Written in Stone San Diego's last
million years
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San Diego is the jewel of southern California. People from all over the
world come here to enjoy the surf
the sun
the culture of southern
California. They come here to do business
they come here to live
and
they come here to retire in a virtual paradise.
Hi, I'm Dr. Pat Abbott, and I'm a geology professor at San Diego State
University
this allows me the great good fortune to live and work right here
in San Diego.
When we begin to think about why people are so attracted to this
place.
What makes it so desirable?
Well, we have a world class zoo
we have a deep-water bay that welcomes
ships of the world
and we have a cultural and ethnic blend that puts energy,
business and life into the community.
But thousands of years ago
long before San Diego ever had a name, people
still came to this place.
They came because it offered an abundance of food
from the sea and the
land
it had a moderate climate. It was a good place to live and, with any
luck, a place to grow old in relative comfort.
But the core of San Diego's success lies with the actual shape of the land
and as geologists, we have pulled back the layers to discover the forces that
created a land with so many attributes?
It is exciting to look around a rich geological terrain like San Diego and
know how it came to be.
How did Mt. Soledad form
Where did Point Loma come from
or Mission
Bay? or the Silver Strand?
Well, we're going to go back into time and speed up the geological time
frame and watch what happens
before there was a Mission Beach
before the
cliffs at Torrey Pines
before Mt. Soledad
before there was even
a
bay
We're going back to when San Diego
was just a beach.
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The landscape of San Diego has been shaped by large-scale processes - sea-
level rise and fall, regional tilting of the land, and shifting and warping
caused by movements of the Rose Canyon fault system.
Changes in the landscape are barely perceptible in any given year, but when
we apply a geologic measurement of say
a million years
the changes
become readily apparent
and this is the backbone of geology
that small
changes
day by day, year by year
viewed over millions of years
reveal major geologic effects.
We can understand the shape of the land in San Diego by dividing the last
million years into 3 acts.
The first act opens with high sea levels retreating westward from a rising
landmass.
In Act 2, the Rose Canyon fault alters the land - it changes the outline of
the coastline
it raises hills
it lowers land which fills with water to
create bays.
And In Act 3, about 11,000 years ago, as glaciers in the Northern Hemisphere
rapidly retreated, the sea level rose, setting the stage to form some of San
Diego's most remarkable terrain.
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We're standing on the top of Mt. Soledad . .. to the west is La Jolla and
the Pacific Ocean
to the south is Mission Bay and Pt. Loma
and at the
base of the
mountain is Interstate 5 where it connects to highway 52.
But if I sweep my eyes northeast to southeast, I can see a broad plain cut
through with valleys running east to west. That's the Linda Vista Mesa.
In the middle of the mesa is the Miramar Marine Air Station surrounded by
thousands of homes, businesses, roads
everything that a human being might
need to survive in today's world.
But a million years ago
it was all under water.
If you know what to look for, you can actually see what we geologists call
the 1 million year old sea cliffs.
They're right here.
And if you want to know what they looked like in their prime, go just off
today's shoreline and look back at the sea cliffs at Torrey Pines State Beach.
Towering cliffs up to three hundred feet tall with the surf pounding at
their base.
But behind the scenes, enormous forces were at work
because today, the
million year sea cliffs are more than 9 miles from the ocean. How did that
happen? How did the sea cliffs and the ocean get that far apart?
Well, several things began to happen at the same time
and remember,
we're talking in geologic time.
The sea floor began to rise
uplift
and as the land began to rise in
the east, the sea,
simultaneously, began to retreat in a westward direction. As each bit of sea
floor, in its turn, became "beach"
the ocean began to scrub and
abrade the sea floor creating a hard platform covered with sand and gravel
several feet thick
that was later cemented together by salts crystallized
from groundwater. The result is that, today, we have nearly flat ground that
makes an excellent natural foundation for building homes, schools, and
businesses.
The uplifted and drained sea floors are dominant features in San Diego
but . what happens to land that has been raised and exposed to the atmosphere?
It meets the great equalizer
Erosion.
Bad for the land
good for us, because it creates the valleys that
provide a different habitat for the plants and animals
opportunities for us.
This is a "rain chamber." And it's located here, in the civil
engineering department at San Diego State University. The reason this particular
machine is here
is to find ways to prevent erosion from damaging
California's roadways. And it does that by eroding different kinds of earth on a
much smaller scale.
