The Earth Story

It’s a modeling photo shoot on the edge of Erta Ale and Dallol volcanoes in Ethiopia. Original caption:

A whole lot of model Tyrannosaur skulls on display at Ohio University

This is a structural geology “Sand table” experiment. Geologists can use simple experiments like this to see how complicated factors interact in the building of a mountain range. They use different materials to create strong and weak zones in their experimental box, compress them so that there’s an added force, and then compare final results to the types of faults and folds found in large mountain ranges like the Himalaya.

Why are there mountains here?

This photograph shows part of the front range of the Tian Shan Mountains in Kyrgystan. The Tian Shan range is found almost literally in the center of the Asian continent – the farthest point on Earth you can get from any ocean is found close to this range. Interestingly, this description seems to violate one of the things we describe as fundamental to geology; that plate tectonics is the driving force to build mountain ranges.

Major mountain ranges, like the Alps, Himalayas, and Andes, are supposed to form where two plates come together. The Tian Shan Mountains are 1000 kilometers from the zone of collision between India and Asia, yet they’re still deforming as part of that same collision. How does a plate collision build a mountain range a thousand kilometers away? A newly published paper suggests that these mountains, and many others on Earth, owe their origins to movements in the mantle.

The Tian Shan Mountains contain a pair of ancient volcanic arcs. China and East Asia were assembled out of smaller blocks of crust that slammed together and stuck to the growing continent over the last few hundred million years. Much of the mountain range that grew when the original blocks assembled has been eroded, but today the force of the collision with India far to the south has reactivated this area.

For an area like this to be “reactivated” as is happening today, something at this site must be weaker than the surrounding crust. If the boundary between these ancient crustal blocks was strong and secure, then the force of the collision to the south would transmit completely through it, without deforming this area at all.

The new research led by scientists at the University of Toronto proposes that there is a weakness in the mantle beneath these mountains and that’s why they’re here. That weakness has allowed stress from the India-Asia collision to travel here, where it pushes up these mountains.

To understand this process, they created a model that allowed them to simulate how rocks of the crust and upper mantle respond to stress. They were able to vary the strength of the rocks and also insert ancient zones of weakness into the rocks. When they inserted weaknesses only in the crust, such as ancient faults and fracture zones, the faults and folds did reactivate, but they didn’t link together into a single mountain range. Instead, ancient features only in the crust produced wide zones of uplift; these types of low plateaus aren’t what we see in areas like the Tian Shan.

Instead, when the scientists simulated a weak zone in the mantle, the position of this mantle weakness controlled what happened above. A thin but present zone of weakness in the mantle focused all the deformation in the crust right above it, creating sharp mountain ranges or major faults.

A mountain range like the Tian Shan, therefore, likely owes its existence to some weakness in the mantle leftover from the assembly of Asia. The Tian Shan isn’t the only feature in Asia likely created by this process; other features such as the large Altyn Tagh fault that runs to the east of the Himalayas are also potentially reactivating ancient mantle structures.

Finally, what exactly does it mean for there to be a zone of “weakness” in the mantle? The mantle is a warm solid, able to deform on geologic time because of its heat content. Something that changes how the mantle responds to stress will change how rapidly it deforms. If the minerals at a spot in the mantle are finer grained, they will deform more rapidly; minerals are often finer-grained in areas with lots of deformation, so this type of weakness even makes sense with continental collisions. Adding water to mantle minerals can weaken them as well – so ancient subduction zones, where water sank into the mantle on an oceanic plate, could produce these zones of weakness as well. Conveniently, a continental collision usually involves a subduction zone and produces deformation that would shrink the size of mineral grains, so the ancient subduction zones of the Tian Shan range are perfect examples of the type of geologic province where reactivation could occur.

-JBB

Image credit: Thomas Depenbusch https://flic.kr/p/aaXJPg

Original paper: http://go.nature.com/28qyBdJ

Press release version: http://bit.ly/1XdSj8F

Author on Twitter: https://twitter.com/philipheron

An Ice Free Arctic

2012 saw the lowest Arctic summer sea ice extent in recorded history, offering a visual indicator of global climate change- but the worst is yet to come. New research suggests that in as little as 20 years’ time there will be no ice extent to record at all during the Northern Hemisphere’s summer months.

The research, which was published in the journal ‘Geophysical Research Letters’, demonstrates how major ice loss in the Arctic Circle could become a reality within a decade or two. The research team led by James Overland of NOAA and Muyin Wang of the University of Washington, analysed data from three common modelling techniques to produce the best forecast available for when we can expect the Arctic to be ice free during the summer. The three models take different approaches which result in different dates, but all three suggest that the Arctic to be ice free during the summer before the middle of the century.

