Geology Reference
In-Depth Information
ture of the seafloor. For that it was useful to have a map of oceanic magnetism. The navy's
program to map the magnetic signature of the seafloor produced another surprise. As re-
searchers surveyed away from mid-ocean ridges, they found alternating bands of different
magnetic polarization—stripes with normal magnetic polarity alternating with stripes of
negative polarity. The scientists who first reported the odd zebra-hide pattern of magnetic-
ally striped seabed sections noted that they had no idea how to explain it. A decade later,
the stripes were understood to be like a strip-chart recording of global reversals in Earth's
magnetic field. Imprinted with the planet's magnetic field—positive or negative—when
they first cooled, the newly formed rocks at the mid-ocean ridges slowly moved away from
the ridge as new crust squeezed up at the ridge and pushed older crust out of the way.
The discovery of how and where earthquakes are produced provided the final critical
piece of the tectonic puzzle through efforts to verify nuclear test ban treaties. The discovery
that underground nuclear tests produced seismic waves that could be distinguished from
the seismic waves produced by earthquakes fueled substantial government investment in
seismology in the 1950s and 1960s. Ratification of the Limited Test Ban Treaty in 1963
made continuous seismic monitoring and locating of earthquakes critical to verifying treaty
compliance. Establishment of a global seismograph network led to a rapid increase in the
global catalog of earthquake locations, revealing a striking pattern. Most earthquakes oc-
curred in the uppermost several hundred miles of Earth's crust. But there was no good ex-
planation for mysterious deep earthquakes triggered more than four hundred miles below
ground. The rocks that far down should be too hot and mushy to sustain the rigid deform-
ation needed to produce earthquakes. As seismologists refined their methods, they found
that these unusually deep earthquakes outlined slabs of crust sinking down into the planet's
interior.
These three seemingly unrelated observations—the birth of new oceanic crust at spread-
ing centers in the middle of the oceans, the slow movement of new crust away from the
spreading centers, and the sinking of crust beneath continents in subduction zones along
the ocean margins—defined a full cycle. New crust was rising up in the middle of the ocean
and sinking back down to be recycled in the deep trenches at the edge of ocean basins.
Crust produced at mid-ocean ridges was being pushed aside until it ran into a lighter con-
tinent and got shoved back down into the mantle. Continents rafted along on mobile plates
