Common Ground for Ketchup and Earthquakes

A bottle of America's commonest tomato sauce can tell a lot about what happens in some of Alaska's relatively uncommon great earthquakes. It's the feature that physical chemists call thixotropy.

Back in high school chemistry, most of us learned about colloidal systems. Put most simply, that means fine pieces of one substance are dispersed evenly through something else so that everything wants to stay put. Whipped cream is a tasty example; the butterfat bits are held in place--suspended in a dispersion--by little air bubbles. Gelatin makes another edible colloid; the protein suspended in the liquid provides a semisolid Jello dessert. Commercial ketchup is another such system, behaving like a kind of gel with fine bits of solid tomato and seasonings dispersed through the liquid.

Not all gels behave the same under different kinds of stresses. Gelatin desserts don't hold up well when they're heated, for example. And thixotropy appears, as the dictionary puts it, as a "Property of certain gels which liquefy when subjected to vibratory forces, such as ultrasonic waves or even simple shaking, and then solidify again when left standing."

Commercially prepared ketchup is really thick stuff; in fact, some advertisements use the slow-pouring feature as a sales pitch, implying that good ketchup is worth waiting for. But to speed up the sauce, all one has to do is shake the bottle vigorously (and to keep the walls clean, shake it with the top still fully screwed onto the bottle). That's the thixotropic feature of the stuff. Jostled severely, its structure weakens. For all practical purposes, it becomes more liquid.

Earthquakes enter the picture not because they make pouring ketchup easier for lazy people, but because all gels are not edible. Thixotropic substances lie underfoot as well as on the pantry shelf.

Notorious among them is thixotropic clay, and probably the most notorious of the thixotropic clays is Bootlegger Cove clay. That's the slippery stuff that became instant underground mud during the Good Friday earthquake in 1964, turning many otherwise well built Anchorage structures into jumbled wrecks.

One way to envision what happens is to imagine the flat little clay particles as constructing what amounts to so many tiny houses of cards. In the card-house spaces, water is contained, and the water actually helps support the microscopic structure. When the clay is shaken violently, as it was during the earthquake, the house of cards comes tumbling down, its movement eased by the lubricating effect of the now-free water.

It's possible to experience the startling effects of thixotropy underfoot in less dramatic ways. Friends of mine were scanning the far side of the river for signs of moose antlers in the brush one cold autumn afternoon. They'd beached their boat on a firm, dry silty shore, but they were dancing around a bit to keep warm--and suddenly discovered they were bopping in a new bog. Their foot-stomping antics had rearranged the structure of the damp silt, releasing enough water to get their boots uncomfortably wet.

Presumably--going back to the dictionary definition of thixotropy--if they'd kept still long enough, the ground beneath their feet would have solidified again. They didn't carry out that portion of the experiment. But if you prefer your ketchup stiff and slow, let it rest on the shelf for a while. That experiment, at least, is easily done.