Sunday, November 17, 2013

Mexico 2013 field trip


I was thrilled when I found out that the Fall 2013 Ge 136 field trip would be going to Baja California again. Regular readers will know that this is my favourite class by far. I have now done it 6 times. In fact I have done it so much that we're beginning to repeat material. For grad students like myself who lack a car, time, and silliness, the field trips are a perfect opportunity to find out about the land from those who know it best!

The previous trip to Mexico occurred in November 2011 (http://caseyexaustralia.blogspot.com/2011/11/ge-136-field-trip-to-baja-california.html), and was the last time my old camera and lasers worked properly. There was a violent rainstorm that caused everything to get wet! The forecast this time was somewhat better. 

Thursday evening. I stumbled out of a class on matrix beam theory and, collecting my gear, headed for the trucks. Lined up gleaming in the dark purple twilight, each had a full tank and an empty trunk, but not for long. The TA H had allocated me as the driver of all the undergrad girls. Punishment or reward, I wasn't certain.

We hit the road. After an hour we emerged from the traffic jam and, cruising south on the 610, made it to the campground near the Tecate border crossing in just under 4 hours. My gamble with the route had, for better or for worse, had no effect - we arrived within 5 minutes of everyone else! I ditched my bag on the party tarp and sat around the campfire. By 10pm, Jupiter had risen, so I cracked out my binoculars and showed people the moons. Three were visible as specks of light around Jupiter's impossibly bright disk. The moon was also visible and cratery.

Aware of the long drive the following day I headed to my luxurious bed - a blanket folded over 3 times under my sleeping bag - leaving the rest of the class to tend the fire and their collective raging thirst. As something caught fire, the leaves above me twinkled in reds and oranges.

During the night, I woke with a start. A roaring sound and the earth shook. Leaves fell from trees - could this be another earthquake? I sat up and looked around. I was surrounded by dark seal-like sleeping bags on the ground. Then the roaring started again. Someone was snoring next to me. I lay back down and strategically accidentally-on purpose sleep-kicked them, and they rolled over. Crisis averted.

I was greeted by a bright clear morning. I retrieved my supply of naan bread and quickly warmed it over the gas cooktop. Sunnies firmly in place, we convoyed to the Mexican border and drove under a boom gate. Ten minutes later we were outside Tecate on the open road. The road snaked between massive outcroppings of boulders. The mountains here are the eroded roots of a batholith, an exhumed granite mass of rock. Periodically we stopped for talks, and before long we had sped to Ensenada and Cicese, where we resisted the pull of roadside stalls and picked up O and J, two geology students from the University of Baja California. To the south of Ensenada, Punta Banda sticks out into the Pacific Ocean. A narrow spit of land broken out from the peninsula and thrust to the west, it has some particularly excellent rudist remains, as well as a series of paleo sea terraces. But today we were not going to Punta Banda, we were going to its mother fault, the Agua Blanca fault. Lunch was set up, and people took photos of the picturesque mountain range. At some point someone said 'that's an interesting off-set'. Indeed, the mountain featured a series of straight v-shaped valleys which abruptly kinked near the base, marking the location of the Agua Blanca right-lateral strike slip fault.

Punta Cina looked like an unlikely bet. There's a fabulous fossilized reef being turned into cement in a factory, and the owners are less friendly than before. Fortunately our Mexican guests knew of another exposure in a canyon behind the next campsite. We cruised south toward Santo Tomas, site of the oldest vineyard in North America, started by Franciscan monks as part of their perhaps well-intentioned and certainly highly effective program of cultural and racial obliteration. We struck out towards the coast and our first campsite. 2 years ago, it was under water and I didn't see that much of it. This time though, gently rolling plains swooping down towards herds of cattle and the sea. We stopped by a lighthouse for some talks, then proceeded to our camp site. 

A quick scramble down the sea cliff and we were on the beach between gently sloping cretaceous sediments and the gentle waves of an ebbing tide. Of course the cretaceous sediments contained more than a few fossils (though you'd have to know what to look for), giving a real insight for how the environment changed over a few million years as we walked down the beach. Generally speaking marine deposits shallow over time as more silt accumulates, however subsidence or sea-level changes can rapidly change the character of species and even the rock type in one strata. I even managed to skim a few chunks out through the breakers, though missed the seagulls in the process. Half way up the sea cliff is an cretacious-paleogene unconformity. Though there are a few tens of millions of years missing, somewhere in between the dinosaurs died out. Today this gap is marked by a narrow ledge, covered with the seabird descendants of the only surviving dinosaurs. Even better, the next bay has a few hadrosaur ribs here and there...

