Daily Archives: May 15, 2011

Ages of Rock

Wow, Bryce, you're looking positively Cenozoic! (photo by Sally Reeder)

The other day I pulled an old college geology textbook off my bookshelf and made a surprising discovery:  nothing in it seemed the least bit familiar to me.  It was as if I had retained nothing from an entire semester!  Then the realization hit me that I had not taken any geology classes when I was in college.  This was my wife’s college textbook which had somehow found its way onto my shelf.

While I was browsing through her book I made another surprising discovery:  a note in Sally’s handwriting, addressed to someone named Dave, that said “we’re through.”  Interesting as that was, it didn’t deter me from my quest, which was to find out when the Jurassic Period of earth’s history was.

As you may know, geologic time is divided into eons, eras, periods and epochs.  Scientists believe that our planet is 4.5 billion years old, give or take a few million (factoring in leap years).  The oldest era was the Precambrian; then you have your Paleozoic, your Mesozoic and your Cenozoic, where we currently reside.

It turns out that the Jurassic Period, when there were dinosaurs and flying reptiles, was roughly from 199 million years ago to, oh, 145 million years ago.  That was just a portion of the Mesozoic Era, which went from 251 million years ago to late August of 65,000,000 B.C.

These dates are determined primarily by a) studying sedimentary strata, including fossil remains; and b) chemical analyses of radioactive minerals.  The latter are known as “radiometric dating techniques”.  It may have been Dave’s radiometric dating techniques that repelled Sally, who knows?

Anyway, by analyzing how much of the material has decayed — its so-called “half-life” — scientists determine the age of, say, a shrimp fossil and the rock in which it is embedded.  For example, the half-life of Carbon 14 is 5,730 years, according to one of my sources who asked to remain anonymous.

There is general agreement among scientists on the half-life of Carbon 14, but they have other issues.  In fact, relations between some factions within the geologic community have become downright… well, rocky.

You and I and some scientists have been blithely referring to the segment of time after the Cretaceous Period as the Tertiary Period.  We reminisce fondly about its Miocene Epoch, with its grasslands and grazing mammals — and how about the appearance of whales, huh?  How could you not get nostalgic about the Tertiary Period? 

That’s what I thought, but that term is so twentieth-century, apparently — it is no longer used by the International Commission on Stratigraphy.  They sneer at Tertiary (and Quaternary, too), preferring to think of the period from 65 – 1.8 million years ago as two distinct periods, which they call the Paleogene and the Neogene.

Maybe it’s just as well that I’ve forgotten this stuff I never really knew in the first place, since I guess I was wrong.  But why these academicians think that particular change in terminology is worth fighting over remains shrouded in mystery.  So, by the way, does the true identity of a long-ago geology student named Dave.