Thursday, February 16, 2012

Hogbacks of the California coast

I recently wrote a blog post examining "The unbelievable bathymetry of Mavericks."

In that post, I noted the amazing underwater features (which others have also discussed, and which in this particular stretch of the Pacific Ocean help to focus the wave energy that makes Mavericks one of the biggest and most powerful waves in the world) and suggested that conventional explanations for these features have some problems.

In particular, I noted that conventional explanations such as the suggestion that ancient uplift may have curved layers of strata, which were then tilted and sheared off (the "plunging folds" explanation), did not seem to fit the evidence (for one thing, I pointed out that "the curves are clearly further apart at the point of greatest 'hairpin turns' rather than closer together as we might expect if they were actually caused by tectonic uplifting"). I then suggested that the graceful curving patterns found not just at Mavericks (off of Pillar Point at the north end of Half Moon Bay) but also for hundreds of miles along the Northern California coast could more likely be the product of volcanic lava flow, or even of glacial carving.

In fact, although I did not make this point in the previous post, the fact that these graceful parallel patterns appear intermittently along a stretch of coast for hundreds of miles is yet another argument against the "plunging folds" explanation.

However, towards the end of the post I did admit that I was not certain of "the exact mechanism that carved the graceful channels in the original deeply-scored washboard," although I did believe that either lava or glacial ice were the two most-likely candidates. I noted that these two mechanisms could easily fit within the framework of the hydroplate theory of Dr. Walt Brown, while the conventional explanations seemed to be somewhat vague and inadequate in light of the evidence on the ground.

Since then, however, I have been in contact with Dr. Brown and he suggested that based on several pieces of evidence, he does not believe that either lava or glacial flows are a likely explanation for the curving ribs that are so evident in the seafloor near Mavericks and along the coast to the north and south. In fact, he wrote that "for a variety of reasons I don't believe those features were produced by glaciers, icebergs, reefs, faults, or volcanic eruptions" (the reason he mentions reefs, icebergs, and faults in that list is that other conventional explanations try to explain these distinctive features by referring to the proximity of the San Andreas Fault, although how the fault could produce such swirling parallel curves is not explained; the other candidates in the list have also been mentioned, including the possibility that the bottoms of passing icebergs floating above might have carved out these patterns, which is so unlikely I did not even take the time to refute it as a possibility in my previous post).

Dr. Brown offered a very different explanation -- he believes that these features found at Mavericks and up and down the California coast look like classic "hog backs." He cautions that "several tests would need to be done before definitely saying those features at Half Moon Bay are hog backs." One such test would be to find similar hogbacks east of the beach (we will return to that line of testing in a moment).

Hogbacks are geological features formed by steeply tilted strata which become exposed, revealing long parallel ridges that resemble the sharp spine of a wild boar (in the US, wild pigs are often called "razorbacks"). The mechanics behind the hogback feature are somewhat different from those being argued for the tilted uplift process that supposedly produces "plunging folds" and which some believe may be responsible for the curves beneath the waves at Mavericks. A hogback is created when forces lift and then shear an entire stack of sediments, generally for many miles: see the diagram below.

I adapted the above diagram (my own rough sketch) from a diagram found in this excellent photo-essay examining a famous hogback feature, the San Rafael Swell (in Utah). The San Rafael Swell in one of many dramatic hogback features in that part of the country which are associated with the uplift of the Colorado Plateau (which Walt Brown explains in his book in the extended discussion of the formation of the Grand Canyon).

The drawing above shows that hogback geology is associated with a formation called a "monocline," which is a place in which the strata have been bent by the uplift of a portion of the earth below them, such that they resemble (in Walt Brown's analogy, see note 47 on this page of his book) a handkerchief draped over a large book sitting on a table (the handkerchief represents the strata, and the large book represents the uplifted geology).

The fact that there are many places in the world today in which the strata gracefully bend up over an uplift in this manner is a piece of supporting evidence for the hydroplate theory, which holds that the strata were all laid down rapidly during the flood event (in which they were sorted into their characteristic layers by liquefaction on a massive scale). If the strata were laid down over millions of years successively, with each becoming dry and brittle in the ages before another layer was laid down, then we would expect the strata to all bend and break at every monocline.

