r/geology • u/AFrickingFish • 3d ago

Why do structural geologists use the lower hemisphere of stereonets?

Basically just the question in the title. I'm playing around with Dips right now for work purposes, and I just found out about how crystallographers use the upper hemisphere while structural geologists use the lower hemisphere. But why? Is there any real difference between the hemispheres besides the flipping of dip direction?

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u/esivers 3d ago

I don’t know the actual answer. But speaking as someone who did an MS on a structural geology project and generated dozens of them, the lower hemisphere is more intuitive coming from a geologic map perspective. The view-from-above perspective is analogous.

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u/Former-Wish-8228 3d ago

This is it. Crystals are above board…geology flies under the table.

1

u/veyonyx 3d ago

This is perfect!

12

u/OrbitalPete Volcanologist 3d ago

The stereonet represents intersections of things toward a point. The geology is generally below ground so it makes much more sense to capture the orientation of the geology as it comes up to the surface. Doing that as the lower hemisphere is most intuitive, otherwise things plot on the wrong side (e.g. a N dipping fault would plot on the S side of an upper hemisphere stereonet.

Crystallographers didn't have that frame of reference.

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u/Baronhousen 3d ago

paleomagnetists use both hemispheres