Travel Reference
In-Depth Information
now (unless they have a real time machine) trying to deduce the sequence of
events. But just such chaos appears to be what formed the west coast of
North America 50 to 100 million years ago, and pity poor
us
now trying to
work it out.
A second key unit is the
terrane
(more properly a "tectonostratigraphic
terrane," but let's keep it simple). Terranes are collections of rock showing a
particular stratigraphy that is separated from other rock types by faults, or
breaks in the rock. Microplates become terranes when they collide with
other rocks and are fused onto continents.
Terranes (and the suspect ones among them) have several disparate
origins. They can be (1) fragments of continents, long since split off, com-
posed of old sedimentary rock and perhaps granites and old metamorphic
rocks; (2) fragments of continental margins, which are usually composed of
sediments shed from a continental margin); (3) fragments of volcanic arcs,
made up of accumulated lava and sediment; or (4) fragments of ocean basins,
and thus made up of basalt.
Terranes may begin their existence by being "born" in some oceanic
setting (such as through the accumulation of lava in a submarine setting), or
they may form as they are "calved" off of some already existing land mass,
through a process known as rifting. In each case the accumulated rock is
buoyant relative to oceanic crust, so it "floats" on heavier crust. Its fate,
sooner or later, will be to collide with some other buoyant crust. Such colli-
sions are the process through which continents often grow larger, for more
often than not, the colliding pieces accrete onto the larger land mass. But
how can we deduce the antiquity of such an event, and how can we tell what
the colliding pieces looked like, were made up of, or came from before the
collision? Stratigraphy is one powerful tool, as we saw in our discussion about
the first recognition of the Gondwana supercontinent. But an even more
useful tool comes from paleomagnetics.
There has long been an uneasy alliance between the two great pillars of
the earth sciences: the branches known as geology and geophysics. In the
simplest sense, geology addresses history—
when
the observable features on
and in the earth formed—whereas geophysics deals mainly with the physical
