Page 86 - 2008 DT 12 issues

Reading the Rocks South America and wondered if all sometimes fracturing off a chuck that
continents could have been one huge becomes Australia, and occasionally
eology is a science of ob- landmass at an earlier time. What re- colliding. Smaller pieces of land car-
servation and measurement, sulted was the theory of plate tectonics, ried along with ocean plates can ram
Ginference, interpretation and which was accepted in principle in the the mainland full speed ahead, wrecking
joint-busting detective work. The field late 1960’s, opening up an entire new havoc over eons. All of these enormous
is your laboratory, the rock hammer and chapter in history of geology. The sub- stresses compress and deform plate mar-
collecting bag the tools of your trade. sequent discovery of a 40,000-mile-long gins, producing volumes of magma.
Mass spectrometers, lasers, ground system of underwater mountain ridges Orogeny, or mountain building, is
penetrating radar and other sophisticated that pattern the ocean floor illustrated most often the result of compression.
technology delve deep into the land’s that the earth is, indeed, dynamic. The Continental collisions between North
chemistry, structure and elemental pro- ridges define the plate blocks that, America, Africa and Europe created the
cesses to understand our planet’s past, moving inch by inch, create and de- Alleghenies and Appalachians. India
adding details and time lines. Rocks stroy oceans, join and split continents bulldozed into Asia and raised the Hi-
are the keepers of history. Their min- and shape and reshape the very face of malayas. Pangaea, itself, was the result
erals record events, transformations, the planet. of a similar process, a suturing of one
catastrophes. Radioactive elements Magma erupts from the mantle in plate to the other, until ultimately break-
are timekeepers. deep ocean trenches and quickly cools ing apart again. Several times during
into solid rock, enlarging the ocean earth’s early history, plates have come
floor. Land masses of the early together and parted again, each time
earth are thought to have been a with consequences.
collection of many “terranes,” The American West, once home to
wandering and driven by spread- an inland sea, dried out and eventually
ing ocean plates. Some formed became prime real estate. Nevada was a
island arcs, like the Aleutians. tidal mudflat, until then, at the western
Others coalesced into larger continental margin. It was a land of sand
pieces. As a plate spreads, it and gentle waves, ebbing and flowing
impinges upon the margins of with the sea. But it didn’t last long.
the loosely interlocking conti- Clusters of island arcs, micro-continents
nental plates. Its leading edge and other itinerant masses rafted in
is consumed as it dives beneath, and stayed put, birthing the coastal
One day in the autumn of 1911, (subducting) on its way back into the mountains, giving rise to California
Alfred Wegener, a young lecturer in earth’s mantle. Friction heats the lead- and snuffing out Nevada’s dreams of a
astronomy and meteorology at the Uni- ing edge of the over-riding continent. western Rivera.
versity of Marburg in western Germany, The resulting magma rises and forms If compression is the prime mover
chanced upon a scientific paper propos- a continental arc of volcanic mountain in mountain building, how, then, do
ing that a land bridge had once existed ranges—a gross simplification of a you explain the Rockies? They aren’t
between Brazil and Africa. Although the complex process. The coastal ranges at the continental margin. They are
idea was not new, Wegner was surprised of California and Oregon resulted from relatively young. The Ancestral Rock-
to read descriptions of identical plant the subducting West Pacific Plate. These ies, high mountains uplifted over 300
and animal fossils that were found on margins can continue to be violent million years ago, were about 1,500
opposite sides of the Atlantic. Rocks places, wracked by earthquakes and km from any active margin. These were
found on the margins of one continent volcanoes as movement persists. composed mostly of shale, siltstone and
precisely matched those on the mar- Continents are passengers carried sandstone (sedimentary rocks) deposited
gins of another. His surprise became along by the dynamism of tectonic in an ancient sea about 2 billion years
an obsession. plates, moving all the time relative ago. Yet sometime between 1.7 and 1.6
Wegener had noted the apparent fit to each other, sometimes breaching billion years ago, these ancient rocks
between the coastlines of Africa and to allow in the sea (think Red Sea), were squeezed and deformed, caught

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