The San Andreas fault makes a great bend around the city. It leaves its citizens well separated from the real America. On the rim of the Pacific, Los Angeles has become the ultimate laid-back city. The beach-and-body life, the Pasadena coffee culture, the dancing on the highways (ok, that was only in that movie, I know), it makes for an eclectic albeit distinctly non-angelic mix. Here is Hollywood, the ultimate virtual reality show, where every story has a happy ending and where every teenager is beautiful, socially apt, and a certain winner against any evil foreign mass murderer. A British accent is a sure sign of ill intentions: the battle of Bunker Hill is re-fought here every day. This city where our future is being created lives on the stereotypes of the past. Perhaps brash New York is, in a way, more grown up than smooth L.A.
The only volcano in Los Angeles is of course also virtual, made up for that other movie, imaginatively called ‘Volcano’. In the movie, the eruptions comes from the La Brea tar pits, an entirely non-volcanic (but fascinating) feature. The tar pit is named after the ranch which was here, and which in turn was named after the tar pit, leaving a name that is a case study in tautology. But rest assured: Los Angeles may be shaky, it is not volcanic, even if this feature at one point was called Los Volcanes de Brea. (Apparently there used to be geysers of tar here!) The nearest recent volcanic area is Lavic Lake, a 20,000 year old lava field, which is over 100 kilometer away.
But in fact there is one volcano that is closer to L.A. It has been inactive for over 20 million years, is unrecognizable, unimpressive, and inaccessible. It is not something Los Angeles would be proud of, and is unlikely to feature in any Hollywood blockbuster (Return of the Volcano, anyone?). But it has history. Once this was a real Californian volcano: it was going places. This volcano was moving upward in the world.
(Conejo volcano is slightly younger and is even closer to L.A. But let’s not let minor facts get in the way of our story. This is Hollywood, after all.)
It is one of those small towns no one has ever heard off. From Palmdale, there is a choice of two routes to get there. One is the straight and rather boring 138 road, starting from Lancaster, on a wide and featureless plain with ridges distant on both sides. The plain is called the Antelope Valley but this promises too much – the local pronghorn have long been wiped out. A few dreamy Joshua trees provide the only scenery. The second route follows a slower but more pleasant road starting from Palmdale, through a straight narrow valley, just behind one of these ridges. The road crosses from one side of the valley to the other, runs between nice-looking houses and goes past Elizabeth lake which was largely dried up and looked sadly unappealing. The valley narrows to just a few tens of meter in places; the slopes beyond are steep but not cliff-like. After a while you begin to wonder about this peculiar valley. Eventually the road takes you up into the hills, past wonderful trees, before dropping down onto those boring plains. In the distance the fields shimmer blue, a strange colour in this semi-desert. Get closer and the blue turns into acres of solar panels. There is real blue too: the road crosses over a wide, sinuous canal which carries water from distant mountains to thirsty Los Angeles. The canal, better known as the California Aquaduct, runs parallel to the narrow valley, although on the other slope of the ridge, overlooking the plain. Flocks of ducks happily congregate on the canal, but sadly I didn’t see any geese (otherwise I could have used the medieval British term ‘gaggle of geese’ in this post, turning the poor geese into messengers of the evil imperialists). (Strangely, the word ‘gaggle’ was also used for a group of women – but I better stop here or I’ll be at risk of being me-too’d.) (But maybe not in the US where it seems people run for office who in other countries would walk into prison.) (Thinking about it, perhaps Washington is in fact a secret luxury prison so they are running to get incarcerated.) In front of you, on the plain, is the unassuming town of Neenach. And the hills behind you on the other side of the canal, through which you just descended, are the eroded remnants of Neenach volcano.
There isn’t much to see. The area where the Neenach volcanics are found covers 40 km2, west of the Mojave desert, but the only visible remains are a few outcroppings on hills, comprising red-coloured andesite. Being on private land, they can only be viewed from some distance.
The Neenach volcanics have been dated to around 22 million years ago. At that time it would have been a substantial mountain, perhaps only a little smaller than St Helens, close to 3 kilometers tall. The volcanic layers it left behind consist of rhyolite, dacite, andesite, rhyolite agglomerate, and a pumice lapilli-tuff. It is quite a shopping list. The idealized drawing below shows how the layers are build up: this shows a cross section of depth along a line southwest-northeast. The horizontal axis extends about 5 kilometer, and the vertical axis goes down about 2.5 kilometer.
The depth of the layers already show that this volcano was not a one-off midget. It started at the bottom, with an explosive phase, giving the tuff layer. After that, 2 kilometer of flowing rhyolite build up. Above that, a layer of volcanic glass, followed by another tuff layer, a less extensive layer of andesite, finally topped off with a dusting of dacite. What happened?
