Highly Allochthonous http://all-geo.org/highlyallochthonous NEWS & COMMENTARY FROM THE WORLD OF GEOLOGY & EARTH SCIENCE Wed, 11 Jan 2012 22:08:47 +0000 en hourly 1 http://wordpress.org/?v=3.2.1 How I (mostly) slept through the one of the largest earthquakes to hit NW Europe in 200 years http://all-geo.org/highlyallochthonous/2012/01/how-i-mostly-slept-through-the-largest-european-earthquake-in-200-years/?utm_source=rss&utm_medium=rss&utm_campaign=how-i-mostly-slept-through-the-largest-european-earthquake-in-200-years http://all-geo.org/highlyallochthonous/2012/01/how-i-mostly-slept-through-the-largest-european-earthquake-in-200-years/#comments Tue, 10 Jan 2012 12:30:27 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7195 Continue reading →]]> A post by Chris RowanIn the early hours of 13 April 1992, the border region in western Europe where Germany, Belgium and the Netherlands meet was shaken by a magnitude 5.4 earthquake, caused by northeast-southwest extension in the Roer Valley Graben. The shaking was severe enough to damage buildings 30-40 kilometres from the epicentre. 40 kilometres to the south, in the small town of Valkenburg, a group of British schoolchildren on a European excursion were sleeping peacefully in their beds, a 13 year-young version of this geoblogger amongst their number.

Location of the Roermond earthquake on 13th April 1992; location of Chris on 13th April 1992. Source for map: Geluk et al. 1994 (click for pdf).

I distinctly remember waking up to the room shaking and (rather woozily) thinking, “Earthquake!” But I also distinctly remember then thinking, “You don’t get earthquakes in the Netherlands. I must be dreaming,” before turning over and going back to sleep. Imagine my surprise the next morning when I discovered that not only had everyone else in our hotel shared my “dream”, but that the shaking had been strong enough to toss some of them from their beds. And that is how I semi-slept through what remains my only up close and personal encounter with the tectonic forces that shape our planet: forces that I would end up studying for a living, little did I know it at the time.

I normally tell this story for humourous effect, but a part of me always cringes a little inside when I recall these events, because it also illustrates how, despite me already having ambitions to become a scientist at this age, I was still a long way from thinking like a scientist. Firstly, I assumed in the arrogance of precocious youth that I already knew everything worth knowing about earthquakes. I had read that earthquakes were associated with the boundaries of tectonic plates, and I also knew that the Netherlands was nowhere near such a boundary. I had read nothing, or at least remembered nothing, about the earthquakes that occur within plates, but I wasn’t yet mature enough to realise how much I didn’t know.

Perhaps 'Mount Unknowingly Ignorant' would be more accurate. Source: Saturday Morning Breakfast Cereal (click for full comic).

Perhaps more seriously, when confronted with a contradiction between what I thought I knew and actual reality, I responded with reflexive denial rather than curiousity. Even if it was 100% established that earthquakes only ever occurred at plate boundaries, a true scientist would not respond to an apparent earthquake in the Netherlands with a cry of “that’s impossible!’ A true scientist would think “hmmm, that’s funny…” and start investigating.

Narrative causality would demand that I close this story by telling you that not only did realising my mistake the next morning set me on the path to developing a proper scientific mindset, but also that it inspired me to take an interest in geology and tectonics. Sadly, neither is true. It took a few much more painful lessons before I became properly humble about my knowledge of the world; and before I learnt to stop bending the world to fit my preconceived notions, and actually observe and think more carefully about what was going on around me. And although I can’t completely rule out some subconcious effect of the Roermond earthquake on my later career choices, it took me at least six more years before I truly discovered the wonders of geology.

Now I know better than to dismiss the earthquake potential of places like central Europe. The Netherlands might be a long way from a true plate boundary, but there is a historical earthquake record going back centuries in this region, and paleoseismic studies following the 1992 earthquake found several fault scarps in the area with signs of Quarternary displacement. The Atlantic coast of the US has recently been described as a ‘passive-aggressive’ margin, hosting faults that can build up significant elastic strain over centuries and millenia before rupturing in earthquakes that could reach magnitude 7. Likewise, its European counterpart is not without its seismic dangers.

