Arenophile genetics, Jedi, and another submarine canyon

Geology doesn’t exactly run in my family. My parents weren’t geologists, I
married someone to whom dip and strike will eternally remain an arcane concept,
and neither of my kids are geologists. Geology has, however, touched, so to
speak, them all, and rubbed off most on my daughter. She’s the one who has always
loved the outdoors and adventures. She became, involuntarily, intimately
familiar with the major faults of the Atlas Mountains during our trek there
together when she turned fifteen (she was, according to our guide, the youngest
western female to climb Jebel Toubkal, the highest mountain in North Africa).
And her recent activities would suggest that arenophilia may be genetic.

Earlier this summer, on a well-deserved vacation/holiday in southern
California, where was her first  destination after arrival? The Imperial Sand
Dunes - in the distinctly uncomfortable temperatures of July. The area, also
known as the Algodones Dunes, is the largest landscape of sand in California,
covering its intersection with Mexico and Arizona. And there she collected,
secured, and labelled in detail, plastic bags of sand for her arenophile father

• mad dogs, Englishmen, and their daughters. En route, she came across the
  strange sight of a small group of Star Warsfigures, for it was here, in
  1982, that a massive set for Return of the Jedi was constructed. And,
  in the same way that the archaeology of the Ten Commandments has become
  an attraction of the dunes at Oceano (see my earlier
  post), so the Imperial dunes are a place of pilgrimage for devoted Star Wars
  fans who re-enact and dig. This phenomenon was covered by a highly
  entertaining article earlier this year in Harper’s Magazine, "Raiders of the lost R2:
  Excavating in a galaxy far, far away." As one of the pilgrims is quoted as
  saying, “As a kid, you can only go so far playing with action figures. As an
  adult, you don’t play with action figures anymore. You become the
  action figure.”

Escaping from the desert heat, my daughter then sought refuge on the beaches
around San Diego - and continued to assiduously collect and catalogue sands for
her deranged father. And, in the process, she made an interesting and
geologically significant, observation. The beaches of La Jolla Shores and La
Jolla Cove are immediately next to each other, the Cove being located in the
rocky headland that juts out at the southern end of the Shores. Yet their sands
are dramatically, visibly, different. Between your toes on La Jolla shores is a
fine sand (upper image, below), quartz grains shot through with dark rock and
mineral fragments; at the Cove (lower image, same magnifications), the sand is coarse but poorly
sorted, made up of a wide range of grain sizes, with shell fragments clearly
visible, along with dark minerals, oranges, pinks, and purples. This
contrast seem bizarre - until we look at the geology and what’s going on along
this stretch of coast.

First, the setting of the Cove is distinctive. It’s backed by sandstone
cliffs shown in the photo, weathering and eroding away at a rapid rate. The
sandstone was formed around 70 million years ago as the tectonic turbulence that
has long characterised southern California raised the Sierra Nevada mountains.
The Sierras were formed as a series of gigantic bodies of originally molten
rock, plutons, emplaced deep within the earth’s crust, but rapidly jacked up to
the surface by the tectonic turbulence. Once there, they were worn away by the
relentless forces of weathering and erosion, the sedimentary debris forming the
sandstones of La Jolla Cove. Now those sandstones are themselves providing much
of the sedimentary debris for this segment of the coast - you can almost sense
the sand pouring off the lower part of the cliff onto the beach - local
ingredients.

Second, and even more importantly, if you were to pull the plug in order to
see the landscape beneath the ocean along this stretch of shoreline, then an
extraordinary topography would emerge. Just a few hundred yards offshore from
the southern end of La Jolla Shores is the head of a huge submarine canyon.
Similar to the Monterey Canyon further north (that I wrote about here),
this feature is deep and sinuous, winding its way far out across the Pacific
Ocean floor. And it’s a sand-sucker - any sand that is carried down the coast
along La Jolla Shores from the north is inevitably and irrevocably swept down
the canyon and out to sea. The La Jolla canyon forms the boundary between two
segments of coastline that are distinct with respect to their sedimentary
processes and the transport and deposition of sand - two different littoral
cells(see my “Beach
Nourishment and Sediment Budgets” post).

No wonder the sands of the Shores and the Cove are so different. This is a
dynamic stretch of coast and the submarine canyon has long been a laboratory in
which we have learned something of the dramas that are played out in these
landscapes. And it’s perhaps no wonder that one of the world’s foremost centres
of oceanographic research, Scripps
Institution of Oceanography, is located (on land) at the head of the
canyon.

[The image of the canyon, along with many others, can be found at http://walrus.wr.usgs.gov/pacmaps/sd-index.html;
Scripps put out a news release a couple of years ago that describes some
fascinating research on sand movements and budgets along this part of the coast:
http://scrippsnews.ucsd.edu/Releases/?releaseID=785;
the department of geology at San Diego State University have, brilliantly, put
together Google Maps overlays of the geological maps: http://www.geology.sdsu.edu/kmlgeology/kmz/la_jolla/la_jolla.html;
and see the comprehensive study of California’s coastal processes and littoral
cells, Littoral
Cells, Sand Budgets, and Beaches: Understanding California’s Shoreline. The photo of the Star Wars set is from http://www.desertusa.com/sandhills/du_sh_star.html. And,
finally, thanks to my daughter for her genetic peculiarity and for prompting the
basis for this story.]

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Comments

Samuel Kazungu Muramba (2017-06-29):

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Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/2009/09/arenophile-genetics-jedi-and-another-submarine-canyon.html