That evening we had a fire constructed, fold-out chairs deployed, and mountains of the only appropriate dinner (pasta) duly produced. I was feeling slightly skittish, so took my camera and tripod up a nearby hill to take photos of the camp and so on with long exposures. My plan was to capitalize on the excellent stars and produce my first all-sky night-time panorama, but unfortunately some clouds blew in and the setting moon washed everything out a bit. Still, I found a nice cactus to sit the camera on. After a few minutes as my hill-climbing heat leaked away, I heard the youthful voices of some undergrad friends making their way up the hill. We took a few photos together and I headed down. At about that moment some fog blew in, but my friends were determined to finish their climb without lights. Soon after their absence was noticed, which caused quite a lot of excitement. But half an hour later around the campfire, all (and then some) was thoroughly forgotten.

I found a stretcher in the back of one of the trucks and, setting it out on a nice flat patch, curled up inside my sleeping bag as the stars ground past overhead and, occasionally, a shooting star shot across the sky. Fortunately, there were no tremors that night.

The next day I took myself out of my sleeping bag, my trousers out of my sleeping bag bag, then placed myself in my trousers and my sleeping bag in its sleeping bag bag. This is not as technical as it sounds. Soon I was packed, eating breakfast, and running around like a epileptic monkey with my camera.

The convoy headed for the homestead, where we met the ranch owner, admired his collection of derelict cars, and headed for the canyon upstream, where, legend has it, an ancient coral reef is exposed between rocks with pouring water. We made our way up canyon, climbing up several waterfalls and balancing on logs across muddy springs until we popped out in the head of the canyon, finding a cave full of ash and rat droppings. In the roof of the cave was preserved the shells of rudists, ancient reef-building clams that died out a long time ago.

Next stop was a rock outcrop to the south. Strata dipped at 30 degrees, with a thin layer of limestone preserving several excellent fossils of coral. Above and below, volcanic ash. Indeed, 100 million years ago, these rocks were the shores of a volcanic atol, and we even found chunks of reef broken off and entrained in basalt lava flows.

Soon after we found a beach highly worthy of a long lunch break. Long mainly because two of the vehicles became stuck in sand, providing much mirth and hilarity in between a short dip in the rather fresh ocean to provide some free space for fresh layers of dirt to accumulate. From here the road continued along the top of a precarious cliff and we, now radioless, drove south to San Quintin, where we parked behind a sand dune and ferried our camping gear to the top of the hill. 

Once again a fire was duly lit, only to realise that we were perilously low on firewood. After a stunning dinner of tortillas with enough left-over rice to keep hunger at bay for whole hours, I went for a walk down the beach to find some wood. I returned with a respectable bundle of sticks, only to find in my absence an enterprising geology grad had sourced a few palettes, which, for the record, burn really well. Later this was augmented by a half dozen tree stumps, for the best camp bonfire I've ever seen. Initially there were about 8 of us on our knees around the fire pit, close enough to high five over the top. By the end, we could have sent signals to Mars.

Then it was time for the shovel-olympics. Being errant geologists, we had no fewer than four shovels. 7 nationalities were represented, though sadly, as always, I was all alone representing the southern hemisphere. The with a large representation of Americans, there was a few defections, in particular to China. The events consisted of shovel javelin, shovel discus (which was more like shovel evasive action), digging a hole, dizzy shovel (shotgun a beer, run around shovel 10 times with face on shovel, then run to the water and back), shovel limbo, and shovel possession knockout. I even remembered to fill in the holes afterwards so noone died.

Next morning we packed the vehicles then prepared for our most distant talks. Just behind the dune was a microbial mat, one of the more ancient types of environments on earth, today preserved only in saline environments. Consisting of bottomless black ooze topped with a 1mm thick rainbow layer of living phototropic bacteria, it's one of the most productive biomes (in terms of biomass accumulated) in the world. It's also thought to be one of the main mechanisms by which oil is generated. My foot-width to height ratio was just high enough that I could walk across the surface without punching through the crust, but many people punched, and some punched gloriously, sinking to the waist. S attempted a body slide, with scratchy results. 