Corrective note: This post title originally referred to me sleeping through the ‘largest European earthquake in 200 years’. I think I originally meant to write ‘one of the largest’, but even then that’s a bit of an over-reach if you’re including Italy and Greece’. The title has now been appropriately modified.

This post was written in response to Ron Schott’s Accretionary Wedge call for stories of the most memorable or significant geologic event that we’ve directly experienced. Even if I sort of slept through it, I should at least get partial credit, right?

]]> http://all-geo.org/highlyallochthonous/2012/01/how-i-mostly-slept-through-the-largest-european-earthquake-in-200-years/feed/ 5 Stuff we linked to on Twitter last week http://all-geo.org/highlyallochthonous/2012/01/stuff-we-linked-to-on-twitter-last-week-39/?utm_source=rss&utm_medium=rss&utm_campaign=stuff-we-linked-to-on-twitter-last-week-39 http://all-geo.org/highlyallochthonous/2012/01/stuff-we-linked-to-on-twitter-last-week-39/#comments Sun, 08 Jan 2012 16:41:56 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7191 Continue reading →]]> A post by Chris RowanA post by Anne JeffersonWelcome to the first Highly Allochthonous Sunday link-fest of 2012. We realise that technically this is the second Sunday of the new year, but we trust that you’ll forgive us…

Other posts on All-geo

Earthquakes

Volcanoes

(Paleo)climate

Water

Environmental

General Geology

Women in Science

Interesting Miscellaney

]]> http://all-geo.org/highlyallochthonous/2012/01/stuff-we-linked-to-on-twitter-last-week-39/feed/ 0 How useful are lectures, really? http://all-geo.org/highlyallochthonous/2012/01/how-useful-are-lectures-really/?utm_source=rss&utm_medium=rss&utm_campaign=how-useful-are-lectures-really http://all-geo.org/highlyallochthonous/2012/01/how-useful-are-lectures-really/#comments Wed, 04 Jan 2012 18:11:34 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7181 Continue reading →]]> A post by Chris RowanThere has been an interesting discussion amongst the geologists on Twitter, that I’ve archived over on Geotweeps Discuss…, over the role of the lecture in undergraduate education. This was in response to an NPR story claiming that in physics at least, lectures are very bad at enabling students to conceptually grasp the material being presented. That sort of understanding requires a more interactive style of teaching, with demonstration, and small-group discussions.

I’ve read a few of these in the last few months, and in some ways I feel they set up a false dichotomy between ‘all lectures’ and ‘no lectures’, when the reality is usually ‘some lectures’. I certainly feel, as did most people who joined the discussion on Twitter, that lecturing should not be the be-all and end-all of the teaching experience, but it still has a place.

My experience of lectures back when I was an undergraduate was that some were good, some were bad. Some were very bad. But they were almost always a good starting point, in the sense that you came out with an idea of what the person teaching your course thought were the important concepts you needed to grasp, and usually a couple of useful example problems or case studies. Attaining true understanding might have only come after practicals, and reading, and talking it through with my classmates and teachers, but the lecture was where I learnt what I needed to understand.

Of course, I’m perhaps a little bit of an outlier: I’ve stayed in the academic system and have spent time at the front of the classroom as well, and I went to a University which had no compuction in telling us that perfect regurgitation of lecture notes would not impress the examiners – whilst also giving us plenty of non-lecture contact time with the faculty.

In a time when Universities are having to accommodate more students with less money, and staff are finding their schedules ever more loaded, perhaps we are heading to a place where the lecture is often the only real point of contact between teacher and the students. Combine this with the fact that students are coming to university ever more accustomed to ‘I’m going to tell you what’s in the exam’ than to ‘Here’s what you need to learn about. Go forth to the library!’, and perhaps we have a problem.

Anyway, I’d be interested to hear other peoples’ thoughts on this. If you went to university, did you love lectures, or hate them? Get all you needed out of them, or nothing at all? Have nothing but lectures, or a bit more balance?

