PDA

View Full Version : cool early earth

Jens
2005-Nov-01, 06:11 AM
There has been a lot of talk recently about the possibility that the earth was cooler much earlier than was previously believed. This article from Scientific American gives a good introduction:

http://www.sciam.com/print_version.cfm?articleID=0005FA5D-5F7C-1333-9F7C83414B7F0000

There's something that bothers me, but I'm not sure if it's a valid question. What I'm wondering is, I believe our current assumption is that the earth was once very hot, and the heat that the earth has now is a remnant of that initial heat. So in other words, the earth is slowly cooling from that early Hadean period.

So the obvious (to me) question is, if the earth was cooler at an earlier date, what does that do to the equations about the current temperature? Wouldn't the current temperature also be lower? I'm thinking maybe that the cooling off is reverse-exponential, if that's the correct term, so a large difference at the beginning would only lead to a negligable difference today.

aurora
2005-Nov-01, 08:46 PM
A few points,

I think the article is talking about the temperature at the surface of the Earth, which is only a tiny fraction of the volume of the Earth. So when one is talking about the temperature of the Earth, or the rate of cooling, it makes a difference if one is considering the surface or the interior.

Second, it isn't just a linear cooling of the Earth from formation to the present. The Earth also generates heat from several sources, including radioactive decay, tidal forces, gravitational differentiation (not sure about the right word, but I mean dense materials settling toward the core). So the interior of the Earth cools slower than you would expect.

Jens
2005-Nov-02, 04:07 AM
I think the article is talking about the temperature at the surface of the Earth, which is only a tiny fraction of the volume of the Earth. So when one is talking about the temperature of the Earth, or the rate of cooling, it makes a difference if one is considering the surface or the interior.

I understand that, but isn't the temperature at the surface of the earth a function of the temperature at the interior? I would assume that most of the energy for the heat at the surface comes from below. Or are you saying that the temperature at the surface is raised by meteor strikes, for example, so that the surface could be hot even with a cold interior?

aurora
2005-Nov-02, 09:06 PM
More likely that the surface would be cool when the interior was hot.

I think the article was saying that the surface apparently cooled faster than previously thought.

Hugh Jass
2005-Nov-05, 07:03 AM
Basically the crust of the earth is an insulating layer protecting the surface/atmosphere from the temperature of the mantle. The generally held thoughts about how the crust formed and our atmosphere developed is that they kind of helped each other along. As one cooled so did the other and influenced each other until we got oceans which sped things up greatly to get to where we are now. But it was thought that as the first rocks of the crust formed, the atmosphere was still something akin to Venus. The Zircons they found appear look like really old subduction zone basalts. Well that would mean A) rocks that formed closer to the surface than would have been expected. Rocks that old are (If I remember right) all granitic, fairly deep in formation. B) to get geochemistry similar to subduction zone basalts according to all examples we know of you need liquid water to interact with the rock, and for the rock to re-melt, which should not have occurred due to the extreme heat of the atmosphere at the time and another ~few 100 million years.

I’m not sure but I’m thinking maybe the water was from a Comet, maybe a direct impact. The chances are next to zero for something like that, but liquid water on the surface, and a mechanism for re-melt is even more remote. It is still interesting that Crustal rocks seem to have formed a couple 100 million years prior to previously thought, and be maybe more widespread, But I think the jury is still out on the temperature of the atmosphere.

PraedSt
2008-Nov-27, 11:31 AM
Early plate tectonics: more evidence for a cooler Hadean

First water, now this: Plate-tectonics may have started over 4bn years ago (http://www.physorg.com/news146924511.html)

"We are proposing that there was plate-tectonic activity in the first 500 million years of Earth's history," said geochemistry professor Mark Harrison, director of UCLA's Institute of Geophysics and Planetary Physics and co-author of the Nature paper. "We are reporting the first evidence of this phenomenon."

Hopkins analyzed the zircons with UCLA's high-resolution ion microprobe, an instrument that enables scientists to date and learn the exact composition of samples with enormous precision. The microprobe shoots a beam of ions, or charged atoms, at a sample, releasing from the sample its own ions, which are then analyzed in a mass spectrometer. Scientists can aim the beam of ions at specific microscopic areas of a sample and conduct a high-resolution isotope analysis of them without destroying the object.

"The global average heat flow in the Earth's first 500 million years was thought to be about 200 to 300 milliwatts per meter squared," Hopkins said. "Our zircons are indicating a heat flow of just 75 milliwatts per meter squared — the figure one would expect to find in subduction zones, where two plates converge, with one moving underneath the other."

"The data we are reporting are from zircons from between 4 billion and 4.2 billion years ago," Harrison said. "The evidence is indirect, but strong. We have assessed dozens of scenarios trying to imagine how to create magmas in a heat flow as low as we have found without plate tectonics, and nothing works; none of them explain the chemistry of the inclusions or the low melting temperature of the granites."

