Hike the John Muir trail and you find yourself constantly in landscapes formed from the c. 85 Ma intrusions that marked the end of one of the longest stretches of volcanic arc activity in the past several hundred million years. For a long time, this was taken as simply the end of an era; the absence of activity that followed was the big message of something strange going on. But increasingly there are signs that this final pulse was itself out of the ordinary and may well have carried the seeds of its own demise.
Looking beyond the Sierra, chaos quickly reigns. While some papers and textbooks describe an orderly eastward shift of the volcanic arc, the rocks on the ground present a more confusing account. While the arc did shift to the east after 85 Ma in southern Arizona and eventually part of New Mexico, no such orderly shift is evident farther north. Magmatic activity actually approached the trench in the Mojave Desert just before volcanoes popped up in Colorado, skipping much intervening terrain. The few plutonic rocks in Nevada and Utah look like melts derived from the crust, not typical arc rocks.
This crazy quilt of magmatism then takes a toll on geodynamic models. The shutdown of the Sierra has been taken as a sign of the slab flattening under North America–but the clearest evidence of the slab flattening is under the Mojave, where igneous activity continued well after flat slab deformation began nearby. Models that predict that slab flattening should shut off igneous activity have trouble with some of the intrusions in the Colorado Mineral Belt–and their absence to the north and south. And if impinging oceanic plateaus are responsible, just how does that timing work out?
We can try to tear this apart in some ways. Here let’s consider these questions:
- Why was there a flare up of activity in the Sierran arc near 85 Ma?
- What could have shut down the Sierran arc?
- What do we know about the relationship of magmatism to schist emplacement in the Mojave region?
- Are the two-mica (peraluminous) granites really just crustal melts?
- What can we say about the Colorado igneous activity?
Two facets of the human history of the Sierra that GG (mostly) left out of his book came together recently. One was the genocide of California Indians during and after the Gold Rush, an outrage now officially apologized for by the governor of California, and the other was the immigration and emigration of Chinese associated with the Transcontinental Railroad, now addressed in Gordon Chang’s recent book, Ghosts of Gold Mountain. Both events and the stories of marginalized peoples are tied to the presence of the Sierra, but neither in such a unique way as to make each story a centerpiece in GG’s book. As this would seem to continue marginalizing these peoples, GG would like to explain a bit why these stories were not more prominent in the book.
It is a rather rare thing when a rather pedestrian-seeming experimental geomorphology paper makes it to the New York Times, but that is the case with a recent study showing that waterfalls can be generated without any special events, like a change in climate or tectonic uplift. Quotes in the Times article indicate that this might require reevaluation of histories of areas that were defined by waterfalls.
Now GG is not a geomorphologist but he does pay some attention to that literature, and it isn’t so much waterfalls that have been dictating interpretations over the past few decades in places he cares about (western U.S.), but there is work that depends on the grade of rivers relative to drainage area (and rainfall and bedrock competence); are these impacted by this?
There is a hint near the end of the Nature article suggesting that the answer could be yes:
In the Big Tujunga Creek catchment, California, USA, many tributaries, such as Fox Creek (Fig. 1c), have a knickzone that has been linked to an increase in uplift rate3. Each knickzone is composed of numerous waterfalls that lack known origins, are consistent with autogenic formation, and have probably changed erosion rates of the broader knickzone in a manner that is inconsistent with fluvial incision models. [GG bolding]
Now if all that was happening was that rivers of a certain grade developed a sort of ramp-flat (waterfall-flat) geometry without changing the overall grade, current styles of interpretation probably would remain robust. But if the development of this style of “self-formed” waterfalls messes with that, then all bets are off for streams with such features. And that includes virtually all of the rivers and streams in the Sierra…
GG kind of wonders how this compares with Clyde Wahrhaftig’s old hypothesis of stepped topography originating from variable bedrock erosion of granite. By stepped topography, he meant the repetition of steep bare granite slopes alternating with vegetated flats as one moved up the west side of the Sierra Nevada. In that hypothesis, the lip of the steep slopes (where the waterfalls are) should have a low erosion rate while the flats near the base of falls should have higher rates. In essence, the exposed bedrock created local knick points that did not move downward much while the upstream areas were eroding to match that elevation. But recent work from Jessup et al (2011), who measured erosion rates with cosmogenic isotopes, found “the pattern of erosion rates is one in which steps erode more quickly than treads, in direct contradiction to Wahrhaftig’s (1965) hypothesis.” But this is precisely the pattern seen in the new experimental work: the flats at the base of the waterfalls stop eroding once a plunge pool gets deep enough that the falling water doesn’t have any erosive power, while the lip of the waterfall gradually erodes down, eventually erasing the waterfall over some thousands of years. The flat acts as the knickpoint, not the top of the fall.
