The Science of Sustainability

A Submarine Grand Canyon Offshore Central California

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A few months ago, the QUEST television program included a segment on various gelatinous marine animals, including jellies, found offshore of central and northern California and featured at the Monterey Bay Aquarium. The beginning of the video segment briefly showed one of the most important habitats for these animals — Monterey submarine canyon. Not unlike on land, features of the undersea landscape have a significant influence on the quantities and diversity of animals. Science involved in understanding the relationship of this “seascape” with submarine ecosystems requires quality maps and the technology to create those maps is improving all the time.

For this post, I simply wanted to show a few images of the canyon and beyond. As readers on my regular blog know, I just love showing images of our planet’s seafloor. We are currently in an age of exploration and discovery when it comes to mapping the seafloor — in many ways it’s like sending probes to another planet. Many of the images I show in this post are from the Monterey Bay Aquarium Research Institute (MBARI), which has not only done a lot of the mapping and science, but has also designed and built the technology to do it. See their page on marine geology, including Monterey canyon, here.

The first image (below) shows the deepest part of Monterey canyon, near the shoreline, cutting across the flat and relatively shallow continental shelf. The yellow arrow represent river sources of sediment that contribute to the longshore currents (red arrows) that funnel the sediment into the canyon and, ultimately, into the deep sea.


In fact, it is the movement of sediment through the canyon that is responsible for creating and sculpting the canyon. Not unlike the Colorado River incising into the Colorado Plateau to create the Grand Canyon, these underwater “rivers” of mud, silt, sand, and water rush down the canyon eroding the canyon little by little. Since I’m bringing up the Grand Canyon, I always like to point out the scale of Monterey submarine canyon with the map below, which compares the two canyon systems at the same scale (also from MBARI).

Grand Canyon (top) and Monterey submarine canyon (bottom); credit: MBARI

Finally, I’d like to point out that there is even more to discover and learn beyond the canyon itself. As the continental slope transitions to the much flatter open ocean floor (greater than 10,000 feet deep) the deep canyon transitions to a subtler feature. The Monterey submarine fan, shown in the map below, is a depositional feature — it’s where all the sediment that cut the canyon (and much more that simply traveled through it) ended up. This is the submarine equivalent of a delta in some ways.


Next time you are standing along the coast in Santa Cruz or Monterey and looking out into the ocean remember that there is an entire landscape on the seafloor as beautiful and complex as what we see on land.

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- Image comparing Grand Canyon and Monterey Canyon courtesy of this MBARI publication (link opens a PDF)

- First and last images created in GeoMapApp, a free web-based software for creating topographic/bathymetric maps

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Category: Geology

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Brian Romans

About the Author ()

Brian Romans is the author the popular geoscience blog Clastic Detritus where he writes about topics in the field of sedimentary and marine geology and shares photographs of geologic field work from around the world. He is fascinated by the dynamic processes that shape our planet and the science of reconstructing ancient landscapes preserved in the geologic record. Brian came to the Bay Area in 2003 and completed a Ph.D. in geology at Stanford University in 2008. He lives in Berkeley with his wife, a high school science teacher, and is currently working as a research scientist in the energy industry. Follow him on Twitter.
  • Chris Bauer

    Great post Brian. I can look at maps all day. And these latest underwater bathymetry images are especially amazing. It’s great that we are finally getting around to properly mapping the sea floor. (Also see: http://www.kqed.org/quest/television/sea-3d-charting-the-ocean-floor )

    When I previously saw images of the Monterey Canyon I had assumed that it was created during the great ice ages when the sea level was significantly lower and much of the present day sea floor off the California coast was exposed. (As you explained so well in this post: http://www.kqed.org/quest/blog/2010/09/02/the-importance-of-studying-the-history-of-sea-level-change-in-san-francisco-bay/ )