However, in some places, where intense shearing took place or where the uplift may have taken place after the strata had some centuries to harden following the flood, the monocline will produce a sheared-off layer of upward-tilted strata, such as can be seen at the left side of the drawing above. These layers will be exposed in great parallel lines running for many miles -- in some cases for hundreds of miles.

Note that this mechanism takes care of one of my objections to the "plunging fold" explanation -- I noted that at the hairpin curves, the parallel ribs at the bottom of the sea near Pillar Point and the Mavericks break are often further apart, rather than closer together (we would expect them to be closer together if they were caused by tectonic uplift at one point, tilted over and then sheared off). Because hogbacks are caused by an entire "layer cake" of strata being stretched upwards by an uplifting monocline and then shearing, rather than by tectonic folding which would compress the layers together at the point of greatest bending, the layers may well be further apart at the curving places, unlike what we would expect with the "plunging fold" explanation.

Hogbacks can also be found in conjunction with rings around domes that resemble a volcanic crater, such as in Sonoma, California and in Sundance, Wyoming and in the Dakota Sandstone Hogback.

In the drawing above, I have modified the diagram found at the website discussing the San Rafael Swell, because at the San Rafael Swell the Colorado Plateau which caused the uplift is to the west (left side of the diagram). In the above diagram, which is also looking to the north, the Pacific Ocean is meant to be to the left, and the shoreline and geology of the Peninsula and points further east are to the right.

Note that (as the caption below the drawing explains), some of the layers will be lost along the uplifted terrain, although small sections of them might remain. This fact may well explain the geology of Pillar Point that is shown on Slide 7 in this excellent Surfline special feature depicting the bathymetry which create the waves at Mavericks, which was written in November of 2011 by Surfline founder Sean Collins, shortly before his death in December of 2011. The text in Slide 7 states that Pillar Point looks the way it does because it is a "pull-apart basin" created by the San Gregorio Fault (a small fault intersecting with the larger and more notorious San Andreas Fault), but it may well be that Pillar Point is a remnant of what I label as "layer C" in my diagram of a hogback formation in the drawing above.

The diagram above horribly oversimplifies the terrain between the Pacific Coast and California's Great Central Valley (depicting all of it as a single massif, while in reality it consists of a series of complicated ranges and valleys and higher ranges and wider valleys before you reach the highest range that separates you from the Central Valley). However, the main point of this oversimplification is to show that, if the swirling patterns off the coast at Mavericks (which seem to be part of a much more extensive complex of such parallel ridges stretching for hundreds of miles up and down the NorCal coast from Half Moon Bay, emerging only intermittently from the silt that blankets them and hides them) are actually a system of hogbacks, then there should be a corresponding and corroborating series of hogbacks on the other side of the uplifted area (just as there is on the right side of the drawing above).

As Walt Brown told me, if we find such hogbacks to the east, then we can be fairly certain that these mysterious features off the California coast are also hogbacks (although diving down and inspecting them would still be the ultimate test).

When we take a look along the western rim of the Great Central Valley, what do we find? Not only do we find hogbacks, but we find an enormous length of hogback formations, stretching just about as far as the deeply-scored and swirled terrain features that we located off the coast along the same latitudes. (There are also hogbacks along some of the highest mountains in the Bay Area east of Half Moon Bay, such as on Mission Peak and Monument Peak, as shown in photographs on this web page from a Bay Area resident).

Below is a diagram showing one small segment of the hogback series along the western edge of the Central Valley -- this is from an area just east and north of Clear Lake, one segment of which is called Devil's Elbow.

Note that this series of hogbacks is roughly parallel to the similar ridges at Point Arena pictured in the image at the top of this post. Point Arena is about as far north as I have been able to see these underwater hogbacks (if they are in fact hogbacks, which is looking more and more likely), and it is notable that the hogback lines also come to an end at roughly the same latitude in the northern area of the Central Valley (and that they do so with some swirling flourishes on the Valley side, reminiscent of some of the curves found in their undersea counterparts off the coast).

