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The next sequence of stops is along Lake Mead Boulevard,
which passes the north face of Frenchman Mountain. Driving east along
the road, one passes progressively younger rocks. The rocks on the
right are Precambrian granite (about 1.7 billion years old). The rocks
on the right, beginning with the white rocks exposed along the slope
of the low hill, are the Cambrian Tapeats Sandstone (about 500 million
years old). The contact between the two units of vastly different
geologic ages is known as the Great Unconformity. |
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The Vishnu Group, below the Great Unconformity, includes
garnet-biotite schists and granites. The granites here have a dark
surface stain from a weathering processes that creates a coating called
desert varnish. The schists originally were sedimentary rocks that
were buried under other sediments and then metamorphosed deep in the
crust along what was the southwest margin of North America about 1.7
billion years ago. Extreme heating of the crust led to partial melting
of the schists to form the granite bodies. This is illustrated by
several pathways in the rock cycle, below. |
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In the rock cycle, sediments that are deposited along
the continental margin may be buried to depths of thousands of feet
below the earth's surface (Pathway A). It takes tens of millions of
years for sediments to move from the surface to such depths. If the
continental margin is deformed by the squeezing action that occurs
when plates converge, the sedimentary rock may be metamorphosed into
schist or similar rocks (Pathway B). When subsurface temperatures
exceed the melting points of minerals in the rock, magma is generated
(Pathway C). Cooling of magma results in crystallization, producing
igneous rock like granite (Pathway D). Generation of magmas and cooling
of large molten masses is estimated to take tens to hundreds of thousands
of years. |
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The Great Unconformity, the contact between the Vishnu
granites and the overlying Tapeats Sandstone, represents an erosion
surface that was covered by the sand deposits. During a time of exposure,
the granites weathered to form a soil-like residue. Granite fragments
have been reworked into the overlying sandstone. The granites must
have formed deep in the crust (as much as 10 miles deep) in order
for the magmas to have cooled slowly enough to form its coarse crystals.
Then the rock had to be moved to the surface by gradual uplift (long
before the uplift that formed Frenchman Mountian). This happens as
mountains are eroded and the "roots" or basement rocks become exposed.
Apparently, this process took over one billion years, the difference
between the age of the granites and the age of the sandstone. |
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The Great Unconformity is also illustrated by the rock
cycle. First, the deeply burried granites were uplifted and exposed
at the surface (Pathway E). Weathering of bedrock releases mineral
sediments that are removed by erosion and sediment transport via water,
wind or ice (Pathway F). The sandstone is composed of quartz grains
that were derived from the weathering of granite elsewhere and carried
to the site of deposition. Sand from buried river or beach deposits
may be transformed into hard sandstone by compaction and cementation
during burial (Pathway G). For example, the Mississippi River has
been depositing sand along the Gulf Coast for tens of thousands of
years. Some of the sands are hundreds of feet deep, but not deep enough
or old enough to have become hard sandstones. However, oil exploration
along the Gulf Coast has revealed over 20,000 feet of older sands
and muds that were deposited over the past 100 million years or more,
and are now solid rock. |
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