Travel Reference
In-Depth Information
new oceanic crust is formed. These same ocean basins that yielded the mag-
netic lineations on the bottom of the sea, showing that ocean crust is ever
more ancient as we move away from the spreading centers. It was the ocean
basins that revealed the presence of subduction zones, the linear regions
where the conveyor belt of crustal movement dives under continents, to be
consumed by the earth's interior heat and then move back toward the
spreading centers along enormous lithic convection cells that power conti-
nental drift. From the continents far less information is available. At least as
far as proofs of continental drift are concerned, the best evidence comes
mainly from the time machines of stratigraphic analysis and paleomagnetics.
Stratigraphy is the simplest branch of the earth sciences, at least in
principle. Stratigraphers study the sequence of rocks as they pile one upon
another through time. Sometimes a stratigraphic succession is created by the
simplest action of one sand grain falling upon another over time, leaving be-
hind sedimentary beds, oldest on the bottom, youngest on top. Sometimes
things are far more complicated, and great slices of earth's crust are thrust
over the tops of other huge slices. Yet even in these complex tectonic cases,
the order of things is deciphered via stratigraphy.
One of the best ways to detect whether one hunk of rock on a conti-
nent is related to another—or even to rocks on a different continent—
derives from stratigraphy. For example, one of the first (and still among the
most powerful) lines of evidence used by early believers in plate tectonics
came from stratigraphic studies conducted in the Southern hemisphere. In-
vestigators such as Alfred Wegener and Alexander du Toit recognized that
similar successions of sedimentary rocks were observable on the now widely
separated expanses of Australia, Africa, Asia, India, South America, and
Antarctica. Long before any believable
mechanisms
had been proposed to ac-
count for continental drift, Southern hemisphere geologists defined a diag-
nostic, quatter-billion-year-old succession of strata (which they named
Gondwana succession) and argued (mostly to deaf ears) that such similar
stratigraphic successions on continents now-separated could not be coinci-
dence. All of the continents bearing these rocks
must
have been part of a sin-
