Thursday, January 9, 2014

Geo 730: Jan. 9, Day 374: Chilled Margins

I posted this photo last May as a "teaser," but I think it's worth revisiting in context. The chilled margins of the dike under the hammer are quite apparent in this photo, and give visiting geologists the ability to relatively date these dikes- that is, determine which are younger and which are older, relative to each other. In the case above, this dike is younger than the material on either side, as shown in the annotation below.
I created the following schematic in much the same way that sheeted dike complexes actually form: starting with a single dike (1), then pulling it apart to inject (2). As it turns out, the two dikes labeled (2) don't butt up against each other, so you'd have to guess which one came first- there's no way to say for certain. Then the dike (2) on the left was split and intruded by (3). I'm not sure this has any practical scientific value, but as an intellectual curiosity, pondering the immensity and time scales of earth processes, and getting a gut feeling for spreading ridge dynamics, I find this sort of puzzle terribly rewarding.
Photo unmodified. May 8, 2013. FlashEarth Location entirely conjectural. Indexed

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