Gray Earth - The Story of Earth - page 77

ings aboard HMS Challenger and reported hints of great mountains on the floor of the
mid-Atlantic—a tantalizing result that some contemporary romantics associated with the
lost continent of Atlantis. Primitive echo sounding technology, first developed for iceberg
detection after the 1912 Titantic disaster, enjoyed rapid improvement during World War I,
as German submarines began to prowl. The 1920s saw the first systematic application of
sonar to mapping the ocean floor and the rapid realization that great mountain ranges lay
hidden beneath all the Earth's oceans. However, the geological implications of these pion-
eering ocean surveys were little noted, and oceanographic efforts were largely curtailed by
the Great Depression and the looming Second World War.
Followingthewar,oceanographerswerearmedwithanewgenerationofhigh-sensitivity
sonardetectorsthatcouldnotonlymapthetopographyoftheentireoceanfloorbutalsode-
tect reflected sound waves from deeper rock layers. General features of the Atlantic Ocean
floor were easily confirmed. For example, continental shelves deepen gradually as you
moveawayfrommostAtlanticcoastlines,fordistancesuptohundredsofmiles.Theedges
of these continental shelves are marked by a sudden drop-off to an abyssal plain two miles
deep and a thousand miles across—a feature much wider and flatter than any on dry land.
And the ocean is bisected by an extensive mountain range, the Mid-Atlantic Ridge.
All that was in accord with earlier discoveries, but the thickness of the ocean crust came
as a huge surprise. Geologists had predicted that the oceans would be less deeply rooted
than land, with ocean crust gradually thinning away from shore. What they found, instead
of this gradual transition, was a remarkably abrupt contrast from thick to thin. Unlike the
tens of miles of crustal rocks beneath the continents, ocean crust was only about five or six
miles thick: the sharp transition occurred right at the drop-off at the edge of the continental
shelf.Suchanarrowdemarcationbetweencontinentsandoceanswasatoddswithisostatic
models.
Year after year, back and forth across the wide ocean the scientists sailed, hundreds of
times. Each and every crossing yielded the same result. A vast mountain chain more than
twenty thousand miles long, the largest on Earth, lay beneath the waves, precisely bisect-
ing the Atlantic. The same sweeping curves of the continental shorelines were mimicked
in the hidden Mid-Atlantic Ridge crest. What's more, if the edge of the continents was
takenasthesharpunderwaterdrop-offtotheabyssalplain(asopposedtotheshiftingsandy
shoreline), thenthematch between continents wasuncanny,asifabrokenchinaplate were
fit neatly back together. No longer could science dismiss the conformity of coastlines as
mere coincidence.
As scientists completed more traverses of the Atlantic and compared more details, new
patterns emerged. The Mid-Atlantic Ridge was no ordinary mountain range. On land, most
mountain chains have a line of the highest peaks down their axis, but at the very center-
line oftheMid-Atlantic Ridge wasawidetroughabouttwenty miles wideandmorethana