Geologic processes at work on a small coastal lagoon on the San Andreas
Fault near San Francisco, California
Long, long ago--about 200 million years ago--almost all the land on earth was clustered together in a giant supercontinent that geologists call Pangaea (pronounced pan-g e-a). Surrounding Pangaea was a single ocean covering the rest of the earth. Two hundred million years ago is not long in geologic terms --about 95% of the earth's history had already happened by then.
At the time of Pangaea, there was no Bolinas Lagoon. None of the land surrounding the lagoon was here either. There was only deep water. This story tells how the land got here over millions of years, much of it from hundreds or thousands of miles away.
The lagoon is very young, geologically. Many things had to happen before it could come into existence. The story begins with plate tectonics: sea floor spreading, moving plates, subduction, and plate collisions. It continues with other processes: rocks being formed and eroded away, earthquakes, sea level changes, land being uplifted and subsiding. The last chapter is about how sand and silt first created the lagoon and then immediately started to destroy it.
The story will unfold in the following steps:
- Land coming in from the sea extends the edge of North America
- The rocks wear down and create new rocks
- The plates start sliding past each other
- The Point Reyes peninsula moves north
- The rest of Marin County forms a different way
- Land moves along the San Andreas fault
- Sea level rises and falls
- Finally, the Lagoon
- Human settlement has its effects
- The Future
2 - The land on the plates is also constantly changing. Some parts are rising as others subside. At the same time rock is eroding off the high places and filling the low places. Sea level falls and rises with the growth and melting of glaciers, revealing and covering dry land. Volcanos erupt and create new land on the continents and under the sea.
3 - The plates move very slowly, but there has been enough time in earth history for very small movements to add up to big changes. For example, Los Angeles is currently moving toward San Francisco about two inches per year. At that rate, it would have traveled about 300 miles in the last 10 million years. If that rate continues, Los Angeles will become a suburb of San Francisco in another 10 million years.
4 - The movement of the plates and the rise and fall of land is not actually continuous. Most of it occurs in small jumps in different places at different times. Los Angeles is actually moving toward San Francisco at an average rate of two inches per year. But during the San Francisco Earthquake of 1906, one of those small jumps, the west side of Bolinas Lagoon moved 13 feet north of the east side in 45 seconds.
What follows is an outline of the processes and events that formed the landscape we see today. Geologists generally agree on the big picture, but they argue about the specifics and new theories are constantly being formulated to provide better answers. For some questions, there simply are no answers yet.
In one place after another, over millions of years, the crust of Pangaea cracked open. Some of these cracks, or "rifts," developed into "spreading centers" when lava from deep in the earth pushed up to the surface. The lava came from the partially molten layer under the crust called the "upper mantle." It cooled and hardened into new crust, which then cracked and moved away from the spreading center as more lava came to the surface. This created two plates moving away from the spreading center in opposite directions. Eventually, ocean waters spilled into the rift and covered the new crust.
In this way, Pangaea broke up into many plates. The North
American Plate separated from the Eurasian and African Plates at a
mid-Atlantic spreading center. New crust created at the spreading center
became the floor of the Atlantic Ocean. As the Atlantic got wider, the
North American Plate was pushed slowly westward. That movement is
continuing today.
North America collides with the Pacific and the ocean floor goes under
As it moved west, the North American Plate met an eastward moving
plate created at a spreading center in the Pacific Ocean. This plate was
called the Farallon Plate (although the Farallon Islands were never part
of it). The Farallon Plate was ocean crust, composed of dense lava from
the mantle. The North American Plate was mainly continental crust, with a
large proportion of light granitic rock. Where the two plates met, the
edge of the heavier Farallon Plate dove under the North American Plate and
down into the mantle in a process called "subduction."
A trench thousands of feet deep developed where the Farallon Plate
sank into the mantle. Sand and mud eroded from the North American
continent was deposited in the trench and eventually compressed into rock.
Some of this rock was scraped onto the edge of North America as the
oceanic plate slid under it, and some went down into the mantle with the
subducting Farallon plate.
When the subducting slab of the Farallon Plate reached the hot
mantle, some of the ocean floor melted and mixed with continental rock to
become molten magma. The magma rose up through the North American Plate
like a solid bubble. It erupted as volcanic lava in places where it
reached the surface, and solidified as granite where it cooled
underground. The granite of the Sierra Nevada formed from magma during the
subduction process. Today, the same process is building volcanic mountains
in the Cascades of Oregon and Washington and in the Andes. Mt. St. Helens
is one of those volcanos. As part of the magma cools slowly deep under
ground, the subduction process is also creating granite that may only
appear on the surface of the earth millions
All of this activity reduced the amount of estuarine habitat as mud flats
changed to salt marsh and salt marsh to meadows. The lagoon is much
shallower today than 150 years ago. If sedimentation continues at the
current rate, there may not be enough tidal circulation to keep the
channel open to the sea. In the absence of a major earthquake to lower the
floor of the lagoon, the channel could close intermittently by 2040.
The 1960s saw a great burst of development proposals that would
have ended any possibility of preserving the lagoon as a rich estuary
habitat. The plans included making Highway 1 into a freeway, dredging a
small boat marina, and piling fill on Kent Island to create land for
hotels, stores, and houses. A four-lane highway was proposed for the top
of Bolinas Ridge. These plans were defeated only with great effort by the
founders of Audubon Canyon Ranch and many others.
The Future
Today, many people are working to keep Bolinas Lagoon alive. A
task force that includes all the government agencies with jurisdiction
over the lagoon is developing a management plan. The Army Corps of
Engineers is conducting studies. The main question is whether and how to
use dredging to extend the life of the lagoon, without causing too much
damage to the habitat that is so important to egrets, herons and other
shore birds, pelicans, harbor seals, fish, and to all the small creatures
that make the lagoon such a rich feeding ground for them.
Diana King may be reached via e-mail: Diana King: kinghicks@sprintmail.com