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Earth just doesn’t make crust like it used to… at least, not according to new research by a team of scientists in the UK.
Researchers with the Universities of Bristol, St Andrews and Portsmouth have studied elements trapped within zircon samples gathered from all over the planet to peer billions of years back in time at how Earth’s crust was being produced.
Zircon, a mineral found in granite, can be dated with precision and is thus an accurate measure for geologic timescales.
What they found was that 65% of our planet’s current crust had already existed 3 billion years ago. Since rocks older than 2.5 billion years are rare on Earth today, this means that some process began to take place that either reworked — or destroyed — a large portion of the older crust, and changed how new crust was formed.
During the first 1.5 billion years of Earth’s history, the team reports, the rate of crust formation was high — approximately 3 cubic kilometers was added to the continents each year. After that the rate dropped substantially, falling to about 0.8 cubic kilometers per year for the next 3 billion years — right up to the present day.
The cause is yet unknown, but it may be the result of the onset of plate tectonics driven by subduction — the process by which sections of Earth’s crust (“plates”) slide beneath other sections, sinking into the underlying mantle to be liquefied into magma by pressure and heat. New crust is created when the magma rises again where the plates separate… Earth’s current “conveyor belt” of crust formation.
Whatever process was in place prior to 3 billion years ago, it was much more efficient at creating crust.
“Such a sharp decrease in the crustal growth rate about 3 billion years ago indicates a dramatic change in the way the continental crust was generated and preserved,” said Dr. Bruno Dhuime of the University of Bristol’s School of Earth Sciences. “This change may in turn be linked to the onset of subduction-driven plate tectonics and discrete subduction zones as observed at the present day. The next challenge is to determine which tectonic regime shaped the Earth’s crust in the planet’s first 1.5 billion years before this change.”
The team’s paper “A Change in the Geodynamics of Continental Growth 3 Billion Years Ago” (Bruno Dhuime, Chris J. Hawkesworth, Peter A. Cawood, Craig D. Storey) was published March 16 in Science.
Read more on the University of Bristol’s press release here.
Don’t let the Greens read this story. They will somehow find a way to blame the change as a result of evil humans.
How tiresome.
The article doesn’t even remotely mention anything to do with AGW. I wonder if you just saw the word “Earth” and thought that was good enough.
These comment sections too often get thrown into AGW debates, but I don’t mind so much if the article is actually about Earth’s climate.
I think I’m going to make a habit of reporting these comments, the only purpose of which that i can see is to troll.
It is worst than we thought!
AGW, for sure.
Sorry, but this is not funny. Worse, seeing the next comment, it is a sad mirroring of today’s science climate.
This article is puzzling to me.
This article is puzzling to me. 3 Billion years ago (roughly Meso-Archean), the accretion process had stopped and the earth was cooling and changing in surface matter from a mantle to actual crust. In other words, the entire earth’s crust was created during in this era. We cannot possibly regenerate that much crust without melting our current crust and letting it solidify all over again. No additional matter has been created or destroyed. Am I missing the point here? While we really can’t pinpoint what processes took place at any specific time, I think we had a pretty good idea prior to this research. Is this just an effort to try to roughly pin point the formation of earth’s crust more exactly? Are we actually “running out of crust” or is the generation of new crust just decreased since 3Bya? We can’t be running out, we have about as much as we did when the planet originally solidified.
I have to be missing the point of this story. If anyone has any insight after reading this, throw this confused puppy a bone. Thanks.
Was the accretion process still taking place 3 billion years ago? Is that the point when plate tectonics began as we know it today? What was the driving process of crustal development for the first 1.5 billion years that resulted in a larger net increase? I think these are the questions posed by the paper.
I don’t think there is any good handle on the formation time of the first thin crust. It may have been tens or, more likely and promoted by most, hundreds of millions of years. It looks from the Moon composition that Earth had a crust by the time the Earth-Moon impactor created the Moon (or so I believe when I am writing this).