Evidence for water on Earth during the planet's first 500 million years is now overwhelming, according to Harrison.

"You don't have plate tectonics on a dry planet," he said.

p.s. seeing as this strictly matches the title but not the OP, plus the fact that there seem to be many geologists here, I might mirror this as new thread. If no-one minds.

jlhredshift
2008-Nov-28, 11:46 AM
A brief and non comprehensive history of Earth:

Accretion (protoplanetary disc) > Big Whack (Moon formation) > differentiation (core formation) > surface cooling (retention of volatiles)

cbacba
2008-Nov-28, 01:54 PM
I'm wondering if some of this has something to do with the warming sun thought to be significantly lower in output long ago compared to now. Most of our understanding has already been posted here. It's really hot deep down - like surface of the Sun hot, 6000K. Dirt/rocks are a major insulator keeping us from being evaporated by the heat. Much of this is the heat of formation of the planet. Best scenario for the moon is we were slammed by a Mars sized object quite some time back. That added a lot of energy to the Earth and evidently created the debris that combined into our moon. Thermal energy leakage is not enough to keep the surface anywhere close to warm. That's all solar radiation doing that and even this keeps us above freezing at least in some areas only because of our atmosphere.

PraedSt
2008-Nov-28, 02:15 PM
How did the surface cool so fast, is what I'm confused about? If the interior was so warm, that is.

Is rapid cooling of just the surface normal? Is there a well-known mechanism? :confused:

cjameshuff
2008-Nov-28, 05:06 PM
How did the surface cool so fast, is what I'm confused about? If the interior was so warm, that is.

Is rapid cooling of just the surface normal? Is there a well-known mechanism? :confused:

Poor thermal conductivity of rock. Heat couldn't be transported from below fast enough to keep up with the heat radiated away into space. A thick blanket of rock makes an effective insulator, without the sun's input, the surface would be a lot cooler...most of our surface heat comes from the sun.

It's also not jut thermal inertia keeping the core warm. First, the same insulating effect means a relatively low amount of radioactive decay would keep the core warm for a considerable amount of time. Second, the inner core is very compact solid iron, the outer core is liquid. Removing heat doesn't cool the outer core, it just allows more of it to solidify, similar to how ice water at the surface stays at the freezing point until it has completely frozen or melted. So while the core is losing heat, it's not necessarily dropping in temperature.

PraedSt
2008-Nov-28, 06:09 PM
Poor thermal conductivity of rock...
Sorry cjameshuff, what you said makes perfect sense, but I worded my question badly. My fault. :doh:

I should have asked: how do we account for the faster than expected rate of surface cooling? How did the initial insulation of rock appear so fast?

As I understand it (and this is where I may be going wrong):

1. We used to think the surface of Hadean Earth was hot.
2. We now have reasonable evidence to think that it might have been cooler.

aurora and Hugh Jass referring to the OP article:

I think the article was saying that the surface apparently cooled faster than previously thought.

It is still interesting that Crustal rocks seem to have formed a couple 100 million years prior to previously thought, and be maybe more widespread...

Do you know what we've changed in our models to accommodate (2), instead of (1)?

Or is this still a matter of dispute? I haven't gone and blundered into ATM have I? :)

JohnD
2008-Nov-28, 10:45 PM
This debate parallels that between Darwin and Lord Kelvin in the 19th century. Kelvin calculated from the properties of Earth rocks and estimates of the initial temperature that the Earth could be no more than 100 million years old, which was far too little time for Darwin's theory to take effect. Extraordinarily, when his theory was attacked, pricipally for ignoring convection ("Are you mad,Sir? In the solid Earth?!") in favour of conduction, he revised his estimate - downwards! To 20 million years!

See: http://www.usd.edu/esci/age/content/failed_scientific_clocks/kelvin_cooling.html

John

PraedSt
2008-Nov-28, 11:18 PM
That's an interesting link. Thanks JohnD. :)

dgavin
2008-Nov-29, 03:00 AM
Early plate tectonics: more evidence for a cooler Hadean

First water, now this: Plate-tectonics may have started over 4bn years ago (http://www.physorg.com/news146924511.html)

"You don't have plate tectonics on a dry planet," he said.

That's true for small planets from Murcury up to about 2 times earth masses. With some notable exceptions such as IO thats Tectonicly active due to Gravitational Stresses.

From about about 2 to 3.5 Earth masses it's guessed you only need a large moon, or oceans, but not both to be Tectonicly active. Earth needs both.

And from about 3.5 to 8 Earth mass neither a moon nor oceans are needed for a techtonicly active planet.

PraedSt
2008-Nov-29, 03:33 AM
That's true for small planets from Murcury up to about 2 times earth masses. With some notable exceptions such as IO thats Tectonicly active due to Gravitational Stresses.

