Today's NOVA Geoblog (http://nvcc.edu/home/cbentley/geoblog/2009/11/two-different-ways-of-getting-r... has a nice discussion with illustration of how to help geology students think about two different ways that something can get to be spherical: an oolite gets round by accreting on all of its sides as it rolls around and experiences chemical precipitation, but a quartz sand grain gets round by being abraded on all of its sides as it rolls around and experiences physical weathering. The observable result (a spherical grain) is similar, but the processes is different.
Callan's well- stated and well-illustrated example with the oolites and the clastic grain could be the basis for a new "hypothesis template."
If you observe something spherical or near spherical, possible causal hypotheses are that:
* it got that way by building up in layers, building up evenly on all sides (e.g. hailstone, oolite.)
* it got that way by starting out irregular in shape and had its corners and protrusions abraded off (e.g. quartz sand grain.)
* it formed under conditions where the internal gravitational attraction of the body itself was more than the gravitational attraction of external objects (e.g. a planet, ball bearings made in the space shuttle www.enotes.com/how-products-encyclopedia/ball-bearing.)
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