While an interesting speculation, at this moment the notion that autogenic waterfalls explains the stepped topography of the Sierra deserves a shaker full of salt. First, the scale of stepped topography is about an order of magnitude larger than the kinds of self-generating waterfalls discussed in the latest work. Second, stepped topography extends well away from the modern stream courses, something the new experimental work does not explore at all. Thus there are serious issues that would need to be solved before this could move beyond speculation. But while the new waterfall work might complicate the current style of landscape evolution analysis, it might also hold a clue to solving a more cryptic landscape puzzle.
Living in the west, sad stories of families losing their houses–or their lives–in wildfires is an all-too-common occurrence. And as a geophysicist, GG is familiar with the old geoscience adage that earthquakes don’t kill people, but built structures failing in earthquakes kill people. It would seem we should adjust the second adage for the first situation: wildfires don’t kill people; flaming buildings kill people.
Have you noticed how often the pictures of destroyed houses includes green trees nearby? How often the description of the burned down house includes the oddly unburned things nearby? Although there are certainly fires so intense they take everything in their path, all too often it seems like houses burn when little else does. And this applies even to the devastation in Paradise California this week. The Los Angeles Times has a piece noting those still-standing trees and finds that the devastation in Paradise was because the fire became an urban fire. Houses were igniting other houses.
Basically the issue is that western houses catch burning embers with things like debris-filled gutters, exposed eaves and ventilation grills, and wooden porches. Once lit, houses tend to go up all at once. This is not new news–anybody with a house in the forest hears about it from their insurance and local fire officials. Yet new houses continue to be built with the same weaknesses, even in fire-prone areas. (At least most areas ban wood-shake roofs). Clearly more thought should be given to eliminating the ways houses catch burning embers.
Does this mean we’re off the hook on forest health? Well, probably not, though exactly what that means looks to be up for grabs more than ever. What seems certainly true is that frequent, low-intensity fires reduce the risk of intense damaging fires: the experiences in both Yosemite and Sequoia National Parks is that major fires lie down (lay down?) when they hit areas previously burned in a controlled manner.
But controlled burning isn’t a great option within the rural subdivisions now present in many forests. Thus many advocate for other kinds of treatment, ranging from wholesale clearcutting to selective logging to mechanical thinning of understory (to…raking???). One study recently highlighted in a CNN op-ed concluded rather strongly that more heavily managed forests are forests that burn more intensely–the opposite of what is usually claimed. And GG can attest to how mundane it can be to encounter a natural wildfire in the unmanaged backcountry, having hiked through or right next to such fires on at least three occasions. But there are confounding factors in play, some of which are noted in the study. Certainly one is ignition: wilderness areas usually see fires starting from lightning strikes. Such fires occur under conditions less apt to drive monster fires: the forest has often been wetted, and long periods of strong, dry winds are less likely. In contrast, managed forests are in more heavily used areas where neglected campfires, power lines, driving over dry grass, and sparks from machinery or gunfire are capable of starting a fire when strong, dry winds are present.
[As an aside, the above is all in reference to forests; Southern California chaparral is nearly immune to controlled burning, and unlike the forests, any such burning would leave the ground bare and hydrophobic. Chaparral is a whole different ballgame.]
What remains disturbing to GG in the forestry studies he’s perused is that the assumption remains that “pre-settlement” (apparently the currently favored term for c. 1840s western U.S.) is equal to “natural”. In some places, this will prove true, but in the Sierra foothills it is almost certainly a false equivalence. Pretending that Native American management was “natural” is likely to lead to poor decision making. Better if land mangers simply sought to restore pre-settlement fire frequency and intensity rather than assuming it was natural. The reality is that many of the places most at risk in the Sierra foothills were occupied by people who had many generations of experience in burning the landscape. We might just want to recognize that as, in some instances, their management goals might not match ours, but when they do, odds are pretty good that their management schemes would be a good place to start.