From about about 2 to 3.5 Earth masses it's guessed you only need a large moon, or oceans, but not both to be Tectonicly active. Earth needs both.

And from about 3.5 to 8 Earth mass neither a moon nor oceans are needed for a techtonicly active planet.

EDIT: Wait. The website in your sig?

timb
2008-Nov-29, 03:41 AM
This debate parallels that between Darwin and Lord Kelvin in the 19th century. Kelvin calculated from the properties of Earth rocks and estimates of the initial temperature that the Earth could be no more than 100 million years old, which was far too little time for Darwin's theory to take effect. Extraordinarily, when his theory was attacked, pricipally for ignoring convection ("Are you mad,Sir? In the solid Earth?!") in favour of conduction, he revised his estimate - downwards! To 20 million years!

See: http://www.usd.edu/esci/age/content/failed_scientific_clocks/kelvin_cooling.html

John

Kelvin is Lord! (http://zapatopi.net/kelvin/) (Though he did get rather pompous in his old age).

JonClarke
2008-Nov-29, 06:31 AM
I wouldn't mind later (in ATM). Right now I just want to know how we've accounted for the quicker cooling (mainstream).

I aam yet to be convinced that there is a problem that needs to be accounted for. The lines of evidence for anything in the Hadean are very slim and very much assumption driven. I would be extremely sceptical of any strong claaims about anything about surface conditions on the Earth before 3.5 GA (which are the oldest well preserved suprcustal rocks).

dgavin
2008-Nov-29, 06:35 AM

EDIT: Wait. The website in your sig?

The numbers are basically guess work, but theroretical models show the heavier a planet gets the less assistance it needs for techtonics.

Here is one article about it, http://news.softpedia.com/news/Newly-Discovered-Earth-like-Planets-Must-Also-Have-Plate-Tectonics-71669.shtml

BobtheEnforcer
2008-Nov-29, 06:10 PM
How did the surface cool so fast, is what I'm confused about? If the interior was so warm, that is.

Is rapid cooling of just the surface normal? Is there a well-known mechanism? :confused:

The interior of the earth doesn't directly impact the heat of the earth's surface. It does indirectly, by greenhouse gases being emitted volcanically. The surface of the earth is primarily controlled by solar radiation and climate/heat transfer. The important point here is that in the past the sun was less strong, and its getting stronger as time progresses. As helium builds in the core, the amount of solar radiation increases. So, early in earths history, the sun was about 6% less luminance.

PraedSt
2008-Nov-30, 06:13 AM
I aam yet to be convinced that there is a problem that needs to be accounted for. The lines of evidence for anything in the Hadean are very slim and very much assumption driven. I would be extremely sceptical of any strong claaims about anything about surface conditions on the Earth before 3.5 GA (which are the oldest well preserved suprcustal rocks).
Oh ok. You mean: 1) assume for now the hot surface Hadean is correct and 2) the cold surface claim is still weak? I'm ok with that.
I would however, be interested to know how the cold surface people decide to account for this early cooling.

The numbers are basically guess work, but theroretical models show the heavier a planet gets the less assistance it needs for techtonics.Thanks.

PraedSt
2008-Nov-30, 06:15 AM
The interior of the earth doesn't directly impact the heat of the earth's surface. It does indirectly, by greenhouse gases being emitted volcanically. The surface of the earth is primarily controlled by solar radiation and climate/heat transfer. The important point here is that in the past the sun was less strong, and its getting stronger as time progresses. As helium builds in the core, the amount of solar radiation increases. So, early in earths history, the sun was about 6% less luminance.
Thanks, I'm ok with what you describe. I actually, and stupidly, framed my question wrong. Post 15 (http://www.bautforum.com/1375560-post15.html) is a better version! :) Thanks again.

JonClarke
2008-Nov-30, 07:04 AM
Oh ok. You mean: 1) assume for now the hot surface Hadean is correct and 2) the cold surface claim is still weak? I'm ok with that.

I think hot or cold, the evidence is luke warm:)

The problem is it is all so indirect, based on isotope ratios in a few tiny mineral grains taken rocks that are incredibly metamorphosed to start with. Plus theoretical models that are vased on assumptions, reasonable perhaps, but still assumptions.

IMHO!

timb
2008-Nov-30, 11:47 AM
I think hot or cold, the evidence is luke warm:)

The problem is it is all so indirect, based on isotope ratios in a few tiny mineral grains taken rocks that are incredibly metamorphosed to start with. Plus theoretical models that are vased on assumptions, reasonable perhaps, but still assumptions.

IMHO!

