2017-09-26

Did the evaporation of the Mediterranean trigger widespread volcanism?


Artistic interpretation of the proposed lowstand
of the Mediterranean level during the salinity
crisis. Authors: Pibernat and Garcia-Castellanos
We are 130 years ahead of the recognition of a hypersaline, evaporitic stage in the Mediterranean (now known to roughly correspond to the Late Messinian, 6 to 5.3 million years ago);
50 years after documenting widespread submarine and riverine erosional features that suggest a subaerial exposure of parts of the Mediterranean Sea;
40 years after the first abissal drilling reaching the top of a salt layer thicker than 1 kilometer...
And yet, the most intriguing and debated question around the Messinian salinity crisis remains whether there was a large sea level fall during the crisis, more than a few hundreds of meters, perhaps more than a kilometer. Evidence in favor and against is piling up on the desks of scientists. 

We now publish a new piece of evidence that supports a Yes answer to this long-standing question. A fall in the level of the Mediterranean Sea about 6 million years ago may have increased volcanic activity over the entire region (Sternai et al., 2017, Nature Geosc.).

Geoscientists inspecting the Realmonte mine in Sicily,
where Messinian salt is commercialized. 
A layer ranging from 1 to 2 km of salt (halite) spreads below much of the Mediterranean seabed, formed when the Mediterranean Sea became isolated from the Atlantic Ocean about 6.0 to 5.3 million years ago, leading to evaporation and sea-level fall in an event known as the Messinian salinity crisis. The rate and amount of sea-level fall in the Mediterranean during this time is strongly debated. However, if the sea-level drop was dramatic and rapid, it could have unloaded Earth’s surface, decompressing the mantle below. Such mantle decompression can enhance magma production and, in turn, lead to volcanic eruptions at the surface.

Pietro Sternai and the rest of us test this idea using a combination of geological data and numerical modelling. Dated magma intrusions and volcanic eruptions in the region show that there was a pulse of increased volcanic activity towards the end of the Messinian salinity crisis. By calculating changes in the surface load caused by a kilometre-scale drop in sea level, and taking into account the counter weight of the increased density of the remaining highly saline water and accumulating salt deposits we verify that such changes in sea level are sufficient to unload and decompress the mantle, triggering a significant increase in volcanism over the Mediterranean.
Decompression and vertical rebound of the lithosphere
in response to a sudden evaporation of the sea. 

The results provide independent support for the idea that sea-level fall during the Messinian salinity crisis was rapid and occurred on a dramatic scale, and also highlights the sensitivity of Earth’s solid interior to changes at the surface.

Check also the News & Views article by Jean-Arthur Olive: “This proposed link will motivate the collection of high-resolution field data that better constrain the timing of volcanism in the Mediterranean, along with the development of novel approaches for coupled lithosphere–magma dynamics.”

Original paper:
Sternai et al, 2017, Nature Geosc. http://dx.doi.org/10.1038/ngeo3032








2016-10-04

Tomanowos - the rock that went through planetary collisions, megafloods, and idiocy

Present display of the meteorite at the at the at the AMNH. My photo.
Last week I had the opportunity to face the rock with probably the most fascinating story on Earth: 
Tomanowos, which meant the visitor from the sky in the extinct Clackamas language, also known as the Willamette meteorite. 
Supernovas spread throughout space the
iron produced in heavy stars. This ejected iron
ends up in particle nebulas that eventually form
new stars and protoplanets. [Image: NASA] 

After being seen by white men near Portland, more than a hundred years ago, Tomanowos inevitably went through one of the most hilarious and silly geological stories that I know of, surely driven by the fatal attraction that a weird rock like this irradiates on humans. But before going to that: what do we know about this weirdness?

Tomanowos is a rare 15,500-kg meteorite made of iron and nickel (Fe 91%, Ni 7.6%). As in other metal meteorites, these Fe and Ni atoms formed at the core of stars that shattered the space with the sub-products of nuclear fusion when ending their lives as supernovae. These materials eventually formed the nebula that clumped together as protoplanets in the Solar System, and Tomanowos was part of the core of one of these protoplanets, where the heavier metals accumulated. 


