Geology Reference
In-Depth Information
Dinosaurs, fl ying reptiles, and marine reptiles evolved
during the Mesozoic.
Birds evolved from small carnivorous dinosaurs,
whereas mammals descended from mammal-like
reptiles.
Mesozoic extinctions resulted in the demise of dino-
saurs and some of their relatives and several types of
marine invertebrates.
The Mesozoic Era (251 to 66 million years ago) was an
important time in Earth's history. The major geologic event
was the breakup of Pangaea, which affected oceanic and cli-
matic circulation patterns and infl uenced the evolution of
the terrestrial and marine biotas. Other important Mesozoic
geologic events resulting from plate movement include the
origin of the Atlantic Ocean basin and the Rocky Moun-
tains, the accumulation of vast salt deposits that eventu-
ally formed salt domes adjacent to which oil and natural
gas were trapped, and the emplacement of huge batholiths
that account for the origin of various mineral resources, in-
cluding the gold that fueled the California gold rush of the
mid-1800s.
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Approximately 150 to 210 million years after the emplace-
ment of massive plutons created the Sierra Nevada (Neva-
dan orogeny), gold was discovered at Sutter's Mill on the
South Fork of the American River at Coloma, California.
On January 24, 1848, James Marshall, a carpenter building
a sawmill for John Sutter, found bits of the glittering metal
in the mill's tailrace. Soon settlements throughout the state
were abandoned as word of the chance for instant riches
spread throughout California.
Within a year after the news of the gold discovery reached
the East Coast, the Sutter's Mill area was swarming with more
than 80,000 prospectors, all hoping to make their fortune. At
least 250,000 gold seekers prospected the Sutter's Mill area,
and although most were Americans, they came from all over
the world, even as far away as China. Most thought the gold
was simply waiting to be taken and didn't realize that pros-
pecting was hard work.
No one thought much about the consequences of so
many people converging on the Sutter's Mill area, all intent
on making easy money. In fact, life in the mining camps was
extremely hard and expensive. The shop owners and traders
frequently made more money than the prospectors. In real-
ity, only a few prospectors ever hit it big or were even mod-
erately successful. The rest barely eked out a living until they
eventually abandoned their dream and went home.
Although many prospectors searched for the mother lode,
the gold that they recovered was mostly in the form of placer
deposits (deposits of sand and gravel containing gold particles
large enough to be recovered by panning). Weathering of gold-
bearing igneous rocks and mechanical separation of minerals
by density during stream transport forms placer deposits. Pan-
ning, a common method for recovering placer deposits, is per-
formed by dipping a shallow pan into a streambed to capture
sediment. The material is then swirled around and the lighter
material is poured off. Gold, being about six times heavier than
most sand grains and rock chips, concentrates on the bottom
of the pan and can then be picked out.
Although some prospectors dug $30,000 worth of gold
dust each week out of a single claim and some gold was found
practically sitting on the surface of the ground, most of this
easy gold was recovered very early during the gold rush. Most
prospectors barely made a living wage working their claims.
Nevertheless, during the fi ve years that constituted the gold
rush proper, from 1848 to 1853, more than $200 million in
gold was extracted.
THE BREAKUP OF PANGAEA
Just as the formation of Pangaea influenced geologic and
biologic events during the Paleozoic, the breakup of this
supercontinent had profound geologic and biologic effects
during the Mesozoic. The movement of continents affected
the global climatic and oceanic regimes, as well as the cli-
mates of the individual continents. Populations became iso-
lated or were brought into contact with other populations,
leading to evolutionary changes in the biota. So great was
the effect of this breakup on the world that it forms the cen-
tral theme of this chapter.
Pangaea's breakup began with rifting between Laurasia
and Gondwana during the Triassic (
Figure 22.1a). By the
end of the Triassic, the expanding Atlantic Ocean separated
North America from Africa. This change was followed by
the rifting of North America from South America sometime
during the Late Triassic and Early Jurassic.
Separation of the continents allowed water from the
Tethys Sea to fl ow into the expanding central Atlantic Ocean,
whereas Pacifi c Ocean waters fl owed into the newly formed
Gulf of Mexico, which at that time was little more than a
restricted bay (
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Figure 22.2). During that time, these areas
were in the low tropical latitudes where high temperatures
and high rates of evaporation were ideal for the formation of
thick evaporite deposits.
During the Late Triassic and Jurassic periods, Antarc-
tica and Australia, which remained sutured together, began
separating from South America and Africa. Also during this
time, India began rifting from the Gondwana continent and
moved northward.
South America and Africa began rifting apart during
the Jurassic (Figure 22.1b) and the subsequent separation
of these two continents formed a narrow basin where thick
evaporite deposits accumulated from the evaporation of
southern ocean waters (Figure 22.2). During this time, the
eastern end of the Tethys Sea began closing as a result of the
clockwise rotation of Laurasia and the northward movement
of Africa. This narrow Late Jurassic and Cretaceous seaway
between Africa and Europe was the forerunner of the present
Mediterranean Sea.
By the end of the Cretaceous, Australia and Antarctica
had detached from each other, and India had moved into the
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