Friday, November 26, 2010

Any Old Neighborhood

An adventure in Watching for Rocks can be anything you want it to be.  It can involve exotic geologic destinations and months if not years of planning.  You can devote endless hours perusing cheap-flight websites, we-be-the-best-deal car rentals, and hope-this-isn’t-a-fleabag-motel-dot-com until you start hyperventilating with excitement.  Or, you can merely slip on a jacket, grab your camera, and talk a walk in any old nearby neighborhood.    If you live where the rock outcrops are exposed and there isn’t all that pesky vegetation to obscure them, so much the better.  Otherwise, you might consider bringing along a machete.

Nearly every square foot of southern Utah’s geology, generally unhindered by that pesky vegetation, has been examined over the past hundred years or more by tens of thousands of people on foot, on horseback, by boat, in autos, in airplanes, and remotely by satellite.   A couple of years ago an interim geologic map of the St. George area was published and I became the proud owner of a copy  –  included are page after page of descriptions of all the mapped stratigraphy including that of the hills behind where I live.  I remain delirious whenever I have cause to study this colorful map.

I don’t know how many readers of WATCH FOR ROCKS know how to read a geologic map.  It isn’t that difficult, really; once you figure out that the top layer on some particular patch of ground is the one that is represented on your particular section of map, you’ve got it made in the shade.  However, applying the map scale to what you see on the ground can be a bit of a sticky wicket.  Trying to find the hill behind your house on a map with a scale of 1:100,000 (≈5/8 inch on the map equals ≈ 1 mile on the ground) is just slightly less difficult than trying to pronounce Eyjafjallaj√§kull volcano.     

I discovered that these are volcanic rocks on the hills immediately behind where I live, and two low hills are mapped as two distinct units:  the Quichipa Group and the Condor Canyon Formation.  These are mainly tuffs, or cemented volcanic ash.  Their age dates range from the late Oligocene Epoch (≈26-23 million years ago) to the early Miocene Epoch (≈23-16 million years ago).  These rocks tell of violent volcanic eruptions which produced immense quantities of ash and rubble that were blown across eastern Nevada and western Utah.  

How immense?  Some of the individual layers of ash (and there were plenty of them in the 7 million years spanning the late Oligocene to early Miocene), representing a single volcanic blast, can be traced across areas of nearly 15,000 square miles.  These sheets of ash are fairly uniform in character and so can be traced with little difficulty across vast areas.

As material is deposited on the earth, such as happened with our ash layers, the iron and other magnetic minerals in the layers are aligned in the same direction as the earth’s magnetic field.    Scientists discovered decades ago that the earth’s magnetic field has reversed periodically in the past – the magnetic poles of north and south were switched or “flipped.”  

 In the Condor Canyon Formation mentioned above there is a layer of ash called the Swett Tuff member.  If you happen to find yourself standing, say, 20 yards or more away from this Swett Tuff while holding a good compass, the needle will point north as expected.  But move ever so slowly closer and closer to the Swett Tuff and your compass needle will start to quiver.  Keep moving closer, and closer, and closer until you are within inches of the tuff at which point your compass needle will have flipped.  Your compass needle will now be pointing to the south.  

I had hoped that this Swett Tuff would show up as an outcrop in the hills behind where I live.  Sadly, it does not.  I know where to find it, though, and the location isn’t very far away.  I need to get back there to see it again; the magnetic reversal it displays is nothing short of astonishing.

Note: 
Utah Geol. Survey Open-File Report 478 was my source for map information. 
“The Broken Land – Adventures in Great Basin Geology” by Frank L. DeCourten was my source for information on Oligocene and Miocene tuffs.  I refer to it often and recommend it highly. 

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