noting the emphasis on might, that's a fair summary ...

beginning assumptions about Earth in Hadean time stem from our ideas of formation, which lead to three phases of "lots of energy to dissipate" -
1. the sum of protoplanetary accretion with collisions ...
2. the Theia impactor event ...
3. the Late Cometary Bombardment ...

without direct samples (and perhaps because of that), conclusions involved planet-wide oceans of molten lava, and/or Venusian-style volcanic landscapes dominating the Hadean (ie, ~4.6 -3.8 GA BP) ...

as Jon pointed out, what direct evidence has been found (after the above line of thinking was widely accepted) mostly amounts to variously altered (zircon) fragments within later (but still very old) rocks ...

the fragments are important limiters, because they show that solid minerals did form on and/or near the surface, and did so in the presence of fluid water - the terms "liquid water" or "surface water" are often used, but the potential conditions also allow for the existence at the time of supercritical water (the important unknown here is the atmospheric pressure/density at the time) ...

the temperatures of crystallisation of these zircon fragments is determined to be less than 800C which, whilst "cooler" than expected for the time, still leaves a lot of room to be hotter than we'd find comfortable ...

ETA:
it's not unreasonable to expect that overall cooling would not be linear, but a type of exponential curve ...

however, there are factors beyond those originally considered (by Lord Kelvin) in the equation -
add any internal heat derived from impacts/accretion, and differentiation (density-driven separation),
to nuclear decay - initially quite high from short-lived radionucliides - this type of energy was only just being realised at the time (Becquerel and Rutherford?);
and rotational and tidal friction ...

modern surface temperatures have more to do with insolation (incoming solar radiation) ...

__________________

Great post, thanks Cran.

Slight tangent. Would it be correct to assume that:

1. The Moon's surface cooled a lot faster than Earth's.
2. The oldest rocks found on the Moon so far: ~4.4bn yrs.
3. Therefore, the absolute earliest time for a cooler surface here, would be ~4bn yrs ago?

__________________

Yes, Thanks Cran.

As to # 3:

The action of plate tectonics, subduction, is facilitated by water. Mars is an example of what happens without water, the volcano erupts in one location. I do not know the location of every outcrop of ancient rock that is convenient for our sampling, but it seems that they are primarily part of accretionary complexes formed by collisional tectonics. The first activation of plate tectonics is a subsequent marker for when the retention of volatiles, water, occurred, and its identification could constrain the time of the cooling of the planet. Not an easy task and luck of emplacement is definitely involved. The search continues.

__________________
(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"

jlhredshift, could you explain those sentences please? In layman's terms? You mean we had many more volcanoes here? Or that volcanoes over here moved? Or both?

Thanks for your post by the way.

__________________

Yes, we have many more volcanoes. Our planet is bigger and had more heat to start with and will take longer to cool, internally, plus radioactive and tidal generation of heat. There are two major sources of volcanics, hot spots, such as Yellowstone and the Hawaiian chain, and subduction zones such as the Pacific "Ring of Fire." As the plates are driven around the points of eruption have changed. This is a simplistic explanation and is not comprehensive. Does this help?

__________________
(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"

Almost! One more if you don't mind.

Do you mean:
1. The points of eruption are fixed relative to the plates, and so they move wrt the Earth?
2. Or the points of eruption are fixed wrt the Earth and the plates move above/through them?

I'm probably not being very clear. I think what I'm asking is whether or not you can trace plate movement by following the trail of the eruption point as it moves along the surface? If it's case (2) I mean.

Sorry- stupid question!

__________________

Not stupid, only a beginning. Look at a topographical map of the Pacific centered on the Hawaiian chain and you can observe the path of the Pacific plate over the Hawaiian hot spot.

__________________
(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"

Good grief. I've actually learnt something today. Thanks jlhredshift! I owe you one.

__________________

Neat huh! Your welcome.

__________________
(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"

Isn't the 'kink' in the Hawaiian chain the result of the North America plate encountering the Farallon plate, which threw off the dynamic forces?

Or something like that.

And to add: Have the seamounts of the chain been dated?

EDIT: I found this.

EDIT AGAIN: When I first read the article I thought to myself "wow 50Ma that's recent." I spending too much time with rocks!

