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Author Topic:   Growing the Geologic Column
Posts: 3849
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001

Message 10 of 740 (733672)
07-20-2014 1:39 AM
Reply to: Message 6 by Faith
07-19-2014 11:16 PM

You seem to want to divorce the Geo Column from the Geo Time Scale but the funny thing is when you google the column it comes up either in connection with the time scale or in some cases as identical with it.

I think to a geologist, the geologic column is the geologic time scale, especially if the context is that we are referring to THE geologic column. I don't have a lot of real world experience, and what I did have was quite a few years ago, but I don't recall geologists referring an area's rocks as being "the geologic column". The rocks themselves are "the rocks", "the geology", or "the stratigraphy". A column is a graphic representation on paper.

But even if you wish the "geologic column" to mean the rocks, there is no single "THE geologic column".

And no, an areas geology is not inherently tied to the geologic time scale. A geologist could look at an area of rocks, map them out, do cross sections, etc., totally independent of the time scale. As a student, I've mapped a number of areas where I never did know the ages.


Added by edit:

I don't like the forums glossary definition of "geologic column":

Geologic column - A diagram representing divisions of geologic time and the rock units formed during each major period.

That makes it sound like each time period has characteristic rock types. They don't.

Edited by Minnemooseus, : Added by edit.

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Posts: 3849
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001

Message 250 of 740 (734294)
07-27-2014 5:52 PM
Reply to: Message 245 by Percy
07-27-2014 5:22 PM

Re: Layer / Sill
I'm reading a fair amount of "Yes, but.." material from the evo side.

Basalt, diabase (also called dolerite), and gabbro all have the same (general) chemical composition. I say "general", in that there is a considerable amount of variation.

The difference is grain size, fine to coarse for the above. And yes, there are dikes and sills that are considered to be basalts (or basaltic).

Shallow and/or small intrusives can be quite fine grained. On point about intrusives that didn't seem to be covered very well, is that they would tend to have chilled margins. In other words, the margins cooled faster and are finer grained, and the interiors cooled slower and are coarser grained.

...here are many articles out there that aren't careful in their terminology and refer to sills and dikes as just "basalt" without the "intrusive" modifier.

That they are sills or dikes automatically means "intrusive". Intrusive dike would be a redundancy.

Gotta go.


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Replies to this message:
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