GeezerHiker

February 23, 2010

The fair ophiolite of Fidalgo Island

Filed under: Geology, Hiking, North Cascades — geezerwriter @ 10:18 pm

This posting is an addendum to one about our hike last week to Mounts Erie and Sugarloaf on Fidalgo Island. Most of that island, which includes the city of Anacortes, gateway to the San Juan Islands, is built on a bedrock base that is called the Fidalgo Ophiolite. Ophiolites have played an important role in a part the history of geology that has some special meaning for me. so the amateur geologist in me, who always likes to rattle on about geological points of interest, got carried away on this one. So I pulled out most of the general talk about ophiolites so that it wouldn’t completely submerge the hike.

So be warned: This is almost entirely about geology and science history, and my experiences with them, and not about hiking.

Another warning: I have absolutely no credentials in the field of geology, just a lifetime of interest in same and a one year sequence of college courses in 1958-9. My main reason for writing all this is to test my own understanding – one of the best ways of testing your understanding of a subject is to try to explain it to someone else. [If you are a REAL geologist, I would appreciate your letting me know if there is any serious BS in here.]

If you can tolerate some pretty heavy geology lingo, the Wikipedia entry on ophiolites has some nice drawing and photos. It was the first place I looked when I started this investigation, and the density of the jargon also was factor in making me want to explain this stuff in simpler language.

Structure

An ophiolite is not a kind of rock, but a series of different kinds of rocks appearing together in one mass. It consists of all or most of these elements:

  • Sedimentary rocks
  • Pillow basalts
  • Basalt dikes
  • Gabbro
  • Mantle rocks

How did all these very different kinds of rocks come to be clumped together in one mass? There are two situations that could account for it, and there seems to be some disagreement about which is more prevalent, but either will do for our purposes:

  1. Rift zones like the Mid-Atlantic Ridge, where the seafloor is being stretched; It cracks and new material wells up from the earth’s mantle to fill the crack and form new crust.
  2. Volcanic island arcs like Hawaii, where a hotspot in the mantle causes volcanoes which grow up into islands.

But in either scenario, the main point is that they originated somewhere far out in some ocean, and that is widely agreed upon.

I find it easier to picture the island arc situation, so I’ll base my explanation on it.

Volcanic Island Arc Ophiolites

So picture Hawaii. Not so much the hulas and Waikiki Beach but rather the big volcanos that have grown, and continue to grow, thousands of feet above the surrounding seafloor, all the way up to the surface of the ocean and beyond. They emit lots of lava, and lava that cools in water, whether it is extruded under water or runs down off the newly formed land, forms into hummocky mounds called “pillow basalts”. And remember that most of the mass of the islands is below the surface, so there could be a lot of pillow basalts down there.

Some lava doesn’t gush out onto the surface, but rather seeps into cracks in the nearby crust, forming flattish plates called dikes. And after millions of years, there would be a lot of those, too.

And the main culprit in the island formation is a big, fat ball of magma (a “magma chamber”) extending down into the mantle, below the crust. (“Magma” just refers to molten rock that has not reached the surface; it is not called “lava” until it erupts.) Magma is a mixture of a number of different minerals, and over time the heavier (and coincidentally darker-colored) minerals tend to sink to the bottom; the lighter ones rise to the top and are more likely to erupt. Oceanic rocks don’t generally have huge amount of the lighter minerals, but the magma does get darker and heavier with time. When and if the volcanic activity quiets down and the magma cools, it forms a rock called “gabbro” which is coarse-grained and crystalline like granite, but much darker in color.

And the magma chamber is either in the mantle or resting on it.

So all but one of the components of the typical ophiolite are accounted for by undersea volcanic activity. What about the sediments? Remember that all of the volcanic activity takes place over an enormous period of time, likely with lots of time between eruptions, during which time the new rocks that are at a higher elevation would begin to erode and wash sediments down onto lower ones.

But nothing goes on forever, so at some point the hotspot cools down or moves away, erosion and sedimentation continue, and the whole thing becomes a docile island archipelago.

The plot thickens

Now as long as all this stuff stays on the bottom of the ocean, it would just be run-of-the-mill ocean bottom that no one would ever have reason to comment on, or even see. One of the key aspects that I neglected to mention earlier is that the term “ophiolite” refers not to just any old clump of oceanic crust, but one that shows up on dry land as part of a continent.