Update: Thanks for all the great comments. There were also a few responses on Twitter that I thought were worth adding for posterity.

@ I think YouTube adds a lot of value to them, since anyone from anywhere can watch lectures from the best schools & teachers.
January 4, 2012 8:09 pm via webReplyRetweetFavorite
@coreburn
Jason R. Hunter

@ if done well lectures can be very stimulating, but not every lecture can "wow" everyone in the class... a mix is best
January 4, 2012 8:16 pm via Twitter for AndroidReplyRetweetFavorite
@smknipe
Stephen M. Knipe II

@ When lectures teach beyond the textbook that's when they come alive. Complementary + new instead of pure repetitiveness.
January 4, 2012 8:58 pm via Twitter for MacReplyRetweetFavorite
@markhilverda
Mark Hilverda

@ @ I go to Uni and I LOVE my lectures..There should be more but I guess that's not going to happen :/
January 4, 2012 10:06 pm via Twitter for iPhoneReplyRetweetFavorite
@Janinemarshal
Janine Marshall

@ Question should be how to deliver impt concepts to a cohort of 200+ ugrads not using a lecture? Logistical constraints there.
January 5, 2012 5:06 am via webReplyRetweetFavorite
@palaeo_isotopes
Dr Andrew Henderson
]]> http://all-geo.org/highlyallochthonous/2012/01/how-useful-are-lectures-really/feed/ 17 Geological mayhem and destruction in 2012: not the end of the world, just business as usual http://all-geo.org/highlyallochthonous/2012/01/geological-mayhem-and-destruction-in-2012-not-the-end-of-the-world-just-business-as-usual/?utm_source=rss&utm_medium=rss&utm_campaign=geological-mayhem-and-destruction-in-2012-not-the-end-of-the-world-just-business-as-usual http://all-geo.org/highlyallochthonous/2012/01/geological-mayhem-and-destruction-in-2012-not-the-end-of-the-world-just-business-as-usual/#comments Sun, 01 Jan 2012 22:33:31 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7164 Continue reading →]]> A post by Chris RowanAs earth scientists and science bloggers, Anne and I can’t help but feel a little trepidation as we enter 2012. The 21st of December this year marks the end of the 13th bak’tun of the Maya Long Count calendar. In the physical world, this date has about as much significance as the turn of the millennium does (or did) in the Gregorian calendar most countries currently use: in other words, none whatsoever. But it is also a date that has long been seized upon by apocalypse-mongers of all stripes as a prime candidate for the day that our world will be swallowed up in fire and chaos. Sadly for us earth scientists, many of the ‘theories’ as to how our doom will unfold involve geology: The poles will flip! The earth will tilt on its axis! The poles will flip and the earth will tip on its axis! Lands sinking into the sea! Earthquakes! (Super)volcanoes! Rains of curry!

Not. Going. To. Happen.

The world will not end in 2012, but it is likely to be a high-water mark for earth science-related hysteria. Even more than usual, any natural disaster, anywhere, risks being pronounced as a dire omen of the greater catastrophe to come. As we gear ourselves up to fight this torrent of nonsense, one thing that is surely needed for the coming twelve months is a proper baseline. Given that natural disasters occur all the time, what sort of geological activity should we expect if it is just a normal year on Planet Earth? The following list of expectations comes with a side-order of snark and a warning: this list of ‘predictions’ is based on a historical record that is often shorter than we ideally need (particularly for earthquakes) and is covering systems that are naturally variable in their behaviour over time. Consider it a starting null hypothesis against which the significance of your theories of imminent doom should be tested.