    I pictured some powerful rivers at the time had carved this deep canyon, just as the Colorado River dug and digs out the Grand Canyon. And then, as the sea level rose to its current depth, the MontereyCanyon remained essentially undisturbed under the ocean. I figured it was a geologic memory or testament to those ancient rivers. So it was really interesting to learn that such a huge canyon could be cut into the earth by “underwater rivers” of sediment. And that then led me to wonder- Are there any other places in the world where currents merge like this and carve out off-shore undersea canyons? How strong are those longshore currents that are carving this canyon? And would changing the amount of river flow into the ocean over the last couple hundred years change the amount of sediment and thus slow down or stop this geologic process? Anyway, things to ponder on a Monday morning. Thanks for the enlightenment and the reminder not to assume!

  • Brian Romans

    Chris,

    Thanks for the comment and link (again!).

    Although sea level was significantly lower during the numerous ice ages (as much as 425 ft lower), Monterey canyon cuts several thousand feet into the continental shelf bedrock.

    To your questions:

    (1) "Are there any other places in the world where currents merge like this and carve out off-shore undersea canyons?"
    There are numerous deep-sea canyons around the world, most of the world's major river systems extend offshore and have submarine canyons (e.g., Mississippi, Indus, Amazon, Ganges-Brahmaputra, Rhone, and so on). In terms of other systems where the longshore currents converge at the head of the canyon as in Monterey Bay, I'm not sure off the top of my head. Most of the other systems do receive sediment from the currents that parallel the shoreline, but it may only be from one direction and not from both sides like in Monterey.

    The longshore currents are what rework and redistribute sediment in shallow water. Once the sediment gets into the head of the submarine canyon it may sit there for some time until a large mass failure occurs. This creates the turbidity currents (picture an avalanche, but with mud and sand instead of snow) which carve the canyon. So, it's really two different processes. MBARI's site has some great articles on this topic.

    (2) How strong are those longshore currents that are carving this canyon?

    As I mention above, it's actually the turbidity currents and not the longshore currents that carve the canyon (I could've explained that more clearly now looking at the post). The strength/velocity of the turbidity currents range quite a bit. One thing that MBARI found out is when they attempt to put equipment on the canyon floor to measure these currents, the equipment (some of which weighs hundreds of kilograms) gets ripped up and transported many miles down-canyon! In other words, these currents are immense and can come ripping down the canyon (again, picture an avalanche).

    (3) Would changing the amount of river flow into the ocean over the last couple hundred years change the amount of sediment and thus slow down or stop this geologic process?

    That is actually the subject of current research in my field. For example, for my Ph.D. I studied systems offshore southern California investigating the record in the deep sea and its relationship to El Ninos over about 7,000 years. In this case, the Santa Clara River (near Ventura) is very sensitive to El Nino times and pumps out a lot of sediment. We were able to see the record of this in the deposits offshore (which made their way through Hueneme submarine canyon).

    I could go on, but I won't, ha! I hope that kind of answers your questions.

  • Chris Bauer

    Thanks for answering my questions and giving further insight. This is really interesting. It’s amazing that those longshore ocean currents can throw such heavy hardware around. Made me think of a story I heard about the building of the Golden Gate Bridge, where hardhat divers were dodging massive boulders that were bouncing around like snooker balls in the incoming and outgoing currents of the San Francisco Bay.

    Had I given it deeper thought (pardon the pun) I should have figured that the surface of the Ice Age ocean was still far above the lowest parts of the Monterey Canyon, and thus the canyon could not have been etched out strictly by the river. But now we can better picture how it might have looked during those Ice Ages. Instead of appearing like the Grand Canyon, a desert landscape cut deeply by a raging river, my guess is it probably looked more like a deep fjord, a narrow ocean inlet or calm bay, surrounded by a very lush environment. Imagining the steep walls of this “fjord” also lined by tall coastal redwood trees, the effect could have been quite dramatic. I’m sure the first people arriving to that place would have been quite impressed… if they weren’t too busy chasing mammoth or running away from saber-toothed cats.

    Thanks again for sparking my curiosity.