Below is a short pamphlet I wrote for my parents, concerning a large granite boulder decorating their front yard in Grinnell, Iowa.  Michelle and I printed the text and figures on a color laser printer and she bound it in a four-leaf booklet with a cover of marbled French paper.

 

A TREATIES ON THE FORMATION

OF

RICHARD AND SYLVIA VOGEL'S

BIG GRANITE BOULDER

FOUND NEAR

GRINNELL IOWA

WITH NOTES ON ITS

GLACIAL TRANSPORT AND MINERALOGICAL COMPOSITION

INCLUDING A SPECIAL SECTION ON

TIME AND THE COSMIC CALENDAR

BY

GREGORY VOGEL, BA, MA, RPA, PH.D., ETC., ETC., ETC.

WITH BINDING AND EDITORIAL ASSISTANCE BY

          MICHELLE BERG VOGEL, BA, RPA, MPA, ETC., ETC., ETC.
 
    
COPYRIGHT 2006
OAK ST. & CO, INC.
NEW YORK, LONDON, AND KAMPSVILLE
ADDITIONAL EDITING BY MSSRS. ORLOFSKY AND G. CAT,
AND MS. DAILA AND MR. HOUDINI, FLD., C.A.T., ETC., ETC., ETC.
  
 
 
 
Your rock is granite. 


 More specifically, it is a glacial erratic igneous boulder with intrusive sheet dikes. 

 
Granite is an intrusive igneous rock.  Its formation begins several miles underground as magma rises and then cools near the top of the earth's mantle.  Most granite forms within the subduction zone of convergent tectonic plate boundaries, where one plate is being pushed underneath another.  Rocks in the subducted plate (the one that is being pushed underneath) become very hot from the depth and pressure they experience – up to 1,600 degrees C.  This melts them into magma, which then rises upwards because it is hot and not as dense as the material surrounding it.  As the magma rises it cools and lithifies (become solid rock again), at about 650 degrees C.  Rocks formed in this way are classified as granite if they have a visible crystalline structure, and are composed primarily of quartz, feldspar, and mica.  Granite's color comes from many different minerals.  The pink hues in your boulder come from potassium-rich feldspar or iron.
 
 

    The formation of granite within a subduction zone.
(Figure credits below) 

 
The crystals in granite are quite large compared to those in most igneous rocks, indicating that it cooled slowly, over a very long period of time.  The larger the crystals, the longer it took the liquid minerals to cool.  The size of crystals in most granites indicate that they took at least a few hundred thousand years to cool. 
 
Sometimes large pockets of granite crack from pressure while they are still deep underground, and new magma is squeezed from below into the cracks.  This forms sheet dikes.  The sheet dikes in your boulder have a different color composition and crystal size, indicating that they formed long after the original, more gray parts of the rock.  The sheet dikes also appear to be made of granite more resistant to weathering, because they protrude slightly from the surrounding material.
 
Your granite boulder was formed near the base of a mountain range that extended across much of the North American mid-continent in Precambrian times, about 1.3 billion years ago or more.  It was later exposed to the surface as the mountains weathered away, and was originally located somewhere far to the north.  It may have been first exposed in Minnesota or Wisconsin, or even farther to the north in Manitoba or Ontario.  It probably sat near the earth's surface for close to 1 billion years.
 
Sometime within the last 2 million years, your rock was moved to central Iowa by glaciers.  Glaciers form in cold climates when the amount of snow that accumulates each year is greater than the amount that melts.  When the snow reaches a depth of about 60 meters, it deforms from its own weight and begins to flow downhill as a glacier. 
  
 


Glacial Geology of Iowa
(Figure credits below) 

 

Glaciers that grow taller than the mountains from which they originate and therefore flow in directions controlled by their own dynamics are called continental-scale glaciers.  These glaciers grow to be several miles thick, scour the earth down to bedrock as they move, and leave behind great quantities of rocks and soil when they eventually melt.  Within the last 2 million years, the earth has experienced about 19 major cycles of large-scale glaciation.  The continental-scale glacier that moved your rock was centered north of the Great Lakes.  The latest version of this glacier (lasting from about 90,000 to 18,000 years ago) is named the Laurentide Ice Sheet.  Rocks moved in this way are called glacial erratics, because they have been moved far from their original source by glaciers.  Your boulder was deposited near the southern tip of one of the later glacial advances, within a glacial region known as the Iowan Surface.  
 

          Extent of the Laurentide Ice Sheet during the last glacial episode.
(Figure credits below)
 
 
 

                                               A NOTE ON THE AGE OF THE ROCK 

Consider the "Cosmic Calendar" of the age of the Universe, which compresses time from the Big Bang until now into one calendar year.  The Big Bang occurred about 15 billion years ago (immediately after the stroke of midnight on January 1 of the Cosmic Calendar).  Cosmic dust coalesced into the primordial Earth 4.4 billion years ago, on September 13.  The formation of your rock probably began around 1.3 billion years ago, sometime in early- or mid- November.  It took a few hundred thousand years to cool, becoming solid rock sometime around Thanksgiving.    

Diagram of the Cosmic Calendar with significant dates noted.  Click image for larger version.
 
 
The rock was moved to central Iowa by glaciers between about 10:20 and 11:58 pm on December 31.  Anatomically modern human beings first appear about 160,000 years ago: 11:55 pm on December 31, and you were born about 11:59 pm and 59.95 seconds – only 1/20th of a second before midnight.  Just now, as this treaties is being written (2006) the clock strikes 12.  Consider this when you contemplate your boulder, and please don't take your rocks for granite.  (Isn't that gneiss?)
 

 

 

 

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Figure Credits

 Subduction figure modified from San Diego Supercomputer Center, Geoinformatics Lab Discover Our Earth Teacher's Guide: http://discoverourearth.org/instructor/tectonics/subduction.html.

Glacial geology of Iowa modified from Landforms of Iowa by Jean C. Prior, University of Iowa Press, 1991.

Extent of the Laurentide Ice Sheet during the last glacial episode modified from Ice Ages In New England by Tammy Marie Rittenour: http://www.bio.umass.edu/biology/conn.river/iceages.html.