In 2012:

  • The Earth’s tectonic plates will continue to move across the mantle at a few centimetres a year. Earthquakes and volcanoes will result in the usual fashion.
  • Based on the last century of seismic activity, there will be 10-20 earthquakes with a moment magnitude greater than 7.0, with a good chance that at least one will exceed magnitude 8. Most of the largest are likely to be associated with subduction zones, with the consequent risk of a tsunami if they rupture at shallow depths.
  • There will be more than a hundred earthquakes between magnitude 6 and 7, which have the potential to cause considerable damage if the rupture occurs close to a major city.
  • As Erik Klemetti will tell you, volcanic eruptions happen all the time, and in 2012 they will continue to do so. Some will fairly harmlessly ooze lava; others will behave more explosively, mimicing this year’s Pliny-winning Puyehue-Cordón Caulle in Chile. But unless something really spectacular happens, akin to Mount Pinatubo’s eruption in 1991, all of these eruptions will be eclipsed by breathless speculation about any slight seismic activity that occurs beneath Katla, the Yellowstone caldera, or any volcano in the Pacific Northwest.
  • In a number of places, too much rain will fall in too short a time, and flooding will result. When this occurs in a rich western country, there will be relatively low casualties, a large bill for damages, and lots of media coverage; when it happens in a poorer country, casualties will be higher and many more people will be displaced, but the media will pay far less attention.
  • The Earth’s magnetic poles will migrate a few tens of kilometres from their present position. This may lead to a few airports re-marking their runways, but is not a sign that the field is about to reverse. If you hear talk of the dipole field’s recent weakening and this being a sign of the field gearing up for a reversal – and in my opinion, it’s at least a few centuries to soon to tell – remember that the whole process takes 5,000 to 10,000 years. Come December 22, your compass will still point north.
  • Scientists will continue point to the serious consequences of climate change as a result of anthropogenic greenhouse gas emissions (which will continue to inexorably rise). The climate itself will continue to push up past the ‘normal’ range in which our civilisation developed and is tuned to flourish in. The resulting extreme weather events – be they heatwaves, floods, or unusually powerful storms – are far more likely to be attributed to mystical cosmic cycles than our disruption of the carbon cycle.

Every year, some parts of the world are suddenly struck by natural disasters, taking lives and destroying property. 2012 will, unfortunately, be no different. But just as in every over year, there will be nothing more cataclysmic going on in the next twelve months than the Earth’s normal geological and meterological processes – although, in the latter case, the planet is getting an increasingly large helping hand from us.

]]> http://all-geo.org/highlyallochthonous/2012/01/geological-mayhem-and-destruction-in-2012-not-the-end-of-the-world-just-business-as-usual/feed/ 6 Scenic Saturday: Year End Reflections http://all-geo.org/highlyallochthonous/2011/12/scenic-saturday-year-end-reflections/?utm_source=rss&utm_medium=rss&utm_campaign=scenic-saturday-year-end-reflections http://all-geo.org/highlyallochthonous/2011/12/scenic-saturday-year-end-reflections/#comments Sun, 01 Jan 2012 00:04:34 +0000 Anne Jefferson http://all-geo.org/highlyallochthonous/?p=7151 Continue reading →]]> A post by Anne JeffersonThe last day of the year saw me doing field work in my very favorite spot in North Carolina, a short drive from Charlotte which takes me to a place that feels worlds away. I was collecting the final dataset for a paper I’m writing, and the afternoon found me musing on beginnings and endings and the questions that continue to tease me in this landscape. On reflection, I’m proud of what I’ve learned and done here, but I’m excited to see where the streams lead me next.

Deep Creek reflections

Deep Creek, Redlair, North Carolina. photo: Chris Rowan, 31 December 2011

]]> http://all-geo.org/highlyallochthonous/2011/12/scenic-saturday-year-end-reflections/feed/ 0 Our Highly Allochthonous travels in 2011 http://all-geo.org/highlyallochthonous/2011/12/our-highly-allochthonous-travels-in-2011/?utm_source=rss&utm_medium=rss&utm_campaign=our-highly-allochthonous-travels-in-2011 http://all-geo.org/highlyallochthonous/2011/12/our-highly-allochthonous-travels-in-2011/#comments Thu, 29 Dec 2011 22:06:05 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7100 Continue reading →]]> A post by Chris RowanA post by Anne Jefferson As 2011 draws to a close, ’tis the season for retrospectives, and we’re surprised that no-one this year seems to have started up the travel meme that has been so popular in the geoblogosphere in the past. After all, it gives us all the chance to (a) boast about all the cool places we’ve been in the past 12 months, and (b) show everyone pretty pictures – what’s not to like? Perhaps it just needs someone (i.e. us) to get things moving with a month-by-month summary of our travels in 2011.

