Archive | geology RSS for this section

Exodus

Geologists have for a long, long time been telling people not to build things in certain places.  Barrier islands? They move and evolve, which means property comes and goes. Not good.  Floodplains? They, um, get flooded.  Landslides? Only if you want a mobile home with a mobile yard. Sometimes we get heard, but usually we don’t. And the more subtle stuff, like recognizing how paving large areas can make floods worse? Lots of luck there. Doesn’t matter if the communities are rich or poor, building in bad places seems a national habit.

Maybe that is changing.

Even as the national media seems to just be noting that flood insurance is encouraging building in vulnerable spots, Politico has a big story on Louisiana’s program to consider how some communities will be forced to move and how to prepare to absorb that exodus as it occurs. For the Grumpy Geophysicist, this is a moment of actual hope, a ray of sunshine in the currently clouded over world of using science to guide public policy. [If you want more darkness, consider that politicians are rewarded for disaster relief and not disaster preparedness.]

The basic point is that people don’t like getting hammered by really bad weather (you know, like floods).  And so they leave–and this isn’t typically a slow migration but instead a real wave of refugees from hurricanes or floods or other such unpleasantries. They don’t often go really far away, so neighboring communities suddenly are flooded with people. There are two main forks to preparing for this: one is to try and get the vulnerable communities to start to think about how they will evolve in the face of the next storm, and the other is for those neighboring communities to prepare for the eventual migration of their neighbors. The state is actively trying to do this kind of work.

While there are uncertainties in our future, there are a few things that will happen.  There will be sea level rise.  There will be bigger rainfall events. These are both so clearly tied to the basic physics of increasing CO2 in the atmosphere that there really is no avoiding them; the best we can do now on that side of the ledger is to try and keep the magnitudes lower than they might otherwise be (and some areas also see land subsidence, which is unrelated to global warming but also causes problems). So we need to prepare, which means surrendering land we cannot defend and defending land we dare not surrender.

That Louisiana is starting to consider this landscape triage may just mean we’ve moved off the “we will rebuild it” mantra of the past century. As the article makes clear, this won’t be easy–but it should be much better than letting the chaos of the next disaster drive change.

Courting Disaster

For those of us in earth science, this past week has highlighted an awful lot of potential “told you so” moments. Like how warming climate and a warming ocean will lead to higher precipitation events. Like how you really do need to plan for floods. And we just missed hearing more about the barrier island/marsh protection talking point. And almost at the same time we’ve been greeted with ever more evidence that the Trump administration has little or no use for scientific input–not even choosing to ignore it, they seem more eager to simply not have any scientific input at all. Just as it is ever clearer that we are facing real decisions in trying to prepare for a warmer world, we seem the have a government yelling “la la LA LA” with its fingers in its ears.

But that isn’t the point here today.

One aspect of the tragedy in Houston is that the absence of any sensible planning has led to more flooding (the worst example might well be letting houses be built within the basin and below the spillway elevation of flood control dams); this is exacerbated by the combination of government subsidized flood insurance and the out-of-date or inadequate flood zone maps. Of course some now point to the zoning-free and laissez-faire approach to building in Texas as the bargain they made with the devil, implying that other places where strict zoning has been enforced will be safer.

Dream on.

If GG has noticed one thing about strict zoning (and Boulder has a pretty heavy hand on building), it is that it is rarely used to prevent building in stupid places–it is mainly used to keep people from building on land other people enjoy as it is. Some years ago when Colorado Springs was approached by a developer who wanted to build houses on an active landslide, the city council had to look away from the evidence they were given in order to approve this ongoing disaster. You can find similar stories elsewhere. Yes, fear of flooding is brought up when a new development is proposed…but mainly as part of the larger arsenal serving Fort NIMBY (sometimes there is a legitimate fear, but sometimes it is greatly exaggerated). California has the Alquist-Priolo act to prevent construction near active faults, but it only moves buildings 50 feet from an active fault.  Direct destruction of a building by a fault being directly under it is one of the least likely modes of destruction (even some dams do OK on faults: the Upper Crystal Springs dam survived having several feet of offset in the 1906 earthquake). Earthquakes do most damage by shaking weak soils: recall the Marina District in San Francisco, far from surface faulting, where shaking from the Loma Prieta earthquake damaged dozens of structures. What strict zoning clearly does is raise housing prices.

The main exceptions to non-use of zoning as a disaster preventative is in the wake of disasters. Even then, the most common refrain after a disaster is “we’re going to rebuild and bring it back better than before.”  After a tornado, this makes sense.  After a flood, whether storm surge or heavy rain? Not so much. The harder statement? “We learned a lesson and we aren’t going to make that mistake again.” It is very hard to say, but if we are going to avoid paying to rebuild over and over again in increasingly vulnerable places, risking the lives of inhabitants in the meantime, it’s time to start saying it and then walking the walk.