__________________
(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"

__________________
Never attribute to malice what can be adequately explained by ignorance or stupidity.
Isaac Asimov

1. without any blanketing atmosphere to speak of, that's a fair assumption ...

2. OK ...

3. well, ~ 400 million years is a long, long time ...
the oldest Jack Hills zircon fragment was dated to around 4.38 GA BP - which means it should predate the Late Cometary Bombardment ...
the current thinking seems to be that crustal microplates formed relatively quickly (on the order of a few hundred thousands years), but were almost totally destroyed (twice) by the major impact episodes ...
so, you could imagine that the surface temperatures varied significantly over the first half billion years ...

-------

I believe the oldest rocks and strata found thus far are in Greenland and Canada, and they've been similarly dated to around 3.8 GA BP - or just after the Late Cometary Bombardment - and they show characteristics interpreted to be associated with water and subduction-related activities ...
bear in mind, that these are interpretations of rocks which have undergone all sorts of alterations short of outright melting ...

--------

of course, the most extensive form of volcanism is rift-related -
its products cover ~70% of the surface now ... even more in the past ...

--------

I believe the seamounts have been dated, if only by their positions on the Pacific plate ...

and all determinations of movement are relative and arbitrary, because everything is moving ... it's difficult, if not impossible, to find a true "fixed point" on or within the Earth ...

__________________

Again an excellent post.
Cran you said: "but were almost totally destroyed (twice) by the major impact episodes ... " my bold; really, I want to know more.

__________________
(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"

well, the conditions surrounding the Theia impact event are about as speculative as any scientist would want to get (about anything) ...
but the ongoing debate seems to favour sufficient time (just) to form relatively thin (and highly mobile) crustal microplates ...
no definitive evidence of such has been found as yet ...

The Late Cometary Bombardment (dated circa 3.9 GA BP) post-dates a number of zircon fragments (within younger zircons) found in Western Australia, Greenland and Canada, providing at least a little evidence that at least some solid minerals did survive the episode ...

keep in mind that:
in general, the further back we investigate, the fewer (and smaller) testable samples we have or are likely to find -
much more is likely to have been recycled beyond recognition ...

and ...

with all of our investigations, we have barely scratched the surface -
the potential for older surviving fragments therefore exists, perhaps buried deep within (or otherwise associated with) the oldest cratons ...

the confidence one places in a "here begins" or "here ends" statement is based purely on the probability of having found the first or last example under consideration ...

__________________

The New York Times has an article about plate tectonics during the Hadean. Seems there's more zircon evidence (reported in Nature).

http://www.nytimes.com/2008/12/02/science/02eart.html
http://www.nature.com/nature/journal...ture07465.html

- from the abstract in the Letter to Nature (linked above)

Harrison has done a lot of work in this area - good source ...
and you can see the key points in the discussion -

the samples fall into the younger part (~4.0 - 4.2 GA) of the time of interest (~3.9 - 4.4 GA),
and the 700C (@ 7kbar) is consistent with other studies ...

which is roughly a quarter of estimates (made when? based on what?) of global heat flow ...

compare with modern similar settings ...


now, how we get from that to this:
-from the NYT article (linked above)

is something I've only ever seen in public media ...

__________________

One would assume the full article has the appropriate citations.

Just want to orient myself here. How deep down do you have to go to get pressures like that?

__________________

not doubting that, nor suggesting it ...

Harrison et al make the point (not just in these articles, but also elsewhere) that the findings arising from studies of the Jack Hills zircons are significantly at odds with ("explode the myth", "cast doubt upon") earlier consensus views of conditions in Hadean time (summarised earlier) -

the same views, derived (as Jon indicated earlier) from theoretical considerations of formation processes ("reasonable assumptions"), which provided the bases for the cited (ie, earlier-published) estimates ...

it's a point that Harrison, Mojzsis, etc, frequently make ...
but which tends to fade in the excitement of discovery, and lead journalists to write about Hadean Earth in modern terms -

it's still a very long stretch to go from envisaging subduction zones on a tectonically active young planet to cool, modern oceans and frozen landscapes ...

__________________

that depends a bit on when ...
and what comprises the pressure column (ie, rock, water, atmosphere) ...

in modern continental settings, it can be pretty deep (down to 20km @ 600C) though still within the crust ...

this might help -
Geol 1303: Physical Geology

__________________

Thanks Cran!

