I finally figured out why obsidian is the way it is, thanks to Hans-Ulrich Schmincke's fine Volcanism. Obsidian is supposed to be a fast-quenched lava, a volcanic glass, but most localities don't look any different from ordinary lava domes, and we've watched lava domes and they don't cool super fast. Now basalt glass you need to quench pretty hard, for instance in a lava pillow in the deep sea or in the hyaloclastite layer of a tuya. But the viscous high-silica lavas (usually rhyolite) that become obsidian turn glassy so easily because their solidus is so close to their liquidus. Most molten rock solidifies over a range of temperature, allowing high-temp minerals to crystallize before low-temp ones. But not rhyolite. It doesn't cool fast, it hardens fast. Obsidian is fascinating stuff — have a look at some.
_{Obsidian tools — Geology Guide photo}

This has been a tumultuous week, and today my city has been struck with a debilitating heat wave. Enough of disaster, enough of politics, enough of everything. Today I rummaged around in my photo collection and added a few new items to the slickenside gallery. I think I'll do some more rummaging.
_{Slickenside in limestone — Geology Guide photo}

In the wake of the Chengdu earthquake earlier this week, the Chinese Seismological Bureau had to issue a statement yesterday telling people to ignore rumors of a supposed aftershock prediction. There was no prediction, because aftershocks can't be predicted any more than mainshocks can. Aftershocks are more predictable than regular earthquakes only in this respect: aftershocks of a particular earthquake, considered as a whole, can be described by three numerical laws. That underlies the US Geological Survey's daily 24-hour aftershock forecasts for California.

There is a line of thought among earthquake researchers that maybe, if you back away far enough, the earthquakes in a large tectonic province or even the whole planet can be analyzed as if they were a great self-generating aftershock series. That underlies some interesting basic research tied to earthquake prediction, but it doesn't provide the planners and responders of today any useful information.

The Times of London reports that China's bloggers are sharing rumors and conspiracy theories in the wake of the Sichuan earthquake. Because earthquakes are such unsettling and mysterious events, widespread irrational thinking is common everywhere they happen. I've heard Californians make the same claims in connection with earthquakes — animals did strange things; scientists predicted a quake but the authorities suppressed them; the government won't accept evidence that contradicts its dogma, and so on.

Let me say first that no one predicts earthquakes today in government or academia. China tried for a while starting in the 1960s. Offices opened all over the country to collect stories of unusual animal behavior, natural phenomena and seismic signals. In 1975 an earthquake warning issued to the city of Haicheng was actually followed by an earthquake, and many lives were saved. That is the only significant time the system has worked, and today the general scientific opinion is that it was coincidence. The Chinese model may be flawed by its cultural/political origin at the height of Maoism, the apotheosis of the proletariat. Large networks of amateur observers were relatively easy to establish, and the prediction algorithm relied on folk traditions. But human sensory observations are notoriously unreliable, and the folk traditions do not hold up against actual data.

The eruption of rumor in the Chinese blogs surely matches what everyone on the street is saying, the same things people everywhere have always said. The history of amateur observation probably adds nuance to the Chinese response. But it's a good inoculation to read the Chinese expressions of panicked thinking now, in hopes that we will be less susceptible when our turn comes in our own lands.

It is literally human nature to turn events into stories imbued with meaning. People who claim to have open minds are often the fastest to shut them down, grasping at an explanation and clinging to it against all evidence. One of the hardest things a scientist can do is resist human nature and remain open to new explanations, but that's what they train for.

A large earthquake struck the heart of western China Monday afternoon local time near the city of Chengdu, killing thousands of people. The US Geological Survey's master page for this event has a page generated by the new population-centered risk software PAGER, showing that more than 15 million people were exposed to damaging levels of ground shaking. It was a shallow thrust event related to the spreading of the Tibetan Plateau against the Chinese craton. Shaking was felt very widely, and news continues to filter out from the region.

