Chris Rowan is a geologist specialising in the dark arts of paleomagnetism, and getting people to pay him to travel to exotic destinations for fieldwork. Having drilled up New Zealand during his PhD, and South Africa in his first post-doc, he now works at the University of Edinburgh.
Anne Jefferson has a love of all things water-related and blends hydrology, geomorphology, geology, and climate change in her work. She has a Ph.D. from Oregon State University and is now an assistant professor at the University of North Carolina at Charlotte.
The winds of change have caught a-hold of our little blog, cast it out into the wider internet, and dropped it down into it's own domain. We can now be found at:
We have both greatly appreciated the chance to write here at Scienceblogs. We hope that the friends that we have made here - both our fellow bloggers, and our smart and exceedingly kind commenters - will continue to read and contribute to the discussions at our new home.
After some discussion, Anne and I are putting Highly Allochthonous on hiatus whilst we consider our future here on Scienceblogs.
This decision is not made lightly. But the events of the last 24 hours have forced us to consider whether we can continue to contribute here without damaging our public and professional credibility.
Seed Media run this space and can therefore invite whoever they want to contribute to it; but allowing the employees of a major multinational to blog, not in a personal capacity, but on behalf of that multinational, raises serious questions about impartiality and conflicts of interest that, as scientists, we find hard to ignore. However, regardless of how the current furore is resolved, it has merely added to our feeling that although we have both enjoyed, and benefited immensely from, our time here at Scienceblogs, the network is moving in a direction that we may not want to follow.
Hutton's Unconfomity, Siccar Point. Here, deposition began again after some time and upheaval...
No long-term decision has been made on future blogging, except there will be future blogging, at some place on the Internet. Hopefully you will join us again when we work out where that will be.
On this hot, hot day, when much of the eastern United States is beset by a record-breaking heat wave, what could be more refreshing than a nice cold, fresh bottle of water?
Drink more fluids (nonalcoholic), regardless of your activity level. Don't wait until you're thirsty to drink. Warning: If your doctor generally limits the amount of fluid you drink or has you on water pills, ask him how much you should drink while the weather is hot. Don't drink liquids that contain alcohol or large amounts of sugar-these actually cause you to lose more body fluid. Also, avoid very cold drinks, because they can cause stomach cramps.
But, maybe, before you pass by the sink on your way to the fridge to get that nice bottle of water, you should watch this video...
This video was brought to mind this morning as I filled several liters of reusable water bottles with tap water in preparation for heading out into the field. It was also brought to mind by the newest Scienceblogs advertorial blog, Pepsico. In addition to being a major manufacturer of those sugary drinks the CDC is warning you not to drink on hot days, Pepsi is also a major producer of bottled water. Their Aquafina brand of bottled water is filtered, municipal tap water. But while muncipal water supplies are required to report their water quality and comply with federal drinking water standards, much fewer regulations exist around the quality of bottled water. So while there are some places where there are legitimate reasons for people to drink bottled water (e.g., lead pipes, pollution from coal mining or natural gas extraction), for the vast majority of Americans, there is no health benefit to drinking bottled water over municipal tap water.
Honestly, though, bottled municipal tap water doesn't bother me as much as bottled spring water, where the springs and the aquatic ecosystems that depend on them can be destroyed in pursuit of the mythical pureness (and retail power) of spring water. While the bottled water industry will assure you that their groundwater consumption is much less than 1% of the national total groundwater withdrawals, the effects of those withdrawals are localized and not distributed around the country evenly. Finally, it doesn't take much analysis to understand why buying bottled water from Fiji, an island in the tropical Pacific where a shallow freshwater lens will be irrevocably contaminated by salt water intrusion by overpumping of the aquifer, is a ridiculously bad idea.
One of the blogging commandments should probably be: know thy readers! Therefore we are following the example of Janet, DrugMonkey and various others (who are themselves riffing from Ed Yong's original idea) and asking you, our readers, to tell us a little about yourselves.
Who are you? Academic or professional geologist, student, enthusiastic rockhound, general browser?
