With Every Fault is there an Earthquake?

There is nothing that motivates a teacher more than a student asking a serious question. Perhaps even more motivating is a teacher who, like us, wants to help a student.

Q: My 5th grade student Awva asked this question. Thank you!

With Every Fault is there an Earthquake? 

 
Amy S

A: With every fault there once WAS an earthquake - that's how the rock broke in the first place. With most faults there are many earthquakes over time. 

However, there are active faults (like the San Andreas) and there are (usually temporarily) inactive faults. There are faults mapped in Precambrian times that seem to have been completely inactive for over 600 million years. However, a fault implies a zone of crustal weakness, and it is not at all uncommon for a fault to be re-mobilized by a later tectonic event. Because the zone is already weakened, it is preferentially broken open again - it's easier to break than the surrounding rocks. I've seen billion-year-old faults in Venezuela that were activated again as the Atlantic Ocean opened between North America and Europe the first time, again when it closed, and then yet again when the modern Atlantic reopened a second time.

To make things even more complicated, however, there are faults that have "slow creep" or "silent" earthquakes. The continental-margin subduction fault off the coast of Vancouver Island, Washington State, and Oregon is an example. These events are movements of the underlying oceanic slab beneath the continental crust. Shallower parts of these faults are fluid-lubricated, and motion on them is so slow that they are detected only by continuously-recording GPS instruments - they are not felt. These silent earthquakes can take more than a day to move two fault surfaces past each other, instead of the milliseconds that would be involved for a more typical earthquake. Thus the strain is slowwwwwly released. However, the strain on rocks deeper down is then increased - this is where the surfaces are locked because fluids no longer reach them.

When one of these huge subduction earthquakes happen, it's a real attention-getter. The Great Tohoku earthquake of 2011 was a magnitude 9 event. It caused massive damage, and the Fukushima nuclear reactor complex is just getting more dangerous every week as time goes on since the original tsunami destroyed the cooling system.

~~~~~

Rock Ages and Distance from Volcanoes

Sometimes questions come in that start off all “rong” – It then becomes necessary for us to set the record straight. Sincere efforts, however, should not go unrewarded. The following question is an example.

Q: My name is Mike W.  I'm a HS science teacher from Sibley, Iowa.  I'm really trying to change my labs and activities so they're more realistic and I have an idea.

I've attached an image of the graph I want the kids to eventually make (image showed  a fake graph with dots lining up along a diagonal line, one axis being distance and the other age attached), where they plot age of igneous rock v. distance from active volcano.  Obviously, it's a generally linear graph, which is great b/c they then can get an equation.  But, I want to do more than just give them a data table.  

I was thinking...

Could I give them rock "samples" (15-20) and each rock sample that they get has an identification number and distance from volcano.  In addition, each "sample" would have a visual representation of the remaining radioactive/non-radioactive isotopes left....getting them to use radiometric decay to date their rock sample.  For instance, sample B would be 2.5 km from the volcano and it would have 5 blue circles (parent) and 35 green circles (daughter).  They could deduce that three half lives have passed and the rock is x number of millions of years old.

I want this to be accurate, yet doable in the classroom.  Do you have any thoughts or suggestions???

Thanks so much!!! 

--Mike W

A: Ummmm. I think you have started from a false premise - that rocks are older the farther away they are from a volcano. I can think of many exceptions to this "rule" -- in fact so many exceptions that it cannot possibly be true.

However, I certainly understand and support your hope to get your students actively involved. Real-world, really cool examples are what drew me from a physics PhD track to geosciences. 

I hope soon to have a publication on the web that your students might be able to make use of. It concerns magnetic and gravity and radiometric data acquired around Newberry Volcano in Oregon (Newberry's footprint is at least 20 times larger than Mount St Helens). If you look at the potassium-40 image, you can see that the youngest eruption (~1,300 YBP) blew tephra out eastward at least 100 km: there is a bright red trail extending from Paulina Peak to the east. Careful sampling has shown that it thins the farther east you go. The problem: there are older tephra underlying this blast that didn't reach as far away. 

Volcanoes are generally messy creatures - many of them may look like neat stratocones, but when you unravel their history with careful mapping, you find that they have very complex histories, and the symmetric cone represents just the latest eruptive episode with the first stage of erosion still well underway. Dating volcanic flows and tephra layers is very difficult - when I served a 5-year tour as chief scientist for volcano hazards, I poured must of the extra funding I could lay hands on into telemetry bottlenecks and a dating laboratory. When you want to know how dangerous a volcano is, you need to see how many times - and how violently - it has erupted over time. It turns out that radioelement dating is extraordinarily complex. First, you have different half-lives that limit their applicability: C14, argon-argon, rubidium-strontium, potassium-argon, uranium-lead, etc. Then you have different availabilities - there is almost no potassium or uranium in mafic and ultramafic rocks, for instance. You must painstakingly look and hope for sphene crystals to do any argon dating. THEN you have the contamination issue: when you are doing argon-argon dating, you have to deal with the fact that the air has more argon in it than the grains you are trying to date. The statistics involved are mind-boggling, but it turns out that where you sample is the most critical part affecting success. 

Where you CAN get a good age-vs-distance curve is with an ocean-floor spreading center. The symmetric magnetic banding across the Mid-Atlantic Ridge is a case in point. Deep-sea drilling has sampled the different magnetic-polarity bands and has verified that the farther you move away from the spreading center the (a) older the rocks are, and (b) the deeper the rocks are. You can get a VERY clear age-vs-distance pattern there, because this is a relatively steady-state, on-going process. 