The most likely scenario is this: Two sections of the crust are moving towards one another. If one side is composed mainly of lighter weight continental rocks and the other of heavier oceanic stuff, the oceanic plate will sink and be shoved under the continent, making a “subduction zone”. This is what is currently going on just off our coast, where the Pacific Ocean is giving the west coast a massive wedgie.

When two pieces of continental crust collide, however, there will likely be a huge smashup and the crust will crumble and fold and make a great big mess of everything, such as when India crashed into Asia, forming the Himalayas.

Suppose we are looking at a subduction zone with the ocean floor sliding under a continent. [Of course “sliding” is a ridiculously gentle word to describe a process that is enormously violent and goes on for thousands of years.] Now suppose that an old island arc or an old piece of mid-oceanic ridge is carried by the oceanic plate toward our subduction zone. Being at least partially composed of lighter material, it might not slide willingly down into the trench but rather just jam up the works. The forces that are causing the plates to move toward each other are not likely to give up so easily, causing a smaller (but still huge) scale version of the Himalayan thing.

Many times the offending arc is probably forced down under the continent, where it will be remelted, maybe appearing later in volcano somewhere on the land. But now and again the arc might break off from the oceanic plate and be shoved up onto the continental plate and we have an “emplacement” of an oceanic formation onto dry land – an ophiolite is born.

Plate Tectonics and Me

When I took Physical Geology in 1958, the whole idea of “continental drift” (now called “plate tectonics”) was dealt with in my textbook in one paragraph and treated as a childish fantasy; ten years later it was well on its way to being accepted as the principal mechanism of landscape formation on earth. Structures like the “Ophiolite of Fidalgo Island” were an important part of that major achievement in the history of geology and all of science. Ophiolites had long been identified and studied in mountain ranges like the Alps, Andes and Himalayas, but once their mid-oceanic origin had become clear the question became, “How in heaven’s name did they come to be in mountain ranges, especially ones far from any ocean?” Of course I should have said “any present-day ocean” but in those ancient times, it was believed that oceans and mountains tended to stay put and didn’t go gallivanting about the planet! The idea that a chunk of oceanic crust could break off in a collision between continents and end up on dry land would have been quite fantastical.

I was one of those childish types for whom drift made a whole lot of sense. I found the conventional schemes for explaining mountain formation to be contrived and even preposterous. They would have required that rocks go up and down, forming synclines and anticlines, but with no mechanism to explain that movement. Why was it respectable to suppose that rocks were going up and down, but childish to think that they move from side to side? Especially given the whole “gravity” thing? I was more than a little insulted by this. This caused a lack of respect for geology and geologists that contributed in no small way to my changing my major from geology to mathematics. (The fact that I was studying geology in northern Indiana, where they don’t have any geological formations more exciting than a sand dune, also contributed to my disaffection. For our big field trip, we had to travel almost 100 miles to find an exposure of bedrock: a limestone quarry.)

You can imagine my surprise when, just about 10 years later, I was browsing in the Natural Science Library at the University of MIchigan (trying to avoid working on my thesis) when I saw the words “Continental Drift” in large letters on the spine of a thin volume, and found that some of my fellow “childish fantasists” were actually able to get books published on the subject. And there was no small amount of undeserved smugness involved, as you can also imagine. Much later I learned that a very distant relative of mine (about a 7th cousin, with the common ancestor living in the 18th century) named Bruce (or Bruus) Heezen was an oceanographer who had led the first oceanic expedition that actually succeeded in measuring the expansion of the sea floor in the Atlantic Ocean.

Epilog

Scientists make lots of mistakes, but science generally gets it right. Eventually. There is no particular reason to think that any scientist’s opinion is any better than anyone else’s, and even a well designed scientific study can sometimes result in bad conclusions, especially in “soft” sciences like sociology but even in “hard” sciences like geology where it is often difficult or impossible to run controlled experiments, because of the enormous scale of the phenomena being studied. But the broad body of scientific investigators, testing and bickering and fighting and challenging each other’s results, ultimately filters out the nonsense. The process is often not very pretty (e.g., the recent hullaballoo caused by some climate scientists questionable handling of data) and is often confusing to the lay observer (e.g., the aforementioned hullaballoo) and can take a very long time (Galileo is a good guy, now. Right?), but the results are ultimately more reliable than reading Tarot cards or chicken entrails.

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1 Comment »

  1. You said this was going to be about geology, but it’s really mostly about you. I loved it, Al! I now know how come you majored in math and how smart you were even as a younger man. This was fun to read, even if a little technical, but I could stop reading when my eyes glazed over… 🙂

    Comment by DJan — February 24, 2010 @ 7:35 am


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