Strange how topics seem to come in spurts. In the ITWire: according to an Australian ancient-earth researcher, about two to three billion years ago most of the Earth was buried under six to nine miles (10 to 15 kilometers) of lava from a series of volcanic explosions. (http://www.itwire.com/content/view/21983/1066/) The abstract of the article they cite (http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4TKXDBX-2&_user=10&_coverDate=11%2F15%2F2008&_alid=832616083&_rdoc=2&_fmt=high&_orig=search&_cdi=5801&_sort=d&_docanchor=&view=c&_ct=35&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=4af33c0bc53da28144272b880e2692b8) doesn't directly support their lead-in, and parts of the ITWire article don't inspire confidence in their scientific literacy ("the volcanoes went dormant but eventually repeated the cycle after about 100,000 to 150,000 million years" :surprised).

lomiller1
2008-Nov-30, 04:22 PM
The interior of the earth doesn't directly impact the heat of the earth's surface. It does indirectly, by greenhouse gases being emitted volcanically. The surface of the earth is primarily controlled by solar radiation and climate/heat transfer. The important point here is that in the past the sun was less strong, and its getting stronger as time progresses. As helium builds in the core, the amount of solar radiation increases. So, early in earths history, the sun was about 6% less luminance.

It’s more then 6% for the timescales we are looking at, closer to 25%

timb
2008-Nov-30, 10:13 PM

PraedSt
2008-Nov-30, 10:24 PM
Strange how topics seem to come in spurts. In the ITWire: according to an Australian ancient-earth researcher, about two to three billion years ago most of the Earth was buried under six to nine miles (10 to 15 kilometers) of lava from a series of volcanic explosions. (http://www.itwire.com/content/view/21983/1066/) The abstract of the article they cite (http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4TKXDBX-2&_user=10&_coverDate=11%2F15%2F2008&_alid=832616083&_rdoc=2&_fmt=high&_orig=search&_cdi=5801&_sort=d&_docanchor=&view=c&_ct=35&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=4af33c0bc53da28144272b880e2692b8) doesn't directly support their lead-in, and parts of the ITWire article don't inspire confidence in their scientific literacy ("the volcanoes went dormant but eventually repeated the cycle after about 100,000 to 150,000 million years" :surprised).
Just curious as to your views on this paper. Other than 'they're probably scientifically illiterate' :)

Just asking because I found it interesting, but I don't yet have way of judging the geological feasibility/credibility of what they say.

timb
2008-Nov-30, 10:49 PM
Just curious as to your views on this paper. Other than 'they're probably scientifically illiterate' :)

Just asking because I found it interesting, but I don't yet have way of judging the geological feasibility/credibility of what they say.

I defer to Jon on all questions relating to geology. Saying (as the abstract does) there were dramatic burst(s) of magmatism lasting a few million years before 1.8Ga ago doesn't bother me, but claiming (as the media report does) the Earth was covered in lava that recently seems wrong. How could life have survived?

PraedSt
2008-Nov-30, 11:33 PM
I defer to Jon on all questions relating to geology. Saying (as the abstract does) there were dramatic burst(s) of magmatism lasting a few million years before 1.8Ga ago doesn't bother me, but claiming (as the media report does) the Earth was covered in lava that recently seems wrong. How could life have survived?
Thanks. The article says:
This explosive activity happened about two to three billion years ago, and each episode lasted for approximately one hundred million years.
[...]
Each episode destroyed most of the microscopic life on Earth, but the life that remained, only the strongest species, was able to recover and survive during the dormancy of the volcanoes.
[...]
The episodes ended about three billion years ago, and are not expected to occur again, according to Davies.
Stopped 3bn yrs ago. That would be ok, I think?

Jim
2008-Dec-01, 10:22 PM
The georeactor is definitely ATM, and if we want to discuss this we need to move the thread.

Jon

That there might be a lot of uranium in the Earth's core is standard theory. And it's a proven fact that fission has taken place near the surface at the oklo natural reactor in Gabon, I think it was. So it's not much of a stretch to suppose that fission could also take place within the Earth's core. ...

This post and the replies to it have been moved to ATM, http://www.bautforum.com/against-mainstream/81859-georeactor.html.

(Warren, you should know better by now.)

dgavin
2008-Dec-02, 01:24 AM
I defer to Jon on all questions relating to geology. Saying (as the abstract does) there were dramatic burst(s) of magmatism lasting a few million years before 1.8Ga ago doesn't bother me, but claiming (as the media report does) the Earth was covered in lava that recently seems wrong. How could life have survived?
Dramatic outbursts of Magma have happened even in recent geological history, although I would not say these came close to ever covering the earth in Lava.

About 17 Million years ago a fissure type eruption from mid Arizona to eastern Washington occurred, the results of which are many of the high desert plains volcano fields.

Eventually it settled down into a split hot-spot system, the southern portion migrating from Arizona into Oregon to what is the Newberry Caldera region. And the Northern part migrating from Eastern Washington south then curving east then back to north east, to the Yellowstone Caldera region.

