Two different ways of getting round grains
On my Historical Geology field trip to the Massanutten Synclinorium, we observed oolites (ooids) in Cambrian-aged limestones deposited in the Sauk Sea. Like these:
On the left, you see marine deposition in warm water becoming more concentrated in its load of dissolved ions as evaporation removes water (but not ions). Accordingly, chemical precipitation of calcite begins. Little grains on the bottom get a layer of calcite deposited over them. These grains are within reach of the wave base, so they get rolled in one direction, then rolled back again. As they oscillate back and forth, they expose "both their back and their belly" to the precipitating calcite, so they get concentric layers deposited: oldest in the middle, youngest on top. Like a gobstopper! Or a hailstone. They literally grow more spherical over time.
On the other hand, the right side of the diagram shows a chunk of rock, broken off from its source area, and tumbling downstream. As it travels, the sharp corners are most susceptible to being snapped off or abraded away, meaning that as it loses volume, it takes on a more and more rounded shape. This is physical weathering at work, not chemical precipitation. Note that even if it's very well rounded, that doesn't mean that it's necessarily spherical.
On the left, you see marine deposition in warm water becoming more concentrated in its load of dissolved ions as evaporation removes water (but not ions). Accordingly, chemical precipitation of calcite begins. Little grains on the bottom get a layer of calcite deposited over them. These grains are within reach of the wave base, so they get rolled in one direction, then rolled back again. As they oscillate back and forth, they expose "both their back and their belly" to the precipitating calcite, so they get concentric layers deposited: oldest in the middle, youngest on top. Like a gobstopper! Or a hailstone. They literally grow more spherical over time.
On the other hand, the right side of the diagram shows a chunk of rock, broken off from its source area, and tumbling downstream. As it travels, the sharp corners are most susceptible to being snapped off or abraded away, meaning that as it loses volume, it takes on a more and more rounded shape. This is physical weathering at work, not chemical precipitation. Note that even if it's very well rounded, that doesn't mean that it's necessarily spherical.
Labels: art, cambrian, field trips, limestone, primary structures, sediment, teaching, weathering
3 Comments:
Do you happen to have a thin-section of these oolites? Might look pretty cool. My thesis revolves around oolithic limestones. From my experience a lot of students also mistake the process of ooid formation for the accretionary snowball effect.
Sorry, no thin sections handy!
I've been toying with the idea that it should be possible to help geoscience students learn to make causal interpretations from spatial information (like shape) by giving them (or helping them construct?) "hypothesis templates" that provide an array of explanation-types for common spatial configurations.
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 http://www.enotes.com/how-products-encyclopedia/ball-bearing.)
Some of my ideas about "hypothesis templates" are at: http://serc.carleton.edu/earthandmind/posts/more_hypothesis.html
and
http://serc.carleton.edu/earthandmind/posts/hypothesis_temp.html
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