexas.edu/research/projects/barbados/sed.deform/sed.deform.htm so it does more than just 'lean' on it via accretion.
Merulos, After reading the interesting link: I'm certainly not expert in this stuff but I wonder: where the energy comes from in the explosion of Mt Helens? The sediments that were drawn under explode but do they explode because of stored chemical energy that was left in there during sedimentation or, is it a mere explosion of superheated light gases that push up, while the energy comes from earth's interior? I mean, if sedimentation plays an "active" role, at least I would expect that energy was added by the sediments sucked under an subduction zone. This would be a mechanism to prolong and sustain big tectonic activity, as you may have suggested. I suspect it is not.
It is all heat energy transfered from the mantle and originating, for the most part from radiooactive decay.
Naturally, since the bulk of all living things are made of water, we find more life where there is more water. So you have two dependent variables (biomass and erosion/transport/sedimentation) and one independent variable (precipitation). Obviously this is something of a simplification: thus, plant cover will reduce erosion [not increase as you are trying to suggest]; the presence, or absence, of plant cover will effect precipitation; but these are generally effects of smaller magnitude. I understand your meaning here. Again, sedimentation is not the motive force for plate tectonics. I take it that you are referring to accretionary wedges. These are a consequence and not a cause of plate tectonics. Of the other terrestrial planets Mercury is too small and is almost ‘all core’; Mars is too small and cooled early; Venus, as you noted, is just plain different. Why? There is a very good chance that the answer is visible to you if you step outside today and look up. The moon likely formed when a Mars sized planetoid collided with the proto-Earth. This stripped away much of the early sialic crust. Now, as you are aware, sialic crust (that forms the continents) is a lot thicker and harder to push around than simatic crust (that floors the oceans). If Earth had not lost that proportion of sial at its birth the plates might well have locked up a couple of billion years ago: the result could well have been a true sister for Venus. You appear to be changing your argument, switching between cause and effect. Earlier your argument was that life promoted/initiated/powered plate tectonics. This is erroneous. In this last statement you are saying the reverse, that life may be favoured by plate tectonics. That is absolutely true. Without plate tectonics the sedimentation rates would have eliminated most land mass. Plume volcanics would have produced large islands (picture Olympus Mons), but no continents. So , I am now confused as to what you are saying. [There is one way in which life might just have saved, though not caused, plate tectonics. Without life to remove carbon dioxide from the atmosphere, Earth could have suffered the greenhouse fate of Venus. Without water to lubricate subduction zones the whole process might have locked up.] .
Summarising, then: - plate tectonics probably predate life, so 'cause' was an unfortunate choice of words, so 'maintain' or 'propagate' would have been more to the point. - Venus has a form of plate tectonics that seems to fit Ophiolite's bill more closely, as in caused by mantle convection ONLY. - the presence of the Moon and water are interesting factors, but a side-track, so I'll not cover them now So the question remains: can biogenic sedimentation be a factor in keeping the process going simply because of the weight of those accretions, and occasional globs getting sucked under? Maybe we could reverse the whole setup: if you cannot have enough sedimentation WITHOUT life's contribution to it, are the current processes MAINTAINABLE over a longer period? Seems pretty straightforward: If not, case closed, if so, life would seem to make no difference, also case closed. How would you calculate it, tho? Are there any numbers divvying up bio/inorganic sedimentation? "The depositional process that resulted in the cherts and porcelanites of Unit III, as well as the carbonates of Unit IV, supplied SiO2, Ba, Sr, and P to the seafloor to a greater extent than if sedimentation had occurred only by terrigenous or metalliferous inputs. The cherts and porcelanites of Unit III are identified by the high SiO2, with respect to average shale (Fig. F58). Normalizing SiO2 to Al2O3 indicates that even though carbonate is the dominant lithology through Unit IV, there is still enough biogenic silica in these sediments to elevate the SiO2/Al2O3 ratio significantly above that of average shale." http://www-odp.tamu.edu/publications/185_IR/chap_04/c4_10.htm#572838 added: maybe some people have a problem with this because they think they need life to have some 'reason' doing this. It doesn't. If it benefits, that's enough reason, just like ducks prevent ice from locking up by huddling together in winter. They don't know what they're doing either, but effectively save themselves notwithstanding.
