Many of the rock formations visible around the Valley of the Sun
are very, very old. They
date from nearly 2
billion years ago, from
what geologists call the era.
Prior to that, our planet was a fiery, violent, almost lifeless
ball of rock, finally
having cooled enough
from its formation to
form thin crustal
layers.
Surface rocks developed from a great basin of sediment -- there was
liquid water and oceans
by then -- and life was
just getting started.
Small, simple organisms
lived and died in
hostile seas.
At
various times, until
around 1.4 billion years
ago, these rock
formations buckled up,
molten rock from deeper
in the earth intruded
them, and then they
cooled slowly.
Mountain chains and
valleys had formed, now
part of a continent all
of its own.
Among other places, rocks which form the remains of the old
land mass are to be seen
at
Squaw Peak,
Black Mountain, the
McDowell Mountains,
the
Sierra Estrella, and
the majority of the rock
forming
Camelback Mountain.
Most of its rocks and landscape -- a thickness of over eight miles
-- then eroded away to
flatness!
What we
now call the Pacific
Ocean came and went
across the remnants, and
subsequent highlands did
not rise again until
dinosaurs roamed a
different landscape,
1200 million years later
and on through the , , and periods.
The land crumpled again, and the ,
as we know them, began
their birth to the east
about 75 million years
ago, causing the future
Arizona region to
experience the forces of
strong geological unrest
for the next 25 million
years!
Some of the world's biggest copper deposits are in Arizona because
of all this activity.
(continued below)
Moss on
Precambrian granite
Twenty million years of quiet times then passed, before more
mountain masses began
their violent growth to
the east of the present
day Phoenix area, now
during the period.
In places they became so steep that mud and rock and even
catastrophic landslides
rushed from their
slopes.
The evidence of these is the fragment-laden reddish rocks of
Papago Park, Red
Mountain, and the west
end of
Camelback Mountain.
Great volcanic soon
exploded from the crust
of the earth. Their
thick, layered ash
deposits form part of
the
Usery Mountains and
the famed
Superstition Mountains,
which pierce the skyline
east of Phoenix.
For reasons not yet well understood, large domes of young molten
rock in turn pushed
upwards through the
surface rocks,
throughout the new
continent from northern
Mexico to southern
Canada.
A number of these dome structures are prominent in Arizona. One of
the classic ones,
studied by geologists
from around the world,
is
South Mountain at
the end of Central
Avenue!
Geologists call them
"", and what caused their arrangement is
still a mystery.
All of these previously mentioned formations have now set the stage
for a few final geologic
episodes -- those that
give the Valley its
distinctive look.
(continued below)
Driven
by giant convection
cells in the of the Earth
below, the crust around
us began to rip apart in
a roughly east-west
direction about 20 to 15
million years ago.
In this torn fabric, from Montana to Mexico, great blocks of
crustal rock started to
settle downward and form
vast valleys -- the
Valley of the Sun is
one.
Among the mountains left to stand between them are what we see
today as the
Phoenix Mountains,
the
Sierra Estrella, and
the
McDowell Mountains.
Geologists call this immense zone the .
In Arizona, the edge of the great continental rift is the , north and
east of Phoenix.
Beyond it lay the still relatively undisturbed and flat-lying rocks
of the ,
visible to all in the
magnificence of the
Grand Canyon.
Also,
during this time, deep
fractures let loose
dark, fluid lava flows
we now see along the
Black Canyon Freeway
(I-17) and capping some
of the mesas north of
Phoenix.
faulting continues
today, though its
effects are much
diminished now in the
Phoenix area, making for
low earthquake
potential.
Its activity is stronger, however, to our north in Nevada and Utah.
Finally, the valleys began to fill with layer upon layer of sand,
silt, gravel, and salt
beds, giving them the
flat expansive nature we
see today.
In places, near Luke Air Force Base, for example, these deposits
total about 2 miles in
depth.
And underneath Glendale sits a body of
nearly pure salt
thought to be about 15
cubic miles in volume!
Phoenix,
Scottsdale, Mesa,
and most of the other
cities of the Valley are
sitting on thousands of
feet of fill.
The mountains and around us are
the mere tips of what is
underneath.
Think of digging straight down for a mile or two, through an ocean
of sand and gravel,
before you hit bedrock!