We can come up with a rough outline of the events. The granite basement wasn’t that old to begin with, perhaps 100 million years. But 23 million years ago, proper history began. It started with a bang, when the pressure from below caused an explosion which spread rock fragments over a wide area. This enlarged the opening sufficiently for the lava to come out. This was rhyolite, forming a slow-moving, viscous lava which builds steep volcanoes. Agung produces this kind of lava. Many of the eruptions may have been explosive. The steep flanks of the volcano regularly collapsed, forming thick landslides. Some entered into the adjacent ocean, forming turbidity flows which extended the breccia layers further. Towards the end of this phase, the eruptions became smaller. The thinner layers allowed the rhyolite to cool faster: if this happens it can’t form crystals and solidifies into obsidian. If the cooling happens under water, the obsidian becomes perlite, and this formed the layer on top of the breccia. And finally, there was another explosive phase forming a tuff layer.
But now the volcano had another lease of life. Andesite began to erupt: this formed when some melted crust material was added to the rhyolitic magma chamber. The andesite eruptions covered a smaller area, perhaps because it was less prone to flank collapses. The andesite eruptions were followed by dacite, which is intermediate between andesite and rhyolite. It seems that the supply of melted crust had been exhausted and had not been replenished.
The final phase saw cooler magma reach the surface. This formed the porphyritic layer, meaning containing crystals. And at this point, the magma supply finally cut off completely.
All the different layers have in common that they lack iron. This composition is typical for subduction zones. During the subduction of the oceanic crust, the mantle wedge located between the crust and the subducted plate starts to melt, and this gives rise to long-lived magma reservoirs. Over time, the magma cools a bit, and this allows iron and magnesium to solidify as they have the highest melting temperatures. It leaves a silicon-rich magma. Rhyolite is most evolved, dacite less and andesite least. In the case of Neenach, the later layer of andesite is actually less evolved than the previous rhyolite. This requires some new melt, which rejuvenated the magma reservoir.
This evidence for subduction rings a bell. The west coast of the US is a mess, where various bits of Pacific plate meet a sticky end underneath the North American plate. The Farallon plate subducted here. The time of the Neenach volcanics is approximately when the old, extinct spreading centre underneath the Pacific began to be subducted. This is the equivalent of a 2 kilometer tall linear mountain, and one can imaging that it was not digested easily! If I am being very speculative, a spreading ridge contains two ridges, on either side of the rift valley which may be 10 kilometer wide. At a subduction rate of 5 cm per year, the two ridges are separated by about two hundred thousand years. Perhaps magma rejuvenation was caused by the second ridge passing by underneath. But this is a complete guess.
So we can understand the Neenach volcanics well enough. The Coneja volcanic centre north of Los Angeles formed similarly, a little later, although with some rotation to add complexity. Both are safely extinct. I read one newspaper from that area where the reporter told a real-estate agent that it had just been discovered that their local Coneja mountain was an old volcano. She responded with ‘After all the disasters we have had, now this!’ I do wonder how she phrased it in the brochure. ‘Potential for redevelopment’, perhaps?
That funny valley
That funny valley I had been driving along turned out to be the scar of California’s nemesis. Unknowingly, I had followed the line of the San Andreas fault along one of its less active segments. It was disconcerting to realize that those expensive looking houses with beautiful yards were located only meters from the most famous fault in the world. At least, none were put on top of it. A later look at the map showed that the path where I had been running earlier that day was also on the San Andreas fault. In fact it followed the fault precisely. It was a bit of a run-down area, apparently used mainly for fly-tipping, and the fault line had not been obvious apart from a small scarp along the track. The San Andreas hasn’t ruptured here since 1857 and in consequence there is little direct indication of the upheavals of the past. Erosion has become the main mover and shaker. California makes it too easy to run in ignorance.
If you are interested, or at risk, http://geology.com/san-andreas-fault/ provides an interactive map of the San Andreas fault. You can find out whether it is your house that is straddling the fault!
Coming from the north, the San Andreas forms a straight line pointing directly at Los Angeles. But near Santa Barbara, it changes its mind, bends eastward and starts to wiggle (faults shouldn’t wiggle but I can find no other word for it). This lasts until Neenach, where it straightens out again, heading towards San Bernardino. There, a second bend takes it further to the east, recovering its linearity at Palm Springs. Near the Salton Sea it gives up. The system is not that old: the main San Andreas fault is no more than 30 million years old, and the southernmost part perhaps only 5 million years. Here, the fault may still be getting organised and perhaps one day it will connect to the Imperial fault running into Mexico, completing the dissection of California.
The Neenach volcano is located just to the east of the San Andreas fault. The breccias and other rocks are confined to that side of the fault. Cross the fault to the Los Angeles side, and they suddenly disappear. It is as if you are jumping into another earth.
The daytime heat is relentless. Summer days of 100F are normal, with a sun blazing from a deep blue sky. Few people are around: it is too hot in the day and too cold at night. There are better times to visit, in autumn or spring. The famous forest of stone pillars can wait.
