Quicksand (a.k.a. Liquefaction)

I had a great question a little bit ago on the Ask GeoMan section of my GeoMania educational website from Elizabeth in Lake Oswego, Oregon. She’d been walking on Nye Beach on the central Oregon coast after a “heavy rainstorm,” took a step off the hard sand, and promptly sank up to her calf in some kind of “liquified mush,” with water bubbling up around her lower leg. Her concern was that she had stumbled into quicksand. Could GeoMan explain?

Yep, and as usual with earth processes, the answer is simple.

The scientific term she was looking for is “liquefaction” — what happens to sand when it is saturated with water and then disturbed (in her case by stepping on it). Liquefaction is the scientific name for what has been colloquially called quicksand ever since the first Conestoga wagon tried to cross the Missouri at the wrong place (and maybe even before that).

Loose sediments have a physical property called porosity: how much of the material is solid grains (like sand at the beach) and how much is the empty space surrounding the solid grains. This empty space can be filled with just about anything that will fit; including air, oil and gas, or water (the source of the groundwater we pump from a well).

Porosity is expressed as a number that represents the amount that is empty space — the bigger the value, the less solid the material. But… porosity is a funny thing, and kinda/sorta works backwards, at least to my way of thinking. As it turns out, the coarser the particles (like in a conglomerate), the lower the porosity, while finer-grained material (like clay) has a greater percentage of open space. This may sound counter-intuitive, but that’s how nature does it.

(It may be instructive to consider that when standing on clay — or the sedimentary rock called shale that is made from lithified clay — you are supported by 80% or more empty space. This is one of the reasons that “oil shale” and the fracking of shale for natural gas can produce such a large amount of petroleum — there’s so much room for the good stuff. Also consider this: your coffee cup, if made of clay, is 80% or more empty space. So why doesn’t it leak? This leads to a second physical property of sediments called “permeability” — how easily the fluid will move through the material. I promise more on how porosity and permeability affect groundwater — and our petroleum resources — in a later post.)

Anyway, in the case of rounded beach sand, the porosity is about 50%. What this means is that, when standing on wet sand, you only seem to be on a solid — 50% of what is below your feet is water! As long as the sand remains undisturbed the grains are self-supporting, and the beach acts like a solid. But (as is the case with “quicksand”), if the grain-to-grain contact is broken, the 50% that is water takes over for a bit, and the beach turns into a liquid.

It will turn back to a solid once the disturbance stops and the grain-to-grain contact is restored. Unfortunately, anything that might have sunk into the sand when it was a liquid is now stuck!

A fine example of liquefaction in action: There was a large earthquake in Port Royal, Jamaica in 1692. This was literally a Pirates of the Caribbean setting, with Port Royal being described at the time as the “most wicked and sinful city in the world” and “one of the lewdest in the Christian world”. The quake was estimated to be around 7.5 on the Richter scale, which was surely enough to cause liquefaction to occur (click here for an earlier post about earthquakes and the Richter Scale). The town was built on a sand spit next to the ocean, and it literally sank into the beach!

The image, above, was drawn by a reporter who happened to be in Port Royal for the festivities (this was before the days of cell phone cameras). During the main shock the sand was said to have formed waves. Fissures repeatedly opened and closed, crushing many people. After the shaking stopped the sand again solidified, trapping hundreds of victims. In the immediate aftermath of the earthquake, it was common to ascribe the destruction to divine retribution on the people of Port Royal for their sinful ways. Of special interest in the image: note the people buried up to their necks in the re-solidified sand (and the feral dogs chewing on their faces). Must have been an awful time for the locals!

Anyway, liquefaction is most likely what happened to Elizabeth at Nye Beach (although on a much smaller scale).