Here's how it works.
Sediment mixtures are packed down forming horizontal surfaces in this 12 ft.
by 30 ft. bed. The entire apparatus is tilted and artificial rain is applied
with resultant erosion. In a small scale and in
a short time, it gives clues to how a real area will erode.
Just as the these experiments simulate the larger-scale and longer-time
erosion of any given type of soil, it also demonstrates how the uplifted Linda
Vista Mesa
the ancient sea floor
has been eroded by the runoff of
rainwater to create the landscape we know today.
We don't see it very readily, because the sheer scale of the erosion is so
great, it has actually turned gullies into valleys.
As we drive uphill and downslope traveling about town, it is easy to get
overwhelmed by the scale of the landscape. We lose perspective of the relatively
simple origin of the mesa and valley topography.
But the uplift and erosion aren't the only forces at work here.
As we near the modern coastline
we notice something else has happened
the landscape has been shifted, warped, and dropped.
For instance, if you look at San Clemente Canyon, that's the canyon that
houses Highway 52, you'll notice that as it runs west toward the ocean, it
abruptly
makes a 90o turn to the south-southeast. It then travels down Rose Canyon
into Mission Bay. The land has been so distorted by active faulting
that the
very rivers that cut these canyons were forced to change their courses. They
created new canyons across the regional slope of the land.
They had to change direction
because they met up
with the Rose
Canyon Fault.
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If we go back a million years
to what is now Mexico, to the Baja
California peninsula
we find that the basin of the Gulf of California is
about 20% narrower. Over the millennia, the Gulf of California basin has
continually widened
because the sea-floor has been spreading apart . .. and
a result of that, is
the Pacific Plate carrying San Diego, slides northwest
past the westward moving North American Plate.
Now, In southernmost California we have many active faults
the San
Andreas,
San Jacinto, Elsinore, Rose Canyon, Coronado Islands, the San Clemente
Island fault
and each of these faults has one thing in common
their
western side moves to the northwest
faster than it's eastern side. It's like
two great ships on a collision course. They're both moving
but the one to
the west moves faster and scrapes by North America
western California
all the way down through Baja California
is travelling northwest on the
Pacific plate.
All of these moving faults have major effects on southern California land,
but the one that most affects the topography of San Diego
is the Rose Canyon
Fault system.
It begins along the east side of San Diego Bay and travels north, through La
Jolla, then runs offshore for 65 miles
until the curving coastline causes it to come back onshore where it is then
known as the Newport-Inglewood Fault.
And that end of the fault
the northern end
in 1933
caused a 6.3
magnitude earthquake that still reigns today as the 2nd deadliest earthquake in
California history. We know it as the Long Beach Earthquake.
It killed 120 people and caused damages exceeding 600 million in today's
dollars!
On the plus side, it also was the earthquake that was responsible for
changing construction standards for schools and brick buildings in the state of
California. Fortunately for us, OUR end of the fault has not generated a large
earthquake
at least, not in historic time.
But in GEOLOGIC time
the Rose Canyon Fault has been responsible for
shaping much of the San Diego terrain.
And it's done it in several ways
One way is called "offset."
What I have here is a single layer cake.with a hard crust made of patching
compound.
We push on one side
just the left side
of the cake
the pressure
builds up until
it cracks. It moves.
The left side
the west side
has moved to the north. If this west
side is viewed as a coastline, then the land
the shoreline
has pushed
out into the ocean.
This explains the shape of the coastline at La Jolla. The Rose Canyon Fault
has moved about 4 miles
and pushed the land out into the ocean.
If we remove the million years of movement on the Rose Canyon fault
the
La Jolla coastline straightens out.
Visualize a fault tearing miles deep through the earth. Does it tear in a
single fault? Does it tear only in a single line? Let's follow the trend of the
Rose Canyon Faults on this photo.
Straight along San Diego Bay
straight through Old Town
straight
along Mission Bay
straight up Rose Canyon
but just before reaching
highway 52 and Ardath Road
the faults bend sharply to the left
what
effect does this left bend have on the land?
These sheets of paper represent rock layers. They are cut along a line
a
fault line with a left bend. If we push the left side north
like the Rose
Canyon Fault moves
look what happens at the bend in the fault
the paper
rises
the land rises
this is Mt. Soledad.
And if we keep pushing
the paper also warps down
this is Mission Bay
down below sea level.
