We recently revisited the Coast Range Ophiolite, which is found on Mount Diablo in Contra Costa County.

An ophiolite is a section of oceanic crustal rocks, a sequence that typifies ocean crust wherever it exists. One might be led, then, to ask what ocean crust is doing on Mount Diablo (40 km outside of San Francisco), 300 meters or so above sea level. Good question. It happens that when a terrane docks with a continent some of the ocean floor is often included in the suture. This particular parcel of ocean floor was formed about 165 million years ago, in the mid to late Jurassic. Subsequently it was buried under 10,000 meters of sediment, and then was faulted to the surface as the Franciscan subduction (~145 - 30 million years before present) of the Farallon plate came to a close, and the right lateral faulting associated with the San Andreas system began.

A cross section of sea floor is typically ordered like this, from top down: chert, pillow lavas, sheeted dikes, gabbro, peridotite. One can see in this set of photos chert, pillow lavas, a diabase (gabbro) quarry, and a manzanita ‘barrens’ (which usually grow on serpentine sourced soils; serpentinite being hydrothermally altered peridotite).

Clicking on any of the images below takes you to a set of images for that day, and away from this web log.

Chert:

Chert (closeup):

Chert (even closer):

Pillow lava:

Diabase (Gabbro) quarry (the next photo is from an earlier visit):

Manzanita barrens:

Over spring break we visited Point Reyes, and happened upon Drakes Beach just as a front was clearing. After Drakes Beach, we went to the light house, and we finished the day at Chimney Rock. Clicking on the photos below brings you to a contact sheet slide for that day.

We visited Fort Funston on a beautiful, warm February day (trust me, this locale is never this warm — summer or winter), and above is a panorama taken from the cliff top. The promontory peeking out from behind the cliff in the photo’s the far right and left sides is Point San Pedro, which is the seaward foot of Montara Mountain. Mt. Tamalpais, in Marin county, is in the photo’s center. Barely visible to the left of Mt. Tamalpais is Point Reyes National Seashore. The San Andreas Fault runs from the gap between Mt. Tam and Pt. Reyes offshore to Mussel Rock, which is visible at the foot of Montara Mountain (it’s the dark sea stack visible to the right of the apex of the shoreline’s bend). San Francisco is visible just to the right of the large, sandy, grassy hummock in the photo’s center. Fort Funston is located in San Francisco’s southwestern most corner, and is part of the Golden Gate National Recreation Area.

The recent spate of winter storms in Northern California brought to mind photos I took at Baker Beach three winters ago (below). In the first photo one can also make out the lighthouse at Point Bonita in the Marin headlands on the image’s left side, just above the wave. If I recall correctly, this was an exceptionally late storm, occurring as it did in June, when the storm door has typically long slammed shut, and the Pacific high has begun blocking the march of mid-latitude storms for duration of the summer.

For weather, tide, and wave information at Ocean Beach: Ocean Beach United States Surf Forecast and Surf Reports from Surf-forecast.com.

San Francisco Bight Coastal Processes Study.

The USGS is conducting a study that documents and analyzes the processes that control the sand transport and sedimentation patterns of Ocean Beach, a National Park site within the Golden Gate National Recreation Area. This area encompasses a complicated coastal setting that is impacted by the tidal influence of San Francisco Bay, as well as the southwest and northwest Pacific swell. High-energy conditions at this site have restricted comprehensive field surveys in the past, but recent innovations in field techniques now make it possible to perform detailed analysis of the physical processes operating on high energy coastlines, such as Ocean Beach.

Ocean Beach USGS webcam.

Wandering around also brought me to this USGS page on the 1700 Cascadia subduction zone earthquake and tsunami, The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America.

Two storms, one with six meter waves (as measured at the San Francisco Buoy), removed about a meter of sand from the beach at Fort Funston in the span of ten days. Don’t worry — this happens every year, and then the gentle waves (comparatively gentle) of summer put the sand back by season’s end.

By the way, the blackness of the beach in these photos has nothing to do with the Cosco Busan oil spill of about a month ago — what you are seeing are magnetite grains that have been sorted to the surface of the beach by the storm.

The presence of magnetite, along with the absence of a local source rock, plus the medium grained sand at Ocean Beach, of which Fort Funston is a southern extension, indicates that this sand is relatively young. Curiously, there is also no local riverine source for this sand, so one is naturally led to ask of its origin.

The San Joaquin / Sacramento rivers, which drain the 700 km length of the Sierra Nevada, do not empty sediment into the ocean at the Golden Gate, as one might think. Instead, they drop their sediment load near Sacramento into one of the world’s rare inland deltas, the product of a sea level rise 8,000 to 15,000 years ago, at the end of the last ice age.

So what explains the mystery of the sand field at Ocean Beach? I say sand field, because Ocean Beach is actually in the middle of one of the largest sand fields on Earth. It extends 40 km east to west, from the continental shelf near the Farallone Islands (which lie about 35 km due west of San Francisco) to half way across the city of San Francisco.

