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:

A web search using key words “” led me to a UNR class (Geology 100: Earthquakes, Volcanoes, and other Natural Disasters) page titled Earthquake Effects, which focussed on the 1995 Hyogo-Ken Nanbu (Kobe) quake. I took two key pieces of information from the page: 1. Wood frame structures, if they lack adequate shear strength, can actually fail more dramatically then adjacent concrete structures; and, 2. Do not, under any circumstance, run outside.

Regarding the first point:

Behind this completely collapsed wood-frame house is a house of reinforced concrete that suffered no structural damage. The number of wood versus masonry buildings that collapsed in Kobe astonished most observers, as wood-frame structures are usually thought to be much better at resisting shear forces. Possibly the concrete house was better-designed and stronger even for its greater weight. The proportionally heavier tile roofs on wooden houses also might have been a factor.

A picture tells of what happens at street level during an earthquake. Trust me, you’re safer inside:

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.

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

The wikipedia entries earthrise, the blue marble, and pale blue dot are pretty good at giving the significance of photos of earth from various NASA missions.

The photographs provided humanity with an epiphany equivalent to revelations such as Darwin’s theory of evolution, the Alvarez team’s discovery of an iridium layer at the K/T boundary (and comet Shoemaker-Levy’s subsequent impact on Jupiter), and Hutton’s observation of deep time, as revealed in the Jedburgh unconformity.

For the first time, we had not merely an abstract understanding of the scale of our planet; rather, we also had photographs that showed the planet in the context of its local environment.

Earthrise

The blue marble

Pale blue dot

NOAA Paleoclimatology slide sets.

Via Highly Allochthonous, a nice explanation of crossbedding.

Mantleplumes.org

The debate about whether plumes exist or not, and what other mechanisms could cause melt anomalies became increasingly popularised in late 2002 and early 2003, as observations conflicting with the plume hypothesis, or unexpectedly failing to confirm it, reached proportions that could no longer be ignored. Journalists began to take notice and write popular articles about the controversy. However, there was almost nothing on the world wide web about the subject. Thus, www.mantleplumes.org was born in March 2003.

The University of California Museum of Paleontology’s pages on foraminifera and radiolaria. I’ve been curious to learn if sampling fauna from core samples has been automated, and how one would go about doing so, and was looking for a little background info.

Time will tell if Kenneth Deffeyes is right in his hypothesis that global oil production peaked in December of 2005. Deffeyes:

February 11, 2006

In the January 2004 Current Events on this web site, I predicted that world oil production would peak on Thanksgiving Day, November 24, 2005. In hindsight, that prediction was in error by three weeks. An update using the 2005 data shows that we passed the peak on December 16, 2005.

Me, just to be safe, I’m taking my grand auto-tour of the continent this summer. Headlines today show oil prices spiking again due to Nigerian revolutionaries declaration of total war on the oil export business. I still don’t completely understand the more alarmist of the peakists’ certainty that the ‘non-negotiable’ way of life is going to evaporate: can’t coal be substituted for oil? Doesn’t high-pressure chemistry make it possible to synthesize what we need — fuel, fertilizer, plastics — from coal? Didn’t Germany and South Africa do just that when faced with a cut-off of their oil supplies, during WWII and the apartheid eras, respectively? Somebody, please, set me straight on this.

Between taking geology this semester and reading of the scathing report issued by Congress on FEMA’s performance during the Hurricane Katrina disaster, I’ve been thinking about earthquake preparedness. Here are a few links on the topic:

• San Francisco’s earthquake preparedness page
• A San Francisco Chronicle article (16 February 2006) House panel rips administration on Katrina response
  BAY AREA CONCERN: Feds can’t respond quickly to big quake
• Firefighting equipment supplies
• San Francisco Neighborhood Emergency Response Team

A couple of articles I’ve found in reading about coal as an alternative to oil:

Mine tragedy a reminder of coal’s role

More than half this country’s electricity is supplied by coal. About 130 new coal-fired power plants are on the drawing boards for the next few years, and that could be just the beginning.

With the price of power sharply higher, the U.S. - long known as the Saudi Arabia of coal - is likely to be relying on it for generations to come.