Let's deform another cake with "icing" and see what results.
If you look closely at the way the compound broke, you can see that it did
NOT crack in a straight line
In fact, not only is the line crooked, there
are many fractures. What it is doing
is following the inherent weaknesses in
the compound
in the compound we call earth.
The cake has pulled apart at a right bend
the cake has dropped below sea
level
this is San Diego Bay.
As the land drops, it breaks from the stable land to the east
in a
series of down-to-the-bay faults
these are the La Nacion Faults that
separate Otay Mesa from the bay.
Faults crossing the bay from the Rose Canyon Fault to the Descanso Fault
offshore
have lifted up Coronado and North Islands.
So we look backwards, and study what has happened. And as we begin to see a
clearer picture of the processes that created Mt. Soledad, or San Diego Bay or
Coronado
then
we can also begin to appreciate the numbers and sizes of
earthquakes that accompanied these deformations of the earth.
The hills and bays that help make coastal San Diego such a special place are
due to the effects of the active Rose Canyon fault system. Compression on the
west side of the Rose Canyon fault at the fault bend south of Ardath Road has
warped up Mt. Soledad, downwarped Mission Bay, and upwarped Point Loma. Pull
apart in the zone between the ends of the Rose Canyon and Descanso faults has
downdropped San Diego Bay which severed from the land to the east along the La
Nacion fault system. Cross faults in the transfer zone lifted up the land to
make North and Coronado Islands.
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(San Diego)
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This is the San Diego National Bank building. From here, We can see San Diego
Bay with its marine traffic
And it really is a superb bay for its commerce
its recreation
and of course, its beauty.
|
But 120,000 years ago, had this building been here, it would have looked
like this.
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AVI Animation
|
For more than 2 and a half million years
the Earth has been locked in an
Ice Age. In fact, sheets of ice up to 2 miles thick
once covered 25% of
North America
It was very much like Antarctica and Greenland are today.
Where did the ice come from?
And where did it go?
Well, the Earth constantly changes. In geological time, continents drift
and
when the continents drift they may divert warm equatorial ocean water to the
polar latitudes.
Warm seawater evaporates into the atmosphere. That moisture is carried aloft
and crystallizes as snow in northern latitudes
It begins an ice-building
cycle
When Earth's orbit and tilt cause less solar energy to reach us, much
of the winter snow does not melt during the summer
It simply becomes
deeper
and
heavier. So the lower layers are compressed into solid ice.
And because ice is weak
it deforms
and then flows farther across
continents
it is not returned to the ocean.
Simply stated
The evaporated seawater is stored on land
as ice.
Which means
the sea level
drops.
And as Earth's orbit and tilt continue to change
a warming cycle returns
just
the opposite occurs. The ice melts
and the sea level rises.
And this cycle of rise and fall, rise and fall
happens many times
throughout geologic history.
In fact 14,000 years ago, The global sea level was 400 feet
lower.
What did San Diego look like then?
At University of California in San Diego, is one of the largest computers in
the world
a super-computer. It can stack and burn the edges off pieces of
paper to make 3 dimensional models
such as the topography of San Diego
14,000 years ago
when the sea level was 400 feet lower.
People lived here when sea level was much lower
and they knew a very
different place than we know today.
There was no San Diego Bay
no Mission Bay
no Silver Strand. They
have all formed in the last 9,000 years.
The latest retreat of the continental ice sheets occurred rapidly
in
less than 5,000 years. As sea level rose, the San Diego shoreline looked
different
one huge bay
with three islands
Today, Mission Beach is a unique and lively beach community enjoyed by
residents and tourists alike.
But 9,000 years ago
it simply
was not here. Now, Mission Beach was
not created by uplift
and it was not created by the tremendous forces of the
Rose Canyon Fault, even though it lies just 2 and a half miles west of the fault
zone.
Mission Beach was created
one wave at a time
day by day
. year by
year
century by century.
Waves arriving from the North Pacific Ocean hit most West Coast beaches at
an angle. Because they hit at an angle
and because most of the waves came
from the Northwest
they pushed tons of sand southward along the coast. As
the sand collected, it built southward into what we now call
Mission Beach.
It is interesting to note that Mission Beach was actually formed before
there was a Mission Bay. But we'll discuss that a little later.
At the same time Mission Beach was being formed, there were three islands
off the coast of San Diego. And each was formed by uplifts associated with the
Rose Canyon Fault system.