__________________

I don't think there is a definitive answer on that. Yet.

That is one explanation.

The period of direction change has been dated to occurring over a span about 8 million years, from 50 M BC to 42 M BC.

Other explanations include:

The Pacific plate overcoming a smaller 'Jaun de Fuca' like plate on it's western edge, which was directing the Pacific's spreading northward up until the point the larger pacific plate slide completely over that smaller plate.

Also it Coincides with the Eocene-age Multiple Low-Impactor Events, so it is possible that, that the 5 impactor bombardment may have had an influence on plate motion. (It's felt this was an event similar in nature to Shoemaker-Levi, in that a comet broke into five pieces then impacted)

It could of been a combination of all of the above events.

Edit to add:

Also the Hawaii hot spot is not stationary, it is slowly moving south ward, so the direction change might also be due to the hot spot changing motion.

So there are a lot of culprits for it, but no definite answers.

__________________
There is no problem that cannot be solved by a suitable application of high explosives - US Army Demolitions School

I just saw Hayley's comet, she waved, Said "why you always running in place? Even the man in the moon disappeared, Somewhere in the stratosphere" - Shinedown

http://worldsofothersuns.home.comcast.net/

Late Veneer question

While on my 'Early Earth' quest (), I came across a couple of articles/papers disputing the 'Late Veneer hypothesis'- the late bombardment as I understand it. One of them was this May08 Astrobio article about Munir Humayun's experiment with platinum.

Earth's Late Veneer (the astrobio article)
The paper at the NHMFL (hotlinked pdf!)

Excerpts from articleI'm interpreting this as (possibly) something else that I can put the side of 'early Earth might have been cooler than thought'. Would this be ok to do? Thanks.

__________________

As I see it, the biggest problem with the "late veneer hypothesis" is that it sets up an either/or scenario, which is unsupportable ...

it's saying that the material which impacted during the LCB (Late Cometary Bombardment) was somehow fundamentally different to the material which went into protoplanetary formation ... that's a difficult position to maintain when the evidence thus far suggests that the materials were essentially the same ... so, the LCB can be said to have added more volatiles, rocky bits, and metals, to the surface ... but not all of them ...

the revolution in thinking over the past two or three decades is that accretion, and differentiation, can (and probably did) occur in much smaller timeframes than previously believed ... similar could be said for initial global cooling - the key unknown (as I indicated before) is the nature and density of the outer environment (enveloping gases and dust; primordial atmosphere; microplate densities, thicknesses, mobility; etc) ...

__________________

Oops. Do I have my understanding of the LCB/LV wrong? I thought that one of the first problems with the LV hypothesis was finding out that the water in 3 comets was qualitatively different to what we have here. In other words, that, amongst other things, places a very low limit of the proportion of materials that we could have gained.

__________________

Not sure what you mean by "qualitatively different" -
terrestrial water, even connate water, has had a few billion years of recycling and interesting thermal and chemical interactions with a whole host of mineral and biological stuff ... whereas cometary water should represent the primordial product, modified only by interactions with the solar wind and other primordial stuff (mostly dust) -
is that what you mean?

estimates have been made of the duration, intensity, and therefore the total amount, of materials dumped on Earth by the LCB - but I couldn't tell you off the top of my head what those estimates are ... finding direct samples of LCB material on Earth would be as close to impossible as can be managed - our environment has a habit of contaminating and assimilating primordial volatiles in a relatively short time - so, gems like carbonaceous chondrites need to be found shortly after impacting, if they are to be found at all ...

__________________

Hi cran. Sorry, I meant different hydrogen isotope ratios.

But don't worry about it. I've decided to have a serious crack at geology, and relying purely on Q&A is a very unsystematic way to learn. I'll have bits of unrelated facts swimming around my head.

So, I've decided to start with A History of Geology on wiki, and go on from there.

If you ever feel the need to reciprocate, A History of Economic Thought (1) and A History of Economic Thought (2) are good places to start.

__________________

I tried that once - it gave me a headache
(but an even bigger headache to a self-proclaimed economist) ...

these days, I limit my economic thoughts to -
"where did all my money go?"

__________________