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The Accretionary Wedge #2: Gravity’s relentless onslaught against humanity

October 15, 2007

The second edition of the geoscience carnival The Accretionary Wedge, is hosted by Kim up at All of My Faults Are Stress Related. The theme is Earth hazards…or, how our planet might kill you. My contribution to this is not as grand as I’d envisioned…this whole finishing-the-PhD thing is taking a significant bite out of my blogging.

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Gravity’s relentless onslaught against humanity

gravityapple.jpgAh yes…gravity…it’s responsible for the coalescence of material to form our planet, it is a fundamental force in our universe (it also makes alpine skiing so much more fun than cross-country), yet it is also constantly trying to destroy us and the stuff we build. It is relentless. Sometimes slowly and imperceptibly, sometimes abruptly and spectacularly. Gravity, with the aid of weathering and erosion, is constantly battling the tectonic forces attempting to push our crust upwards.

When tectonics is winning the war (i.e., there is high relief), gravity really steps up its attacks and can win some major battles from time to time. Although these events are occurring all the time all over the planet, we typically only hear about them when they are (1) of large magnitude, and (2) directly affect human life and/or property. Just a couple weeks ago, residents atop San Diego’s Mount Soledad experienced some gravitational “adjustments”.

slide-laconchita.jpgThese gravity-induced processes, known as mass wasting, are subdivided into numerous types of slides, falls, slumps, creep, flows, and so on. The geology of the bedrock and climate play a huge role in determining what types of processes, the location, how often, and how severe these events are. The photo at left is the recent La Conchita slide in southern California (go here and here for more). In this case, heavy rains, weak Holocene sediments, and high relief all converged and created this relatively small slide. In other cases, however, entire villages can be wiped out by mudslides.

The slides and falls are obviously the quickest and most violent, but gravity never tires and will work over long periods of time if it has to. Soil creep and subsidence are slow processes that work to destroy our property and infrastructure.

And don’t think that the ocean is off limits to gravity’s onslaught. Submarine slides and flows threaten our sea-floor infrastructure as well (there’s a lot down there). The image below outlines the deposit of a single submarine slide near Palos Verdes, California. Some of the blocks in the ‘blocky debris’ are as big as large office buildings. Read more about this event here.

aw2palosverdes.jpg

Finally, this video, which could not be found on YouTube for a while, has returned. It is of a landslide in Japan. If the embedded video is broken, go here.

We will never beat gravity. Gravity is relentless. But, if we continue to work to understand the processes and patterns related to these events, we can do a much better job of, at the very least, knowing where not to put our house.

Apple cartoon above from here

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