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Seeing Geologic Maps

Bedrock geologic map

They will show you the rocks beneath your landscape and make it newly strange, in a good way.

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Geology Spotlight10

Do We Need Hadean Eras?

Saturday April 19, 2014

In geology, the rocks have a way of messing with our pretty schemes. One instance I'm thinking of involves the base of the geologic time scale. The Earth itself is about 4.5 billion years old--but the time scale starts at the base of the Archean Eon with a time unit called the Eoarchean Era, running from 4.0 billion years ago (4 Ga) to 3.6 Ga. Like most of the Precambrian time periods (and unlike the more familiar Phanerozoic time periods), the Eoarchean is based on arbitrary numbers rather than notable geologic events.

When this part of the time scale was put together, we figured, from long experience, that there weren't any rocks older than 4 Ga. So much for experience: now we have rocks in hand that are older than the official time scale, and some zircon crystals that are reliably dated at 4.4 Ga. Today, Hadean time is no longer a matter of conjecture. So I hope someone is thinking about setting up signposts in deep time for the Hadean Eon. I suggest the Paleohadean for 4.5 to 4.4 Ga, the Mesohadean for 4.4 to 4.2 Ga, and the Neohadean for 4.2 to 4.0 Ga.

Here's an oddity: in the Cenozoic Era, the Paleocene Epoch comes before the Eocene, but in the Archean the Eoarchean comes before the Paleoarchean. Why is that?

It's Frisco Quake Day

Friday April 18, 2014

1906 earthquakeIt was 18 April 1906, 5:12 local time when the rumbling began. (Today that hour would be 6:12, a quarter-hour before sunup.) More than a full minute later, the shaking was still going on, and hundreds, maybe thousands of San Francisco's buildings had fallen, broken or caught fire. With the water supply rendered useless, fire raged over the city for three days and left half its people homeless. And that was only the beginning of the story of the great San Francisco earthquake, 108 years ago today. The legend lives on, even if no one alive can remember that day, because artifacts of the time still stand in Baghdad-by-the-Bay. This fire hydrant was connected to a cistern that continued to supply water near Mission Dolores and saved the neighborhood. Ever since that day, the hydrant has been painted gold. Folklore and ceremony are good ways to keep awareness alive.

If you've ever given serious thought to how earthquakes would affect where you live, the 1906 quake is as timely a lesson as ever, and there are several good books I can recommend—and one to steer away from.
The miracle fireplug — Geology Guide photo

The Code of Hammering

Thursday April 17, 2014

hammer outcropAs field season returns in my part of the world, I'm gearing up for some nice outings. So it's time again to present my code of hammering. There are guidebooks that touch on matters of professional practice, and every rockhound group teaches its members hammer safety. But as an amateur geologist I practice a game between that of the professional and the rockhound. It incorporates a respect for the rock as something with its own aesthetics and right to exist as nature made it. See if you agree.
Outcrop practicing satori — Geology Guide photo

How to Look at a Rock

Tuesday April 15, 2014

I try to put up basic information here, but even so I'm way ahead of most people. Fortunately they keep writing me and visiting the Forum with pictures of rocks, and that tells me they need information that's even more basic. They say, "I've looked all over and can't find a picture that looks like this." That's not how to do things. Don't look at pictures, look at your rock. Look closely at it. Study it and take notes. Start here with Rocks 101: How to Look at a Rock. That said, I've added some pictures to the article just because.

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