Ultra-clean fuels from coal liquefaction: China about to launch big projects - Brief Article

Pending final government approvals, Shenhua Group — China’s largest coal producer — just announced it aims to build a 50,000 barrels/day refinery to make ultra-low sulfur diesel and gasoline from direct coal liquefaction.

The $2 billion plant, to be built adjacent to coal mines at Majata, Inner Mongolia, will use coal liquefaction technology developed by U.S.-based Hydrocarbon Technologies Inc., (HTI) a division of coal-synfuels developer, Headwaters.

HTI developed the process in part with U.S. Department of Energy “clean-coal” liquefaction research in recent years, HTI president L.K. Lee told us in an interview. Shenhua spent the last five years evaluating technology options from vendors and conducting feasibility studies, before signing technology licenses with HTI last week.

Assuming that the Chinese government grants final approval, construction of Shenhua’s first reactor train would start in early 2003, followed by plant start-up in 2005. A total of three licensed reactor trains would process about 12,800 tons/day of the local coal.

Petroleum and Coal

At the time this text was written, coal was the most cost-efficient fuel for heating. The cost of coal delivered to the Purdue University physical plant was $1.41 per million kJ of heating energy. The equivalent cost for natural gas would have been $5.22 and #2 fuel oil would have cost $7.34. Although coal is cheaper than natural gas and oil, it is more difficult to handle. As a result, there has been a long history of efforts to turn coal into either a gaseous or a liquid fuel.

[…]

The first step toward making liquid fuels from coal involves the manufacture of synthesis gas (CO and H2) from coal. In 1925, Franz Fischer and Hans Tropsch developed a catalyst that converted CO and H2 at 1 atm and 250 to 300C into liquid hydrocarbons. By 1941, Fischer-Tropsch plants produced 740,000 tons of petroleum products per year in Germany.

Fischer-Tropsch technology is based on a complex series of reactions that use H2 to reduce CO to CH2 groups linked to form long-chain hydrocarbons.

You decide what it all means:

Source: Dr. C.J. Campbell, Petroconsultants

Franciscan Cherts of Marin, a page from the Marin College’s To See a World project, looks like it offers a good exploration of the geology of the Marin Headlands (excerpted below).

It is part of the To See a World project’s “Geology of California’s Golden Gate: A Virtual Field Trip” site.

Of particular interest to me is the Baker Beach field trip, given that I live nearby.

These rocks, like the pillow lavas they cover, clearly formed underwater, definitely in very deep water, possibly at the Equator and during the age of dinosaurs (Marin’s wet Jurassic Park?). This is an image of an exposure of these ribbon cherts in the Marin Headlands. A example of a modern environment in which the majority of Marin’s cherts were formed is seen in this seismic profile collected during the voyage of the USNS Kane in 1968.

Microscopic radiolarian skeletons, are today, forming extensive sediments in the Equatorial regions - they seem to like warm water and thrive in the nutrient rich Equatorial upwelling region. Radiolarians are protozoans which form skeletons of glass (SiO2). Their skeletons are quite well preserved within the hard chert, even though there has been extensive alterations of the originally soft oozes they comprise. This scanning electron micrograph of a radiolarian actually comes from similar-aged limestones in the San Andreas Fault zone near Olema.

Some cherts resting immediately upon the pillows, have been significantly metamorphosed by hot seawater and converted into “pretty chert” or “jasper”. This photo shows a thermal front within chert in close proximity

to highly altered pillow lavas. The cherts close to the pillow lavas commonly have significant accumulations of metallic oxides within them, commonly manganese within Marin, but some small amounts of copper have been mined from the western slopes of Mt. Tamalpais. This photo shows interbedded manganese and chert from Red Rock Island in San Francisco Bay. All of these metallic deposits are likely ancient “black smoker” deposits, like those reported photographed from the East Pacific Rise where it enters the Gulf of California (see pillow lava page) and in the Gorda Ridge off of Northern California and Oregon.

While Ring Mountain it self does not have any good chert exposures, there happens to be abundant chert on the top of Ring Mountain. This chert was imported for the construction of the Nike missle site that occupied the ridge during early parts of the cold war.

Excellent…I’d actually like to take one of the “Field courses that feature the geology of the Pt Reyes Peninsula
and Eastern Marin … offered regularly by the College of Marin.”

USGS table of epochs.

Ah, disaster: it’s so much fun. I’ve long had an interest in geology, and that led in turn to related areas, such as glaciology, paleoclimatology, and the like.