The reader is invited to spend some time looking at Google Maps to check out the distinctive terrain along the western edge of the Great Central Valley of California for himself or herself. The best way to see these hogbacks is to click on the "maps" square in the upper-right corner of the Google Map, and then select "Terrain." The best way to see the hogbacks under the water off the coast, however, is to select "Satellite" and then zoom down to the water's edge (the parallel ridge spines seem to emerge near promontories such as Pillar Point, Point Reyes, Point Arena, Morro Rock, Bolinas, and other similar points).

The hogback features can be traced all the way from the dramatic hogbacks at Devil's Elbow near Clear Lake down to the town of Coalinga and even a bit further south of that, roughly parallel to Morro Rock. The fact that the terrain features off the coast extend about the same distance north and south as the hogback lines along the western side of the Central Valley is another clue that Dr. Brown is correct in suggesting that the curved reefs that produce the surf at Mavericks are really submerged hogbacks.

As mentioned in passing above, the hydroplate theory has a much better explanation for hogbacks (and especially for the dramatically curved hogbacks in the seafloor near Mavericks) than the conventional theories of geology. The hydroplate theory maintains that the strata were all laid down rapidly as a result of a global flood, and that they were soft and pliant for some time after the floodwaters drained into the ocean basins.

They hydroplate theory also maintains that the ocean levels were much lower for centuries after the flood. This would allow the layers to harden, including those that are now submerged in twenty to a couple hundred feet just off the California coast. The sinking of the continents and the rising of the ocean floors took place over the next few centuries, due to the increased weight of the continents that thickened during the flood event (as a result of the buckling that pushed up mountain ranges such as those along the California coast and further east such as the Sierras and the Rockies -- these mountain ranges also sank down and pushed up plateaus such as the Colorado Plateau in the process of sinking down into the mantle). It is quite possible that the severe forces surrounding this "settling in" process in the centuries after the flood were the forces responsible for shearing the layers that now form the hogbacks under the ocean. It is also possible that these hogbacks (and the corresponding hogbacks on the Central Valley side) were bent and sheared during the compression event itself.

Note that there are also dramatic hogback lines on the eastern edge of the Central Valley, where it meets the uplifted hills that eventually lead up to the Sierra Nevada.

It is also possible that the rapid draining of the Central Valley some centuries after the flood created conditions that led to further uplift and the shearing that created some or all of the hogbacks of California. In his discussion of the evidence surrounding the formation of the Grand Canyon, Dr. Brown explains how the escaping water from trapped inland lakes on the Colorado Plateau stripped off layers of limestone, allowing upwarping of the basement rock which sprang upwards when the extra weight above it was removed.

Take a good look at diagram 113 on this web page of Walt Brown's book (about 7/8ths of the way down the page) -- it shows a cross-sectional drawing of the Kaibab Plateau (which he explains should really be called the Kaibab Upwarp or the Kaibab Uplift, as it is not technically a plateau). Note the layers on the right side of the diagram -- the East Kaibab Monocline. This terrain feature and the mechanism which produced it are both very similar to hogbacks and the forces that produced them.

As we noted in the previous blog post about the bathymetry of Mavericks, there are "grand canyons" stretching down into the Pacific just west of Mavericks and Half Moon Bay, and there are others up and down the coast. These canyons were clearly not carved after the oceans rose to their current levels, but before. It is possible that the forces which created the hogbacks of the Northern California coast were unleashed in conjunction with the draining of the Central Valley basin some centuries after the flood.

Thus, upon careful further consideration, it appears quite likely that the dramatic ridges below the surface at Mavericks (and further north and south along the California coast), which have only become clearly visible recently thanks to new underwater imagery released in 2007, are submerged hogbacks. This explanation is certainly consistent with the details of the hydroplate theory.

It is a possibility that deserves much more study from geologists.