One model, which relies on past sea ice trends predicts an ice free Arctic by 2020. Another, which takes a more stochastic approach, suggests the area to be ice free by 2030. The last model, which focuses on global climate information to gauge Arctic warming predicts sea ice loss by around 2060- it is noted in the paper that this is a rather optimistic forecast.

No matter what model we choose to focus in on, it is reasonable to conclude that an ice-free Arctic summer will be likely to occur before 2050 best case scenario, or even in as little as a decade.

So, what implications will this have on our planet? The answer is: many- we will see effects in the Arctic food-chain, from plankton to polar bears. There will be concerns with regard melting permafrost and of course the disruption to the global energy balance. The Arctic sea ice, as a result of its white colouration, has a high albedo; it reflects the sun’s energy back into space- without this effect in place, more sunlight will be absorbed by the oceans which will likely disrupt the oceanic conveyor belt which distributes cold and warm water around the planet. Without this thermodynamic cycle in place, the Northern hemisphere’s climate could become very different to what we have grown accustomed to. Very different indeed.

-Jean

For full journal access go here: http://onlinelibrary.wiley.com/doi/10.1002/grl.50316/abstract

Photo courtesy of NSIDC

Why does this seamount chain bend?

The Hawaiian-Emperor Seamount Chain is the longest continuous chain of seamounts on the planet Earth. Each of the seamounts in this chain represents an ancient volcano like the ones found on Hawaii today; many are smaller than the current islands but like Kilauea they all poured out large amounts of basaltic magma. As the Pacific plate moves off the site of the hotspot, the volcano dies and the plate begins to cool, causing the volcano to sink. Eventually it submerges beneath the waters of the Pacific, forming first a coral reef before it is lost from view as a seamount.

Obviously if you look at a bathymetric image of the Pacific Ocean floor, one feature stands out; the chain bends. That’s not something you normally expect from our hotspot model; a plate moving in one direction over a hotspot should have all its volcanoes and seamounts form a line, so what gives?

The first thing to understand is that plate motion directions aren’t always fixed. The bend in the Hawaiian Emperor chain occurred about 47 million years ago. At this time, there were two other plates out in the Pacific Ocean: the Kula plate and the Farallon plate. The Farallon plate is mostly gone, small chunks of it remain as the Juan de Fuca plate and the Cocos Plate. The Kula plate is even more completely subducted; we can infer its presence because magnetic stripes in the crust of the Pacific Plate turn, recording the presence of a mid-ocean ridge between the extant Pacific Plate and a plate that is no longer there, the Kula plate.

When a plate fully subducts or a major piece of crust runs into a continent, the forces that move the plates change. The last traces of the Kula plate and the first interaction between the Pacific Plate and North America both occurred at about the time of the Hawaiian Emperor Bend, so a change in plate motion direction has been the most common explanation for the bend. Change the forces tugging on the Pacific Plate; turn the plate by tens of degrees.

Oddly though, there are other island arc chains on the Pacific Plate. They are caused by hotspots too, even if those aren’t as active as the Hawaiian volcanoes. Some of these chains bend as well, including the Louisville Seamount chain in the Western Pacific, but that chain bends much more gradually than the Hawaiian Emperor chain. If plate motion changes caused one part to change direction suddenly, why does the other one happen over millions of years?

A new study led by researchers at the University of Sydney proposes that the plate motion explanation may not be the cause of the Hawaiian Emperor bend. Instead, they suggest the cause lies in the mantle.

Deep in the mantle beneath the western Pacific and Africa, there are two large regions distinct from the rest of the mantle seismically. They’ve been given the name LLSVP (Large Low Shear Velocity Provinces) or more colloquially, superplumes (https://tmblr.co/Zyv2Js1pESuuK). Previous work has argued that these zones are either hotter than the rest of the mantle or made of different stuff, and therefore they can be linked to a number of hotspots around Africa and the Pacific.

The Hawaiian hotspot today is close to the edge of one of these LLSVP. If this hotspot always stayed at the edge of the Pacific LLSVP, then anything that pushed into the lower mantle could cause the hotspot to shift position.

The Kula plate, Farallon Plate, and Pacific Plate have been subducting beneath the surrounding continents for hundreds of millions of years. Seismic imagery today can track plates all the way into the lower mantle. If a subducting plate moved into the lower mantle, it could serve as a bulldozer, pushing the LLSVP and changing the direction of the Hawaiian Emperor Seamount chain.

That bulldozer is the short explanation for this new model. The subduction zone that forms the Aleutian Islands has migrated south over the last hundred million years, implying that the subducting plate has moved to the south. As this plate moved south and sank, it would eventually run into the edge of the LLSVP and push it to the southeast, causing a change in the seamount trend not related to plate motions above. Similarly, the gentle bend in the Louisville Hotspot Chain could be a distant reflection of this same pattern; material being moved in the LLSVP that is farther from the actual bulldozer.

