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Mining the Data Dumps

GG is hunting around for some information related to the little trainwreck series of posts, and has noticed some issues that bear on the broader business of (upbeat music cue here) Big Data.

Now Big Data comes in lots of flavors.  Two leap to mind: satellite imagery and national health records. Much satellite imagery is collected regardless of immediate interest; it is then in the interests of the folks owning it that people will find the parts of interest to themselves.  So Digital Globe, for instance, would very much like to sell its suite of images of, say, croplands to folks who trade in commodity futures. NASA would very much like to have people write their Congressional representatives about how Landsat imagery allowed them to build a business. So these organizations will invest in the metadata needed to find the useful stuff.  And since there is a *lot* of useful stuff, it falls into the category of Big Data.

Health data is a bit different and far enough from GG’s specializations that the gory details are only faintly visible. There is raw mortality and morbidity information that governments collect, and there are some large and broad ongoing survey studies like the Nurses’ Health Study that collect a lot of data without a really specific goal. Marry this with data collected on the environment, say pollution measurements made by EPA, and you have the basis for most epidemiological studies. This kind of cross-datatype style of data mining is also using a form of Big Data.

The funny thing in a way is that the earth sciences also collect big datasets, but the peculiarities of them show where cracks exist in the lands of Big Data.  Let’s start with arguably the most successful of the big datasets, the collection of seismograms from all around the world. This start with the worldwide standardized seismic network (WWSSN) in the 1960s.  Although created to help monitor for nuclear tests, the data was available to the research community, albeit in awkward photographic form and catalogs of earthquake locations. As instrumentation transitioned into digital formats, this was brought together into the Global Seismographic Network archived by IRIS.

So far, so NASA-like. But there is an interesting sidelight to this: not only does the IRIS Data Management Center collect and provide all this standard data from permanent stations, it also archives temporary experiments. Now one prominent such experiment (EarthScope’s USArray) was also pretty standard in that it was an institutionally run set of instrument with no specific goal, but nearly all the rest were investigator-driven experiments.  And this is where things get interesting.

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The 85 Ma Trainwreck: Schists

OK, so GG got distracted from this project, but it is high time to look at the pieces of the late Cretaceous puzzle, and first up are the schists of oceanic and forearc affinity. These are often classed as the Pelona/Orocopia/Rand (or POR) schists after the three more voluminous exposures, but there are many individual schist bodies that have similar lithologies and age constraints that are usually included in any examination of the origins of these bodies.

Generally speaking, these are metasedimentary bodies with occasional metabasalts and far more rarely, pieces of deeper lithosphere. They are broadly aligned today in a northwest-southeast swath more or less along the San Andreas, with the major exceptions being the Catalina Schist on Santa Catalina Island, the Rand Schist well to the east along the Garlock fault, and recently recognized bodies in central Arizona. When Cenozoic deformation is restored, these exposures generally land in a gap between the fairly undeformed Sierra Nevada Batholith and the Peninsular Ranges Batholith. As such, they seem connected to the more chaotic Mesozoic geology of the Mojave Desert, where these rocks underlie in fault contact middle crustal plutonic rocks. As such, these are widely interpreted to represent the product of some kind of very low-angle subduction event.

Dating the emplacement of the schists is important to all the stories of the Late Cretaceous in this part of the world; for a long time ages were hard to come by (the schists were often originally mapped as Precambrian), but the ability to date individual zircons broke open the problem.  Basically individual detrital zircons in a schist must predate the metamorphism and creation of the schist.  Cooling ages from more traditional geochronology then documents either the emplacement of schists at higher levels in the subduction zone or its cooling down while sitting in a cooling upper plate environment. A good example of this approach is shown below.

JacobsonSchistTiming

Age constraints on individual schist exposures as represented by a maximum age of emplacement from the detrital zircons in the schists and the metamorphic ages from cooling of the schists.  Organized roughly north (left) to south (right) in a palinspastic sense. From Jacobson et al., GSA Bull, 2011.

There are a number of questions that arise, and we’ll see what we can say about them below (this is apt to get long):

  1. How robust are these ages?
  2. What does an emplacement age represent in terms of the broader tectonic environment?
  3. What does the spatial and temporal variation in emplacement ages telling us?

 

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Getting Colorado High: So What?

