Ask a Geologist

[Orogeny]

I can understand how layers of salt could be formed in a dried up lake bed, but shouldn't the salt dissolved when the next layer of sediment is being laid down? Or are layers above salt always wind borne?

Look back at the scale of the diapir diagram and the scale of the salt walls in the Paradox basin. These structures can be tens of kilometers long and kilometers high. We're not talking about dry lake bed salt, we're talking about salt thousands of meters thick and thousands of square kilometers in area. Typically these deposits form in early rift basins (although neither the Gulf of Mexico or Paradox happen to be rift basins) where the basin floor is essentially at sea level, so that minor rises in sea level (fluctuations on the Milankovitch scale) inundate the basin, and then minor drops cut the basin off from the global ocean, causing the basin to dry up and precipitate salt. These salt deposits take hundreds of thousands to millions of years to form, so they won't be completely dissolved by a bit of water.

It's important to understand that the salt is typically not pure; there are usually thinly interbedded carbonates, silts, and shales, as well as the occasional volcanic deposit. When the salt is exposed to water, the top of the salt may indeed dissolve, but this leaves behind a residue of whatever is interbedded with the salt. This residue forms an impermeable (or weakly permiable) cap over the underlying salt, protecting it from further dissolution. Juve is partially right as well, in that sediments deposited over the salt will also have an insulating effect.

 

[Assyrian]

thanks

 

[Orogeny]

There is a lot of salt in layers under the great lakes. Back when they were salt water the salt just settled to the bottom.

The salt was there before the lakes were. The lakes are thought to only be as old as the last glacial maximum, which ended about 10,000 years ago, and are thought to have been carved out by glacial action. The salt is Silurian in age, and was deposited in restricted basins similar to those I describe above.

 

[Orogeny]

How common is it to find fossilized coral in limestone in Britain? I'm sure I found some while digging up the garden.

I suspect it's not uncommon- corals are an important constituent of many limestones. If you can be more specific about where you are, I can try to find some literature on the area.

If you can post a picture, I'd very much like to see what you found.

 

[Orogeny]

Of course the big question has to do with age. Now to be sure you have unrelated ways to verify the age of the earth. For example we have the receding rate of the moon.

This is an inaccurate and difficult measure of the Earth's age at this point, because the physics of the system are incredibly complex. Please excuse the 'biased' source, but this is the best description I could find of earth-moon physics for the layman (me!).

We have things like Niagara Falls where we know the erosion rate and we can see how much erosion there is, so we can determine age from that.

As already noted, the Great Lakes are only as old as the end of the last glacial maximum, so any waterfall emptying into them must not be older than this. At best Niagara Falls give us a minimum age of the earth.

Still they change this stuff all the time. They just recently changed the age they think the universe is.

Who? And where? And why is this bad? Shouldn't we modify our theories with the acquisition of new data?

One thing it all comes down to is an assumption of what is consistent. We do not know for sure that it is all that consistent. So how do we know we can depend on you to give us the right time frame for the various geological ages?

The best that we can do is observe nature and develop models from these observations that attempt to mimic what natural systems. Are these models perfect? Never. Are the models close? Sometimes, and we (try to) discard those that aren't. As new data is gathered, the models will change to incorporate the data, hopefully bringing us asymptotically closer to a perfect model. This constant barrage of 'well things keep changing!' from creationists isn't informing us of something we don't already know. We're getting closer, and that's what is important.

Now it seems that you're trying to address radiometric dating. As there are separate threads for this topic, I request that we not deal with it here. If you're willing to wade through the piles of muck in those threads, you'll find quite a bit of useful information about how radiometric dating is performed. IMHO, you'd be better served to simply google 'radiometric dating' and spend a couple of days reading through wiki and .edu sites with an open and curious mind. If you're coming in with no background you're not going to understand it in two days, but you'll be closer. It's worth the effort.

 

[Naraoia]

Apparently it was derived from the Greek word diapeirein, meaning to pierce, since salt diapirs were originally thought to have forcibly intruded into overlying strata due to buoyancy and/or geopressuring.

And I see the etymology has nothing to do with diapers

Wow, thanks for the lecture on their formation. I never would have thought salt, of all things, can flow.

I'm not an astrogeologist by any stretch of the imagination, but my understanding of the issue is that surface temperatures are so hot that there has been significant crystal fractionation (alluded to in an earlier post) in the crust, making the lithosphere much more buoyant than the mantle. This buoyancy inhibits subduction, which requires crustal rocks (on earth, old, cold oceanic basalts) approaching the densities of the upper mantle.

