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The following picture (Painted wall cliff in the Black Canyon of the Gunnison National Park, Colorado, USA) shows a pegmatite dike: is it possible to tell what was the flow direction? From thin (see below the printed 1 in the picture) to thick (2) or from thick to thin?

enter image description here

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    $\begingroup$ Who said that they had to go up or down? They could also go laterally, either "forwards" or "backwards". This is most likely what you're seeing here, and the "branches" are not the dyke combining or splitting, it's just parallel lateral movements of the magma. $\endgroup$ – Gimelist Sep 12 '19 at 1:18
  • $\begingroup$ @Gimelist I am not a geologist, I just have questions that can be silly. When you say most likely you mean that the flow was probably "entering" or "exiting" from the plane of the picture, am I correct? $\endgroup$ – Alessandro Jacopson Sep 12 '19 at 6:20
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    $\begingroup$ Yes, exactly. Magma rarely flows directly "up". It flows in oblique sheets, with the flow direction overall being up, but it can have a large "sideways" component. A nearly-vertical outcrop cuts along this plane in an arbitrary orientation, and it is very challenging to tell which way the magma was going just from this image. $\endgroup$ – Gimelist Sep 12 '19 at 6:22
  • $\begingroup$ @Gimelist I would love to upvote your answer $\endgroup$ – Alessandro Jacopson Sep 12 '19 at 8:03
  • $\begingroup$ that’s fine, I already have more than enough rep 😀 $\endgroup$ – Gimelist Sep 12 '19 at 8:51
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I am not sure those are pegmatites. They may simply be granitic dikes. But in either case, you would need to assess the regional geology to look for potential sources of this material, which would be a granitic type magma of younger age than the host rock. You could then map out the size, orientation, and frequency of these dikes to tie them back into their parent unit. Geochemical fingerprinting and age dating could help, too. But there is no way to look at that picture, or a given outcrop, and tell the direction. Remember that the geometry of the third dimension, the one in and out of the picture or outcrop, always needs to be kept in mind and is typically unknown.

I am pretty sure it was not top down, though!

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  • $\begingroup$ Geologic Principles—Cross-cutting Relationships at nps.gov/articles/… says Pegmatite dikes exposed in Painted Wall cliff face. The light colored dikes are younger than the dark rock. I understand that the dike is the light colored veins, I am correct? $\endgroup$ – Alessandro Jacopson Sep 10 '19 at 18:38
  • $\begingroup$ Yes, you are absolutely correct. No doubt. $\endgroup$ – Daniel Sep 11 '19 at 4:11

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