Q–What is quicksand? Is there any in the Bay Area? Is it true that an earthquake can turn some land into quicksand? Has anyone ever been sucked into such soil? [Chantal, San Francisco]

I have had two personal encounters with quicksand and neither happened in the Bay Area. The first was along the Amargosa River near Death Valley and later on the bank of the Colorado River in the Grand Canyon. Fortunately like everyone else who steps in quicksand, I survived! Classic Western movies, of course, have portrayed an entirely different outcome. Usually bad guys are sucked down into the muck with only their black hat remaining on the surface as witness to their justifiable and righteous death.

“Quick” in this sense does not mean fast but means alive which is the original definition of the word. The use is similar to the word –quicksilver- for the metal mercury’s liveliness in the liquid state. Basically quicksand is made up of small particles (technically not sand but finer grains referred to as silt by geologists) that are saturated with water. The friction between the silt particles is so reduced that the entire mass is a semi-liquid that cannot support any weight. Most quicksand is only a few feet deep but if you happen to find yourself floundering in it, don’t panic (yeah, right). Your body is less dense than the quicksand and you can swim to the nearby bank. All of your movements should be slow and deliberate like you are swimming in a big vat of molasses.

However there is a closely related phenomenon, which is present in the San Francisco Bay area – liquefaction. You can demonstrate this on any beach. Find some wet but firm sand and jump up and down on it. Soon the entire mass will become more and more liquid and unstable as your feet sink down into it. You have just demonstrated what can happen in certain areas during an earthquake. Much of our infrastructure in the Bay area is built on fill or young sediment, which is very near the water table. During an earthquake the ground shakes and the underlying sediment can quickly become saturated with water. It becomes unstable and unable to support the overlying structures. Buildings, roads and bridges collapse just like we saw during the Loma Prieta earthquake in the Marina District. And this my friends, is not a movie.

Michael Ellis – Ask the Naturalist- Bay Nature Magazine



Posted on

August 22, 2009