Icelandic Plate Tectonics: No Scree-ching Halt: Give It an Inch and It'll Take A Year

     At long last, we are ready for the trip to Iceland now that the Viking Sunstone (optical Iceland spar or calcite) has been found:

Viking Sunstone--Soap-Bar Sized Optical Calcite 

 

     Optical calcite is rhombohedral and refracts light in a way that Norse explorers of 900-1200 A. D. were likely able to locate the sun even after sunset and on cloudy days. In those pre-GPS days, the calcite rhomb was accurate to within 1 degree:

     Optical calcite is found in abundance in the scree of Iceland, though it is now a protected resource since Icelandic tourism has recently blossomed. You may read more about calcite here, but I want to get us to Iceland, the only place on earth (currently) where a mid-oceanic ridge occurs on land:

Calcite Optics

     To repeat, Iceland is the ONLY place on our planet where one may actually witness oceanic crust being created on land. All other parts of the global mid-oceanic ridges are deep beneath the surface of the ocean. These folks are walking between two tectonic plates, the North American plate and the Eurasian plate, in Iceland:

     The theory of plate tectonics describes the large-scale motions of the lithosphere. A good, general overview of the theory may be found here:

 Plate Tectonics Overview

     The divergent boundaries between plates, known as mid-oceanic ridges, are the places where oceanic crust is created. There is a particularly good color, animated graphic in the link below (and shared here) which shows the new basalt (nicknamed MORB for Mid-Oceanic Ridge Basalt) or gabbro being created:

 Mid-Oceanic Ridges

     Oceanic crust is richer in iron and magnesium making it heavier than continental crust, which is richer in lighter silica. The very newest "skin of the earth"  is created at mid-oceanic ridges. As one moves away from the center of the ridge, the oceanic crust is progressively older on mirroring sides of the ridge. Almost all oceanic crust is 200 million years old or younger, fairly young in geologic terms. Then, at the margins with lighter continental crust, the oceanic crust dives beneath the lighter continental crust and is essentially recycled. The major plates and their current movements are shown below:

     In Iceland, one may witness this new "baby earth skin" creation directly. In the land of fire and ice, you can actually touch this brand new earth as the mid-oceanic Atlantic ridge runs right through Iceland:

       The Eurasian and North American plates are moving apart at the rate of about an inch a year. There is no scree-ching halt to the oceanic plate movement due to the convection currents in the earth. The convection is akin to heating up pudding on the stove. The rising hot pudding comes to the surface then plunges back down again as it cools at the surface. 

        To witness new earth being created is, for me, amazing. After seeing the volcanos, ice, spar, and other Icelandic delights, here's that optional side trip to the Lucky Leif bridge in southwestern Iceland. It's a moment to ponder the earth's dynamic nature while straddling the two tectonic plates. Our trip would have been unabridged without it. :-) Enjoy!

        I welcome your comments, insights, and any sparring. 

      Here's to MORB excitement with all of you gab-BRO and SIS enthusiasts,

      Word Woman (aka Scientific Steph) 

 

Petrographic Thin Sections, Deadlines, and Nature's Stained Glass

     Deciding on this week's Partial Ellipsis of the Sun topic was harder than usual. The genetically modified organisms (GMOs) we saw on the western side of Kaua'i, guarded by several people, trucks and barbed wire, was an option. I decided that topic needed more research than driving past miles of corn (in various developmental stages) where sugar cane and pineapples used to grow ten years ago and reading a few articles. I will also read Frankenstein's Cat: Cuddling up to Biotech's Brave New Beasts by Emily Anthes before addressing this topic. (Great suggestion, Jan).

     So...deadlines to deadliness :-), on this Tuesday I decided to write about petrographic thin sections because they are something I know and they are amazing beautiful.

      Here's a piece of olivine, a magnesium iron silicate (called peridot when of gem quality):

      

     Amazingly beautiful, right? How about cutting a nice, clean, thin slice of this and attaching it to a glass slide. And then, how about looking at this thin slice of rock (or thin section) under a petrographic microscope? Still not impressed?

      Well, how about under crossed Nicols filters (see below) under a petrographic microscope, in thin section?

 
      The  two polarizing filters on a petrographic microscope work similarly to polarizing sunglasses. Each filter allows light vibrating at one particular orientation to pass through. One of the two filters is fixed. The other can be moved in and out of the light path. That filter's  polarizing direction is perpendicular to the fixed filter. 

      If you put a thin section of a rock in between the two polarizing filters (referred to as looking at the section under "crossed Nicols") it creates the stunningly beautiful portraits we see above of olivine and below of  zircon: 

     The uncrossed filter photograph of zircon, a nesosilicate, is below:

      
       Here's one more crossed Nicols thin section of basalt, a fast-cooling, fine-grained volcanic rock from Thailand with two large spectacular olivine crystals in the center.

       And, for completeness, a sample of basalt:

           I will end this week's illustration-rich blog with a photo of one of my favorite thin sections of gabbro, coarser-grained (i.e. slower cooling), intrusive igneous rock found near mid-oceanic rift zones (under crossed Nicols):

         The brightly colored orthopyroxene crystals are surrounded by striped plagioclase feldspar. Slower cooling produces some amazing crystals.

          Stained glass: made by nature.

          Enjoy! Next stop, a field trip to Iceland to see the mid-Atlantic ocean ridge on land to see that basalt and gabbro up close!

Petrographically yours,

Word Woman (Scientific Steph)