Vesta, a surviving protoplanet of the 
early Solar System. Due to their large
 size, protoplanets develop a differenciated 
density distribution with heavier elements like 
iron concentrated in the core. Tomanowos is an 
ejected piece of a protoplanet core like this. 
[EPFL/Jamani Caillet, Harold Clenet]
A protoplanetary collision 4 billion years ago sent a piece of that core back to space. Subsequent impacts over billions of years made the orbit of this meteorite eventually go across that of the Earth. As a result of this cosmic billiard, about 20,000 years ago, the meteorite entered our atmosphere at a speed of ~60,000 km per hour and landed on an ice cap in Canada.

Over the following decades, the ice flow slowly brought Tomanowos southwards, towards a glacier lobe that was at the time blocking the Fork River in Montana. The glacial tongue piled ice across the river valley forming a 600-m barrier that impounded the enormous Lake Missoula behind. Tomanowos happened to reach the ice dam at the precise year when it collapsed, releasing one of the largest floods ever documented: the #MissoulaFloods that shaped the Scablands in Washington. This process is known as glacial outburst flooding and it still happens every few years in the Perito Moreno glaciar, for example. Except that the water discharge during the Missoula Floods is known to have been equivalent to a few thousand Niagara Falls. The research of the Missoula floods by Bretz and Pardee in the early 20th century led to one of the most significant paradigm shifts in recent geoscience: the recognition that catastrophic events can significantly contribute to landscape evolution.
Map of the Missoula Floods path, showing Lake Missoula 
(blue), the ice cap where Tomanowos landed (north of the 
lake outlet), and the inundated areas of Washington and 
Oregon (grey).
Source: Washington Univ.

Trapped in ice and rafted down by the flood, Tomanowos crossed Idaho, Washington and Oregon along the overflown Columbia River at speeds sometimes faster than 20 meters per second. While floating up on the flood waters near today's Portland, the ice case broke apart and the meteorite was dropped on the bottom of the flooding waters. Hundreds of other ice-rafted erratics (rocks that do not match the local geology, nor could be transported by rivers or glaciers) have been found along the Columbia River. All are souvenirs from the Missoula floods.

As the flood ceased, the meteorite became exposed to the atmosphere. Over thousands of years, rain mixed with the iron sulfide inclusions producing sulfuric acid that gradually dissolved the iron of the exposed side of the rock:
These cavities were produced by acid dissolution of iron at the exposed side.
A few thousand years after the flood, the Clackamas arrived to Oregon and named the meteorite as the Visitor of the Sky, a heaven's representative that unified earth, water & sky. Did they know that nickel rocks come from heaven? Were they intrigued by the absence of a crater at the Meteorite site? In any case, the name reminds us that pre-scientific cultures were not idiotic, or not more than us today anyway.

To confirm this latter hypothesis, in 1902 a colonist named Ellis Hughes decided to literally move the iron rock to his own land to claim property. The millennia of peaceful rest in the Willamette had to come to an end. But moving a 15-ton rock a distance of 1,200 m without being noticed is not easy, not even in Oregon. Hughes and his son labored for three back-breaking months in secrecy: 

As D. J. Preston hilariously explains, after finally
succeeding with the moving, Hughes built a shack around
the meteorite, announced he had found it on his property
and started charging twenty-five cents admission to view
the heavenly visitor.
It was during this transport that the rock sadly underwent severe mutilations.
Unimpressed by this deployment of idiocy, Hughes' neighbor fabricated a lawsuit contending that the meteorite had, in fact, landed on his property. And to buttress his case he showed investigators a huge crater on his land. The case was dismissed when a third neighbor reported a great deal of blasting only the week before.

IRONically, the owner of the original land of the iron meteorite turned out to be the Oregon Iron and Steel Company, that was unaware of the meteorite but soon hired a twenty-four-hour guard who sat on top with a loaded gun while the case was being appealed. They won the case in 1905 and sold it to the AMNH a year later.
The meteorite in the early 1900s, before being transported to the AMNH.

Today, amazingly enough, the @AMNH exhibition does not yet mention the Missoula Floods as a key part of the meteorite story, in spite of the wide geomorphological consensus. But the descendants of the Clackamas still keep the right to visit the meteorite and talk to the visitor who brought the Sky, the Water, and the Earth together. 

2016-04-27

Glacier retreat in southern Iceland

Looking at old pictures, I realise that I had a first-hand glance at the retreat of the Jökulsárlón glacier (S. Iceland) back in 2013. I took these two pictures from the same spot with an 18-years time lag. Although the first one is taken in August and accordingly shows less snow in the background mountains than the more recent one, the latter does show the glacier front retreated by about 3 km. I pasted the Landsat images for comparison.


