Friday, June 19, 2015

The La Brea Tar Pits, with some igneous notes thrown in!


Two weeks ago my title was MOG: “Mother-of-the-Groom” in US wedding lingo. I was in Los Angeles, California, to celebrate the wedding of my son, whose apartment is just a couple blocks from the world-famous La Brea Tar Pits (http://www.tarpits.org).


On the 1-mile walk from our hotel to son’s apartment, I cut through Hancock Park (https://en.wikipedia.org/wiki/Hancock_Park), which includes the campus of LACMA (Los Angeles County Museum of Art) and the Tar Pits. I walked under Levitated Mass, a large outdoor static-art piece (nod to any igneous geologists out there), opened to the public in 2012*, and then ESE through the grounds of the adjacent Tar Pits.
Levitated Mass in upper left corner, LACMA left and lower center, Tar Pits is green space. The large Lake Pit, with mammoth sculpture group, just to left of red pin; Project 23 boxes and exam space are little white dots in upper center (iPhone screen capture from Maps app).

Levitated Mass by Michael Heizer. Top photo from LACMA website; bottom photo, and all others with no attribution in this blog post, by Malinconico. The art piece includes the rock mass, walkway, and surrounding decomposed granite aggregate. The big rock is a diorite, probably Cretaceous, from Stone Valley Quarry, an aggregate quarry in the Jurupa Mountains near Glen Avon, California*.
Yes, people will tell you that the name, La Brea Tar Pits, is a redundancy since La Brea means “tar” in Spanish. Rancho La Brea was one of the original colonial Spanish land grants (http://www.tarpits.org/la-brea-tar-pits/timeline: History). Oil seeps upward from the Salt Lake oil field, in the northern part of the Los Angeles basin (http://www.searchanddiscovery.com/pdfz/documents/2012/20164gautier/ndx_gautier.pdf.html, slide 6; http://www.ucmp.berkeley.edu/quaternary/labrea.php), and the loss of volatile hydrocarbons leaves behind a tarry “gooey” residue AKA asphalt or bitumen.


Native Americans and later European settlers had used the tar for mortar, glue, caulking, medicine, and fuel (http://www.tarpits.org/la-brea-tar-pits/timeline). These uses were also known 5000 years ago in the Middle East (Daniel Yergin, The Prize, 1991, p. 23-24). The Hancock family, who owned Rancho La Brea in the late 19th/early 20th centuries, initially mined the asphalt, but later went into oil production on the land. Animal bones found in the La Brea tar were originally thought to be those of modern cattle, but, in 1875, it was recognized that the bones were actually fossils. Studies since have been “the core of late Pleistocene North American [paleontological] research” (http://www.nhm.org/site/research-collections/rancho-la-brea/research-studies).
 
Excavating fossils 1913-15 at La Brea, Hancock Ranch, with oil wells in background (http://www.tarpits.org/la-brea-tar-pits/timeline: Excavations).
The present-day La Brea ponds filled with water and a scum of tar are remnants of former fossil (numbered on map) or mined-asphalt (Lake Pit) excavation pits (http://www.tarpits.org/visit/map). Besides these pits, a large number of tar-encased fossils are currently being “released” from Project 23: in 2006, during construction of an underground parking garage for LACMA, new fossil deposits were found. Twenty-three (therefore the name, Project 23) large wooden boxes were built around the masses of asphalt, to preserve the relationship of bones to each other, and removed, along with over 300 buckets of asphalt, for further examination. One can see remaining boxes outside on the grounds (photos below) with preliminary examination workstations and a blackboard describing the latest in what’s been found. Fossil exhibits, laboratories and research facilities are in the Page Museum onsite (http://www.tarpits.org/page-museum).
 
Pit 13
Tarry scum on surface of water in Lake Pit
Project 23 preliminary examination lab: Large wooden box, by blue wheelbarrow, is open, tent behind holds exam table space, other large boxes in background.
The fossils found at La Brea represent those from ~40,000 years ago (Late Pleistocene) into the Quaternary (<11,700 years ago) (http://www.ucmp.berkeley.edu/quaternary/pleistocene.php). Quaternary fauna recovered are like those we live with today, but the Late Pleistocene fossils include large extinct mammals such as mastodons, mammoths, and the saber-toothed cats (http://www.tarpits.org/la-brea-tar-pits/timeline: Pleistocene). Most of the recovered fossils are those of predators and scavengers, assumably packs of predators chasing lone prey, all getting stuck in the tar (http://www.ucmp.berkeley.edu/quaternary/labrea.php). The causes of the extinction at the end of the Pleistocene are not clear. Climate change at the end of the last (Wisconsin) Ice Age, overkill by early man, or a combination of both are suggested (http://web.stanford.edu/group/journal/cgi-bin/wordpress/wp-content/uploads/2012/09/Gibbons_NatSci_2004.pdf). A very controversial hypothesis is climate change caused by impact or low-atmosphere explosion of a meteor over Canada (http://www.nature.com/news/evidence-found-for-planet-cooling-asteroid-1.13661 plus references for and against cited in linked PNAS paper and weblinks; so far I am not convinced of such an impact).


I exited the Tar Pits by the large Lake Pit and its sculpture group of three Columbian mammoths: the mother tragically mired in tar, with panicked offspring and mate on the shore. The sorrowful scenario, although not fine art, is one of my three personal favorite emotive sculptures, the other two being The Dying Gaul (http://www.thehistoryblog.com/archives/28318) and The Peace Monument, with Grief leaning on the shoulder of History (http://www.dcmemorials.com/index_indiv0000310.htm). 
Lake Pit with mammoth family and bubbling methane in foreground and to left behind mired mother. Page Museum is building in background
After the wedding, we drove to Las Vegas. On the north side of highway I-15 right before the California-Nevada state line are the Molycorp Mountain Pass rare-earth-element (REE) mine and processing plant**. The north rim of the open pit can be seen from the highway. The ore body is the Sulphide Queen stock, a 1.4 billion-year-old carbonatite (note a second igneous reference in this post!). Carbonatite is an igneous rock with a large percent of carbonate minerals (so not the organic carbon of this blog’s focus), which at Mountain Pass are primarily calcite, dolomite, and barite. The REE-bearing ore mineral is bastnäsite. Uses for REE include high-efficiency magnets in modern wind turbines and electric motors, and as coatings in compact fluorescent light bulbs, all important technologies in both saving energy and the transition to non-fossil fuel-based energy systems.

*Some Levitated Mass links about the sculpture, rock source and transport:
** Links to information on Mountain Pass REE deposit:
Mine and processing plant:
Geology:

No comments:

Post a Comment