But other natural forces began to factor into the equation
and all three
of these islands would be greatly affected.
These surfers come out here for one reason
to ride the waves. Hard-core
surfers actually study
and know
how global weather patterns affect their
particular part of surfing paradise.
In this instance, they are riding the very forces that created a significant
part of San Diego today.
Over the millennia, as each flood season came and went, the ancient Tijuana
River dumped millions of tons of sand and mud into its delta
which was built
out into the ocean.
During the late summer, hurricanes in the Southern Hemisphere, some as far
away as New Zealand, generated enormous waves
and as they came, they picked
up the sand from the river's mouth and began to redistribute it.
They began to build a barrier.
And as the barrier became larger and longer, over six miles in length, it
connected with two islands .... Coronado
and North
And they became tied to the mainland.
Today, we call this six mile barrier
the Silver Strand
and though we
refer to Coronado Island and North Island, they are now, really part of a
peninsula.
So, while surfers enjoy the late Summer waves on the ocean side of the
beach, we realize that, because of Silver Strand in the South, and Mission Beach
in the North, the coastline is no longer open to the ocean. It is now, for the
first time, a bay
But how did this mega bay get divided in two
with San Diego Bay to the
South and Mission Bay to the North?
And what about the third, and largest, island?
Well, it was positioned just off the coast, opposite the mouth of the
ancient San Diego River. Like the ancient Tijuana River, as each flood season
came and went, the San Diego River dumped enormous amounts of sand and mud into
its ever-expanding delta.
And as the delta grew, it reached farther out into the bay.
A few thousand years ago, it met the island and connected it to the mainland
and it split the mega bay into two smaller bays
to the north is Mission
Bay
and in the south is San Diego Bay.
Today, we call the island
Point Loma. And the land that grew out as the
delta is called the Midway District
which is where I am right now.
Now the ground may appear firm enough, but in actuality, it is made of loose
sand
the river has simply dumped it into the bay. By geological definition,
it is a weak, water saturated mixture but it has become the foundation for some
of San Diego's most widely used structures. Lindbergh Field
the Sports Arena
in fact, all the sewer pipes from San Diego cross this unstable sediment to
reach the sewage treatment plant on Point Loma.
When we have a major earthquake on the Rose Canyon Fault
this property,
the Midway District , will liquefy
and our human-built structures will fail.
No operating Lindbergh Field, no sewage treatment
of course, badly damaged
roads and all the other problems that come with major fault movement.
But the reality is
even if we had a major earthquake
the worst we
could imagine
it would be a simple, chiropractic adjustment in geologic
terms.
Over the millennia, however, these little adjustments cut off the mesas, as
it did to Linda Vista and Otay Mesas
and build a mountain, like Mt. Soledad
and warp land downward to create a Mission Bay.
And the process never stops. The Earth is always shedding its skin.
So lets speed up the last million years and see what happened.
Regional uplift caused the retreating sea to cut Linda Vista Mesa and Otay
Mesas as glacial advances and retreats lowered and raised the sea level many
times.
Movements within the Rose Canyon Fault warped and shifted the land
creating Mt. Soledad, Mission Bay, and the three islands of Coronado, North and
Loma.
Ocean waves from the north pushed sand south to form Mission Beach
just
as Summer waves from the south washed Tijuana River delta sand to the north to
form the Silver Strand that "captured" Coronado and North Islands.
Meanwhile, the San Diego River delta built outward and attached the big
island to the mainland to create Pt. Loma.
So here we are in the NOW.
And knowing what we know about the past, it makes it fun to think about what
the next million years might bring.
Let's take an educated guess.
If present trends continue, Linda Vista and Otay Mesas will continue to
rise. The Rose Canyon Fault will squeeze Mt. Soledad even higher and drop
Mission Bay even lower.
And since the continental ice sheets have been growing and shrinking
melting the ice on Greenland and Antarctica would cause the sea level to rise
200 feet, forming beach front property in Mission Valley and San Ysidro.
Conversely, the sea level will drop 400 feet lower than now several times
emptying San Diego and Mission Bays
leaving the present coastal communities
like high and dry
and making the Coronado Islands part of the mainland.
It is certainly interesting to speculate. But the truth is
every time we
think we have this old world figured out
she
manages to surprise us. And anyway
that's a million years from now. We
might as well go surfing.
May 12, 2000