Soon it was time to head back, with only one more stop along the way. We drove down a wash towards a small but pretty beach, a few ramshackle houses dotting the hill above. On the right, a layer of soft gray sediments was topped by a hard red conglomerate. Nearby, the grey contained ammonites, and the red contained shock-quartz, generated by a meteorite impact. A short erosional unconformity, but otherwise a perfect place to finally give my talk on radioactive space rocks. 

Nearly all rocks are made of minerals with oxygen. Oxygen comes in three stable varieties, of respective atomic weights 16, 17, and 18. The 16 isotope is MUCH more common, so looking at the ratio of 17:16 and 18:16 is illustrative. The relative concentrations get moved around by mass-dependent reactions and symmetry-dependent reactions, forming discrete regions. Different space rocks start out with different amounts of oxygen, because oxygen is made in old dead stars and spread into space on grains of dust and as gasses, and each planet is made of different amounts of gas and dust, so each planet and asteroid family has a different root composition. Put together, you can work out where any meteorite came from. For instance, a thin layer of clay around the world at the K-Pg boundary is a mix of about 90% destroyed coral reef and 10% meteorite that wiped out the dinosaurs, except birds. By counting how much space-rock is in that layer, we can estimate that the relevant asteroid was about 10km in diameter. That sounds small in comparison to the size of a city or even a marathon, but remember that when its face touched the ocean surface, its tail was still in the stratosphere. There's also a growing family of meteorites that are recognised to have come from Mars. Magnetic analysis tells us they never got hotter than 50C or so either during ejection, flying through space, or Earth atmospheric entry. Cosmic ray analysis says they were in space for a few hundred thousand to a few million years. In other words, even today Mars rocks are raining down, and in the past, many, many more would have, more or less guaranteeing that if life originated on either planet, it could now be on both. I think that's cool!


Concerning Mars, MAVEN, an atmospheric satellite is going to Mars in the next few weeks. It's going to try and figure out what happened to Mars' atmosphere. We know it used to have a thick, wet, warm atmosphere, but it's been lost. Either it's frozen out on and in the ground, or it was blown away by the sun. Both are possible, since Mars lacks a large magnetic field (though it certainly once had one). It's thought Mars was warm and wet until 3.8 billion years ago (with possible short warm periods since), while for comparison the earliest life on Earth was 4.2 billion years ago. It's possibly Mars lost its magnetic field, then enough of the atmosphere blew away to start a run-away icebox effect. Of course, the Sun is about 25% hotter today, so maybe we could reverse that effect. My personal suspicion is that oxygen producing microbes evolved on Mars, as they did on Earth. On Earth, the oxygen oxidised minerals on the surface until saturation, producing the banded iron formations. Tectonic and geologic activity ensured a large supply of oxidizable rocks, but eventually things saturated about 2.4 billion years ago. The atmosphere lost much of its CO2 and methane as microbes poisoned themselves with oxygen, and in the process, a lot of the greenhouse effect. Earth went through several phases of mass glaciation, known as snowball Earth, because a nitrogen oxygen atmosphere is not as warm as a CO2 heavy one. My personal opinion, completely unsupported by evidence, is that the same process occurred on Mars, but it occurred faster. Mars has geological processes, but they're about a million times slower. As a result, the time taken to oxidize minerals at the surface was much shorter, at which point the microbes effectively punched large heat-windows in their own atmosphere and froze to death. Unlike on Earth, there wasn't enough time to evolve oxygen-using microbes that could exploit the oxygen and stabilize the atmosphere, at least, not on a planetary scale. We shall see.

But, I digress. We loaded the cars and headed back to the road. K copiloted and provided conversation to keep me awake. In return I tried to teach her to solve a Rubik's cube, with the added complication that I was unable to see it. But we nearly got there.

We spent only 1.5 hours at the border waiting to get through. Fortunately our car had a nice guard who wasn't too mean, but some of the other cars were flashlighted rather aggressively. Then onto the main road, a brief stop in Temecula In 'n' Out, and zooming back to Caltech on the 15.

As always, an extraordinarily fun, rejuvenating weekend.















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