Descent to the Thrihnukagigur volcanic conduit, Iceland

[location map]
A few days ago I joined a descent to the Þríhnúkagígur volcanic conduit in Iceland. It consists of a 150-meter vertical descent along a volcanic pipe. This lava conduit was abruptly emptied of lava during the last eruption in the area, about 4500 years ago. The reason for the emptying may have been that the magma chamber found an easier (lower?) exit to the surface. Thanks to that you can actually walk down there today and see exciting things. Some of them I suspect are the erosive marks left by outflowing rocks. There are also solidified droplets of magma falling along the walls of the crater as this became empty. More interestingly, you can see that the pipe is elongated and at its two extremes there is a near-vertical fault. The magma may have taken advantage of this fault when searching its way to the surface. There is some limited scientific literature that i did not yet check, so take this critically since i am not a volcanologist.

Here are some videos of the event:

Unsurprisingly but nicely, the fault is oriented in NE-SW direction, as usual for the SW Icelandic portion of the Mid-Atlantic ridge. Part of the 2 cm/year of the spreading between Eurasia and North America is sometimes taking place here.

One of the cracks along the North-Atlantic ridge, a few hundred
meters from the crater.
Eurasia to your left, N.America to the right.

Looking upwards towards the entrance to the cave. Note the elongated shape
of the pipe when looking upward toward the surface.
I guess this reflects the fault plane along which the pipe formed. 

The fault in one of the edges of the conduit. Its continuation could be followed behind the camera. 

All together the descent is a very recommendable experience if you are a geoscientist, if you love volcanos, or if you are just rich. It's organised by the consortium Inside the Volcano.

More pictures of the trip here.

Large Pleistocene Floods along the Columbia River

I spent the last weeks travelling through Idaho, Montana, Washington, and Oregon, tracking evidence for the floods that shaped the landscape of the US Pacific Northwest. For most of it I was privileged to be guided by Jim O'Connor (USGS, Portland), world-class specialist in these events (sample references below). The pictures of this geological trip show erosional and depositional features produced by outburst floods during the Pleistocene, including the Missoula ice-dam collapse and the Bonneville flood. 

Snake River gorge cutting the
Flood Basalt layers, through which
the Bonneville flood discharged. Looking downstream. Foto: DGC

Detail of the boulder field in the center of previous figure. Looking upstream. Foto: DGC

One thing that makes this scenario key to understand the importance of large flooding events in shaping landscape is the ubiquitous Columbia River Basalt formation (one of the largest flood basalt regions) and the Yellostone-Snake River volcanic zone. Both covered many of the areas we visited with a nearly flat layer of very fluid basalt, during the Neogene, long before the Pleistocene floods (see e.g. Fouch, Geology, 2011). The simple relief and lithological setting left by the flood basalts enhances the erosional features related to the Pleistocene water floods and the presence of vertical tectonic motions. 

Megafloods, gradualism, and the birth of geology

Altai Republic, southern Siberia, close to Mongolia. All the mountains around are older than 60 million years and we are about 300 m above the Chuja River. And yet we are stepping on recent (Pleistocene) gravels! These gravels are similar to those in every river bed except for their elevation above the river and for the fact that they show no apparent stratification: good indications that they were deposited in turbulent waters at that height, only 15 thousand years ago. The problem is: considering the valley width and the slope along the river in this area (~0.6%), such high water should have moved faster than 30 m/s, implying a discharge of about 100 times the present Amazon river. This event is getting to be known as the Altai Flood (+ info in this pdf).
No meteorological event could explain such a huge water discharge, particularly in a small catchment like that of the Chuja River. Instead, Russian geomorphologists came out during the 80's with this explanation: some tens of kilometers upstream (here), a glacier blocked the Chuja River for some thousands of years and formed a large lake behind. When the ice barrier collapsed, the sudden release of the lake's water produced a gigantic outburst flood with no historical precedent. This is today the most accepted interpretation of features such as the gravels laying along the flanks of the Katun valley (e.g., Herget, 2009).

This picture is taken from the top of the gravel deposits, nearly 300 m above today's river level. The gravels (note their size of a few cm) are interpreted to mark the upper reaches of the flooding waters when they encountered the hill obstacle and then ran up converting kinetic energy into potential energy. Location map. Other photos of this fieldtrip here.

But wait, doesn't all this sound a bit pre-geological? Religions often depict the Earth as being shaped by large floods and cataclysms following Creation. Is geology now acknowledging some truth in those myths?