Columnar Jointing Gives Rise to Natural Wonders
The Giant’s Causeway
The famed Giant’s Causeway in northern Ireland is justly considered to be one of the seven wonders of the United Kingdom. Local legend holds that the causeway is the remnant of a bridge spanning the channel between Ireland and Scotland, built by an Irish giant named Fionn mac Cumhaill (Finn McCool).
The causeway consists of nearly 40,000 interlocking basalt columns, some as tall as 36 feet. The tops of the columns from “stepping stones” leading from the foot of the cliffs before disappearing under the sea. It looks far too regular in terms of its patterned structure to have been caused by natural processes–yet it was.
A fellow of Trinity College named Sir Richard Bulkeley II officially announced the causeway’s existence in 1693, in a presentation before the Royal Society of London. Theories abounded as to how the causeway had formed. In 1771, a Frenchman identified only as Demarest announced that it was the result of volcanic activity.
According to physicist Lucas Goehring of the University of Toronto–who presented a paper at the 2007 APS March Meeting in Denver describing his recent work in this area with fellow Toronto colleague Stephen Morris–the columnar joints that make up the causeway were formed roughly 60 million years ago by the cooling and shrinking of molten lava from a massive volcanic eruption.
When the lava flowed into the sea, it quickly cooled, contracted, and crystallized into the near-perfect hexagonal columns we see today. (In fact, geologists believe there were three major lava flows, giving rise to lower, middle and upper basaltic layers, with the causeway columns occurring in the middle layer.) That kind of shrinkage inevitably causes stresses that fracture the rock.
“The columns are formed as a sharp front of cooling moves into the lava flow, assisted by the boiling of groundwater,” said Goehring. “As the front advances, it leaves behind a crack network which evolves into an almost hexagonal arrangement. This network carves out the columns.”
Similar structures can be created with a simple kitchen experiment: mix equal parts corn starch and water and place into a coffee cup. Dry the mixture by shining a bright light above it. Within a week or so, the mixture will be completely dry and you can break it apart to reveal an interior that is broken up into “starch columns.”
In addition to studying the genuine article in situ in both Ireland and Scotland, Goehring and his cohorts have figured out how to control this tabletop kitchen experiment so precisely that they can study the formation process in much greater detail than scientists could in the past, augmented by X-ray tomography to give the first genuinely 3D imaging of the internal structure of the columns.
Among the more surprising findings: (1) the columns are not quite as perfectly hexagonal as previously believed, and (2) the continuous dynamics of the formations can be found even deep inside the structure, similar to dry foams. Also, the size of the columns depends on the speed at which the cracks advance, and the rate at which the water can move through the starch. That’s why lava-formed columns are 1000 times larger than the tabletop experiment: the analogous properties of lava are much slower.
Sharing the session with Goehring was Meredith Betterton of the University of Colorado, Boulder, who became enthralled by the large icy spikes–called penitentes because they resemble a procession of white-hooded monks–she observed while viewing glaciers in the Andes.
Along with colleagues at the Ecole Normale Superieure in France, Betterton created the first artificial versions of these spiky ice formations, which can be found quite frequently on high-altitude glaciers where the air is particularly dry. She also devised a mathematical model to predict the process. The hope is that this research will yield useful insights into how glaciers evaporate; it may also lead to a practical strategy for preserving glaciers in light of global warming.
Penitentes arise when the sun’s rays evaporate snow in such a way that the ice turns directly into water vapor, without melting into water first. The process is called sublimation. The snowy surface might start out smooth, but it gradually develops depressions as some areas sublimate faster than others, and the resulting curved surfaces concentrate more sunlight and speed up the sublimation even more, leaving behind a forest of towering spikes of ice. Penitentes are nature’s ice sculptures.
Rising temperatures slow the formation of penitentes quite a bit, an especially alarming factor in light of global warming, because fewer ice spikes could accelerate the melting of glaciers. The spikes cast shadows, and serve as a natural cooling mechanism. There’s a working hypothesis that spreading a small layer of dirt over glaciers could help preserve them by fostering faster formation of penitentes.
This turned out to be true with the small-scale versions Betterton created in her lab. She spread printer toner on her artificial snow layer to simulate pollutants common to glaciers around the world, and found that the ice spikes grew more rapidly as a result. It’s a bit counter-intuitive, since carbon-based pollutants actually increase melting rates on glaciers because the ice absorbs more sunlight and therefore heats up more quickly. The formation of more penitentes could offset that damage.