Not that this is a surprise, really: 
But i had to share.
In the meanwhile, I found this other JAXA (Japan) link as well. 

2016-02-24

Extreme Geodynamics at the Tsangpo Gorge

If you aim at understanding what shapes the surface of the Earth, the Tsangpo Gorge (Eastern syntax of the Himalayas) will inevitably become one of your favorite places.

This is the place where bedrock is
being eroded at the fastest
measured rate of nearly 1 mm/yr.
The uncommonly vertical valley
walls adopt this high angle to cope
by landsliding with the incision rates
produced by water. 
This is the place on Earth where one of the the highest bedrock erosion rates, the fastest tectonic uplift, and some of the highest topographic gradients have been measured. Every year, nearly 1 cm of very hard metamorphic rock is dig by the Tsangpo River, which descends from an elevation of >3000 m near the Tibetan plateau, to a mere 1000 m in less than 100 km. An average water discharge above 1400 m3/s, together with the pronounced slope, implies a huge erosion power.
Upstream from this gorge, there are widespread terraces and shore sediments of a lake that used to cover a few hundred kilometers of the river valley and impounded up to 800 km3 of water in a lake. What caused this impoundment is a matter of discussion: Only the tectonic uplift along the gorge? Or also an increase in landsliding from the valley flanks during the Pleistocene? Or glacial moraine accumulations?
The long duration of this competition between uplift and erosion (at least 10 Myr) implies that the region must be approximately in equilibrium, so uplift rates are presumably in the range of a cm per year, only comparable to the post-glacial isostatic rebound of Scandinavia.


A recent study of the infill of those lake sediments concludes that the steepening of the Tsangpo Gorge started about 2 to 2.5 million years ago as a consequence of a faster rock uplift: 
(A) Longitudinal river profile of the Tsangpo River, location of drill cores with observed depth to bedrock (vertical black bars), estimated depth to bedrock (yellow area), and reconstructed valley bottom before uplift of Tsangpo Gorge (dashed line). (B) Hillslope angles at the river flanks, specific stream power, and landslide erosion rates. (C) Erosion rates of close to 10 mm/yr are reflected in the age at which the minerals cooled down while being exhumed towards the surface. From Wang et al., 2014, Science. 
The extreme uplift and exhumation rates have been linked to a feedback effect of erosion on channelizing crustal rock towards the surface (the so called tectonic aneurysm; Montgomery & Stolar, 2006).

In contrast, other studies favor the role of glacial transport from the high surrounding mountains near the gorge in blocking the river with glacial moraines. This may have triggered megafloods sourced at impoundments formed by glacial dams (Lang et al., 2013, Geology), since some of the largest known outburst floods in the world have also been reported here.

Tsangpo Gorge
Hence, the competition between tectonic uplift and erosion at the Tsangpo encompasses many of the big conundrums in present geomorphology and geodynamics: the importance of episodicity in landscape evolution, the implications of the glacial ages on erosion rates, the possible effects of climate on tectonic deformation...

2015-11-16

How do we know that the Earth has a core?

1. A typical depiction of the core
of the Earth.
Many of us have wondered, at some point of our lives, why the cartoons depicting the Earth as a watermelon with a missing portion always show this ball in the center named the 'core'. How do we know that a distinct 'body' is actually down there, 2900 km below the surface?

Let's see: we know the total mass of the Earth through its gravitational interaction with the solar system. In 1797, Cavendish [ref.1] measured the Gravitational constant G and the density of the Earth is ever since known to be about 5.51 times the density of water: nearly twice the average rock density we find at the surface.

In 1898, Wiechert suggested [ref.2] that this high Earth’s density could be explained by a core in the center made of nickel and iron (like many meteorites known at the time) surrounded by a shell, or mantle, of the lighter silicon-dominated rocks that we see in the surface.

2. Inge Lehmann was one of the key
discoverers of the inner core of the Earth
But only in 1906, Richard D. Oldham found that the increasing speed of seismic waves with depth within the Earth holds only down to 2890 km below the surface. Deeper than that, the mechanical waves (sound) propagate much slower (fig. 6), suggesting a different rock nature. Because this distinct material did not transmit shear seismic waves, it became clear that this core is liquid.

In 1936, Inge Lehmann found that the center of the core is indeed nearly-solid, since she detected weak shear waves travelling through it [ref. 4] using highly-sensitive seismometers in New Zealand. This has become known as the inner core.