In January we were both at ScienceOnline in North Carolina, the annual chance to meet in real life all of our online friends. As usual, many interesting conversations ensued.

In February we didn’t travel anywhere, but the 20-odd inches of snow deposited by the Groundhog Day snowstorm made Chicago an entirely different place.

Animation of snowfall accumulating during the Groundhog Day blizzard.

Snow accumulation over 12 hours during the Groundhog Day blizzard 2011. Photos: Chris Rowan, 2011

In March, we explored Mammoth Cave in Kentucky, where you can see groundwater in action and where Anne’s daughter appreciated being the only person on the tour who didn’t have to duck in the low passageways between the giant main chambers. Plus, speleothems!

Frozen Niagara dripstone, Mammoth Cave, Kentucky (photo by A. Jefferson, March 2011)

Frozen Niagara dripstone, Mammoth Cave, Kentucky (photo by A. Jefferson, March 2011)

An April hike in the Smoky Mountains had Chris appreciating not just a pretty waterfall, but the long geological history that contributed to it being there.

Grotto Falls, Great Smoky Mountains National Park. Photo: Chris Rowan, 2011 (click to enlarge).

On a trip to Southern Illinois in May, Anne got to play with stream tables and see the aftermath of the Mississippi flooding that she wrote an award-winning blog post about.

Em4 model at work.

Em4 stream table model at work being played with.

Levee repair along the Big Muddy

Temporary levee repair along the Big Muddy River in southern Illinois, near the Mississippi River.

In June, we took advantage of the summer weather to take a boat trip along the shores of Lake Michigan and the Chicago River. It should come as no surprise than Anne was quite interested in the locks that ensure that the flow of the Chicago river is reversed, with water running out of Lake Michigan rather than into it as it originally did.

The locks between the Chicago River and Lake Michigan. Photo: Anne Jefferson, 2011.

There was also a brief trip to Wisconsin Dells, including an early morning hike at Rocky Arbor State Park.

Rocky Arbor State Park. Photo: Anne Jefferson, 2011.

In July, Anne won the ‘make Chris insanely jealous’ prize by getting to visit the Galapagos Islands for a Chapman conference. It was, of course, complete happenstance that the best lavas to look at were on the beach.

Anne on a ropy pahoehoe flow on the beach

Anne enjoying the scenery on Isabella Island, Galapagos, July 2011

August‘s visit to the UK included ammonite hunting on the Jurassic coast, and a visit to some Wollemi pines that have made their home in Essex, of all places.

Cliffs west of Lyme Regis where Mary Anning collected her fossils

Cliffs west of Lyme Regis, Dorset. August 2011.

Wollemi pines at Mark's Hall, near Colchester, Essex. Photo: Chris Rowan, 2011

In September Chris spent a few days in Montreal for a conference, which has at least one item of geological interest in the form of Mont Royal, an eroded volcanic complex. Chris, starved of topography from all those months in flat, flat Chicago, couldn’t resist climbing to the top of it, but sadly didn’t have time for any real geologising.

In October Anne travelled to Minnesota for the GSA conference, and took the time to take a walk around one of the lakes the state is famous for – one familiar to her from her days as a Masters’ student at the University of Minnesota.

Staring Lake, October 2011

Staring Lake, October 2011, photo by A. Jefferson

November saw very little travel, because we were both busy preparing for the annual December pilgrimage to the AGU conference in San Francisco. Despite spending most of the time in entirely different sessions, hearing about totally unrelated (yet equally cool) new science, we did take the time to nip over the hill to Fisherman’s Wharf, and were treated to a lovely sunset over the Bay.

Sunset from Fisherman's Wharf, San Francisco. Photo: Chris Rowan 2011.

You are challenged to join us in summarising your travels. Go on, make us jealous!