What's your level of science education? Postgraduate, undergraduate, school, dropped it like a hot potato at earliest opportunity?
What originally brought you to this blog, and what keeps you coming back (if indeed, you intend to)?
Which of the topics covered here do you particularly enjoy? Is there anything you tend to skip?
Are there any topics that would you like us to write about more often?
If you lurk rather than commenting, are you content with that? Are there conditions that you think might suck you into commenting?
If you could ask us to write one post explaining one basic concept in earth science, what would that concept be?
And finally, we have to ask: which is better, water or rocks?
This is a bit of an experiment. People seem to quite appreciate my posts that place significant earthquakes in their tectonic context (e.g. #1, #2, #3). However, I can't write a detailed post for every single one. So I'm wondering if it might be worth producing a brief weekly summary of any significant earthquakes, showing their focal mechanisms together with a brief tectonic interpretation. Below I do this for all magnitude 6+ earthquakes reported by the USGS in the past seven days. The format is admittedly a bit unpolished right now, but I'm interested if you'd be interested in me making this a regular feature.
Saturday 26 June: M 6.7, Solomon Islands, Depth 35 km
A transpressional focal mechanism, with mostly NE-SW compression. The direction of shortening is consistent with the NE subduction of the Australian plate beneath the Pacific plate under the Solomon Islands. Historical seismicity suggests that this quake occured near the subduction interface.
(USGS page)
Wednesday 30 June: M 6.3, S of Fiji, Depth 536 km
Focal mechanism indicates extension. The great depth of the rupture, and it's location behind the Tonga Kermadec arc, suggests it occurred far down dip on the subducting Pacific Plate, which is stretching under its own weight as it sinks deeper into the mantle.
(USGS page)
Wednesday 30 June: M 6.2, Oaxaca, Mexico, Depth 20 km
Focal mechanism indicates NE-SW compression. This is consistent with the convergence direction of the subduction zone off the west coast of Mexico (Cocos plate subducting beneath the North American plate), but the rupture is too shallow to be associated with the subduction thrust itself. This earthquake is likely caused by strain transferred across the locked subduction boundary is being accommodated by thrust faulting in the overriding plate.
(USGS page)
Friday 2 July: M 6.3, Vanuatu, SW Pacific, Depth 34 km
Focal mechanism indicates E-W compression. In this region, the Australian plate is subducting to the east beneath the Pacific plate, and the depth of the rupture is consistent with it being a thrust close to the subduction interface.
(USGS page)
The fossil record prior to 550 million years ago is so patchy that every discovery is going to cause some fanfare. That is certainly case with these odd looking things, which have been proclaimed in Nature as the oldest mulitcellular organisms ever found.
A 2.1 billion year old fossil atop the bed it was preserved in. Source: Albani et al., Fig. 2
These flat, dish-like fossils are found at a number of horizons within a black shale unit of the Francevillian Group in southeast Gabon. They grew in a marine delta environment, and following a rapid burial event, sulphate-reducing bacteria got to work decomposing them. One by-product of sulphate reduction is pyrite, and as a result decomposition left a durable, mineralised impression of what was consumed.
To my admittedly inexpert eyes, these things do not immediately scream 'multicellular!' at me (in contrast to, say, the enigmatic Stirling fauna) - they look like some kind of bacterial mat. However, Albani et al. have used high resolution X-ray scans to reveal a complex internal structure, and what they argue is evidence of coordinated growth patterns, both of which suggest a higher degree of organisation than a bacterial colony [Update: Go and read Ed Yong's write-up for a number of semi-sceptical expert opinions].
The age of the Francevillian Group is given as 2.1 billion years (2100 million), and it is this great age that makes these fossils so potentially significant: if they are multicellular, then they are the oldest known large multicelluar organisms by a margin of about 200 million years (it's not clear in the article, but I think that the Stirling fauna is what has been usurped). It seems not everyone is so impressed by this*, but it does push the possible origins of multicellularity back much closer to the oxygenation of the Earth's atmosphere between 2400 and 2300 million years ago, which must have had a significant evolutionary impact on life. If this discovery pans out, a hypothesised connection between the two becomes slightly less tenuous.