3. Images of the tsunami following last week's earthquake in Chile.

Today, detecting the core down there has become a doable task for anyone. Last week's earthquake in Chile, for example, provides a great opportunity for you to check if Oldham did everything right. You only need to get seismograms from seismic stations around the world (many of these stations have their data publicly available), and sort the signals according to the distance from the station to the EQ's epicenter, using the same time of reference, like in this image:
4. Left: Each horizontal line is a seismogram of the Chile earthquake recorded at different locations of the planet (check USGS: 2015-10-16; Mw=8.3). Each seismogram is plotted according to the distance of the measuring station to the earthquake (vertical axis). The red circle shows the signal gap due to the outer core.
Right: Same image, with the identification of the arrivals of the different seismic waves. 'P' waves are the compressional waves, they are first to arrive all around the planet's surface.
The horizontal axis shows elapsed time, measured since the EQ occurred.
The vertical axis shows the distance from the measuring station to the EQ.
The red circle shows the region (around 110 degrees from the source) where the first seismic waves are not recorded. 

5. Seismic shadow produced by an imaginary
earthquake occurring at the north pole. The
outer core, due to its slower seismic velocity,
refracts the mechanical waves of the earthquake,
shadowing a vast region of the planet, as seen
in figure 4.
6. The velocity of seismic waves
changes with depth within the Earth.









In summary: the absence of wave reception in regions around 14,000 km (between 103 and 143 degrees) apart from the hypocenter demonstrates that there is a liquid core where seismic waves travel slow.
Isn't it amazing that nobody realized this before the 20th century?

Finally, remember that the outer core is where the magnetic field of the Earth is generated, by the thermal convection of conductive molten iron around a nearly-solid iron inner core. In fact the changes in the convection patterns in the outer core seem responsible for the rapid historical changes observed in the magnetic field. There is more about the magnetic field in this earlier post.

7. Convection in the iron-dominated outer core is
the most-accepted cause for the Earth's magnetic field.
Update 2015-11: a new study finds that the core (and thus the magnetic field) was formed by the gradual cooling of the Earth only 1 to 1.5 billion years ago.

Update 2015-12: Geophysicists call it the new core paradox: They can't quite explain how the ancient Earth could have sustained a magnetic field billions of years ago, as it was cooling from its fiery birth. Now, two scientists have proposed two different ways to solve the paradox. http://ow.ly/W3eQX

References (thank you nuclearplanet):
1. Cavendish, H., Experiments to determine the density of Earth. Philosophical Transactions of the Royal Society of London, 1798, 88, 469-479.
2. Wiechert, E., Über die Massenverteilung im Inneren der Erde. Nachr. K. Ges. Wiss. Goettingen, Math-Kl., 1897, 221-243.
3. Oldham, R. D., The constitution of the interior of the Earth as revealed by earthquakes. Q. T. Geol. Soc. Lond., 1906. 62, 459-486.
4. Lehmann, I., P'. Publ. Int. Geod. Geophys. Union, Assoc. Seismol., Ser. A, Trav. Sci., 1936, 14, 87-115.

2015-11-03

Conferencias de divulgación geocientífica (50 aniversario del ICTJA)

Con motivo del 50 aniversario de nuestro instituto, cuatro investigadores del ICTJA participamos en el ciclo de conferencias divulgativas en Barcelona: "Las Ciencias de la Tierra en nuestra vida cotidiana", dentro del Cicle Dilluns de Ciència del CSIC-Catalunya.

Lugar: (mapa)
Sala d’Actes de la Residència d’Investigadors,
CSIC-Catalunya,
Barcelona

La conferencias de divulgación son los siguientes lunes:

2 Novembre, 18:30 h,
Charles Darwin, Lord Kelvin, els radioisòtops i el concepte de Temps
Dr. Santiago Giralt

9 Noviembre, 18:30 h
Tambora, 200 años de la erupción que cambió el Mundo
Dra. Adelina Geyer

16 Noviembre, 18:30 h
Megainundaciones, placas tectónicas y la formación del relieve terrestre
Dr. Daniel García-Castellanos

23 Novembre, 18:30 h
Interacció radiació-matèria per a estudiar-ho gairebé tot: nanomaterials, minerals exòtics, obres d’art, cadàvers,...
Dr. Jordi Ibáñez