 

A (nearly) forgotten constraint

Over the past decade or so, a fairly common event has been the publication of a paper taking on a piece of evidence for large magnitude extension in the Basin and Range.  For the most part, this has been going after the various individual constraints used to make Wernicke et al.’s (1988) estimate of 247 ± 56 km of WNW-oriented extension. For instance, there are papers attacking the displacement on (or existence of) the Mormon Peak detachment, on the reconstructions of the fold-and-thrust belt across the Death Valley region, and on the age and amount of extension across Panamint Valley. The net result some would infer is that extension in the Basin and Range here was actually a lot less than 247 km (yes, GG has been told “a lot less”).

Rather than wade into these multiple controversies, GG would prefer to step back and ask, are there other ways of coming at this? There are a couple of approaches.  For instance, one can try to improve the somewhat circular logic of Coney and Harms (1984) and try to use the modern and estimates of the pre-extensional thickness of the crust to get at total extension.  A problem is that the crust has been an open system and constraining the amount of magmatic additions limits this approach.

The other approach comes from a rather unexpected quarter: plate tectonics.  Or more precisely, plate tectonic reconstructions, and is has maybe been overshadowed by these other arguments. It is rather clever, but to see its power, we have to take a moment to understand what is going on.

Read More…

Geohero or geochump?

A comment at a meeting GG was at got him to thinking about the popular view of scientists.  The comment was that scientists in the 19th century were heroes for Americans because they helped open up the West, while in the 20th century they were more thorns in the sides of growth.  Of course, this is so oversimplified it collapses quickly: John Wesley Powell, a hero for his explorations of the Colorado River, was viewed with great disdain when he closed claims for public lands. And post-WWII America fell in love with science in many ways. But still, when are scientists lauded and when are they scorned? An interesting pair of cases in the late 1860s and 1870s may shed light on this.

In both cases a scientist running a geological survey became aware of claims of major mineral finds within the area of his survey.  In both cases, the scientist claimed that these finds were incorrect. In both cases, the finds were not economic. Yet in one case, the scientist in question, Clarence King, was lauded, became first director of the USGS, and was viewed as one of the best and brightest America had to offer.  The other, Josiah Whitney, lost his survey and spent years grousing about the outcome. Why the difference?

Read More…

Rubia Slippers

Last fall GG’s Western U.S. Tectonics class took on trying to evaluate the status quo challenging hypothesis of Robert Hildebrand that the western part of the U.S. (west of central Utah, roughly) was a separate ribbon continent, Rubia, prior to colliding with North America in the early Tertiary, creating the Rocky Mountains. (That status quo holds that the far west was gradually assembled from the latest Paleozoic going on to the Miocene, with an arc being present on the edge of North America from the Permian to the late Cretaceous and again in much of the Tertiary). As Hildebrand’s argument was wide ranging and published as two lengthy GSA Special Papers (457 and 495), it isn’t a casual affair to consider the question of whether Hildebrand has caught western geologists in a huge misinterpretation or not.  Many workers, content with their personal knowledge, have not peered into this abyss, so the class set out to take a swing at this.  Basically, has Hildebrand identified observations inconsistent with our current interpretation of the geology? And are observables more consistent with Rubia than the standard model? A “yes” to the first might show that Hildebrand has put his finger on a problem even if the answer to the second is a “no”.

The class broke the hypothesis into these elements:

  • North America was subducted under Rubia in the late Cretaceous
  • Mesozoic and late Paleozoic magmatism, widespread in Rubia, never extended into “true” North America
  • The magmatic volumes at the end of the Cretaceous in the western arcs are far too voluminous to have been produced by subduction of oceanic lithosphere
  • Much of the classic late Precambrian – Paleozoic Cordilleran miogeocline is exotic to North America (i.e., is Rubia)
  • Deformation from accretionary events is limited to Rubia.
  • Mesozoic thin-skinned thrusts contain too much shortening to be limited to North America and are far greater than found in backarcs of typical continental arcs
  • Magmatism and uplift in the latest Cretaceous and early Tertiary was produced by the oceanic part of the subjected North American plate falling off.

You can go and read the individual assessments made by class members to particular parts of this analysis, but a summary is below.

Read More…

Laramide Times

Traditionally, the Laramide Orogeny starts around 75 million years ago.  Probably most geoscientists would agree with the overall analysis of Dickinson et al. (1988), which is mainly based on sedimentary rocks preserved from that time. So their criteria were that marine sedimentation (diagonal hatch) had ended prior to the Laramide, individual basins shifted from sharing facies with adjacent areas (black square) to having distinctly thicker deposits (circle) and coarse clastic detritus derived from nearby uplifts (black triangle) as the Laramide started:

DickinsonLaramide

It would be hard to argue that the Laramide Orogeny started later than the kinds of dates that Dickinson et al. proposed–but could it be earlier? If you had shallow sea floor covered in muds and parts started to rise up, might the muds simply get entrained in the existing current systems and be scoured down, creating a lacuna that, later on, would erased by even deeper erosion? In other words, is it possible that there was early deformation that wasn’t vigorous enough to overcome the broad subsidence of the region and so failed to produce positive topography? And if so, would subsurface loads have started to create local depocenters that perhaps have escaped recognition?

Read More…

Mountains that Remade America

Jones_comp proofFor those who come hoping to see material related to the Grumpy Geophysicist’s trade book on the Sierra Nevada, The Mountains that Remade America, here are a few quick pointers and some upcoming talks that might be of interest.

Read More…