Mussel Rock
Encyclopedia
Mussel Rock is a rock formation
Rock formations in the United States
The following is a partial list of rock formations in the United States, by state:-Arizona:*Canyon de Chelly National Monument**Spider Rock*Chiricahua National Monument**Duck on a Rock**Organ Pipe**Mushroom Rock**Sea Captain*Monument Valley...

 on the coast of San Mateo County, California
San Mateo County, California
San Mateo County is a county located in the San Francisco Bay Area of the U.S. state of California. It covers most of the San Francisco Peninsula just south of San Francisco, and north of Santa Clara County. San Francisco International Airport is located at the northern end of the county, and...

, offshore from Daly City
Daly City, California
Daly City is the largest city in San Mateo County, California, United States, with a 2010 population of 101,123. Located immediately south of San Francisco, it is named in honor of businessman and landowner John Daly.-History:...

. It consists of one large and numerous smaller rocks of a type known as a stack
Stack (geology)
A stack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, isolated by erosion. Stacks are formed through processes of coastal geomorphology, which are entirely natural. Time, wind and water are the only factors involved in the...

, where a headland is eroded unevenly, leaving small islands.

It is best known for being the closest point to the epicenter
Epicenter
The epicenter or epicentre is the point on the Earth's surface that is directly above the hypocenter or focus, the point where an earthquake or underground explosion originates...

 of the 1906 San Francisco Earthquake
1906 San Francisco earthquake
The San Francisco earthquake of 1906 was a major earthquake that struck San Francisco, California, and the coast of Northern California at 5:12 a.m. on Wednesday, April 18, 1906. The most widely accepted estimate for the magnitude of the earthquake is a moment magnitude of 7.9; however, other...

, and where the San Andreas Fault
San Andreas Fault
The San Andreas Fault is a continental strike-slip fault that runs a length of roughly through California in the United States. The fault's motion is right-lateral strike-slip...

 enters the San Francisco Peninsula
San Francisco Peninsula
The San Francisco Peninsula is a peninsula in the San Francisco Bay Area that separates the San Francisco Bay from the Pacific Ocean. On its northern tip is the City and County of San Francisco. Its southern base is in Santa Clara County, including the cities of Palo Alto, Los Altos, and Mountain...

 from the northwest. An additional minor fault, the Mussel Rock Fault, was identified in 2000.

The area above Mussel Rock consists of steep cliffs with frequent landslides, threatening homes in a subdivision above. The adjacent area is a city park.

Geology

Mussel Rock is a "greenstone assemblage
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....

" which is part of the Franciscan Complex, which is the bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

 under San Francisco, and was transported to the site by a subducting plate
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...

 about 80 or 90 million years ago. It is older than the nearby sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s which are part of the Merced Formation, which are about 3 million years old.

External links



In his 1993 book, "Assembling California", describing the geology of the state, John McPhee, begins at Mussel Rock. According to McPhee, "Mussel Rock is a horse. As any geologist will tell you, a horse is a displaced rock mass that has been caught between the walls of a fault. This one appears to have got away....Green seas slammed against it and turned white. It was not a small rock. It was like a three-story building,standing in the Pacific, with brown pelicans on the roof....After a five hundred-mile northwesterly drift through southern and central California, this was where the San Andreas Fault intersected the sea."

Source: McPhee, John A. , "Assembling California", Farrar, Strauss and Giroux, New York, 1993

See also

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