Because precisely dating such old rocks is a tricky business, I was curious to know exactly how good the age constraints on the Francevillian Group were. As it turns out, they are actually pretty good. By a lucky happenstance, the unit just above the fossil-bearing layers has a zircon-bearing tuff which has been extremely precisely dated as forming 2080 million years ago. This provides a minimum age for the units below - they had to have already been there when the tuff erupted. Another minimum age comes from the Oklo uranium ore deposit, which is found at a slightly lower stratigraphic level, is pretty famous in its own right, and is known to have formed around 2050 million years ago. Because the ore body is the result of later mineral growth some time after deposition of the host rock, the host rock must be older than the ore. Diagenetic clays just below the fossil-bearing layers, which probably formed shortly after deposition, have also been less accurately dated at around 2100 million years ago.
A final chronological clue is provided by the observation that carbonate minerals within the fossil-bearing sequence have elevated levels of carbon-13 with respect to carbon-12. A similar trend (known as the Lomagundi excursion after the place in Zimbabwe it was first identified) has been found in sequences between 2220 and 2100 million years old from Africa, South America, North America, Scandanavia and Australia. Correlating to what seems to have been a global change in seawater chemistry therefore provides maximum and minimum estimates for when the fossil organisms lived and died. When combined with the other age constraints described above, particularly the age of the overlying tuff, an age towards the younger end of this interval seems to be the most reasonable interpretation, hence the 2100 million year estimate given by the authors.
So, whilst the significance of these Gabonese fossils can be debated, they are almost certainly around 2.1 billion years old.
*only people used to working in the Archean and Proterozoic can be blasé about such lengths of time. 200 million years from the present day, dinosaurs were still at the beginning of their reign, and all the continents were collected together into Pangaea, so it's not an insignificant interval in terms of evolution or geology. Although to be fair to Daniel, he's mainly responding to a media write-up that propagates the whole 'nothing biologically interesting before the Cambrian - except THIS!' trope.
Albani, A., Bengtson, S., Canfield, D., Bekker, A., Macchiarelli, R., Mazurier, A., Hammarlund, E., Boulvais, P., Dupuy, J., Fontaine, C., Fürsich, F., Gauthier-Lafaye, F., Janvier, P., Javaux, E., Ossa, F., Pierson-Wickmann, A., Riboulleau, A., Sardini, P., Vachard, D., Whitehouse, M., & Meunier, A. (2010). Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago Nature, 466 (7302), 100-104 DOI: 10.1038/nature09166
What does faulting do to a rock 2 miles beneath the Earth's surface? Thanks to the San Andreas Fault Observatory at Depth (SAFOD) project, which retrieved samples across an active segment of the San Andreas Fault from 3000m below the Earth's surface, we can answer this question: it turns it into fragments a little like this:
Polished and striated rock chip from fault zone in SAFOD borehole. Source: Schleicher et al., Fig. 1B.
Anja Schleicher and her co-authors found abundant fragments like the one above, with polished and striated fracture surfaces formed by strike-slip motion of the San Andreas fault, which collectively make up a fault gouge - or, less technically, a really really smashed up rock - in the fault zone. However, as they report in their recently published paper in Geology, closer examination and chemical analysis of these fracture surfaces reveals something rather interested - a thin coating of clay minerals such as illite. These minerals' growth patterns, and radiometric dating of their time of formation, demonstrate that they post-date the fracturing - they grew on the polished grain surfaces at some point after the rock was broken apart.
What makes this discovery particularly interesting is that the SAFOD borehole was drilled through a part of the San Andreas Fault which 'creeps': there is fairly continuous, slow movement, rather than the stick-slip behaviour that generates large earthquakes. It seems that in this region, the friction between the two sides of the fault is too low for any significant elastic strain to build up before the fault moves. These new observations suggest that the cause of this weak, creeping behaviour is the growth of the illite and other clay minerals within the fault rock; these new minerals then act as a lubricant that reduces friction and allows more continuous deformation. This contrasts with the hypothesis put forward when the borehole samples were first unveiled a couple of years ago, which pointed to the presence of serpentinite (and talc derived from the breakdown thereof) within the units being deformed as the explanation for creep.
