November 8, 2016 | David F. Coppedge

Lyell Loses in a Landslide

Politics is on the minds of Americans today. Landslides are turning geological votes away from Lyell’s uniformitarianism toward catastrophism.

Landslides can cause monumental changes in just minutes. Here are some examples from previous reports:

  • Perhaps you’ve heard of the Heart Mountain Slide in Wyoming. Geologists believe that over 5,000 cubic kilometers of mountain broke apart in prehistoric times and slid 31 miles in just 3 minutes (source: Minnesota State and 1/28/15).
  • Two other landslides in Wyoming in historic times, the Gros Ventre slide (1925) and the Hebgen Lake slide (1959), dammed rivers and formed large lakes that caused (or threatened) major flooding downstream.
  • A Zion rockfall dated to Bronze Age times (5/18/16) enlarged the canyon significantly in 60 to 90 seconds.
  • A terraced river deposit in the San Gabriel Mountains of California, long thought to be built up slowly over years, was reinterpreted to be the work of a single large landslide (7/08/16).
  • In 2013, geologists were jolted out of their uniformitarian slumbers after one storm in 2013 caused thousand landslides and debris flows along Colorado’s Front Range (4/25/15).

Two recent papers reveal some of the changes in geological thinking about landslides. They don’t necessarily need a high slope to get going. Long-runout slides can cover much more distance than previously believed. And dating landslides can be tricky business. The following observations underscore the catastrophic potential of sudden, large-scale events, in contrast to Charles Lyell’s picture of a world in slow and gradual “uniformitarian” change.

Kazakhstan

Four scientists studied landslide escarpments around the Caspian Sea. Publishing their conclusions in Geology, they believe these slides took place in the Pleistocene epoch. Whenever they occurred, these “giant landslides” were pretty amazing: 300 slides on shallow slopes moved hundreds of millions of cubic meters of material in catastrophic fashion.

The history of Quaternary sea-level changes in the Caspian Sea, the world’s largest lake, is partly enigmatic, and so is the geomorphic response of its coasts. Late Pleistocene transgressions during the Early Khvalynian (ca. 40–25 ka [thousand years]) inundated extensive portions of the flat, low-lying semi-desert of western Kazakhstan. Cliffs cut during these highstands form a prominent escarpment tens of kilometers to several hundred kilometers from the present coast of the Caspian Sea. Satellite images, digital terrain analysis, and field mapping reveal that >300 giant landslides intersect with this escarpment. More than 100 of these slope failures mobilized volumes >108 m3 along basal failure planes with gradients as low as ∼5°. All landslides share characteristics of lateral rock spreads involving competent limestones overlying weak and plastic claystones. From relative stratigraphy and new 14C data, we infer that catastrophic slope failure of over 41 km3 occurred mostly during Pleistocene Caspian sea-level highstands, while several landslides may have been reactivated or entirely originated during the Holocene. This largest cluster of terrestrial mass wasting in a tectonically quiescent setting offers an opportunity to understand how landslides erode low-relief landscapes subject to oscillating sea levels.

The cliffs and landslides are visible east of the Caspian Sea using Google Earth. It seems odd that hundreds of landslides would cluster in one region tens of thousands of years ago, only to have “reactivated” or originated again thousands of years later. The main surprises are these: according to the abstract, 41 cubic kilometers of material moved along slopes that in some cases would look almost level to the eye: just 5 degrees. And this happened in a “tectonically quiescent setting.”

Italy

The Bulletin of the GSA (Geological Society of America) has bad news for vacationers in Ancona, Italy. They’ve been told that a big landslide deposit at Portonovo Bay, a popular beach resort, happened in prehistoric times. New carbon-14 dates of fossils under the slide lead the authors to the conclusion that it happened in medieval times around 1320 A.D. That means the area is more at risk of another big landslide than thought.

This paper is a case study about a natural catastrophic event that caused a human disaster in the medieval Benedictine enclave of Santa Maria in Portonovo—a large landslide involving some 5 million cubic meters of rock, which suddenly collapsed from the northeastern flank of the Monte Conero anticline hurling down from 400 m elevation over the littoral zone of the Portonovo bay, ∼9 km south of the Adriatic port city of Ancona. This landslide is the largest of the Conero Riviera, a 16-km-long, seismically active high coast with cliffs overhanging directly over the sea, which is therefore particularly prone to landsliding. While the landslide risk along the Conero Riviera has been evaluated by the local municipal authority, the risk in the Portonovo bay, a very popular beach resort, has been somewhat underestimated inasmuch as the huge landslide was previously considered, by geologists and geoengineers, to be a prehistoric event and therefore unimportant in the context of prevention plans aimed at assessing the hydrogeological instability of this area.

Multiple methods lead the authors to infer that “the landslide occurred in historical times, contrary to common belief that it occurred in prehistory.” After figuring the date and trigger from seismic activity, they worried that similar conditions exist around the bay now.

The same structural and lithologic settings are found today on the slopes adjacent to the western side of the medieval detachment scar, thus representing a serious threat for the stability of the area and a high risk for imminent large landsliding, which may have grave consequences for present-day infrastructure and inhabitants of the popular Portonovo beach resort.

Expert opinion of geologists and geoengineers had been wrong for years, giving a false sense of security to vacationers. Few people were present in the 14th century to witness the medieval event. But now, with tourists crowding the resort, another big landslide could have “grave consequences,” indeed.

Pluto

Landslides are not restricted to Earth. They can happen anywhere where piled-up solid material suffers slope failure, spreading out vertically and horizontally. A big landslide had been detected in 2005 on the edge of a crater on Iapetus, one of Saturn’s outer moons (see Cassini photo). Now, landslides have also been detected on Pluto’s companion Charon. “Four landslides were spotted on Charon in total, all in Serenity Chasma, which is part of a vast belt of deep canyons bordered by cliffs stretching for 1,100 miles (1,800 km) and reaching heights of up to 4.5 miles (7.2 km), according to NASA.” Photos of the landslides on Space.com are accompanied by admissions that scientists do not understand them: “Beyer said scientists have a lot of open questions about the landslides, including how and when they formed.” It would seem if they were ancient, they would have been erased by craters over billions of years. But if they are recent, what processes could have triggered them on a quiescent body so long after the moon’s formation?

So did Lyell lie a little? (7/25/08). Landslides can be scaled up to cataclysmic events, given the right conditions. If strata are weakened by seismic activity, inundation of rising sea levels, or just accumulated strain, they can give way suddenly and cause major changes in a matter of minutes or seconds. Charles Lyell (who influenced Darwin with his vision of vast ages of slow, gradual processes) wanted to restrict his scientific explanations to “causes now in operation.” But how could he draw the line on landslides? It would be his own arbitrary judgment to say that small landslides can occur, but huge ones (e.g., continent-size events) cannot.

A landslide is the opposite of a slow and gradual occurrence. Charlie & Charlie (Lyell & Darwin) put the scientific world to sleep with their concoctions of slow, gradual change. Read again in our 4/25/15 entry how the rumble of reality has jolted some Darwinos awake, at least partly.

 

 

Comments

  • John C says:

    Thanks for the links to some FANTASTIC photos of Iapetus and Charon. The Iapetus photo is especially exciting when you realize the slide escarpment to the right of the debris field is 15 km (9.2 mi) high! Am I too far off the mark as to think the landslide might be due to the smaller impact that has collected the debris? Thanks for the engaging articles. Charles Lie-well must be squirming, wherever he may be.

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