This hypothesis doesn’t explain everything about this seamount trend. The Hawaiian hotspot location today is thought to move and changes in the mantle can cause that, but its extremely difficult for changes in the mantle to produce a sharp bend. To explain that, the authors still must call on features at shallow levels, such as the remnant of older mid-ocean ridge fracture zones or convecting mass in the upper mantle, to allow the bend to occur so suddenly.

We still don’t fully understand the behavior of the Earth’s mantle, or what the LLSVP are. Although this is one model for the Hawaiian Emperor bend, future work may still argue for a crustal plate motion origin for it, or perhaps even some other change in the LLSVP we haven’t thought of yet. This new model offers an intriguing explanation for the Hawaiian emperor bend, but testing the exact origin of features in the lower mantle remains a difficult proposition today, so there remains much more science to be done

-JBB

Image credit: Ingo Wölbern http://bit.ly/1OlQJ1u

Original Paper: http://bit.ly/1TbaOqh

References/Press versions: http://bit.ly/1OlQQtY http://pubs.usgs.gov/gip/dynamic/Hawaiian.html Louisville seamounts: http://bit.ly/1VUSuVY

Climate Change Disrupting Atmospheric Waves

Giant atmospheric waves that are part of the global weather system are being disrupted by climate change. And this disruption is behind the extreme weather seen around the globe. Vladimir Petoukhov, lead author and scientist at the Potsdam Institute for Climate Impact Research (PIK) says "What we found is that during several recent extreme weather events these planetary waves almost freeze in their tracks for weeks. So instead of bringing in cool air after having brought warm air in before, the heat just stays. In fact, we observe a strong amplification of the usually weak, slowly moving component of these waves,"

They developed a complex system of equations and models tested and proven with help from the US National Centers for Environmental Prediction. These models helped substantiate previous research that had linked extreme weather events with climate change, but did not have a mechanism to cause it.

---Adam

Photo: PIK

Further Reading:

http://www.sciencedaily.com/releases/2013/02/130225153128.htm

http://www.pik-potsdam.de/

When did the moon form?

In part 1 of this story, I discussed how planets form from the early solar nebula and how the formation of cores is a key step in their chemistry (see here: https://www.facebook.com/TheEarthStory/posts/669201963140851 ). When cores form, certain elements are taken out of the mantle almost completely and locked away in the core for almost all time.

This process happened many times in the early solar system, even on objects as small as the asteroid Vesta. Every time it happened, it left a chemical fingerprint. The element Hafnium has a radioactive isotope that decayed to Tungsten early in solar system history. Tungsten is siderophile – when cores form, almost all the Hafnium stays in the mantle while almost all the Tungsten goes into the core. Therefore, almost every atom of tungsten in the mantle should have formed after core-formation and counting those atoms can give a date.

Unfortunately, it’s never been that simple. The Earth’s core probably formed in several steps – cores formed on the objects that went into Earth, and when those objects collided, the cores splashed together and mixed in with the mantles. On top of that, other elements have been added back into the mantle after the cores finally formed since meteorites do continue hitting the earth – a process we call adding a “Late Veneer”.

That brings me to this paper regarding Moon formation. The giant impact that formed the moon was likely the last giant impact on Earth – the last time anything was added to Earth’s core. That impact added so much energy that part of the planet vaporized and the mantle melted, forming a magma ocean probably about 2000 kilometers deep.

If, at that time, all the iron-loving elements in the mantle were taken to the core, then everything in the mantle today was delivered to the planet after that impact. The scientists from France came up with a novel idea – to date the Earth, they could simply count how abundant those elements are in Earth’s mantle.

To do this, they simulated how the solar system evolved. To make the current mantle, the moon-forming impact had to happen when there was still some mass remaining in the solar system – the planets can’t have eaten it all beforehand. They modeled how the planets formed and found that the moon-forming impact must have happened around 95 million years after the start of the solar system. With their models, that is the only way to make the Earth’s mantle as we see it today.

Is there any reason why this analysis might not give the right age? Well, there are a couple. Even the moon-forming impact likely didn’t melt Earth’s entire mantle; that’s one of the issues that complicated the earlier measurements. If the whole mantle didn’t melt, then it’s possible to have a lower mantle which retained some elements, altering the simple calculation. It’s also worth noting that this group’s models for accretion in the early solar system differs from other groups; the other models would put the same constraint much later, as late as 150 million years, a number so late it’s actually difficult to explain with the context of moon rocks in our collection.

This is an interesting attempt to explain the timing of moon formation, but it’s only one part of the picture. To give a full picture, we need a structure that explains the chemistry and isotopic composition of the earth and the moon all at once in the context of the early solar system; all complexities scientists are working on to this day.

-JBB

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Image credit:http://en.wikipedia.org/wiki/File:Artist%27s_concept_of_collision_at_HD_172555.jpg Read more: http://www.iflscience.com/space/dating-moon