Examples of Liquefaction
Quicksand. Quicksand is a simple example of liquefaction. Spring-fed water flowing up through sand creates quicksand. The upward flowing water lifts the sand grains very slightly, surrounding each grain with a thin film of water. This cushioning gives quicksand, and other liquefied sediments, a mushy, fluidlike texture.3
Contrary to popular belief and Hollywood films, a person or animal stepping into deep quicksand will not sink out of sight forever. They will quickly sink in—but only so far. Then, they will be lifted, or buoyed up, by a force equal to the weight of the sand and water displaced. The more they sink in, the greater the lifting force. Buoyancy forces also lift a person floating in a swimming pool. However, quicksand’s buoyancy is almost twice that of water, because the weight of the displaced sand and water is almost twice that of water alone. As we will see, fluidlike sediments produced a buoyancy that largely explains why fossils show a degree of vertical sorting and why the world’s sedimentary rocks are usually layered sharply.
Earthquakes. Liquefaction is frequently seen during, and even minutes after, earthquakes. During the Alaskan Good Friday earthquake of 1964, liquefaction caused most of the destruction within Anchorage, Alaska. Much of the damage during the San Francisco earthquake of 1989 resulted from liquefaction. Although geologists can describe the consequences of liquefaction, few seem to understand why it happens. Levin describes it as follows:
Often during earthquakes, fine-grained water-saturated sediments may lose their former strength and form into a thick mobile mudlike material. The process is called liquefaction. The liquefied sediment not only moves about beneath the surface but may also rise through fissures and “erupt” as mud boils and mud “volcanoes.” 4
Liquefaction was captured on film after the 9.0 Japanese earthquake on 11 March 2011. In a city park built over a landfill in what was part of Tokyo Bay, subsurface water that had been trapped in the sediment’s pore spaces is seen erupting through cracks produced by the swelling of the land. [See www.youtube.com/watch?v=I3hJK1BoRak, and www.youtube.com/watch?v=9x_kS3Bm6fA&feature=related.]
Strahler says that in a severe earthquake:
... the ground shaking reduces the strength of earth material on which heavy structures rest. Parts of many major cities, particularly port cities, have been built on naturally occurring bodies of soft, unconsolidated clay-rich sediment (such as the delta deposits of a river) or on filled areas in which large amounts of loose earth materials have been dumped to build up the land level. These water-saturated deposits often experience a change in property known as liquefaction when shaken by an earthquake. The material loses strength to the degree that it becomes a highly fluid mud, incapable of supporting buildings, which show severe tilting or collapse.5
These are accurate descriptions of liquefaction, but they do not explain why it occurs. When we understand the mechanics of liquefaction, we will see that liquefaction occurred globally—for weeks or months during the flood.
Visualize a box filled with many rocks. If the box were so full that you could not quite close its lid, you would shake the box, so the rocks settled into a denser packing arrangement. Now repeat this thought experiment, only this time all space between the rocks is filled with water. As you shake the box and the rocks settle into a denser packing arrangement, water will be forced up to the top by the “falling” rocks. If the box is tall, many rocks will settle, so the force of the rising water will increase. The tall column of rocks will also provide great resistance to the upward flow, increasing the water’s pressure even more. Water pressure will exert a lifting force on the rocks for as long as the upward flow continues.6
This is similar to an earthquake in a region having loose, water-saturated sediments. Once upward-flowing water lifts the topmost sediments, weight is removed from the sediments below. The upward flow can then lift the second level of sediments. This, in turn, unburdens the particles beneath them, etc. The particles are no longer in solid-to-solid contact, but are suspended in and lubricated by water, so they can easily slip by each other.
Wave-Loading—A Small Example. You are barefoot, walking along the beach. As each wave comes in, water rises from the bottom of your feet to your knees. When the wave returns to the sea, the sand beneath your feet becomes loose and mushy. As your feet sink in, walking becomes difficult. This temporarily mushy sand, familiar to most of us, is a small example of liquefaction.
Why does this happen? Below each wave crest, water is forced down into the sand. As the wave trough approaches, that water gushes back out. In doing so, it lifts the topmost sand particles, forming the mushy mixture.
If you submerged yourself face down under breaking waves but just above the seafloor, you would see sand particles rise slightly above the floor as each wave trough approached. Water just above the sand floor also moves back and forth horizontally with each wave cycle. Fortunately, the current moves toward the beach as liquefaction lifts sand particles above the floor. So, sand particles are continually nudged upslope, toward the beach. If this did not happen, beaches would not be sandy.7
Wave-Loading—Medium-Sized Example. During a storm, as large waves pass over pipes buried offshore, water pressure increases below the wave crests. This forces more water into the porous sediments surrounding the pipes. As the wave peaks pass and the wave troughs approach, pressure over the pipes drops, and the stored, high-pressure water in the sediments flows upward. This lifts the sediments and causes liquefaction. The buried pipes, “floating” upward, sometimes break.8
Wave-Loading—A Large Example. On 18 November 1929, an earthquake struck the continental slope off the coast of Newfoundland. Minutes later, transatlantic phone cables began breaking sequentially, farther and farther downslope, away from the epicenter. Twelve cables were snapped in a total of 28 places. Exact times and locations were recorded for each break. Investigators suggested that a 60-mile-per-hour current of muddy water swept 400 miles down the continental slope from the earthquake’s epicenter, snapping the cables.9
This event intrigued geologists. If thick muddy flows could travel that fast and far, they could erode long submarine canyons and do other geological work. Such hypothetical flows, called turbidity currents, now constitute a large field of study within geology. However, there are several problems with this 60-mile-per-hour, turbidity-current explanation:
- water resistance prevents even conventional nuclear-powered submarines from traveling nearly that fast,
- the ocean floor in that area off the coast of Newfoundland slopes less than 2 degrees,10
- some broken cables were upslope from the earthquake’s epicenter, and
Instead, a large wave (a tsunami11) probably radiated out rapidly from the earthquake’s epicenter. Below the expanding wave, sediments on the seafloor partially liquefied. Then, they slowly flowed downhill12 where they loaded and snapped cable segments that were perpendicular to the downhill flow. Other details support this explanation.
We can now see that liquefaction occurs whenever water is forced up through loose sediments with enough pressure to lift the topmost sedimentary particles. Now let’s look at a gigantic example of liquefaction, caused by many weeks of global wave-loading.