Download Geologic Resources at Golden Gate

Geologic Resources
National Park Service
U.S. Department of the Interior
Golden Gate National Recreation Area
The Golden Gate National Recreation Area is truly a park living on the edge, located on the far western end of the North
American tectonic plate and flanking the Pacific plate. Whether formed from the interaction of ocean crust and mantle
rocks under heat and extreme pressure like serpentine, or built by millions of microscopic sea creatures under pressure
and over time like radiolarian chert, our park has anything a rock lover could want.
Geologic Thrusts from the Past
The rocks that compose the geologic basement, or bedrock,
of the Bay Area were formed over 80 million years ago. They
are part of the Franciscan Complex—ocean floor material
erupted and deposited during a 100 million year journey
from a mid-ocean ridge to the edge of the North American
continent and a subduction zone. These rocks were greatly
deformed and partly metamorphosed as the ocean floor was
thrust under the western edge of North American. Some
material was scraped off the subducting plate to form a mélange
or mixture of blocks of serpentine, oceanic crust (basalt and
greenstone), open ocean sediments (chert and limestone),
and submarine landslide deposits (graywacke sandstone) shed
from North America. Today’s landscape around the Golden
Gate is the result of the more easily eroded sandstone and shale
forming valleys, and the blocks of more resistant rocks forming
prominent outcrops on the ridge lines.
Slip Sliding Away
Today, the Golden Gate is located in a seismically active area
along a transform fault plate boundary. About 28 million years
ago the Pacific plate started slowly creeping northward past
the North American plate instead of moving under it. The
San Andreas is the major transform fault in the area, but many
smaller faults also exist. In the park, the San Andreas Fault
extends northwest from Phleger Estate to near Fort Funston,
where it goes offshore and then runs through Bolinas Lagoon
and Tomales Bay. On Average it moves about one inch a year.
This movement is expressed as a violent earthquake occurring
here about once a century. Many earthquakes of lesser
magnitude occur continually along the length of the fault.
Movements on the fault created a “pull-apart basin” in which
the Merced Formation underlying Fort Funston was deposited.
The fault also may have resulted in a 130° clockwise rotation of
the block of rock forming the Marin Headlands, creating the
gaping passageway that is now known as the Golden Gate Strait.
A right bend in the San Andreas offshore of Fort Funston forms a
hole in which the Merced Formation was deposited.
Golden Gate includes six terranes of the Franciscan Complex
including the rotated Marin Headlands terrane block.
Brand New to Really Old
Talking geologic time is hard for most people because brand
new is a few thousand years old in geology! Golden Gate has
a wide variety of rock ages; from the Franciscan Complex,
representing events that took place over a hundred million of
years ago; to the Colma Formation, which tells of a time just
a hundred thousand years ago when San Francisco was an
island; to sand dunes only a few thousand years old containing
sand grains that migrated from the Sierra Nevada Mountains
down the Sacramento River to the coast. In fact, Golden Gate’s
diverse geology has played an important role in the history of
geology; from Berkeley professor Andrew Lawson’s delineating
the San Andreas fault in the late 1800s, priming the world for
plate tectonics; to our Franciscan rocks helping provide the first
real understanding of subduction tectonics in the 1980s.
Ranger Will Elder assists students from the teacher credential program at San Francisco State University.
Park staff in the Interpretive Division have done a great job of sharing our park’s exciting geologic resources with the
public. Whether visitors are downloading rock field guides, or Bay Area middle school students are making a field trip to
the Marin Headlands, the story in the stone is being told. Many employees may be surprised to know that there are some
amazing fossils in the park that are just now being inventoried. Since rocks are the foundation for all of the vegetation and
wildlife that live upon them, it benefits all park staff to have a basic understanding of the earth beneath our feet!
Whether studying serpentine sites for restoration potential or researching fossil clams at Fort Funston, staff are learning more about the park’s geology.
Franciscan Field Guide
Our Park has a downloadable field guide to the geology of the
Golden Gate Headlands on both sides of the bridge. Rock
hounds love this park because of the truly bizarre geology
found here. The Presidio showcases our green and scaly
state rock serpentinite, which is found only in areas where
oceanic crust is subducted and then pushed up again along
fault zones. Serpentinite is high in heavy metals and naturally
occurring asbestos. Also mind-boggling is a rock of biological
origin in the Marin Headlands, radiolarian chert. This chert
is composed of innumerable skeletons of zooplankton that
Rocks on the Move and PARK Teachers
Rocks on the Move provides an opportunity for Bay Area
middle school classes to hike through underwater volcanoes
that erupted millions of years ago. Students study the global
geologic forces that formed today’s dramatic Bay Area
landscape, and then come to the Marin Headlands to conduct
a geologic investigation to unveil how these global forces are
recorded in local rocks. Activities challenge the students to
use observational and critical-thinking skills to speculate
about possible future geologic developments.
slowly rained down to the ocean floor over eons. Basalts of the
Franciscan erupted from volcanoes at mid-ocean ridges and
were altered to greenstone, and the underwater landslides that
deposited graywacke sandstone carried sand, silt, rock fragments,
and plant and animal debris from the continental margin into
deep ocean environments. Take some time to hike parts of Baker
Beach or Rodeo Beach to really see these fascinating rocks up
close. Learn more about the Park’s geology at:
www.nps.gov/prsf/naturescience/geologicformations.htm
The PARK Teachers geology program brings teacher candidates
to the park to experience place-based inquiry learning. It is
accompanied by a web site that helps teachers understand plate
tectonics and the Franciscan Complex of the San Francisco
Bay Area. The content includes classroom strategies, specific
lesson plans and materials, an interactive game of geology, and
downloadable teaching tools, guidebooks and posters. If you
know teachers or classes that would benefit from our park’s
educational resources, get the word out there!
www.nps.gov/goga/forteachers/
Paleontological Resource Inventory
Fossils have taught us much of what we know about the
history of life on Earth. Shells, bones or leaves reveal when
and where organisms lived and died, new species arose, and
changes in climate occurred. On a grander scale, fossils tell us
about the age of the rocks they are in, the movement of land
masses and the formation of mountains and seas. The San
Francisco Area Inventory and Monitoring Network recently
completed a Paleontological Resource Inventory of the park
that has identified a number of intriguing facts. First, the tropical
radiolarian species found in the Headlands chert probably formed
in an equatorial region west of Mexico 100-200 million years ago
and then was faulted northward to the current location. Also
the Merced Formation of Fort Funston contains the bones and
foot prints of large mega fauna of past ice ages such as woolly
mammoth, giant ground sloth, mastodon, early horse, and camel.
http://science.nature.nps.gov/im/units/sfan/reports/
Executive%20Briefings/Paleo_Inventory_Executive_Briefing.pdf