~~~~~

Is this a Meteorite?

At Ask-a-Geologist, we see a disproportionate number of “What’s this rock” questions. Often they are accompanied by a blurry photo, and we find ourselves trying to explain the sometimes complicated path we must follow to identify a rock ourselves. The following query is a bit different – and not just because it had no photo attached. I hope the answer will help encourage readers to remake themselves into amateur scientists. The fact that the question was asked in the first place is encouraging to me - because no questions asked means nothing learned.

As an aside, several fields of science have discovered that they can make a huge progress by engaging interested amateurs in their research programs. Classifying galaxies, folding proteins, and tracking bird species are just some of these. Some of the greatest science of the 19th and 20th Centuries was done by amateur scientists; Einstein was a patent clerk when he published Special Relativity in 1905.

 

Q: I have a rock. I think it's a meteorite. Where can I take it to have it tested? I am in San Bernardino, CA.

Thank you,

Nancy B.

A: Some background first, so you can better understand what you might have in your hand.

Meteorites are not commonly found lying on the ground. They are found disproportionately in the Antarctic and in snow-covered areas. As you might instinctively expect, they fall in equal numbers everywhere on the planet. However, in Antarctica they impact ice and snow, and do not get mixed in and confused with Earth rocks. The ice and snow in Antarctica become glaciers, carrying their meteorite collections along like a conveyor belt. The glaciers tend to sublimate (evaporate) near their lead edges - where the meteorites accumulate. Consequently, dark rocks falling out of white ice are concentrated, easy to see, and have not been mixed in with Earth rocks. For slightly different reasons (among other things, there is no conveyor belt concentrating mechanism), the Sahara and the Empty Quarter of Saudi Arabia are also places where meteorites are more readily found – they stand out sharply in the white-to-beige sand dunes and flat lag-gravel plains.

Almost all meteorites break up in the upper to middle atmosphere of the Earth, and the fragments that don't burn up generally fall to the ground at their terminal velocities in air. This is typically greater than 200 km/hour on average, and they have punched through cars and houses in the past. Some exceptions to the atmospheric breakup rule include the nickel-iron asteroid fragments, which can easily make it through the Earth’s powerfully protective atmospheric blanket. With the Wabar object that I once mapped in the Empty Quarter of Saudi Arabia, we calculated the mass (from crater diameters) to be about 3,500 tons at impact. Compositionally, it was fairly uniformly 94% iron, 5% nickel, and the rest was cobalt, copper, and iridium (a so-called “sidereal” element not normally found on the Earth’s surface). “Irons” represents only about 2% of the stuff floating around in the Asteroid Belt (most of which are stony or chondrite objects). However, iron meteorites represents about 5% of the meteorites found on Earth. Why the higher percentage? Because blackened metal is far more easily identified when picked up than an ablated (burned-looking) rock that falls among other similar-looking Earth rocks.

