Gray Earth - The Story of Earth - page 76

gener's hypothesis we must forget everything that has been learned in the last 70 years and
start all over.”
Even so, a few Earth scientists were sufficiently intrigued by the findings of Wegener
andhissupporterstodevisenovelmechanismsforcontinentalshifts.Oneschoolofthought
posited that Earth is shrinking, perhaps by cooling or by collapse of gas-filled voids in the
deepinterior,andthusportionsofthesurfacemustgraduallyfallinwardlikeabrokenarch-
way. In this untenable model, the continents once boasted a continuous expanse of land
from the western coasts of the Americas all the way to the eastern coasts of Africa and
Asia.Today'sAtlanticOceanwasviewedasagiantarchwayoflandthathascollapsedinto
the mantle. Basic Euclidian geometry foiled this shrinking Earth model: a simple archway
can collapse, but transfer that idea onto a sphere and there's no way a continental volume
covering the area of the Atlantic Ocean could collapse into anything.
Another group proposed the antithetical view that Earth has been expanding, inflating
like a balloon over geological time. Once upon a time, there was only continental crust,
which has cracked and split apart as the planet inflated (by some accounts from the gener-
ation of deep, hot expanding gases). Indeed, if you play an imaginary videotape of a sup-
posedly expanding Earth backward, you can arrive at a state where all the continents slide
neatlytogethertocoveraspherethatisaboutthree-fifthsthediameterofthemodernEarth.
Lacking any other widely accepted formation mechanism for the Atlantic, this hypothesis
persisted in some geological circles from the 1920sto as late as the 1960s,when a compel-
ling new idea took its place.
The Hidden Mountains
Fast-forward to the post-World War II years, a time of tremendous technological innova-
tion and optimism in science. Two developments in antisubmarine warfare, both declassi-
fied and adopted by oceanographers in the 1950s, led to transformative discoveries about
the dynamic Earth.
Sonar, which uses sound waves to measure distance and direction, is a century-old tech-
nology familiar to anyone who has watched Hollywood submarine movies. You hear a
PING, which is answered a short while later by a softer, echoing ping . A sound wave has
bounced off the solid hull of a submarine. (The effect on the viewer depends on whether
the movie's point of view is the hunter or the hunted.) “PING. . . . . . ping ,” “PING . . . .
ping ,” “PING . . ping ”: the echoes come faster as the submarine's location is pinpointed.
The tense music builds; depth charges are released.
The exact same technology can be put to scientific use to study ocean depth and thus
ocean-floor topography. Even the deepest ocean valleys and trenches can be plumbed with
sound waves. As early as the 1870s, British scientists employed crude deep-water sound-