]]> http://all-geo.org/highlyallochthonous/2011/12/our-highly-allochthonous-travels-in-2011/feed/ 1 Two more earthquakes shake Christchurch http://all-geo.org/highlyallochthonous/2011/12/two-more-earthquakes-shake-christchurch/?utm_source=rss&utm_medium=rss&utm_campaign=two-more-earthquakes-shake-christchurch http://all-geo.org/highlyallochthonous/2011/12/two-more-earthquakes-shake-christchurch/#comments Fri, 23 Dec 2011 05:12:50 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7086 Continue reading →]]> A post by Chris RowanJust as it seemed that seismic activity was finally dying down in Christchurch, the city has been shaken by two more earthquakes. The USGS currently has the first shock pegged as a magnitude 5.8, and the second as a magnitude 5.9; the NZ Herald reports that the effects in Christchurch itself include loss of power, liquefaction and flooding, and rockfalls, but only minor injuries so far.

The first shock occurred about 2pm local time. The rupture was shallow (about 5 km deep) and located offshore, about 25 km miles east of Christchurch. The focal mechanism suggests westward thrusting on a north-south oriented fault. This earthquake was quickly followed by a magnitude 5.3 event. The second M 5.9 hit just under two-and-a-half hours later, 5 miles closer to the shore. It was also a relatively shallow rupture, and the focal mechanism also indicates thrusting, this time in a more northwest direction, mixed in with a bit of dextral strike-slip.

USGS focal mechanisms for the December 23rd M 5.8 earthquakes near Christchurch, with the focal mechanisms for the September 2010 Darfield and February 2011 Port Hills earthquakes, and their approximate ruptures, plotted for comparison.

(a primer on interpreting focal mechanisms)

It’s quite early, and the focal mechanism on the first shock in particular is a little poorly constrained by the look of things, but a few things stand out.

  • The area that these earthquakes occurred in is along the trend of the fault – the Port Hills Fault, that ruptured in Feburary’s magnitude 6.3 earthquake. It’s likely that the Port Hills earthquake would have caused the stress in the crust in this region offshore to have increased slightly.
  • Looking at the focal mechanism for February’s quake, it also indicates north-west directed thrusting with some dextral strike-slip.
  • So whilst based on the first, less well-constrained, focal mechanism, I thought that this new sequence was due to motion on an entirely new fault, it is possible this is just stress being released on an eastward extension of the Port Hills fault that ruptured in February, or possibly a parallel strand of the same fault system.

It will probably become clearer as more data is collected and analysed in the next few hours and days. In the meantime, the NZ Herald has a rolling updates page on the situation in Christchurch. My guess would be that the smaller size of these earthquakes compared to the Darfield and Port Hills events, and the fact that they were further away than February’s shock, meant that the shaking from these earthquakes was much less likely to cause catastrophic damage on its own. However, the cumulative effect on already damaged buildings may be an issue, and liquefaction, and the flooding and subsidence that are associated with it, could greatly increase the long-term impact.

]]> http://all-geo.org/highlyallochthonous/2011/12/two-more-earthquakes-shake-christchurch/feed/ 7 Stuff we linked to on Twitter last week http://all-geo.org/highlyallochthonous/2011/12/stuff-we-linked-to-on-twitter-last-week-38/?utm_source=rss&utm_medium=rss&utm_campaign=stuff-we-linked-to-on-twitter-last-week-38 http://all-geo.org/highlyallochthonous/2011/12/stuff-we-linked-to-on-twitter-last-week-38/#comments Sun, 18 Dec 2011 18:35:44 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7081 Continue reading →]]> A post by Chris RowanA post by Anne Jefferson A good crop of links for your Sunday reading pleasure this week – and some new geoblogs to check out, too.