I expect that when this system was in it's fissure stage, that it would of been quite a sight, and anything alive seeing it might -think- the world was covered in Lava. All told though it only covered about 2-3% of the North American continent with lava flows. (Or about 8% of the continental united states)

cran
2008-Dec-02, 01:08 PM
As I understand it (and this is where I may be going wrong):

1. We used to think the surface of Hadean Earth was hot.
2. We now have reasonable evidence to think that it might have been cooler.
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:

What I'm wondering is, I believe our current assumption is that the earth was once very hot, and the heat that the earth has now is a remnant of that initial heat. So in other words, the earth is slowly cooling from that early Hadean period.

So the obvious (to me) question is, if the earth was cooler at an earlier date, what does that do to the equations about the current temperature? Wouldn't the current temperature also be lower? I'm thinking maybe that the cooling off is reverse-exponential, if that's the correct term, so a large difference at the beginning would only lead to a negligable difference today.
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) ...

PraedSt
2008-Dec-02, 01:55 PM
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 ...
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?

jlhredshift
2008-Dec-02, 03:24 PM
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.

PraedSt
2008-Dec-02, 07:11 PM
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.
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.

jlhredshift
2008-Dec-02, 07:25 PM
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?

PraedSt
2008-Dec-02, 08:02 PM
As the plates are driven around the points of eruption have changed.
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!

jlhredshift
2008-Dec-02, 08:08 PM
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.

PraedSt
2008-Dec-02, 08:29 PM
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.
Good grief. I've actually learnt something today. Thanks jlhredshift! I owe you one. :)

jlhredshift
2008-Dec-02, 08:35 PM
Good grief. I've actually learnt something today. Thanks jlhredshift! I owe you one. :)

geonuc
2008-Dec-02, 10:19 PM
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.

jlhredshift
2008-Dec-02, 10:48 PM
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 (http://www.sciencemag.org/cgi/content/abstract/313/5791/1281?ck=nck).

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

Jim
2008-Dec-02, 10:57 PM
This post and the replies to it have been moved to ATM, http://www.bautforum.com/against-mainstream/81859-georeactor.html.

(Warren, you should know better by now.)

Oops, turns out Vallkyn started all this. Sorry, Warren.

cran
2008-Dec-03, 12:08 AM
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?
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 ...

jlhredshift
2008-Dec-03, 12:18 AM
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.

cran
2008-Dec-03, 12:48 AM
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.

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 ...

geonuc
2008-Dec-03, 10:12 AM
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/v456/n7221/abs/nature07465.html

cran
2008-Dec-03, 11:59 AM
Our thermobarometric analyses of 4.02–4.19-Gyr-old inclusion-bearing zircons constrain their magmatic formation conditions to about 700 °C and 7 kbar. This result implies a near-surface heat flow of http://www.nature.com/__chars/math/special/sim/black/med/base/glyph.gif75 mW m-2, about three to five times lower than estimates of Hadean global heat flow. As the only site of magmatism on modern Earth that is characterized by heat flow of about one-quarter of the global average is above subduction zones, we suggest that the magmas from which the Jack Hills Hadean zircons crystallized were formed largely in an underthrust environment, perhaps similar to modern convergent margins. - 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:

... Geologists now almost universally agree that by 4.2 billion years ago, the Earth was a pretty placid place, with both land and oceans. Instead of hellishly hot, it may have frozen over. Because the young Sun put out 30 percent less energy than it does today, temperatures on Earth might have been cold enough for parts of the surface to have been covered by expanses of ice...-from the NYT article (linked above)

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

geonuc
2008-Dec-03, 12:03 PM
which is roughly a quarter of estimates (made when? based on what?) of global heat flow ...

PraedSt
2008-Dec-03, 12:17 PM
and the 700C (@ 7kbar) is consistent with other studies ...

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

cran
2008-Dec-03, 12:44 PM
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 ...

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 ...

cran
2008-Dec-03, 01:01 PM
Just want to orient myself here. How deep down do you have to go to get pressures like that?
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 (http://209.85.173.132/search?q=cache:1fU3_1wBHWcJ:www.tamuk.edu/geo/Mcgehee/physical/CH8_Metamorphism.doc+depth+%227+kilobars%22&hl=en&ct=clnk&cd=1&gl=au)

PraedSt
2008-Dec-03, 10:58 PM
in modern continental settings, it can be pretty deep (down to 20km @ 600C) though still within the crust ...
Thanks Cran!

dgavin
2008-Dec-05, 04:07 AM
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.

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.

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.

PraedSt
2008-Dec-05, 07:52 PM
Late Veneer question

While on my 'Early Earth' quest (:D), 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 (http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=2708) (the astrobio article)
The paper at the NHMFL (http://www.magnet.fsu.edu/library/publications/NHMFL_Publication-4058.pdf) (hotlinked pdf!)

