Geology Reference
In-Depth Information
One of the most influential presentations of this remarkable discovery appeared in 1961
in the
Geological Society of America Bulletin
. British geophysicist Ronald Mason and
American electronics expert Arthur Raff of the Scripps Institution of Oceanography in
California hadcollaborated foralmost adecade, compiling exhaustive magnetic surveysof
the ocean floor off the West Coast of North America. The centerpiece of their publication
was a detailed magnetic map of the Juan de Fuca Ridge, a prominent ridge feature on the
Pacific seafloor just a short day's sail from ports in Oregon, Washington State, and British
Columbia.
The stark black and white map of Mason and Raff—white and black bands representing
normal and reversed magnetic fields, respectively—displays dozens of north-south trend-
ing stripes. Uniformity prevails within large blocks of ocean crust, each hundreds of miles
wide, each with a symmetrical pattern of stripes about a central rift valley. But between
adjacent blocks the pattern is broken, offset by transform fault lines and skewed like a cu-
bist painting. Analysis of offsets along one of these faults, the Mendocino Fracture Zone,
reveals a remarkable lateral displacement of seven hundred miles. Epic internal processes
must be at work to so disrupt Earth's crust.
With similar evidence piling up from ridge systems around the globe, geologists, geo-
physicists, and oceanographers began to talk to one another in a newly integrated effort.
Correlations of ocean-floor topography, seismology, magnetism, and rock ages all pointed
to the same conclusions. Ocean crust is being created around the world at ridge systems,
which are dynamic zones of volcanic activity. The rate of seafloor spreading is recorded in
the symmetrical patterns of magnetic stripes and the ages of basalts.
A flood of influential papers transformed the collective geological mind-set, and by the
mid-1960s almost everyone was convinced of what had once been heresy: the continents
aremoving.TheAtlanticOceanhasbeengrowingwidereveryyearformorethanonehun-
dred million years.
The Case of the Disappearing Crust
The early years of the plate tectonics revolution were a time of rapid discovery, changing
paradigms, and equally puzzling new questions. One unanswered question stood out from
therest:Howcouldamile-wideswathofnewbasalticcrustbeaddedeverythirtythousand
yearsalongmorethanthirtythousandmilesofmidoceanridgesintheAtlantic,Pacific,and
Indian oceans? How did all that new crust fit? Unless Earth was growing larger—and for a
brief interlude in the 1950s and early 1960s, a small but vocal group of geologists, includ-
ingBruceHeezen,didadvocatetheuntenableexpanding-Earthscenario—theoldcrusthad
to go
somewhere
.
