Even More Rock Evidence
Continents are built of blocks of crust varying in age, size, rock composition, structure, and fossil
assemblage (fauna and flora). In general, most continents have stable, older interiors (called cratons), while
the zones bordering the cratons typically consist of younger, structurally more complicated rocks. Some
bordering zones are composed of remnants of ancient oceanic lithosphere, volcanic arcs, or mountain
ranges -- reasonably interpreted to be products of pre-Pangaea plate tectonics -- that have attached
themselves to the cratons. In other zones, however, the geological arrangement of these attached remnants
seemed totally chaotic, defying reasonable explanation by geologists until recently. For example, one
remnant characterized by a specific kind of rock or fossil of distinctive age may lie next to, or be
surrounded by, other remnants characterized by entirely different groups of rocks or fossils, even though
they may be similar in geologic age. With the plate-tectonics model, it is now possible to provide more
rational explanations for these zones of oddly juxtaposed crustal remnants.
Scientists now recognize that continental margins are often a mosaic of lithosphere fragments that have
been added as a result of plates impinging on one another during movement. The process by which the
lithospheric fragments, actually pieces of other plates, became attached to the continents is called
accretion. Such fragments can be either continental or oceanic in origin; if they are sufficiently large and
share similar geologic characteristics, these fragments are called terranes. Terranes that seem out of place
geologically, called exotic or suspect terranes, are composed of pieces of plates that have broken off and
then drifted great distances before attaching (accreting) to some other terrane or continental landmass.
Western North America is an example of a complex geologic region that is best interpreted as a patchwork
of several far-travelled terranes that accreted together after the break-up of Pangaea.
In recent years, the study of terranes (called "terrane tectonics" or "terrane analysis") has become a
specialized field within plate-tectonics research. Such studies suggest that plate tectonics has been
operating in some fashion since very early in the Earth's history, perhaps as early as 3.8 billion years ago.
An intriguing, but sketchy, picture seems to be emerging: There have been several cycles of supercontinent
formation, each followed by break-up and subsequent drifting of the fragmented parts. Pangaea itself may
have been formed by the aggregation of separate continents that drifted back together after the break-up of
an older supercontinent that existed about 550 million years ago.
Dr. David G. Howell (USGS, Menlo Park, California), a specialist in terrane analysis, likens such
movement of continents -- as the plates join and separate again and again throughout the Earth's history -to the motion of "lithospheric bumper cars." There are several important differences, however: this
imaginative comparison ignores the fact that electric bumper cars at amusement parks can each move
independently, rather than being parts of an integrated system. And their average speeds are at least 500
million times faster than those of tectonic plates!
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What are cratons?
What happens at continental margins?
What is this process called?
What are “suspect terraines”? Give an example of one.
What might the earth have been like before Pangaea?
http://pubs.usgs.gov/publications/text/developing.html
Western North America
showing some important
plate-tectonics features
and the mosaic of fartravelled exotic terranes
plastered against the
long-lived, stable interior
of the continent (see text).
(Modified from
illustration provided by
Oceanus Magazine;
original figure by Jack
Cook, Woods Hole
Oceanographic
Institution.)
http://pubs.usgs.gov/publications/text/developing.html