What are the "laws" of geology?
One of the best parts of the study of the earth is that there really aren't any laws that work in all situations and at all times. (Many also consider this the worst part of geology, and I certainly confess to having felt that way myself on more than one occasion.) In any event, there are only a few and they really aren't "laws" anyway, as you'll see.
Uniformitarianism: the present is the key to the past
Some things just work the way they do - usually because they follow the "laws" of the other sciences. Like gravity, and how water runs downhill, and stuff like that. We assume that earth processes that are working now have probably worked in pretty much the same way throughout geologic time. Hopefully, there aren't too many exceptions to this one.
Original Horizontality: sedimentary layers are initially deposited in horizontal layers
The forces of reality beat up on the earth and break it into small pieces, which then wash down rivers to the ocean and settle out on the seafloor. In horizontal layers. Lots of exceptions here!
Superposition: the layer on the bottom is the oldest, and they get younger as they pile up.
This makes a lot of sense if you think about it, especially if the sediments are settling down from above. But what if they aren't? Definitely some exceptions to this one.
Cross-cutting relationships: the thing being cut is older than the thing doing the cutting
My personal favorite, mostly because it usually works. This is the zucchini concept - you can't cut it if it isn't there. Works for things like faults and dikes and veins.
That's about it for the "classical" laws of geology. Really only a set of general guidelines. Kind of leaves one hanging, but at the same time allows limitless freedom for observation and interpretation. Refer to Strickler's Laws of GeoFantasy for additional legal support of the earth sciences.
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