But that crust is not todays granite dominated crust. Granites are eutectics* of siliceous rocks, and luckily they are lighter than the rest. The trick is to form them, they mostly form with the presence of water.
Worse, today’s granite crust is not the first thick crust that could form stable floats, they were more primitive greenstones, so called TTG minerals. “It is widely accepted that most Archaean granite–greenstones are dominated by TTG, … The origin of TTG suites is debated; their chemistry is similar to modern-day subduction zone magmas, but there is disagreement as to whether modern plate tectonic processes operated during the Archaean.[1] Some authors have suggested alternate styles of subduction, while others attribute the development of TTG to direct melting of the lithosphere by mantle plumes.[2]”
What regenerates (and destroys) crust today is plate tectonics, which has the capacity of total reworking over time. (Assuming some granite cores survive as plates, but not necessarily the oldest.) Apparently that is pretty much what has happened.
Are we running out of crust? Not if you look at the paper fig 2B increasing volume of continental crust (instead we are running out of ocean =D).
While the fig 2A shows cause for concern, a varying but increasing trend of crustal rework. Since we will run out of biosphere time in ~ 0.5 – 1.5 billion years due to the increased solar heating of our aging star, our descendants will likely not see a problem here.
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* A well defined composition due to being the composition with the smallest melting temperature. This is the composition that reheated local melts tend to.
To me it would seem, from Paragraph #5, that there is some confusion between “crust” and “continental crust.” The total diameter of the earth has surely remained just about the same since we lost a portion of our mass in the moon’s creation.
So if there is a re-working of our total crust through plate tectonics, of course the older crust would disappear. But I doubt that the actual crust is getting thinner, as lighter materials must slowly be coming up from the deep interior.
This article needed more thought and a better editor before being published, is my take.
Yeah, I thought the article might go into how present day volcanism isn’t creating the same type of crust. Maybe all the basalt is more akin to just hardened magma than actual more typical crust. Clearly, we don’t need to be making it like they used to. The world is only so big and it IS all covered in crust.
Very interesting! There is, as far as this non-geologist knows, a debate between a majority (I think) of non-uniformitarians and uniformitarians, as we can call them descriptively, and this paper looks like it goes into that.
The non-uniformitarians takes the crustal data (mainly zircon ages) as reflecting non-uniform crustal growth rates. The uniformitarians takes it as reflecting formation of supercontinents as they would inhibit crustal destruction before plowing plates and crustal formation after in a complex balance.
I don’t have access to the paper yet, but it looks like their fig 1 separates out supracrustal zircons to leave a uniformitarian type of smooth crustal formation rate curve, and their fig 2 sums two ways to a uniformitarian type of smooth crustal volume growth rate trend.
The upshot is that they can confirm earlier suggestions of a late ~ 3 – 3.5 billion years ago start of plate tectonics, with a previous different regime. I have been looking at uniformitarian papers that looks to push plate tectonics back to ~ 4 billion years ago as in the comparison curve of Belousova et al in the fig 2.
However that may be, there was a previous regime where the seeds of crust got formed and recycled. My astrobiology books looks at so called sagduction, forming crustal floats that collapses in the middle due to increased heat as they block convection. But the high crustal formation rate then doesn’t look too compliant with that scheme.
Lex Luthor was right:
“Land – it’s the one thing they don’t make anymore.”
Volcanism doesn’t create crust. Volcanic lava sources are remelted crust, not mantle material. You have to go to the ocean formation ridges for that. (Yes, they are lavas too, just not volcanic lavas.)
Look up plate tectonics and volcanism in Wikipedia, IIRC it is all there.
Reply to Perestroika, my page script seems broken:
Volcanism doesn’t create crust. Volcanic lava sources are remelted crust, not mantle material. You have to go to the ocean formation ridges for that. (Yes, they are lavas too, just not volcanic lavas.)
Look up plate tectonics and volcanism in Wikipedia, IIRC it is all there.