Re: How did this rock get to be the shape that it is?

I am constantly amazed by the astounding variety of rocks that inhabit 
our planet and seem to pop up where we least expect them!  Add to 
that fact the size and shape of this particular rock and no wonder your 
curiosity was aroused!  You have given a great deal of useful 
information in describing this rock, so let me work with that and give 
you my best guess as to what this rock is and how it got where you 
found it.

First off, the rock is almost a perfect sphere.  Right away we know that 
this rock has led a particularly interesting life.  In general, rocks are 
not spherical.  They are broken off of larger rock outcrops and have 
edges, not to mention jagged points and rough surfaces.  The vast 
majority of rocks begin this way and it is only through erosion that the 
sharp, jagged edges break off and the rock becomes smoother.  In 
fact, if a rock undergoes a great deal of erosion, it will become 
spherical.  Think of river rocks or pebbles on a beach.  These rocks 
have been tossed about by waves in a river, lake or ocean.  While 
being tossed around, they have struck other rocks, which tends to 
remove the bits of the rocks that stick out from the rest of the rock – 
the edges and the points.  So the longer a rock is allowed to roll 
about in the water, the more it is eroded and the smoother and 
rounder it gets.

Your rock, however, is far too large to have been smoothed in a river, 
lake or even the ocean.  Waves simply are not strong enough to move 
that rock around enough to make it almost a perfect sphere.  In this 
case, we need a more aggressive and powerful erosion process.  
Since this rock was found in Alberta, my initial reaction is that it was 
smoothed and made spherical by a glacier.  Glaciers are very 
powerful and as they move across the surface of 
the Earth, they can easily pick up large rocks.  These rocks are 
carried along within the glacier where they are constantly being 
ground upon by other rocks as well as the glacial ice.  Thus, a glacier 
erodes rocks very efficiently and the rocks it is carrying can become 
very smooth and nearly spherical.  When the glacier retreats, or melts 
and reverses its direction, the rocks it has been carrying are 
deposited wherever they drop.  Often very unusual rocks are later 
found in unusual places.  These rocks have been carried long 
distances, have much smoother surfaces than we might ordinarily 
expect, and may be unlike any other rocks in the nearby region.  
Sometimes many such rocks are dropped together and other times 
one or two are left behind at a given location.  These rocks are 
commonly referred to as "glacial erratics".  Check out the National 
Snow and Ice Data Center for an image gallery of glacial erratics 
(web link below).  

In particular, a photo taken by Paul Doherty (a scientist and educator 
at the Exploratorium in San Francisco – home page below) while he 
was on a trip to Sweden indicates the uniqueness of just the type of 
rock you have seen.  The photo shows a nearly spherical, granite glacial 
erratic sitting atop a smaller pile of stones, and can be seen here.  There
is almost no granite in this area, and so this rock had traveled a long
distance, brought by a glacier to this spot.  The Iron Age people used these
very special rocks to mark gravesites.  

Based on your estimates for size and mass, a spherical rock 1.2 m in 
diameter with a mass of 2200 kg gives a density of 2400 kilograms 
per cubic meter (kg/m3).  This value is a bit too low for granite, 
although my guess is that this rock is indeed granite.  Granite is a 
very tough rock to erode. It could be transported a long distance in a 
glacier while being constantly ground down without having eroded 
away completely.  The density of granite is typically 2700 kg/m3.  In 
order for your rock to have that density, the diameter would have to be 
1.16 m (only 4 cm less than your estimate!).   Alternatively, the mass 
needed would be 2400 kg (although I expect that your crane 
operator’s estimate for mass is more accurate than this).  

It is my opinion that the rock you are looking at is indeed granite 
transported to this location and rounded by a glacier.  However, to 
really be sure, you could break off a small piece (or possibly even 
take a close-up photo) and bring that evidence to a local museum, 
geology department at a local college, or the local Geological Survey 
of Canada office.  A geologist who could see the rock itself would 
certainly be able to identify whether it is granite and if it is a glacial 
erratic.  They might also be able to tell you from where the rock 
originated and possibly something about the path of the glacier that 
brought it to your construction site.  That would be a very interesting 
discovery – to find out how far that rock traveled to get into your hands 
today!


National Snow and Ice Data Center Gallery of Glacial Erratics:
http://nsidc.org/glaciers/gallery/erratics.html

Paul Doherty’s homepage:
http://www.exo.net/~pauld/index.html

Image of spherical glacial erratic in Sweden:
http://www.exo.net/~pauld/TomTits2000/sweden10.html