Wait, shouldn't a higher surface temperature reduce fractionation? As in, higher surface temperature = smaller temperature gradient as you go deeper? Maybe I missed something, but that doesn't make an awful lot of sense to me.

I haven't any sources for this, just heard it in a lecture a while back. I'm happy to look into it for ya, but right now I'm going to go have some dinner.

Bon appetite!

At least he isn't a mineralogist studying cummingtonite

Oh, that one. GeolSoc in St Andrews have hoodies with "Are you Mg7(Si8O22)(OH)2" on the back. Saw that on a friend, and I was fool enough to ask what it meant

What I've heard is that the surface of Venus is remarkably young (based on impact crater counts) which leads some astrogeologists to think that perhaps the planet's crust has at one time broken up and experienced massive "flood basalts" (planetary wide versions of the flood basalts like you see in Washington state) and essentially re-paved the planet (LINKY)

"Linky" is fascinating!

I think this is still being debated.

I just find it amazing that you have these two rocky planets of near-identical size, and they turned into such completely different worlds. A planet-sized vision of hell versus a balmy haven for life as we know it, one doesn't have plate tectonics and the other does, one doesn't have an intrinsic magnetic field and the other does, one got a complete volcanic facelift while the other didn't... (Of course, if all of those are connected, then maybe there is only one fatal difference )

But what causes a whole (nearly) earth-sized planet to be flooded by lava in the first place? (I hope we don't get that treatment in the near future )

BTW, isn't counting craters a bit meaningless on Venus? I mean, the thing has a thick atmosphere that eats rocks for breakfast. Why would you expect lots of old craters to stick around regardless of volcanism?

I suspect it's not uncommon- corals are an important constituent of many limestones. If you can be more specific about where you are, I can try to find some literature on the area.

If you can post a picture, I'd very much like to see what you found.

Seconded! We like fossils!

 

[thaumaturgy]

Wait, shouldn't a higher surface temperature reduce fractionation? As in, higher surface temperature = smaller temperature gradient as you go deeper? Maybe I missed something, but that doesn't make an awful lot of sense to me.

In petrology fractional crystallization occurs because certain mineral phases come out of the melt at different temperatures than others. If you remove those crystals from contact with the melt you will concentrate the remaining elements in the melt.

There's a simplified geochemical diagram called Bowen's Reaction Series:

If you start off with a melt containing sufficient amounts of Fe, Mg, Si, Al, etc etc. and you let it cool the first minerals to form will be these dark Fe-Mg rich silicates ("Mafic" for magnesium and ferrous/ic), and if you remove those from the melt you'll have a solid that is a mafic rock. Dense and high in Fe and Mg. But if you leave these minerals like olivine in contact with the melt it will react with the melt to form pyroxenes as the melt cools etc. and so forth down the series.

Fractional crystallization is when we remove some of the earlier formed minerals leaving us with much more "felsic" components (Feldspar and Silica). These minerals form granites and are usually "less dense"

Hence on earth we have dense "basaltic" (mafic) ocean floor and less dense continental (felsic, granitic) continental masses. That's why when a continental plate collides with an oceanic plate it "rides up over" the more dense oceanic plate.

This is of course pretty grossly oversimplified and it's been a good 20 years since I did petrology but this is generally the discussion at hand.

 

[thaumaturgy]

Seconded! We like fossils!

Be careful here Naraoia, need I remind you Orogeny is a carbonate petrologist. There are dark corners of geology where some things are too boring even for humans to bear! Some fossils are simply too dull for the human mind to stand!

(Actually I just like making fun of carbonate petrology and corals. In reality I love carbonates and there are some really cool coral fossils out there. Back in unnergrad our paleontology class wasn't about dinosaurs or any of the interesting life forms, no it was almost 100% invertebrates. Corals and brachiopods. Kind of a let down when you realize that 99% of the paleontologists out there aren't digging up dinosaurs but are digging up corals and brachiopods and bryozoans.

When I took my paleo class we had an assignment to go to a local quarry and find several examples of a local fossil (almost all Pennsylvanian in age) and write up a regular paleo paper on it. I spent a couple days on my belly crawling over a shale pile. I found a shark's tooth. A Mesozoic shark was swimming around the waters that were then East Central Illinois millions of years later. Because it was so rare I only had to find one and do my paper on it. It now has a position in my almy mater's fossil collection.

I was just glad not to have to do one of the "usual" brachiopod write-ups which I'm sure the prof of the class had seen about a b'zillion times.