A further implication of these observations is that creeping behaviour is a function of the age of the fault. The polished and striated rock fragments testify to the fact that the rock intersected by the borehole was originally shattered by brittle failure, which produces earthquakes. Schleicher et al. propose that this initial fracturing allows fluids to more easily circulate through the fault zone, speeding up chemical alteration and producing the clay minerals. They initially occur in small, unconnected pockets, but further seismic motion and alteration eventually link the clay-rich regions together into larger fractures that can accommodate tectonic strain by slow creep rather than jerky sticking and slipping. It's a kind of reverse arthritis - rather than seizing up, the older Earth's tectonic joints get,the more freely they may move.
Questions remain, however. Whilst the segment of the San Andreas Fault sampled by SAFOD deforms mainly by creep, it is right next door to rather more seismically hazardous sections that do not. In a press release accompanying the paper, co-author Ben van der Pluijm suggests that the difference is due to activity being focussed on either older, clay-lubricated or younger, stronger, strands of the fault system; however, I'm uncertain why activity would shift away from weaker areas of the crust into stronger ones like that. Another possiblity is that the growth of clay minerals proceeds at different rates in different parts of the fault, being controlled or limited by lithology, or the amount and/or composition of circulating fluids. Interesting as this finding is, there's still plenty more work to do before we truly understand why faults behave the way they do.
Schleicher, A., van der Pluijm, B., & Warr, L. (2010). Nanocoatings of clay and creep of the San Andreas fault at Parkfield, California Geology, 38 (7), 667-670 DOI: 10.1130/G31091.1
Ouch. Spherules cited as evidence for Younger Dryas impact 'fossilized balls of fungus, charcoal, fecal pellets' http://www.physorg.com/news195979458.html
[I have previously blogged about this debate here and here]
Give yourselves a pat on the back: virtually everyone guessed correctly that my fortnight away was chiefly spent exploring Yellowstone National Park, bookended by some time in Grand Teton National Park just next door. The first photo I showed you was of a dead tree standing in a growing expanse of silica deposited by a nearby hot spring*. The spring in question is the Grand Prismatic Spring, which is the third largest hot spring in the world, and even looks pretty from space.
The Grand Prismatic Spring
The second photo is of a rhyolite lava flow in the Firehole Canyon. Rhyolite lavas are extremely viscous, as illustrated nicely by the intensely deformed flow banding in this outcrop - it hasn't so much flowed, as oozed. This flow occured within the Yellowstone caldera some time after it was excavated by the last big explosive eruption 640,000 years ago.
Perhaps it was pretty obvious in hindsight - but I didn't expect you all to be all so North America-centric that you wouldn't guess one of the world's other geothermal areas. Maybe the pine trees were too much of a giveaway. Regardless, I saw plenty of awesome geology whilst I was away - and I'm planning to share the highlights with you all over the next few weeks.
*incidentally, I may just have a big hole in my mineralogical knowledge (which is entirely possible), but I swear I've never heard hydrothermally deposited silica referred to as 'geyserite' before. Is this usage limited to North America, perhaps?
Chris Rowan is a geologist specialising in the dark arts of paleomagnetism, and getting people to pay him to travel to exotic destinations for fieldwork. Having drilled up New Zealand during his PhD, and South Africa in his first post-doc, he now works at the University of Edinburgh.
Anne Jefferson has a love of all things water-related and blends hydrology, geomorphology, geology, and climate change in her work. She has a Ph.D. from Oregon State University and is now an assistant professor at the University of North Carolina at Charlotte.
The winds of change have caught a-hold of our little blog, cast it out into the wider internet, and dropped it down into it's own domain. We can now be found at:
Hutton's Unconfomity, Siccar Point. Here, deposition began again after some time and upheaval...
Source: 
A 2.1 billion year old fossil atop the bed it was preserved in. Source: 
Polished and striated rock chip from fault zone in SAFOD borehole. Source: 
The Grand Prismatic Spring