Meteorites are notoriously difficult to identify - in large part because there are sooooo many rocks that could easily be mistaken for a meteorite. As a rough approximation, probably less than 1 object in 10,000 that people THINK could be a meteorite actually turns out to BE a meteorite. Moreover, most professional geologists don't know how to distinguish a meteorite from a look-alike... This is because most geologists have never seen a meteorite (they are rare), except for a few under-representative examples in a museum somewhere. In other words, they have never been able to handle and examine a meteorite like the rocks they examine in the field areas where they are commonly working. To put things in perspective, there are far more gemologists than real meteorite specialists in the world.

I wish I could help you myself, but I do not consider myself a meteorite specialist. Moreover, the US Geological Survey is not funded by Congress to study meteorites - we have very specific tasks that we are assigned to do, such as monitor volcanoes or carry out mineral resource assessments (like me). Consequently, any studies outside of these assigned tasks we must do on our own time - because we find the subject interesting.

In my case, for instance, I was drawn into the meteorite field almost accidentally when I visited and mapped the Wabar meteorite impact site a number of years ago. Accompanying me on one of our three trips was one of the foremost meteorite impact specialists in the world at the time, Gene Shoemaker (the “Father of Astrogeology”), and he provided a massive data-dump of his experience for me. Many of the few "real" meteorite specialists that I have personally known in or outside the US Geological Survey are now either dead (Gene tragically died in a vehicle accident in Australia) or retired. One transferred to NASA, because he couldn't do what he wanted most to do (study meteorites) in the USGS. There are meteorite specialists in the Smithsonian in Washington, DC, but both NASA headquarters and the Smithsonian are on the opposite sides of the continent from you. There are a few meteorite specialists at the Lunar and Planetary Lab at the University of Arizona in Tucson, and there are individuals who are competent to assess meteorites in the Astrogeology science center of the US Geological Survey in Flagstaff, Arizona, among others.

All THAT said, how DO you identify the sample you have? I will offer you two lines of approach, but both require that you expend significant efforts to learn more. In essence, both require you to teach yourself to become an amateur meteorite scientist:

1.      Easier Path: Search the internet for books on, and photo examples of, meteorites. Look closely at any that show texture (for instance tectites have a distinctive texture but are not technically meteorites). Keep in mind that there are quite a number of different types of meteorites, from Stony to Chondrite to Nickel-Iron, to rare Pallasites and Nakhlites, as well as others. This list, by the way, is in increasing order of how rare they are. First become familiar with the possibilities, then when you see one that looks like your sample, search for other examples of that type. ESPECIALLY search for detailed descriptions of that kind of sample. Buy or borrow a hand-lens (a reasonably good one will cost ~$35), and see if you can see any of these characteristics in your sample. I have seen a few reasonably good self-help guides for identifying rocks - choose one,  and then spend some time in the first part of the book learning rock-identifying principles.

2.      Harder Path: This is more difficult, requiring you to make more than one cold contact; it also requires you to do some significant homework up front. You could contact the geology department in a university close to you. Look for the largest university that you are willing to drive to, because you will have the best chance to find a true meteorite specialist there. Contact the geology department first to make sure you are not wasting your drive. THEN see if you can arrange to meet the geologist there who indicates that she or he feels confident enough to identify a meteorite. Keep in mind that these people have paid work they must do (such as teaching), and may not want to take the time to help. Also, geology departments are commonly inundated with people showing up and asking "what is this rock?"... so it will require patience on their part and on yours to make a connection.

Warning: One thing I would not recommend is sending a photo of the sample. No competent geologist would be willing to identify a rock from a photo alone (even if it was high-resolution, taken with a macro lens, and was crisply focused). The reason for this: to identify a rock, a geologist must be able to handle it, turn it over in sunlight looking at texture and constituent minerals, scratch mineral grains with a knife, crack the rock open to examine a fresh unweathered surface, examine it minutely with a hand-lens, etc. In most cases, a photo conveys less than 10% of the information needed to identify a rock. In many cases, even these techniques leave the identification unresolved, and thin-sections have to be cut and examined in a polarizing microscope, or a chemical analysis must be done, or both, to get a definitive answer.

Like most things of value, this won't be an easy thing for you. However, you will become a smarter and wiser person if you study this subject. Then you could go beyond that and become an amateur (meteorite, or anything else) scientist.

~~~~~