December 29, 2004
Deep structure
Last night my PBS affiliate rebroadcast a NOVA episode about a team of climbers scaling the Vinson Massif, Antarctica's highest mountain, by a route never tried before. It's a spectacular episode. At one point, the team is faced with a difficult decision: to climb an ice wall via a direct but very technical and steep route, or take an easier indirect route that lies under some dangerous hanging seracs.
Seracs are giant blocks of ice that form when glaciers flow downhill and fracture in the process. The ice is being stretched out, and breaks into blocks, which tilt upwards at one end, making a chaotic ridge-valley-ridge pattern. The seracs precarious position means they can break away and collapse at any moment, with little or no warning. The team's dilemma consisted in a Sophie's choice between a route that might kill their less experienced climbers, or a route that might get everyone crushed under tons of falling ice.
What I realized is that seracs have the same basic structure as basin-and-range fault systems, seen in extensional tectonic environments like those in the American Southwest (roughly covering the area from the Sierra Nevada east to western Texas). (The illustration at right is an abstraction of a basin-range "normal" fault.) In this region, and other similar ones like the Persian Gulf, the crust is extending, stretching outwards, as a continental plate divides itself in two. Huge blocks of crust break apart as the earth stretches, and tilt upwards at various angles, producing an extremely regular range-basin-range pattern. The ranges are all about the same height (roughly a mile) because the thickness of crust is roughly consistent. The exception here is the Sierra, which are higher (two to three miles) because they sit on thicker crust.
The odd similarity between seracs and basin-range systems caught my attention at a moment when the movement of earth has been much on my mind. An upcoming trip to Death Valley, as usual, prompted my tectonic ponderings. And thanks to Sunday's catastrophe in the Indian Ocean, tectonic upheaval has been heavily featured in the news these last few days. One of the most interesting pieces was in the NYT, where Simon Winchester reflects on the earthquake and its attendant curtain of water:
In recent decades, thanks largely to the controversial Gaia Theory developed by the British scientists James Lovelock, it has become ever more respectable to consider the planet as one immense and eternally interacting living system - the living planet, floating in space, every part of its great engine affecting every other, for good or for ill.Mr. Lovelock's notion, which he named after the earth goddess of the Ancient Greeks, makes much of the delicacy of the balance that mankind's environmental carelessness increasingly threatens. But his theory also acknowledges the somber necessity of natural happenings, many of which seem in human terms so tragically unjust, as part of a vast system of checks and balances. The events that this week destroyed the shores of the Indian Ocean, and which leveled the city of Bam a year ago, were of unmitigated horror: but they may also serve some deeper planetary purpose, one quite hidden to our own beliefs.
The deep structure of two radically disparate systems: ice and rock, both incomprehensibly huge blocks of solid matter cracked into pieces by slow, slow forces we can't even imagine. The forces that cause the cracking leave the pieces perched precariously above us, while we wait for the tension to become too great. Eventually, the fault will slip, or the block will tumble down the slope, and in neither case will it care that we are in its way.