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...