Over 200 years ago, we began to glimpse the outlines of the great engine that sculpts and maintains the Earth as we know and love it. It took a century and a half to come up with a paradigma body of theory and worldviewthat we could sink our teeth into: plate tectonics. Today the general public is familiar with the basics of plates. But this quiz gets into the deep details that only a Geo-Whiz has mastered. Could you be one? Give the quiz a try.
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 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 recommendand one to steer away from.
The miracle fireplug Geology Guide photo
As 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
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.
Panola Mountain courtesy SixtyTwo Point of View of Flickr
Geologists can be ecumenical about minerals: they're all interesting for various reasons. But some minerals are more equal than others, especially when money is involved. Ores are those big-money minerals that provide the essential metals of civilized life. So jump into this week's Geo-Whiz Quiz on ores and see if you are a truly well-grounded citizen. What could be a better way to get ready for Earth Day-the-way-it-should-be?
Earth history, as recorded in rocks, begins about 3.5 billion years ago (with a few older outliers). Solar system history, as recorded on the surface of solid bodies, ends about 3.7 Ga (few younger outliers). So to study the earliest Earth, between 4.6 and 3.5 Ga, all we have to go on is the history of the greater solar system. And for that, our best tool is a crude one: counting craters. But you know, we made a lot of progress in geology, throughout the 19th and much of the 20th centuries, with crude tools just like it. My new article, "How We Date Planetary Surfaces," goes into the subject in more detail.
Hydraulic fracturing (fracking) is the well-publicized process by which "tight" rocks can be made to produce oil and gas. It involves explosives, and in a handful out of approximately a million cases small earthquakes have resulted as existing stresses in unmapped faults were released. One of those exceedingly rare cases happened in Ohio last month, and the authorities revised the regulations today to ensure greater caution when the signs arise again.
Now, when permits are issued for drilling within 3 miles of areas of known seismicity (magnitude 2 events, which are usually barely felt), the drillers must install seismic monitors. When the monitors detect events of magnitude 1, drilling stops while the cause is investigated. Interested groups approve: the Interstate Oil and Gas Compact Commission calls it "a sensible response to a serious issue" and the Groundwater Protection Council says "these additional standards add even more strength to Ohio's already comprehensive regulatory program."
Today I've been deeply preoccupied with cosmic impacts after reading a new paper that models a truly gigantic asteroid impact. The paper, in the journal Geochemistry, Geophysics, Geosystems, bristles with equations that can show us, as accurately as basic physics knows how, what would have happened when an asteroid 45 kilometers across struck the Earth at 20 kilometers per second, some 3.26 billion years ago. I'm writing up something about how this research helps us understand the early Earth.
This kind of science is way over the heads of most of us, including me. But out in the field, there are features that you or I can see that are considered good evidence of impacts, called shatter cones. See an example in my list of common pseudofossils.
Shatter cones Courtesy Graeme Churchard under Creative Commons license