Vredefort dome, South Africa
Space shuttle image STS51I-33-56AA

I just watched the movie, The Day After Tomorrow, and following that I was browsing the net, checking out what has been found in the attempt to piece together the transition from the last ice age to this most recent of interregnums, our warm period, the Holocene. And of course one finds on the net plenty of scientific information, but also a plethora of wacky obssession with doomsday scenarios. It’s a curious propensity, this fascination with events approaching from the periphery of human consciousness and memory. Shadows from beneath the bed.

The biggest local cosmic train wreck so far? The Vredefort dome.

Where else would one find descriptions as colorful as this (from the link above):

This ring of hills comprises quartz conglomerates as found in the gold-bearing strata of the Witwatersrand reefs.

Village, Vredefort dome, South Africa

Village stones, Vredefort dome, South Africa

Photos from
Hartebeesthoek Radio Astronomy Observatory

The white quartz pebbles are evident. This was once the bed of a fast flowing water course which
deposited grains of quartz and the pebbles. This area was mined for gold in the 1880’s. However the concentration of gold was much poorer than at Johannesburg, and the diggings were soon abandoned. Old mine adits are still to be seen in the hills. This is the Amazon Reef.

The outermost ring of hills was home to a quite different group of people in the 1500’s to 1700’s. These were SeSotho/SeTswana-speaking farmers. This village at Askoppies was a defensive position on the crest of the hill, but it did not save the village from destruction, by the warriors of Mzilikazi. The view shown above left looks east, back in towards the inner rings of the Vredefort dome.

The stone walls of the village are shown above right. They are made of the fine-grained grey Ventersdorp lavas that comprise this ridge. These rocks are 2700 million years old.

of illustrative climate-change related charts is here (I’ll try to include attributions later, but most are from the excellent American Scientist article cited in the graph caption, below).

My favorite of these? The chart that shows anthropogenic induced change in the atmospheric concentration of C02. The two graphs shown are derived from an ice core from Vostok, Antarctica. The top chart is of deuterium, a proxy for temperature. The bottom is of CO2, parts per million. The time scale at the bottom is in thousands of years before the present.

Vostok Ice Core, Antarctica, 0 to 160,000 years before present
Top chart is deuterium (per mil).
Bottom chart is C02 (parts per million by volume).

Graphic from “Rapid Climate Change,” Kendrick Taylor,
American Scientist, July-August, 1999.

The Holocene is the 10,000 or so year-long warm period that we’re living in now. The cold stretch of 125,000 years prior to the Holocene is the most recent Pleistocene era glaciation, know as the Wisconsin ice age in North America. Just before the Wisconsinin is the Eemian, another brief warm spell like our own; and before that lies an earlier glacial period (but still lying within the 2 million year extent of the Pleistocene). The red spike on the left side, bottom chart is the increase in atmospheric C02 caused by humans in the industrial era, now 150 years old.

I’ve been reading a little more about the Younger Dryas, which was an abrupt state change in climate, probably global in scale, from warm back to cold, lasting for about 1300 years, and ending 11640 years before present. This AGU page describes the event:

The Younger Dryas (YD) was the most significant rapid climate change event that occurred during the last deglaciation of the North Atlantic region. Previous ice core studies have focused on the abrupt termination of this event [ Dansgaard et al., 1989] because this transition marks the end of the last major climate reorganization during the deglaciation. Most recently the YD has been redated–using precision, subannually resolved, multivariate measurements from the GISP2 core–as an event of 1300 +/- 70 years duration that terminated abruptly, as evidenced by an 7C rise in temperature and a twofold increase in accumulation rate, at 11.64 kyr BP [ Alley et al., 1993] (Figure 2). The transition into the Preboreal (PB), the PB/YD transition, and the YD/Holocene transition were all remarkably fast, each occurring over a period of a decade or so [ Alley et al., 1993]. Fluctuations in the electrical conductivity of GISP2 ice on the scale of <5-20 years have been used to reveal rapid changes in the dust content of the atmosphere during the same periods and throughout the last glacial [ Taylor et al., 1993b]. These rapid changes appear to reflect a type of “flickering” between preferred states of the atmosphere [ Taylor et al., 1993b], which provides a new view of climate change. Holocene climates are by comparison stable and warm.