Geology Reference
In-Depth Information
year period when all the feedbacks of climate and life were in such perfect harmonious
balance? How could such a thing have happened?
At a leisurely breakfast meeting near the University of Tennessee campus, Linda Kah
patiently explains Earth's dramatic and repeated transformations (and, consequently, why
the Mesoproterozoic wasn't boring at all). She comes prepared with a stack ofblank sheets
of 8½-by-11-inch white paper; while she talks, she draws neat illustrative diagrams in con-
trasting blue and red ink.
“Ihadtheseideasadecadeago,”shesays,describingthefruitsofhergruelingfieldwork
in the hostile Mesoproterozoic terrains of the Mauritanian desert in northwestern Africa.
She'd love to go back now, but a rise in banditry and kidnapping makes such a field excur-
sion problematic, foolhardy. Instead she will be a member of the next Mars rover science
team, a safer choice.
Kah's scientific story is devoted to plate tectonics and the mess they make of Earth's
past—the constant shuffling, colliding, splitting, and suturing of landmasses that cause
our globe to look radically different every hundred million years. Even during the three-
hundred-million-year interval that introduced the boring billion—when the supercontinent
Columbia persisted more or less intact—plate tectonics didn't stop. A notable feature of
supercontinents isthat they continue togrowgradually at the edges, asocean plates plunge
under their margins and new volcanoes rise near the coasts. The modern-day expansion
of the Pacific Northwest coast, where such majestic volcanoes as Mount Rainier, Mount
Hood, and Mount Olympia are still active, is but one recent example of this age-old phe-
nomenon. So it was with the expanding margins of Columbia.
EvenmorecontinentalcrustwasaddedtoEarth'sinventorywhenColumbiaexperienced
episodic rifting and breakup into smaller continents and islands. About 1.6 billion years
ago—the beginning of the Mesoproterozoic—the splitting off of the continent Ur to the
west from Laurentia and the rest of Columbia to the east led to a major intercratonic sea
and the deposition of a massive sedimentary sequence reaching thicknesses of more than
ten miles. This heroic deposit, called the Belt-Purcell supergroup, today forms prominent
outcrops over much of western Canada and the northwestern United States. Thus, even as
supercontinents split apart and eroded, new continental rocks were being produced from
the old.
ThisriftingofColumbiaintotwodiverginglandmasseshadotherconsequences.Lauren-
tia,Ur,andtheothercontinentswerestillallcenteredmoreorlessabouttheEquator,which
means that there were still no continents at the poles, which means that there was still no
thick ice buildup at the poles, which means that ocean levels were still relatively high.
Indeed, shallow seas embayed vast swaths of Laurentia's new west coast; likely less than
a quarter of Earth's surface was dry. For a time perhaps exceeding two hundred million
years, Earth's total land area was dramatically reduced, while thick sedimentary deposits
Search WWH ::




Custom Search