Excerpts from article
The paper provides a direct challenge to the popular “late veneer hypothesis,” a theory which suggests that all of our water, as well as several so-called “iron-loving” elements, were added to the Earth late in its formation by impacts with icy comets, meteorites and other passing objects.
[...]
To test the hypothesis, Humayun and his NASA colleagues[...]used a massive 880-ton press to expose samples of rock containing palladium[...]to extremes of heat and temperature equal to those found more than 300 miles inside the Earth. The samples were then brought to the magnet lab, where Humayun used a highly sensitive analytical tool known as an inductively coupled plasma mass spectrometer, or ICP-MS, to measure the distribution of palladium within the sample.

“At the highest pressures and temperatures, our experiments found palladium in the same relative proportions between rock and metal as is observed in the natural world,” Humayun said. “Put another way, the distribution of palladium and other siderophile elements in the Earth’s mantle can be explained by means other than millions of years of meteorite bombardment.”

“This work will have important consequences for geologists’ thinking about core formation, the core’s present relation to the mantle, and the bombardment history of the early Earth,” he said. “It also could lead us to rethink the origins of life on our planet.”

I'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.

cran
2008-Dec-06, 01:18 PM
Late Veneer question

While on my 'Early Earth' quest (:D), 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 (http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=2708) (the astrobio article)
The paper at the NHMFL (http://www.magnet.fsu.edu/library/publications/NHMFL_Publication-4058.pdf) (hotlinked pdf!)

Excerpts from article

I'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) ...

PraedSt
2008-Dec-07, 02:33 PM
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 ... 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. :confused:

cran
2008-Dec-07, 07:42 PM
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. :confused:

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 ...

PraedSt
2008-Dec-07, 08:12 PM
Not sure what you mean by "qualitatively different" -..
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 (http://en.wikipedia.org/wiki/History_of_geology) on wiki, and go on from there.

If you ever feel the need to reciprocate, A History of Economic Thought (1) (http://en.wikipedia.org/wiki/History_of_economic_thought)and A History of Economic Thought (2) (http://cepa.newschool.edu/het/) are good places to start. :D

cran
2008-Dec-07, 09:33 PM
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 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 (http://en.wikipedia.org/wiki/History_of_geology) on wiki, and go on from there.

If you ever feel the need to reciprocate, A History of Economic Thought (1) (http://en.wikipedia.org/wiki/History_of_economic_thought)and A History of Economic Thought (2) (http://cepa.newschool.edu/het/) are good places to start. :D

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?"

mythusmage
2010-Feb-26, 08:37 AM
Assuming that the Earth had cooled down substantially prior to the Earth-Theia impact, and that evidence of the cooling survived the impact, one wonders if maybe the Earth-Theia encounter wasn't so much an impact as a side-swipe. Perhaps even a near miss.

Scenario: Theia approaches the early Earth. As the two worlds get closer each planet's gravity stresses the other, causing flexing, the fracturing of their crusts, and in the case of Theia the loss of material to space and the transportation of much of that material to Earth. This last producing a bout of impacts of Theia matter serving to partially melt the crust.

The end result is a much reduced Theia, a temporary ring of former Theia material, and an enlarged Earth with a thinner crust than before. Combine this last with the gravitational influence of the new Luna and you get a more tectonically active crust as a combination of liquid water and tidal stresses keeps things moving.

Now contrast Earth with Venus, where no such close encounter took place (so far as we know). The Venerian crust formed quickly, liquid water formed on the surface, and then --- before that water could saturate the Venerian crust, rising atmospheric temperatures evaporated the water the resulting water vapor producing a run away green house effect that doomed Venus to its current condition.

In contrast Earth escaped that fate because of a close encounter with a Mars sized world. An encounter that resulted in a rather lop-sided moon, a rather fragile Earthly crust, and conditions conducive to life.

Makes me wonder. What if Venus had had a close encounter with her own Theia. Let's call this world Aphrodite, and the moon that results from the near miss Eros. What would the Venus/Eros system be like? Would it have life? What would Venerian history be like? Would Venus have had an Ice Venus period considering how much closer the planet is to the Sun? Without an Ice-Venus period what would the most likely course of development for Venerian life?

G O R T
2010-Feb-26, 12:24 PM
It is likely that Venus was hit early on in a way that actually reversed its rotation.

This would mean the Venus collusion would have been more of a sideswipe.

