People are always asking questions about geology, from rocks they pick up in their backyard, to why not use a nuclear device to "pop" a restive volcano. This blog will cover the gamut of the fascinating world of Geoscience. It's sort of a "Geology for Dummies"... but you're 'WAY smarter than that!
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?
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.
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
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 showeda 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
Thanks so much!!!
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
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.
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,