ANd I shouldn't make light of corals since what I studied for my masters was Pb-Zn sulfide deposits some of which were hosted in a dolomitized reef formation in southeastern Missouri.

But I will warn you that carbonate petrology is NOTHING compared to the sheer and utter devestatingly dull, dull, dull world of clay mineralogy. One of my advisors was a clay "god" in college and he kept trying to get me to study clays, I just wanted to work on organic geochem (which he also did). So I did the minimal amount of clay stuff I could and ran as far away from it as I could...only to find myself working for a while almost exclusively with kaolinite when I was working in Georgia and now as coatings chemist I work mostly with carbonates and clays.

I tell myself clays are now "interesting"! Yay clays! Not just dirt! CLAYS! Yay! Look at the exciting octahedral layer! Ooooo!

But I'm just fooling myself. Clays are dull.

Important but dull.

Clays make carbonate petrologists seem like "Indiana Jones" of the soft-rock petrology world.

 

[Orogeny]

And I see the etymology has nothing to do with diapers

Sorry.

Wow, thanks for the lecture on their formation. I never would have thought salt, of all things, can flow.

My pleasure, it's a fascinating subject. Understanding salt tectonics is of significant economic importance as well, since several of the world's largest petroleum-generating basins are salt provinces.

Wait, shouldn't a higher surface temperature reduce fractionation? As in, higher surface temperature = smaller temperature gradient as you go deeper? Maybe I missed something, but that doesn't make an awful lot of sense to me.

Thaum gave a great explanation of crystal fractionation, but here's how it applies to Venus: the high surface temperature means that while the geothermal gradient may be lower(?), the absolute temperatures are higher, allowing melts to remain melts for much longer than they would be able to on Earth, giving more time for dense mafic minerals to settle out of the less dense melt. So while on earth we see a range of igneous compositions from SiO2 ~45% to 80%+, on Venus we may see compositions skewed towards the higher %SiO2, less dense end of that range.

Bon appetite!

Pan fried pork dumplings!

Oh, that one. GeolSoc in St Andrews have hoodies with "Are you Mg7(Si8O22)(OH)2" on the back. Saw that on a friend, and I was fool enough to ask what it meant

How do I get hold of one of those?

 

[Orogeny]

Clays make carbonate petrologists seem like "Indiana Jones" of the soft-rock petrology world.

We ARE the Indiana Joneses of the petrology world! Acid peels WHIPAH! Sand traps galore! Unearthing ancient dead things left and right! Fighting off Nazis to acquire floatstone samp- um... never mind.

Question: Do you ever work with zeolites?
Second question: What the heck is up with zeolites?
Statement: Zeolites look cool in cross polarized light.

 

[Naraoia]

Thaum gave a great explanation of crystal fractionation...

*facepalm* @ me. It appears that I have attention problems. Where did that happen?

...but here's how it applies to Venus: the high surface temperature means that while the geothermal gradient may be lower(?), the absolute temperatures are higher, allowing melts to remain melts for much longer than they would be able to on Earth, giving more time for dense mafic minerals to settle out of the less dense melt. So while on earth we see a range of igneous compositions from SiO2 ~45% to 80%+, on Venus we may see compositions skewed towards the higher %SiO2, less dense end of that range.

Okay, I think that makes sense.

How do I get hold of one of those?

I guess you'd have to get in touch with them and ask if they still get the cummingtonite hoodies and if random people who heard about them on internet forums can get one. I'm sure the question wouldn't earn you any strange or puzzled looks at all

 

[Orogeny]

*facepalm* @ me. It appears that I have attention problems. Where did that happen?

Here

 

[Assyrian]

But I'm just fooling myself. Clays are dull.

Important but dull.

Not like the good old days.

 

[thaumaturgy]

We ARE the Indiana Joneses of the petrology world! Acid peels WHIPAH! Sand traps galore! Unearthing ancient dead things left and right! Fighting off Nazis to acquire floatstone samp- um... never mind.

Question: Do you ever work with zeolites?
Second question: What the heck is up with zeolites?
Statement: Zeolites look cool in cross polarized light.

I have worked a wee bit with zeolites, mostly some synthetic ones for coating applications.

The pore sizes were desirable and even the cation exchange capacity was of some utility. I've got a patent application in using zeolites in one of our processes.

I have never done any optical microscopy on them, but crystallographically they are waaay cool. At one point I bought one of those inorganic compound model sets and built one of the crystals to show around at meetings when I was pushing these for some of our coating applications.

There are times when I think some of the look more like Escher drawings than inorganic compounds:

But I always ended up working with this one:

(Clinoptilolite)

But in reality this is what I saw day in and day out:

Just as an FYI: don't try to take these materials through TSA in your hand luggage! Ship it.