Other posts on All-geo

Newly discovered blogs

Volcanoes

Earthquakes

Floods

Fossils

(Paleo)climate

Environmental

Planets

General Geology

Interesting Miscellaney

]]> http://all-geo.org/highlyallochthonous/2011/12/stuff-we-linked-to-on-twitter-last-week-38/feed/ 0 Scenic Saturday: Mammoth Cave, where surface water and groundwater meet http://all-geo.org/highlyallochthonous/2011/12/scenic-saturday-mammoth-cave-where-surface-water-and-groundwater-meet/?utm_source=rss&utm_medium=rss&utm_campaign=scenic-saturday-mammoth-cave-where-surface-water-and-groundwater-meet http://all-geo.org/highlyallochthonous/2011/12/scenic-saturday-mammoth-cave-where-surface-water-and-groundwater-meet/#comments Sat, 17 Dec 2011 12:19:39 +0000 Anne Jefferson http://all-geo.org/highlyallochthonous/?p=7067 Continue reading →]]> A post by Anne Jefferson It’s that wonderful time of year, as one semester finally gives up the fight and a new one waits in the shadows, pouncing on unsuspecting students and faculty just as they breathe a sigh of that they’ve won the first battle. This past semester I taught a class on river geomorphology, but in January, my teaching takes a subterranean turn as I lead about 18 students through a semester of hydrogeology. At many research universities, surface water and subsurface water are distinct specialties and courses are taught by different faculty, so I appreciate the chance to get to indulge both of my passions. Besides which, it is really impossible to separate surface and subsurface water, as is aptly illustrated by this week’s Scenic Saturday photos.

Stage gage in River Hall, Mammoth Cave, Kentucky (photo by A. Jefferson, March 2011)

Stage gage in River Hall, Mammoth Cave, Kentucky (photo by A. Jefferson, March 2011)

This is a stage gage, which is used to visually measure water height, usually in a lake or river. Here though it’s in River Hall, 250 feet below ground in Mammoth Cave, Kentucky. Normally this area is about 45 feet above the water table, and there are benches here for tourists to rest on. But when it rains hard and the Green River floods above ground, it also backs up into the cave, with the water table rising up into River Hall. The last time it flooded into River Hall was in spring 2010, and you can read more about it from karst hydrologists here.

Digging into the origins of the cave, it’s not surprising that cave water and river water should be so well connected, because their geologic history is intricately tied. As the Green River cut downward over the last four million years, it took the regional water down with it. The multiple levels of dry trunk passages in Mammoth Cave correspond with elevations where the Green River paused in its downwards erosion and groundwater had significant time to dissolve the limestone at that elevation. You can read more about the formation of Mammoth Cave and other similar caves on the Cumberland Plateau in this paper by Anthony and Granger from 2004.

(Adapted rom Arthur N. Palmer, A Geological Guide to Mammoth Cave National Park, 1981)

Cross-section of Mammoth Cave, adapted from Palmer, 1981 and obtained from http://www.nps.gov (click image to link to source)

Most of Mammoth Cave is surprisingly dry, as far as caves go, and some would even call it a “fossil cave.” This is because of the lowering of the regional groundwater, but also because a sandstone cap above the cave-forming limestone limits the infiltration of local rainfall into the cave. Where the capstone is missing, some of the more typical cave dripstone formations are seen.

Frozen Niagara dripstone, Mammoth Cave, Kentucky (photo by A. Jefferson, March 2011)

Frozen Niagara dripstone, Mammoth Cave, Kentucky (photo by A. Jefferson, March 2011)

]]> http://all-geo.org/highlyallochthonous/2011/12/scenic-saturday-mammoth-cave-where-surface-water-and-groundwater-meet/feed/ 1 Friday(ish) Focal Mechanism: a kinky slab beneath Mexico http://all-geo.org/highlyallochthonous/2011/12/fridayish-focal-mechanism-a-kinky-slab-beneath-mexico/?utm_source=rss&utm_medium=rss&utm_campaign=fridayish-focal-mechanism-a-kinky-slab-beneath-mexico http://all-geo.org/highlyallochthonous/2011/12/fridayish-focal-mechanism-a-kinky-slab-beneath-mexico/#comments Sat, 17 Dec 2011 06:27:09 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7053 Continue reading →]]> A quick look this week at the magnitude 6.5 earthquake that shook southern Mexico last Sunday. It caused a fair amount of shaking in Mexico City, and a few deaths, but apparently no major structural damage. The depth of the rupture – around 65 kilometres (40 miles) – means that the seismic energy released had spread out over a wider area by the time it reached the surface, reducing the maximum shaking close to the epicentre.