There is an entire chapter "Tectonics and Sedimentation" in "The Cambridge Encyclopaedia of Earth Sciences". Conclusion This chapter has centred on the role of erosion and deposition of sediment in the vast recycling process that occurs as the Earth's lithospheric plates interact with each other. Tectonic and isostatic processes cause uplift and subsidence while geomorphic and sedimentary processes cause erosion and deposition. A logical scheme of birth, destruction and renewal will continue as long as sufficient heat is produced in the Earth's mantle to drive the plates around the surface of the globe. ... ."
Oops what is this? The 10 fold higher deuterium ratio on Venus strongly suggests that there has been plenty of water on Venus. Water is capable of removing carbon dioxide from the atmosphere by weathering, no life required for that. Actually this process is the base for the "wet greenhouse" hypothesis (Kastings et al) and this part of that hypethesis looks sound. There are some problems though with the complete outgassing and the missing oxygen, but that's besides the point. Correct me if I'm wrong, but the basaltic based simatic crusts seems to have a higher density than the granite based sial continental crust. Why would it be harder to push that around? The thick crust idea of Venus is under heavy investigation. But much more interesting is the investigation of the thermal gradient of Venus crust, that suggests that the interior of the crust has cooled. I posted that somewhere. Time to think of completely different hypotheses? Venus may have no plate tectonics because the planet may have cooled, the same reason why it lacks magnetism.
I'll look into that, see if they have it at the library... Found this table with percentages of biogenic material (same site as earlier link, have to really check it out more thoroughly, it keeps popping up on my searches): "The trends in biogenic silica abundance correlate strongly with sedimentation rate, as the more southerly Sites 1209 through 1212 were characterized by lower sedimentation rates." http://www-odp.tamu.edu/publications/198_IR/chap_01/c1_f40.htm Andre: according to one of the previous links Venus does have plate tectonics, just not the kind we find on Earth, tho... added: finally found some support here... "Carbonate oozes are the most widespread shell deposits on earth. Nearly half the pelagic sediment in the world's oceans is carbonate ooze ." "Pelagic sediments in the Atlantic and Indian Oceans are predominantly calcareous oozes." http://cima.uprm.edu/~morelock/dpseabiogenic.htm
Well I don't know about that: http://www.xs4all.nl/~carlkop/venus.html http://www.sparknotes.com/astronomy/venus/section4.rhtml http://wisp.physics.wisc.edu/astro104/lecture16/lec16_print.html What we see of Venus by the eyes of Magallan is a frozen past. We have very little idea about it's activities the last few ten-million years if at all. We know that the vulcanism was very active in the early period but declined dramatically towards the present. We know that the thermal gradient of the lithosphere was over 25 degrees per km about 0,7 billion years ago and is less than 4 degrees per km some 300 million years ago. Again, the current thermal gradient may be too low for any form of tectonics. A little paradigm shift would answer a lot of questions here.
Yeah, you gave those links last time we discussed it. But please do read that previous link I mentioned: "An oft quoted surprising discovery about Venus is its lack of plate tectonics; this, however, comes from too specific and narrow an understanding of what plate tectonics actually is. When scientists first studied high-resolution images of Venus taken by NASA's Magellan spacecraft, they were looking for examples of spreading ridges, subduction zones and transform faults Ð the "hallmarks" of plate tectonics. The lack of these features led to the conclusion that Venus does not have plate tectonics. Yet neither are these features found on Earth's continents, because they are the hallmarks of a unique type of plate tectonics, found only on Earth's oceans." etc. 'Paradigm shifts' can only begin with a willingness to at least investigate seemingly conflicting viewpoints... https://www3.imperial.ac.uk/portal/page?_pageid=46,73409&_dad=portallive&_schema=PORTALLIVE
Yep, picked it up (Dutch translation): Page 187 (translated back), next to image 10.2: The conventional model of plate tectonics, in which the plates are uncoupled from the asthenosphere, causing flow in the mantle in reaction to plate movement (and is not the cause of such movement). Hmm. Seem *yours* is the alternative theory. Might have mentioned that. Anyway. You asked where the energy comes from when globs get sucked under, added or just gas. Artists impression from the Cambridge would say gaseous: Please Register or Log in to view the hidden image! mmm don't see no picture here (yes it's on in prefs). url is : http://www.aidainternational.nl/subduction.jpg