Universetoday (http://www.universetoday.com/guide-to-space/venus/retrograte-rotation-of-venus/)

Scientific American (http://www.scientificamerican.com/article.cfm?id=double-impact-may-explain)

cran
2010-Feb-26, 02:53 PM
Assuming that the Earth had cooled down substantially prior to the Earth-Theia impact, and that evidence of the cooling survived the impact ...

unfortunately, no such evidence has been found of pre-Theian terrestrial crust;
the few surviving crystal fragments which have been found are post-Theian and pre-LCB (LHB, or LV) -
the only pre-Theian mineral fragments found to date are extraterrestrial (meteoritic) ...
the assumption of pre-Theian crustal cooling is based on mathematical models, and limited studies of other bodies ...

which makes the rest of your idea highly speculative (and unnecessary) ...

neilzero
2010-Feb-27, 12:17 AM
It is generally thought that Earth's surface temperature is the result of the Sun and only a few parts per million due to the hot interior for the past 4 billion years, so the fact that the interior was twice as hot long ago is almost irrelevant. At 4.6 billion years ago the surface was likely molten because the Earth was growing larger. Compression heating is likely minor now as Earth gains mass at perhaps 1 part per billion per century, and the moon is farther from Earth and still receding about one centimeter per year. Radio active heating is also down about by half as some of the isotopes have reached non-radioactive in the past 4 billion years. Neil

mythusmage
2010-Feb-27, 12:40 AM
unfortunately, no such evidence has been found of pre-Theian terrestrial crust;
the few surviving crystal fragments which have been found are post-Theian and pre-LCB (LHB, or LV) -
the only pre-Theian mineral fragments found to date are extraterrestrial (meteoritic) ...
the assumption of pre-Theian crustal cooling is based on mathematical models, and limited studies of other bodies ...

which makes the rest of your idea highly speculative (and unnecessary) ...

How old are the oldest zircons? When was the Earth-Theia encounter? Far as I know the oldest zircons pre date the near miss. While strictly speaking the crust may not have survived the encounter, the destruction of gross structure does not mean the necessary destruction of fine structures.

BTW, speculation may be unproductive, but it is never unnecessary. For even unproductive speculation can still demonstrate unprofitable paths of inquiry.

cran
2010-Feb-27, 03:26 AM
How old are the oldest zircons?
4.38 Ga - rounded up to 4.4 Ga BP...
discovered by my old Curtin uni geology professor - Simon Wilde -
in a 3.3 Ga old Jack Hills (Western Australia) rock ...
http://www.geology.wisc.edu/zircon/Earliest%20Piece/Earliest.html

When was the Earth-Theia encounter? roughly 100 million years earlier than the oldest surviving terrestrial zircon fragments found to date ...
~4.5 Ga BP ...

[

mythusmage
2010-Feb-27, 04:32 AM
4.38 Ga - rounded up to 4.4 Ga BP...
discovered by my old Curtin uni geology professor - Simon Wilde -
in a 3.3 Ga old Jack Hills (Western Australia) rock ...
http://www.geology.wisc.edu/zircon/Earliest%20Piece/Earliest.html

So the oldest zircons were formed just 120 million years after the Earth itself started forming. Which tells us a lot about how and how swiftly planetary formation takes.

roughly 100 million years earlier than the oldest surviving terrestrial zircon fragments found to date ...
~4.5 Ga BP ...

That's not the message I get from this page (http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=446). The author, G. Jeffrey Taylor has this to say on the timing of the impact,

Simulations indicate that the third stage took 100 to 200 million years, about the time estimated from isotopic data on rocks from Earth, the Moon, and meteorites.

That is, between 20 million years before the first zircons, to as much as 80 million after the first zircons. If the latter is the case that means the early Earth had a solid crust and liquid water before Theia bumped into it. If this is the case the pre-impact Earth might have had life, though that depends on how much needs to happen before life can arise and on how long it takes.

Depending on the extent of the destruction of the early crust, complete or incomplete, this early life may have been utterly destroyed. If it was, then our life would be Earth's second domain, and Earth's first life may have been something fundamentally different. Using different nucleotides as the basis of its genetics, and even a different number. If pre-impact crustal material can be found at the Lagrange points we might find evidence of pre-impact life and pre-impact genes with something like 4, 6, or maybe even 8 nucleotide based genes. But all this depends on how things happened and their chronology.

Almost forgot. Do you have contact information for Dr. Simon or could you get in touch with him? I think his input would be helpful here.

cran
2010-Feb-27, 10:36 AM
So the oldest zircons were formed just 120 million years after the Earth itself started forming. Which tells us a lot about how and how swiftly planetary formation takes.
Some early stages are much swifter than that; ~2-4 orders of magnitude less ...

That's not the message I get from this page (http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=446). The author, G. Jeffrey Taylor has this to say on the timing of the impact,

Simulations indicate that the third stage took 100 to 200 million years, about the time estimated from isotopic data on rocks from Earth, the Moon, and meteorites.
the protoplanetary impact phase/stage ... yes, the Theia impact event is included in this stage ...

but also (in the same article):

Calculations indicate that the Moon could have formed from that debris in ten years or less! This implies that the Moon would have formed very hot, possibly entirely molten...