One of the peculiarities of American law is that mineral rights tend to trump everything else. Basically, if somebody owns the mineral rights under your property, good luck keeping them off, as many owners of split estate surface rights have learned.
One irony is that this is not what some of the original court decisions favored. In a California Supreme Court decision in Biddle Boggs vs. Merced Mining Company, Justice Stephen Field (who would later be the longest serving member of the U.S. Supreme Court) wrote
There is something shocking to all our ideas of the rights of property in the proposition that one man may invade the possessions of another, dig up his fields and gardens, cut down his timber and occupy his land, under the pretense that he has reason to believe there is gold under the surface, or if existing, that he wishes to extract and remove it.
And with this, the court found that the surface rights prevented miners from entering John Fremont’s Mariposa Estate to seek and extract gold. (Only later did the courts decide that Fremont owned the gold under terms of his patent). But later decisions eventually upended this, giving the owner of mineral rights the opportunity–indeed, the right–to invade a surface right holder’s land to get at their minerals.
The result of the upper hand mineral rights holders have had has been that oil and gas developers have forced themselves on individuals and communities that are not welcoming their presence. Here in Colorado that has led to efforts from communities to ban development–efforts that have foundered on the supremacy of state law. So now the effort is to impose state-level restrictions on oil and gas development that would probably prevent development in nearly any city or town but would also restrict it in more rural areas where the rights are not split and the person occupying the house near the oil well is the one receiving the royalty checks every month. The fight over this issue has led the oil and gas community to advance their own proposed amendment, one that would cripple everything from zoning laws to fire codes to noise ordinances. Just how this approach by both sides will play out remains to be seen, but it is clear that strong battle lines are drawn and there will be unintended casualties.
An alternative GG has never seen advanced is to use the eminent domain power of government to intervene in mineral development when it is contrary to the public interest. Eminent domain is not itself uncontroversial–while mainly used to do things like build a road, it has also been used to condemn an area to make way for new shopping centers, and so it is the bane of libertarians. But it is a well-known and well-established power in the U.S. So why not use it for retiring mineral rights?
The first and most obvious answer is that the cost is too high. Most communities, and especially those with lower income residents, wouldn’t be able to afford the rights. But maybe this isn’t as clear-cut as it seems. First, it isn’t the value of the oil or gas, it is the net value that should matter. When it costs millions of dollars to sink holes in the ground, the return is relatively meager. So meager, in fact, that many financial people are now viewing the oil and gas industry as one threatening to drown in red ink. When the federal government is getting about $450/acre for oil and gas leases, it would seem that the price of shutting down an oil play in a town could be pretty doable.
The second reservation may well be that eminent domain is limited to surface rights. GG is not a land law attorney and so has no idea. But it might not be hard to change state law to allow it if indeed you can’t do it today.
A third reservation is that this wouldn’t stop imminent drilling. But that in fact can be stopped by municipal action–drillers have face moratoria as local governments have tried to settle on requirements regarding drilling in floodplains, near schools, etc. Simply putting a moratorium in place until an eminent domain purchase is concluded or denied would seem to be in line with current Colorado law.
In a sense the City of Longmont has kind of pursued this already, swapping rights in the city for income from rights the city holds elsewhere. As this included shutting down existing wells, it is a bit harder to relate to the question of using eminent domain, but the $3M price tag would seem to be higher than what would result from precluding drilling rather than shutting down existing and proposed wells.
Odds are that the current scorched earth approaches both sides are taking will be rejected by voters. Coloradans are unlikely to want to so greatly eliminate the ability of rural residents from cashing in on their good fortune, but they are also unlikely to approve of making it impossible for their city or town government from enforcing such basic rules as zoning laws or noise ordinances. Perhaps after the dust has settled interest will shift to other means of addressing the legitimate concerns of drilling neighbors.
GG has been piddling along though the Sierra (ostensibly to give a campfire talk in Mineral King) and in doing so stared a bit longer at a recent paper on the age of a pediment in the Sierran foothills by Sousa et al. in Geosphere in 2017. In a way this is a callback to concepts from far back in the geologic literature, namely the significance of an “Eocene erosion surface.”