The flood disaster in Myanmar was caused by a severe storm, but it might as well have been a tsunami — indeed, the Indian media used the traditional name "tidal wave" for the storm surge. I've argued that geologists should stop objecting to the press using that term because it isn't really confusing, but now I wonder if I'm wrong.

On the one hand, the results of tsunamis and storm surges are the same for human purposes and probably the same in the sedimentary record. When we look for ancient tsunamis, the historical record is ambiguous. Some of history's most deadly tsunamis might have been storm surges. History mentions very deadly "tidal waves" along the South Asian coast, comparable to what we're seeing in Myanmar, and for planning purposes we need to beware both earthquakes and storms.

On the other hand, for geology the two causes must be distinguished, if possible. In some places, such as the American Gulf coast, there are no earthquakes, and the prehistoric hurricane record can be deciphered by experts in paleotempestology. But if the Myanmar cyclone had happened a thousand years ago, could we tell from the sediments what had happened? Hard to say. Without sure signs of earthquake such as sand blows or suggestive signs like mud volcanoes, we're stuck.

But probably arguing about "tidal waves" versus "tsunamis" is immaterial, just as it is when the press talks about the "Richter scale" for earthquakes. The important thing is that geologists get to explain Earth hazards to the public.

A research paper on the planet Mercury is an example of a new discipline: comparative geomagnetic core studies. You see, Mercury is the most Earthlike planet in one respect—it has a large, fluid iron core that produces a magnetic field. The paper, in April's Geophysical Research Letters, analyzes high-pressure experiments on hot iron-sulfur mixtures and suggests that iron crystallizes, leaving behind a sulfur-enriched liquid, and rains—rather, snows—down onto a solid inner core, like Earth's. Not only that, the iron snows from two places in the core; from its top and from a zone partway down. Earth's core is not so exotic; in it, iron snow forms at the top of the core taking nickel along with it, leaving an iron-enriched liquid behind. (Sulfur is not involved.) But Earth has a far more energetic magnetic field, probably related to its much greater mass and much greater angular momentum due to its rapid spin. After all, Mercury is very un-Earthlike in its rotational spin, making its day longer than its year. And let's mention the other two terrestrial planets while we're talking: Venus has a large liquid core but an extremely slow spin (it actually rotates backward) while Mars has a small solid core but a day length uncannily close to Earth's, however neither planet has a significant magnetic field.

I don't let geology keep me Earthbound, not when so much of interest to geologists happens above the ground and out in the rest of the universe. I've written (and newly updated) a seven-part series of articles on various atmospheric and space-related topics, including sprites, stardust and cosmic impacts. It starts with the Sun.
_{The Sun in ultraviolet light — NASA image}

Chaitén Volcano, in Chile, has culminated three days of unexpected activity with a major eruption today that sent ash 30 kilometers high—that is, into the stratosphere. Before this week, its last documented eruption was about 7420 BCE. NASA's Earth Observatory has two satellite images of the volcano from yesterday. In the image a big ash plume crosses the continent, which admittedly is narrow at this latitude, about 45° south.

Not much is known yet scientifically. Another recent eruption in southern Chile, that of Cerro Hudson in 1991, released a large quantity of fluoride and sulfide gases into the stratosphere that subsequently contributed to a large Antarctic ozone hole. Chaitén is about 250 km north of Hudson and 150 km south of Puerto Montt, the end of the road.

Follow the official news from the Chilean geologic agency SERNAGEOMIN. If your Spanish isn't good, Google's translation service does a decent enough job. The agency is conducting daily overflights and is deploying a local seismic network.

Maybe you've enjoyed some of the geological Webcams that I've compiled on my site. Now there's an adjunct that delivers live raw audio/video feeds from news broadcasters across the country, at livenewscameras.com. Of course, weather fanatics can make good use of it, as Rachelle Oblack has noted. But it should be compelling viewing during geological events, too, like eruptions and earthquakes. NASA is there, too. And accompanying chat. In the meantime, watching the trees sway on Catalina Island is . . . kind of . . .