The oldest materials that formed in the Solar System are inclusions rich in calcium and aluminum found within carbonaceous chondrite meteorites. Nicknamed CAIs (for Calcium-Aluminum-rich Inclusions), these objects are thought to have been some of the first solids to form after the cloud of gas and dust began to heat up. CAIs have ages of 4.566 billion years. On the basis of measurements of several isotopes, the Earth and Moon formed about 50 to 100 million years later...
ie, 4.516 - 4.466 Ga ... or 4.491(+0.025) GA ... commonly rounded to 4.5 Ga ...

or, "roughly 100 Ma earlier" than the oldest (4.4 Ga) terrestrial zircons found to date ...

so, where is the conflict in information that you see?

Almost forgot. Do you have contact information for Dr. Simon or could you get in touch with him? I think his input would be helpful here.I do, and I can ... but Simon Wilde, like any busy research scientist who is also a university lecturer, and an administrator of the school of geology, would simply tell you to read the published papers and attend the lectures ...

you can contact him yourself via the School of Geology at Curtin University in Western Australia; the university has a website, and he is listed on the faculty ...

Ara Pacis
2010-Feb-27, 07:41 PM
I'm not sure if this is related or not:

Schmitt estimates that the Procellarum basin formed at about 4.3 b.y and South Pole-Aitken at about 4.2 b.y. If these formation ages are in the ballpark, they suggest an explanation for detrital zircon (ZrSiO4) crystals of about the same ages in very old sedimentary rocks on Earth. Early impacts of the scale of South Pole-Aitken and Procellarum, occurring in water-rich environments such as the Earth and Mars, would create thick sheets of impact generated rock melt on a continental scale. As these magma sheets crystallized, zirconium concentrations may have reached levels that produced the very old zircons.At first I thought he was saying that some older zircons on Earth came from the moon, but after re-reading it I think he means similar impacts may have created them on Earth and that somehow this affects the validity of the Giant Impact Hypothesis.

cran
2010-Feb-28, 12:25 AM
I'm not sure if this is related or not:
At first I thought he was saying that some older zircons on Earth came from the moon,
yes ... I can see how that could happen in the way the lines are structured - I had to read it twice to catch the difference, and realise that Schmitt is arguing that:
large impacts occurred on Earth as well as on the Moon (and Mars, etc), post-Theia and prior to the Late Heavy Bombardment (ie, the relatively* quiet stage following on from the proto-planetary impact stage); and
that the pre-LHB zircons formed in cooling magma sheets caused by such impacts ...
*relative to the pre- and post- times; compared to modern time, still a "sky is falling!" era ...

but after re-reading it I think he means similar impacts may have created them on Earth and that somehow this affects the validity of the Giant Impact Hypothesis.not sure how it would (or could) affect the GI hypothesis - it's pretty well agreed that there were lots of big things falling onto bigger things at the time (for some time prior and following) ...

apart from some desire to create huge numbers of zircons at a given time, there's no need for the mechanism Schmitt proposes - zircons are common enough (and resilient enough) in igneous deposits throughout time, that they are the main targets (especially in Archaean deposits) for radiometric dating - and, like quartz, they are also transported and concentrated into various sand deposits ... which over time become sandstones ...

...........

that said - Schmitt's stronger arguments against the GI hypothesis are:

"The major problem with this hypothesis," says Schmitt, "is that the interior of the Moon is not cooperating. Most importantly, the lower lunar mantle, based on analyses of the Apollo 17 orange pyroclastic glass, has a chondritic, that is, primordial elemental and isotopic imprint. This primordial imprint would have disappeared or have been significantly modified if the mantles of the Earth and the impactor had already formed as required by the current Giant Impact hypothesis.- which is a tricky and somewhat complex argument to work through - the central question would be "how much fractionation and differentiation would have to have occurred - and how much could have occurred within the first ~60 million years of formation, while primordials were continually being added to the young terrestrials?"

and:

Similarly, many scientists agree that the Moon's 50 or so basins greater than 300km in diameter, as well as most other ancient lunar craters, were formed at about the same time by an apparent "cataclysm" 3.9 billion years ago. According to Schmitt, "the primary argument against this hypothesis is found in the sampling sites for Apollo and lunar meteorite samples of impact-created glass for which formation ages have been determined. These samples have come largely from the surface of the Moon most affected by the 14 youngest large basin-forming impacts and debris thrown from them. These 14 youngest impacts are, indeed, 3.9-3.8 billion years old based on the dating of Apollo samples. A variety of volcanic and impact evidence indicates that it is highly unlikely that all the 35 or more older impact basins formed during the same interval.which is basically saying that large to very large impacts also occurred earlier than the LHB (~3.9 Ga) - that's not really a "shocking" revelation; the proto-planetary impact stage was full of such things - the generalisation that all large lunar basins resulted from the LHB, however, may well be worthy of skepticism ...

jlhredshift
2010-Feb-28, 01:36 AM
For a discussion about Schmitt in 2008 and abstract see: