Crustal Uplift Mechanism Discovered

This work adds an important newtool to the geologist’s tool kit on a par with plate tectonics.  A mechanism is now shown to exist by whichthe crust is able to slowly rise and fall through amplitude approaching a fullkilometer through a time span of around a million years.

Most forget that before theadvent of plate tectonics, geology relied on a bizarre land bridge idea forwhich there had to have been some evidence. This shows us a better explanation for such evidence

As an aside, prior to plate tectonicsand a clear understanding of high temperature chemistry, a lot of errantnonsense found its way into geological reports. Enough said.

At this point one such event isrecognized.  Perhaps Yellowstoneis to be blamed for lifting the North American continent up.  There is some reason for all those sedimentsto be so well uplifted.

No longer do we need to wonderwhy areas remote from a plate margin are able to rise so much.

Pulsating planet: Superhot rocks make the Earth roll

07 April 2011 by CarolineWilliams

http://www.newscientist.com/article/mg20928031.600-pulsating-planet-superhot-rocks-make-the-earth-roll.html

An invisible force is creating giant ripples in the Earth's crust – ina geological blink of an eye

BRYAN LOVELL likes to show his fellow geologists an image of a networkof river valleys. "I ask them where they think this might be onEarth," he says. It is, as you probably guessed, a trick question. Theriver valleys are in the North Sea, north of Scotland, beneath a kilometre ofwater and a further 2 kilometres of sediment.

The sea floor here has been slowly sinking ever since it formed. Yet 55million years ago, something very strange happened. In a geological blink of aneye the sea floor was thrust upwards nearly a kilometre, until it was high anddry above the waves. It remained above the sea for about a million years, longenough for rainwater to carve deep valleys. Then it sank down again. It allhappened astonishingly fast.

The big question is why. None of the usual explanations apply. Thecollision of tectonic plates can lift vast regions many kilometres up into theair, but there are no colliding plates under the North Sea.Sea level can fall when huge ice sheets form, but not by a kilometre and not inonly one area. Volcanic hotspots, where hot molten rock rises up from deepwithin the Earth, can also produce uplift, but the nearest hotspot is hundredsof kilometres to the west, where it has lifted up Iceland. What's more, hotspotsproduce slow uplift over tens of millions of years, not a relatively suddenrise and fall.

The North Sea is not the only exampleof a geological jack-in-the-box. Something similar seems to have happened inseveral regions around the world, including the coast of Angola and the Arabian peninsula. In fact, there are many long-standing geological mysteriesof this nature.

Now, at long last, there may be an answer. A team from the University of Cambridge reckons that the likelyculprits are pulses of particularly hot rock or, to give them their highlytechnical name, "hot blobs". These blobs are less dense than thecooler material around them, so when they pop up under the crust theytemporarily lift large areas up in the air. In some cases, they appear tospread out in a gradually subsiding circular ripple. (Check out ouranimatedgraphic to see how it works). What's more, in many places there mayhave been a series of pulses, lifting and dropping the land over and overagain.

And while hot blobs are most likely to occur at or near major hotspots,the Cambridgeteam says, they could pop up almost anywhere. If the idea is correct, it couldsolve a lot of irritating riddles in geology. Maybe even one of the biggest ofall: what triggered one of the hottest periods in Earth's history. "It'sexciting because you can see ancient problems through new eyes," saysLovell, a member of the team and president of the UK's Geological Society."We are re-examining observations we've never understood because we didn'thave this hypothesis to hand."

Hot blobs

That idea that hot blobs exist in the mantle, the region betweenEarth's crust and its core, is not new. It has long been known that some partsof the Earth's mantle are hotter than others. In fact, one of the few thingsknown for sure about the mantle is that its temperature, viscosity and densityvary widely. This keeps the mantle constantly churning, as hotter and lessdense rock rises towards the crust while cooler and denser slabs sink towardsthe core. This mantle convection is what powers the movement in platetectonics, and brings heat to volcanoes and to hotspots such as the plumesbeneath Hawaii and Iceland.

It is also well known that within hotspots, blobs of particularly hotrock occasionally rise up. Models of the Earth's mantle have suggested thatthey exist, and there is also solid evidence in the form of changes in thetypes of volcanic rocks found on the ocean floor. Until recently, however,nobody imagined that these hot pulses could change the Earth's surfacedramatically and quickly, or that this could happen away from major hotspots.

The best evidence to support the Cambridgeteam's claims comes from two areas north of Scotland. By setting off modestexplosions and analysing the echoes, it is possible to create detailedthree-dimensional maps of what lies beneath the sea floor. Seismic images fromthe oil industry have revealed ancient land surfaces, now buried beneath 2kilometres of sediment, with the unmistakable pattern of river valleys. A new,more detailed map is due to be published soon by the Cambridge team.

The seismic evidence is supported by borehole samples. The deepestcores, representing the oldest rocks, contain large numbers of marine fossils.Higher up, and so more recently, coal deposits and pollen suddenly startappearing, along with signs of rivers. Then, a million years later, marinefossils start appearing again.