 

[TemperateSeaIsland]

I suspect it's not uncommon- corals are an important constituent of many limestones. If you can be more specific about where you are, I can try to find some literature on the area.

If you can post a picture, I'd very much like to see what you found.

I found it on Anglesey, North Wales but it came from a pile of spoil so it could have come from anywhere in the North Wales area.

Sorry no picture available, I found this thing years ago and had it on my bookshelf for ages. Sadly when I went to Uni my mother threw it away because she "thought it was ugly". Real shame as it was pretty cool looking and it looked like it has a partial fossil of a fish or something in there too.

 

[Naraoia]

Here

Argh, the stupid forum marked new posts as read again. I hate how it does that when you haven't actually looked at the stupid thread. Remind me never to just click "first unread" again...

Be careful here Naraoia, need I remind you Orogeny is a carbonate petrologist. There are dark corners of geology where some things are too boring even for humans to bear! Some fossils are simply too dull for the human mind to stand!

(Actually I just like making fun of carbonate petrology and corals. In reality I love carbonates and there are some really cool coral fossils out there. Back in unnergrad our paleontology class wasn't about dinosaurs or any of the interesting life forms, no it was almost 100% invertebrates. Corals and brachiopods. Kind of a let down when you realize that 99% of the paleontologists out there aren't digging up dinosaurs but are digging up corals and brachiopods and bryozoans.

I remember being sooooo disappointed that (1) intro geoscience in St Andrews includes next to no palaeontology, (2) what it does include is all invertebrates.

I was so young and foolish, still thinking vertebrates were the coolest creatures ever to crap in the sea...

 

[Naraoia]

I found it on Anglesey, North Wales but it came from a pile of spoil so it could have come from anywhere in the North Wales area.

Sorry no picture available, I found this thing years ago and had it on my bookshelf for ages. Sadly when I went to Uni my mother threw it away because she "thought it was ugly". Real shame as it was pretty cool looking and it looked like it has a partial fossil of a fish or something in there too.

Ow, that's a shame. Parents never understand!

 

[Jazer]

This is an inaccurate and difficult measure of the Earth's age at this point, because the physics of the system are incredibly complex. Please excuse the 'biased' source, but this is the best description I could find of earth-moon physics for the layman (me!).

Perhaps you should try wiki you will see that: "Tidal rhythmites from 620 million years ago show that over hundreds of millions of years the Moon receded at an average rate of 22 millimetres per year and the day lengthened at an average rate of 12 microseconds per year, both about half of their current values."

Now what is so difficult about "22 millimetres per year"?
But we can let the Phd's do the math for us:

"Astronomers think the collision between Earth and Theia happened about 4.53 Ga; about 30-50 million years after the rest of the Solar System formed. However, evidence presented in 2008 suggests that the collision may have occurred later, at about 4.48 Ga.[10]" wiki

Of course this is based on current computer models. New information could come along at any time. But for now this is what we have to work with:

"Theia is thought to have struck the Earth at an oblique angle when the latter was nearly fully formed. Computer simulations of this "late-impact" scenario suggest an impact angle of about 45° and an initial impactor velocity below 4 km/s" wiki

 

[Jazer]

I found it on Anglesey, North Wales but it came from a pile of spoil so it could have come from anywhere in the North Wales area.

I had a pastor named Angley, maybe his family was from that part of Wales.

 

[Orogeny]

Perhaps you should try wiki you will see that: "Tidal rhythmites from 620 million years ago show that over hundreds of millions of years the Moon receded at an average rate of 22 millimetres per year and the day lengthened at an average rate of 12 microseconds per year, both about half of their current values."

Now what is so difficult about "22 millimetres per year"?
But we can let the Phd's do the math for us:

"Astronomers think the collision between Earth and Theia happened about 4.53 Ga; about 30-50 million years after the rest of the Solar System formed. However, evidence presented in 2008 suggests that the collision may have occurred later, at about 4.48 Ga.[10]" wiki

Of course this is based on current computer models. New information could come along at any time. But for now this is what we have to work with:

"Theia is thought to have struck the Earth at an oblique angle when the latter was nearly fully formed. Computer simulations of this "late-impact" scenario suggest an impact angle of about 45° and an initial impactor velocity below 4 km/s" wiki

None of this deals with calculating the age of the earth based on lunar recession, which was your original point. This is a thread for curiosity, not argument, so if you want to play the goalpost shifting game, please play it elsewhere.