We are now 315 kilometers north of Neenach, in the Pinnacles National Park. The area is impressive, rather than beautiful. Like Neenach, it is volcanic in origin, and it has approximately the same age. Clearly, they formed in the same general phase of volcanic activity.
In 1976, Matthews pointed out how remarkably similar the two were. The granite basement is identical, and the composition of the breccia is the same. And the various layers match well both in order and thickness.
There is one small difference (apart from the 315 kilometers north by northwest). Where Neenach borders the San Andreas, the Pinnacles are a few miles displaced from the main fault. However, there is another, parallel fault which does terminate the Pinnacles layers just as the San Andreas fault does to Neenach: the Chalone Creek fault. And whereas Neenach lies east of the fault line, the Pinnacles are to the west. The temptation to make the logical jump became irresistible: Neenach and the Pinnacles were two halves of the same volcano. A mountain had been on the move.
At the time, in the 1970’s, the amount of total movement along the San Andreas was disputed. Different numbers were found, depending on which rock formations people assumed came from the same source. The Pinnacles and Neenach formed by some distance the strongest evidence, and it became accepted that the San Andreas fault has shown right lateral movement of 314 kilometers. The volcanic evidence matched 10 different layers of rock, their origin, and the precise time. It was indisputable.
By why were the halves terminated by different fault? Well, faults with big egos can show a certain lack of fidelity. Often, several parallel faults form, each a place where the rocks have given in to the shear. The main fault is embedded in a wider zone with other faults which take up some of the stress. The main movement can easily jump from one such fault to the next, leaving the land in-between stranded on the other side. This appears to be on-going at the southern end of the San Andreas where there are at least three parallel faults: the San Andreas may well be in process of changing allegiance to one further to the west, so that it can line up better with the Imperial Fault, its most obvious extension into Mexico. Such a jump appears to have happened at the Pinnacles as well. At the time that the two volcanic halves were separated, the Chalone Creek fault was the San Andreas. It is no longer – it lost the contract and became an also-ran. In contrast, at Neenach the San Andreas remained faithful to a fault.
Faults rarely exist in isolation. The San Andreas fault takes up about three quarters of the transform motion. The rest is distributed over other faults. As an aside, for some reason the San Andreas only does big quakes, while the other faults in the zone do the smaller quakes. ‘Smaller’ does not mean small: they can go well above M6. In California, you never know where the next shake is going to come from. All the while, the San Andreas lies quiet, leaving other faults to do the work. When it is ready, it will show who is the boss.
One of those large earthquakes occurred on Jan 9, 1857, at 8:13am. It started with two foreshocks near Palmfield, halfway to San Francisco. This triggered a failure point which rapidly moved south, as far as Wrightwood, 20 kilometer southeast of Palmdale. The red line on the plots shows the length of the rupture. There was only one inhabited location along the entire segment: Fort Tejon, where little was left standing but amazingly only two people died. Further from the fault, there was significant damage in Santa Barbara. The earthquake had an estimated magnitude between 7.9 and 8.2, and caused 9.5 meter of offset along the fault in the Carrizo plains. (That was in the northern part of the rupture zone: the offset may have been less around Neenach.) None of the houses I drove past that day would have stood a chance.
The San Andreas has on average 2.5 cm movement per year. The Neenach area has been affected by large earthquakes around 1470, 1610, 1690, 1812 and 1857. Since the 1857 earthquake, it has been quiet. Over that time, the San Andreas has accumulated close to 4 meter of stress. But the segment remains locked and nothing is actually moving. One day, probably this century, it will rupture again: the risk is highest at the southern end, near and beyond Palmdale.
The 1857 earthquake added as much a 9.5 meter to the distance between Neenach and the Pinnacles. This is still a moving mountain.
The San Andreas fault has split Neenach: it is only half the volcano it used to be. The other half found itself on the opposite, moving side of the transform fault, and became upwardly mobile: it started to migrate north. They say faith can move mountains. Maybe California had only half a faith, since it managed to move only half a mountain. And now, 20 million years later, the Pinnacles are still on the move, trying to reach San Francisco. Given another 10 million years or so, this national park will arrive at San Jose. Its other half will still be near Los Angeles. Who has come off best? One side is a designated area of beauty, a protected park where condors still fly. The other side lies forgotten, hidden and alone. West was best.
There is one more surprise. Once the total displacement of the Pinnacles was known, people started to correlate other rock formations along the San Andreas. And it turns out, they often agreed very well with this 315 kilometer displacement. Which was funny, because those rocks had very different ages, often far older than Neenach. It means that there had been no movement along the San Andreas before Neenach formed. The volcano is older than the San Andreas fault. It would be natural to assume that the magma had traveled up through this fault, but it didn’t: it came too early. When later the San Andreas formed, it accidentally went straight through this volcano, splitting it in half and moving the two sides apart, in the process creating the most bipolar volcano in the world. This isn’t a volcano with a fault. It is a fault with a volcano.
Albert, December 2017