Here, to be brief, low-temperature thermochronology from a low-elevation pediment in the western foothills of the Sierra yields very old ages–in fact, overlapping with the emplacement of plutons in the Sierran crest [this was not a unique observation; Cecil et al., 2006, had a pretty old point in their collection]. Sousa and coauthors model these data and get a cooling to surface conditions by about 40 Ma. Because these pediments abut noticeable topography, this means there was at least that much local relief in the ancient Sierra. While the pediments had been noticed by others, many suspected a far more recent age.
In some ways, this is old news. The Eocene sediments in the northern Sierra have long made clear the presence of significant local relief, and many workers had inferred that such relief was probably higher in the southern Sierra (e.g., Wakabayashi and Sawyer, 2001). But the southern Sierra lacked the Eocene sediments necessary to know what the Eocene landscape might have looked like, so this paper opens up a new window for us.
Where does this lead us? Kind of down a rabbit hole only to come up with no strong and useful statement–though perhaps future work could nail things down. This is more a personal attempt to try and grasp what is going on, so profound errors might exist and insights are few. So, proceed at your own risk….
A favorite shortcut employed by many in trying to decide between hypotheses is to enlist Occam’s Razor–that the simplest explanation for something is most probably right. Now this has strength because humans are pretty good at rationalizing notions they put forward, adding in new ingredients to keep a favored explanation from collapse. But a theory that has probably passed its must-use-by date will have enough extra bells and whistles to discourage Rube Goldberg from trying to get it to work.
However, there is nothing that says Mother Nature had to be supremely parsimonious. In a complex system like Earth, there can be odd coincidences that are meaningless (like the Moon and Sun sharing the same apparent diameter from Earth’s surface) and outcomes that might be highly improbable (taking over 500 million years to get intelligence after making complex animals with hard parts seems like dawdling, especially when burning most of that time on dinosaurs). Even so, Occam can be a help if used with care.
But lots of times you can face competing hypotheses that lack Occam-style clues. For instance, which is simpler: that post-5 Ma erosion of the High Plains of the U.S. was caused by an eastward tilt, or that this was the product of a changing climate? Both are pretty easy to describe; both have issues. Yet many earth scientists feel pretty comfortable arguing that one is correct; what is the basis of such assurance?
Arguably the most common discriminator used by earth scientists is the principle of least astonishment. What surprises you least feels, in an Occam kind of way, like the interpretation that is most likely. The problem is, we all are astonished differently.
If you are a sedimentologist, you might look at the problem of the High Plains as one of depositing the Ogallala Group in the Miocene as crucial. Could you possibly deposit something like that on a slope like that we have today? This seems so astonishing that if can’t be right; the original slope had to be lower.
But maybe you are a geophysicist looking at the ways to create a tilt about 5 million years ago over something like 1000 km. That looks really hard to do, especially if dynamic topography from flow under the lithosphere is ruled out. It would be astonishing if that happened; it must be that the grade was already there much longer ago.
Skepticism from both geoscientists is warranted; either of these seems really hard to do. Data is gathered by both sets of experts. Margaret McMillan and colleagues measure paleogradients in the Ogallala using a widely applied approach and find there must have been a lot of tilting. Will Levandowski and colleagues (including GG) look at geophysical measurements and find support for the elevations comes from within the crust, where changes over the past 5 million years seem exceptionally implausible.
Could these be resolved? Well, you could posit that prior to 5 Ma there was dynamic subsidence holding the western end down and once that was released, the crustal buoyancy expressed itself. But now Occam detectors are flashing red–this feels ad hoc. Of course, there could be mistakes in the measurements of paleogradient, or in relating seismic wavespeeds to densities–each side has poison darts to shoot at the other side.
What makes this frustrating is what makes this interesting. After all, in the end somebody will be astonished–the earth did something they didn’t expect.
And a funny thing, shoes flip feet in the Sierra, where those studying the sediments argue for no tilting despite deposition at an even steeper grade than modern-day Ogallala, while geophysicists feel they have good evidence for a very recent change in the buoyancy structure of the region.
Are you astonished yet?