This vast dune field is the product of glaciation during the last ice age, and was deposited when the San Joaquin / Sacramento rivers reached a shoreline that 18,000 years ago lay near the Farllones. Sea level was 100 meters lower than today, and the rivers were swollen with snow melt, and carried a much larger burden of sediment that they deposited on the then exposed continental shelf west of the Golden Gate. The sand, not having a local source river or rock, is termed ‘relict’ sand by geologists. One can discern this entire story by simply looking with a hand lens at the sand grains themselves — each beach reveals the story of its life and travels through its sand.

A footnote: I was reading in Wikipedia about magnetite, and via a footnote discovered an article titled Ferrous Nonsnotus by Bob Moriarty. Ok, 25 cubic km of 10% magnetite bearing sand is impressive, but what got me were the 2000 meter sand dunes — that’s just insane.

We visited Kirby Cove on 23 November, three weeks after the Cosco Busan struck the Bay Bridge on 7 November. There was some oil on the west end of the beach, but otherwise it looked clean. Some of the shallower tidepool areas looked scoured of life, but I’m not certain of that without checking earlier photos. Deeper tidepool areas (we were there right at an exceptionally low -1.7 meter tide) still had plenty of anemones, feather boa kelp, and other organisms. The spill happened at neap tide, so the lowest tidal areas probably escaped exposure to the oil.

Kirby Cove lies west of the Golden Gate bridge on the north (Marin) side of the strait. One can see downtown San Francisco, the San Francisco Bay, the entire length of the northern, strait facing shore of San Francisco, and the Pacific ocean from it. It is part of the Golden Gate National Recreation Area, and as such is managed by the National Park Service.

Clicking on one of the images below takes you away from this blog to images for that day.

Visible on Rodeo beach in the photos below are a dozen or so hazardous materials suit clad workers (see enlargement, below), who are nearly indistinguishable from the tens of white bags of oiled beach material that line the north end of the beach’s high tide line.

On November 7th, a container ship, the Cosco Busan, struck a glancing blow to one of the San Francisco Bay Bridge’s support fenders. The bridge was unaffected, but the Cosco Busan suffered a 40 meter gash 4 meters above the water line, which ruptured fuel tanks containing viscous bunker oil. Within an hour or so 220,000 liters had spilled into the bay, and within a day or two the oil had reached as far north as Bolinas lagoon (the mouth of which lies just below the northern most ridge visible in the bottom photo), and as far south as Hayward, a distance spanning 80 kilometers.

Kirby Cove, photographed the week before and published in this blog’s previous entry, was heavily oiled, and presumably tidal zone organisms were affected. The bottom photo was taken from a point just north of the lighthouse at Point Bonita (the light house is located on the Golden Gate’s northern, ocean side), and Kirby Cove is about 3 kilometers closer to San Francisco Bay on the same side of the Gate.

All of the coast visible in the picture below was affected by the oil spill, but the only 5% (or less) was actively being cleaned up, as is visible by looking at the coast in the other photos from this set.

Clicking the photo below will take you to the contact sheet for that day (and away from this web log).

Photographs below are of Kirby Cove near a low slack tide (NOAA San Francisco Tide Data). We saw a sail boat tacking, waiting just west of the bridge for slack tide. It was being carried out to sea despite that it was sailing into the gate. In the first picture below, you can see the turbulence of the current in the center of the strait. Impressive.

Equally impressive was the intertidal zone (MBARI’s results for search on intertidal zone) that was exposed. If I were working at Industrial Light and Magic, the giant green anemones (Monterey Bay Aquarium’s Online Field Guide: giant green anemones) would definitely be my model for some sort of alien life form. Totally cool.

Photos 2-5 below show ribbon chert, a sure sign of this particular parcel of sea floor’s 200 million year or so journey from a mid-ocean rift to the trench (now defunct here, but still active in Oregon and Washington) at North America’s Pacific edge.

Kirby Cove is part of the Golden Gate National Recreation Area.

Fort Funston lies on the Pacific Ocean in the southwest corner of San Francisco. I often go there, and photographs from a recent visit follow. An early winter storm had left its mark on every corner of the beach and cliffs; in addition, a nice low tide exposed lovely, reflective expanses.

The image below is a view south from the Marin Headlands towards San Francisco’s western shore. The building perched on the edge of the Golden Gate, near the photo’s center, is the Cliff House, and behind it, to its right in the photo, Ocean Beach stretches south. At the southern end of Ocean Beach lies Fort Funston, and just to the right of Fort Funston’s orange cliff, a mile or two further south, one can see what geologists term a ‘horse’.

This particular horse is a product of movement on the San Andreas fault: one side of the fault has been thrust north, into the ocean. Daly City sits perched unsteadily atop the northern lip of the San Andreas’ rift. I live at the foot of this ridge in the Sunset District of San Francisco, below Daly City and just above the Cliff House in the photo.

The prominent ridge behind the San Andreas’ valley is Montara mountain, part of the Salinian block. The Salinian block is a fragment containing Sierra Nevadan granite that has moved several hundred miles north along the San Andreas.

Click to enlarge

From Strange Maps, an article on the island of California:

Doubts remained, however, and the issue was finally laid to rest only with the expeditions of Juan Bautista de Anza (1774-1776).

It is somehow fitting that California, now home to the entertainment industry in general and Hollywood in particular, itself should be named after a fictional place first mentioned in a novel.