Gargantuan ripple

What this shows is that about 55 million years ago, the sea floor wasmuch closer to the surface, and was subsiding very slowly. Then, abruptly, anarea halfway between Scotlandand the Faroe Islands was lifted by at least800 metres over the course of a million years. Over another million or soyears, it sank again.

There is also evidence that another area several hundred kilometres tothe east also briefly popped up before sinking back beneath the waves.Afterwards, the slow subsidence of the sea floor resumed, so this old land isnow 3 kilometres down.

"Something happened to make a happy marine environment go up by atleast 800 metres," says Nicky White, who leads the Cambridge team. That something, the teamsuggests, was a hot blob coming up the Iceland plume, a few hundred kilometresto the west, hitting the tectonic plate and spreading out like a ripple beneaththe North Atlantic at the rate of 40 centimetres per year (Earth andPlanetary Science Letters, vol 267, p 146).

As the hot ripple spread out it pushed up the land above it, since thelower-density hotter rock took up more space in the mantle than the surroundingrock. Then, as the ripple spread out further, the land behind it wasdropped(see "Ups and downs"). Lovell likens the effect to ratsrunning under the carpet. "As the rats run under the rug, it is lifted upthen down again. As they get to the edge there is early Scotland, whichis lifted higher out of the ocean and then dropped back under the waves,"he says. Something similar would have happened all around the ripple and Lovellsays there is evidence for a similar uplift on the opposite side of the positedcircle, in Greenland.

As if this weren't dramatic enough, newer and as yet unpublishedseismic maps have revealed even older land surfaces even deeper beneath theseabed in the same area off Scotland.That suggests that this process did not just happen once, 55 million years ago,but several times. "There is a whole series of these landscapes on top ofeach other," says White. "Every 2 to 5 million years you get ratsunder the carpet."

Modelling of a virtual hot blob pulsing up the Iceland plume has convinced the team that, for Scotlandat least, the ripple theory stands up. And there are tantalising hints of"rats under carpets" elsewhere. On the coast of Angola in westernAfrica, for example, a dome 1000 kilometres in diameter was lifted 500 metresin the space of just a couple of million years, starting 5 million years ago.Seismic imaging suggests that there is something particularly hot in the mantleunderneath it, says White, so perhaps this hot region pulsed even hotter 5million years ago. There is also unexplained uplift in the Arabian peninsula that looks a lot like the work of a hot blob, saysLovell.

White and Lovell admit they need more evidence to be sure that hotblobs are at work beyond the North Sea, but say that they are expecting to findmore signs of the effect all over the world. "Now we know what we'relooking for, I predict we will find it," says Lovell. "Something inthe geological record will look odd - it will go up very fast or down veryfast. I've been asking my most experienced colleagues, 'Are there breaks in thegeological record that you don't understand?' and there are lots of them."

Clint Conrad, a geologist at the University of Hawaii in Honolulu, isintrigued. "The idea that we can get uplift in areas distant from theregions of volcanism is fairly new and indicates that whatever is going on isan aspect of either plumes or mantle flow that we don't completely understandyet."

But he thinks that hot blobs will only show these kinds of dramaticeffects where there is already a decent-sized hotspot (see map). Scotlandisn't that far from Iceland, he points out, so it is where you would expect tofind such an effect if it exists.

The Cambridgeteam, however, thinks there is no reason that hot pulses couldn't createdramatic - and transient - uplifts almost anywhere, though White advisescaution: "We can't invoke blobs everywhere."

If this is right, geologists may have to reassess some long-heldassumptions. They have long tried to explain mysterious changes in coastlinesby attributing them to global sea-level change due to some unknown ice age, andthen used these putative events to date landscapes. "It's circularreasoning," says Lovell.

A bigger implication, though, is Lovell's suggestion that the Scottishhot blob helped trigger an abrupt warming known as thePalaeocene-Eocene thermal maximum. Around 55 million years ago, when Earthwas much hotter than today, global temperatures suddenly shot up by 6 °C to 10°C, causing a minor extinction event. The evidence points to a rise in methanelevels, most likely from methane hydrates on the sea floor.

This release could have been triggered simply by a rise in oceantemperatures, but uplift of the seabed would also destabilise any methanehydrate deposits. "What we know is that right at the same time this hotblob passed under the North Atlantic, therewas a major release of carbon into the atmosphere," Lovell says."We're not allowed to appeal to coincidences in science, but it's got tobe a good candidate."

Computer modelling by two other members of the Cambridge team, JohnMaclennan and Stephen Jones, has suggested that up to 2000 gigatonnes of carbonmight have been released in such a way. "A large methane release of even afew per cent of that figure would be enough to trigger catastrophic climatechange," says Euan Nisbet, an earth scientist at RoyalHolloway Universityof London, whohas proposed an alternative source for the methane.

There are several other competing ideas about the trigger for thesudden warming, and pinning down the cause will not be easy. Meanwhile, the Cambridge team is intenton looking for more signs that hot blobs have rippled by, or may be at worktoday. For Lovell, it's all about finding hard evidence. "You can't arguewith rocks," he says.

Caroline Williams is a freelance writer based in London