A tectonic map of the region shows that there is a subduction zone running along the southwest coast of Mexico, where the oceanic Cocos plate is being subducted eastward beneath the North American plate. The rupture depth of 65 km puts it below the crust of the overriding plate, so it must be associated with the subducting slab. The focal mechanism indicates that the earthquake was due to northeast-southwest extension (see my primer on interpreting focal mechanisms).

Map showing location of the M 6.5 earthquake that struck south of Mexico City on 11 December, its extensional focal mechanism and proximity to the subduction zone off the southwest Mexican coast.

This might seem a bit odd at first glance: subduction zones are all about accommodating plate convergence, so a compressional, thrust focal mechanism would seem more likely. However, while that would certainly be the case for an earthquake that occurred on the subduction megathrust itself, this is not the only place in a relatively cold, subducting plate where earthquakes can occur if stress is being applied to the interior somehow. One mechanism that causes extensional earthquakes within the downgoing slab is if it is being bent; for example, you often see earthquakes with normal focal mechanisms on the outer rise of a subducting plate, where the plate is just starting to bend as it is pushed beneath the overriding plate into the mantle. This earthquake was 180 kilometres behind the trench, so was obviously not associated with this initial bending. However, gravity, GPS and seismic studies of this subduction zone have found that there is probably further bending of the slab beneath Mexico. The width of the locked zone associated with the subduction thrust, where elastic strain is being built up that will mostly be released in future megaquakes, is about twice as wide (200 km or so) as it usually is, suggesting that the Cocos plate is being subducted at a very shallow angle beneath southwest Mexico, before it eventually steepens again about 275 km away from the trench. As the image below, taken from Manea et al. (2004) shows, an earlier kink, where there is a temporary steepening of the slab before it flattens out again, has also been postulated, mainly in order to explain the positions of all of the earthquakes that trace out the position of the main thrust.

Cross section showing the inferred geometry of the subducted Cocos plate beneath southern Mexico, showing mostly shallow subduction with a steeper kink about 100 km behind the subduction zone, and slab steep deepening after 250-300 km. Red and yellow circle shows the approximate location of Sunday's earthquake. Grey arrows show inferred bending of the slab. Source: Manea et al., 2004.

Where does last Sunday’s earthquake fit into this geometry? It is located on the second shallowly dipping section of the subducted Cocos plate, about half way between the first steep section and the end of shallow subduction. This is a plausible location for an extensional earthquake; the slab will be starting to get stretched as the plate ahead of it is pulled down more steeply into the mantle. As the figure above shows, there were four other large earthquakes associated with the second bend in the plate; after a very brief search, I found the record of the 1999 6.9, which also has an extensional focal mechanism. So it seems reasonable to interpret last Sundays earthquake as a plate bending event, found where the Cocos plate is starting to transition from shallow to steeper subduction.

Of course, that still leaves us with a mystery: why is the dip of the subducting slab initially so shallow? If you look back to the map in the first figure, you’ll notice that whilst the crust being subducted beneath the west coast of Mexico belongs to the Cocos plate, you don’t have to travel much farther offshore before you cross another plate boundary – this time, a mid-ocean ridge – and find yourself on the Pacific plate. Although the ridge is still active and producing new crust, it is doing so at a slower pace than the Cocos plate is being subducted beneath Mexico. So over time, the ridge is getting closer and closer to the trench, and closer and closer to being subducted itself; a little further to the north the ridge and trench have already collided with each other.

One consequence of this is that the crust being subducted beneath Mexico is not very old; after cooling and solidifying from hot, molten magma at the ridge, it lingers on the surface for just a few million years after being created before it gets pushed back down into the mantle again. And if you’re subducting unsually young crust, that means you’re also subducting unusually warm crust: the way that oceanic crust is created means that it starts hot and gradually cools over tens of millions of years. Because hot, young crust is more buoyant – and therefore harder to subduct – than old, cold, dense crust, this might explain the shallow dip. The additional buoyancy of young Cocos crust makes it want to stay near the surface for a little bit longer, leading to shallow subduction. But eventually the slab is called back to a more proper angle for its return back to where it came from, even if it registers a seismic protest as it does so.

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