So in the previous two installments, we reviewed ideas for how the High Plains got so high and some of the observations out there that bear on this question. Beyond satisfying some curiosity, what does this do for earth science?  Why pay money to do this?

Let’s consider three outcomes: that the High Plains gained their elevation by the end of the Laramide orogeny (say, 40 Ma), that they gained their elevation after the deposition of the Ogallala Group (say about 5 Ma), and that they were high, went down, and rose again. Read More…

Getting Colorado High: Observations

So earlier we saw that there are a number of different ideas for how the High Plains got high, but what matters are the observations. The oldest of these–the classical reason for saying the Rockies are young–is that the Miocene Ogallala Group/Formation has been deeply incised and removed entirely from large areas like the Denver basin. The classical interpretation of a switch from deposition to erosion is uplift and tilting, but another possibility is that the changes in climate going into the Pleistocene changed the ability of rivers to incise. This has opened the door to multiple lines of evidence.  Below is a stab at trying to get a handle on some of this literature (happy to hear what GG missed in the comments). [Note: this is subject to updating]. Read More…

Getting Colorado High: Theories

No, not high in that sense…high like “Mile High City”. This still is a problem GG is interested in and so for grins let’s quickly review the main ideas GG has seen with their pros and cons. The candidates are thickening the crust mechanically or by piling on sediment, thinning the mantle lithosphere, dynamic topography, hydrating the mantle or the crust, depleting the lithosphere, and emplacing depleted lithosphere. Whew! GG’s hot takes on these below the fold… Read More…

Diamonds Or Paste?

Occasionally a paper comes along that rattles you out of your present biases; whether the paper is right or not is less important than getting you thinking.  A paper in Geology got GG thinking about some things he’d ignored…

Kimberlites are rather famous kinds of igneous intrusions as they host most of the world’s diamonds. These eruptions originate at great depths in the earth but seem to pop up rather erratically and their relationship to subduction zones and the like is somewhere between unclear and non-existent. In North America, they seem to pop up in sort of broad swaths of the continent. One band in particular is of interest to those of us studying the origins of the Cordillera: a collection of Cretaceous kimberlites that seem to have erupted almost under the eastern part of the seaway that ran from the Gulf of Mexico to the Arctic.

Most workers have generally sought to connect these Cretaceous eruptions to the subduction of the Farallon plate under North America. This proposal generally seems to work by adding fluids to the deep continental lithosphere, which would then generate the melts that rise forcefully to the surface to emplace the kimberlites (e.g., Currie and Beaumont, 2011).

CurrieKKimberlite

Cretaceous (red) and Tertiary (white) kimberlites and related deeply-sourced magmas as plotted by Currie and Beaumont (2011).

In this view, the easterly positions of the kimberlites in the Cretaceous reflects a fairly low-angle subduction regime that would have had to be established by 112 Ma (the oldest intrusion in Kansas) and continued to about 85-90 Ma in the U.S. and into the Tertiary in Canada.

The alternative in a recent issue of Geology by Zhang and others looks at this in a very different direction, namely with westward subduction of North America under the western Cordillera, an idea put forward in some lengthy publications by Robert Hildebrand. Read More…

Fear of Every Geology Prof…

…is death on a class trip.  Going to places with unstable footing and exposure is often part of seeing geology that clarifies understanding, but it carries real risks. For GG, the most terrifying site is Toroweap Point in Grand Canyon National Park where, every time he visits, he breathes a sign of relief when the same number of students pile back into vehicles that had piled out of them. That site has 3000′ of vertical cliff to punish the unwary, but it doesn’t take that much for a fatality, as an environmental studies class from Briar Cliff University found out when they lost a classmate to a 100′ fall.

While family and friends grieve, another discussion is probably going on, if not now then soon. Should the school curtail field expeditions? Given the growing number of deaths by selfie, what is the role (and responsibility) of the instructor who takes students to places with hazards? Should the school dictate what is and is not an acceptable risk? Should students sign waivers, and if so, are they really enforceable?

Geoscience education benefits immensely from seeing what you are studying in the field. And the greatest hazard in field trips is generally the drive to the field or working on roadcuts near highways. But the drama of a fatal fall is more damning in some ways.  GG hopes that future students will get to experience the field safely, hopefully mainly by recognizing and avoiding hazardous situations on their own and with the guidance of an instructor rather than by being blocked from accessing important or memorable sites by fearful administrators.