In an email to me wwh came close to the answer to my question , so I'd best hurry my explanation of the air root conundrum so I'll get credit for being hip and in the know. Which is...

The largest and deepest (over 1200 feet) of the Great Lakes is rising on the western side. The land is still rebounding from the depressed state it underwent during the last glacial period which ended 12,000 years ago when it was depressed by the weight of several thousand vertical feet of ice. Today the land is rising at the rate of one to four feet per century.

Meanwhile the land to the south and east has achieved an equilibrium of sorts and no longer rises. The effect, in effect, is like tilting a great pan of water and so Lake Superior is slowly draining into the other great Lakes.

This unassailable reality plays havoc with Lake Superior's shorelines and erosion rates multiply, and inadvertantly this was the penultimate cause of the air root of the fir tree of Chapel Rock. To wit...

Sometime during the last 100 years a great thousand ton sandstone rock was removed from beneath the twenty foot tree root by great storm waves and today that rock is sand on the nearby beach.

(wwh had proposed that a commercial sand company had removed the sand supporting the root by strip mining as they have in other parts of Michigan where their dune mining is evidenced by vast and unsightly open pits.)