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Archive for January, 2010

EQ&V Friday #4

This week’s image is a picture taken from a plane looking down at the US.   Students should think about:

  1. what kind of fault is present?
  2. what is the offset on the fault?
  3. where is this fault located?  (either coordinates or name of the closest town & which direction it is)

(I’ll acknowledge where I got the picture from when I post the answer on Tuesday–the info would give the answer away!)

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Because we’re currently discussing microstructural deformation in structure, I’m going to post another photomicrograph today 🙂   This one is compliments of the 2002 Microstructures class I took with Jane Gilotti at UIowa and is of a mylonite in New Mexico.   The photomicrograph was taken with the accessory plate inserted, which is a bit unusual (I’ll explain why we used it in the answer).   The structure students should:

  1. Identify the minerals present to the best of your ability
  2. Identify what the evidence of deformation is & name each type of microstructure present
  3. Explain why I took this image with the accessory plate inserted

NM-99.   (My notes don’t include how wide the field of view is… hmmm)

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Answer Tuesday #3

(Sorry today’s post is so late–I was working on a few other things and time just ran away from me.)

The EQ&V #3 post depicted Iceland, whose volcanism is due to both spreading at the Mid-Atlantic Ridge and a hotspot.   I’ve labeled the various bodies on land on the GoogleEarth image:

For Structure #2, the fold is comprised of two main lithologies: a sandstone and a shale.   The sandstone is distinctly more competent than the shale layer (its the lighter colored layer on the image & the one that stands out because the shale weathers more easily).   I’ve marked a section of it with a yellow X.   As for how many strike & dip measurements a geologist would need to determine the fold axis orientation, there’s the minimum answer (2) and what I would consider the “enough to compensate for error” answer (6).   If only two measurements are taken, they should be on the two different limbs (if six, three per limb would be good).   I’ve marked on the image in yellow with strike & dip symbols where I would likely take the six measurements.   In this case, it was much easier to take measurements on the competent sandstone then the very fissable shale.

Once you have the measurements, you would then plot them on a stereonet and where the traces intersect, that would be the fold axis.

(taken from: http://www.chipr.sunysb.edu/eserc/summereducationalinterns/linda/foldstereo.gif)

(I would have plotted up actual data from the fold itself, but magically, I can’t find my original data at the moment.)

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Mid-Ocean Ridge

Computer-generated detailed topographic map of an area of the Mid-Ocean Ridge. The red, orange, and yellow colors indicate the ridge rising above the seafloor, and the green, blue and purple colors represent lower elevations. The image was taken at approximatley 9° north and is part of the East Pacific Rise. I thought this picture was interesting as well as relevant since the last question on our labsdealt with describing the rheology of a continent collision zone (an area relativley similar to the mid-ocean ridge) except for the whole rifting thing.

USGS

Imagery courtesy of Stacey Tighe, University of Rhode Island

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Today, we’re going to move closer to the Victory Pluton in the same contact aureole as the past two posts.   The increase in temperature has increased the metamorphic grade of the sample and the rocks now contain: garnet + staurolite + sillimanite + biotite + quartz + plagioclase + cordierite.   The latter is a very hard mineral to ID in thin section because it looks very similar to both quartz and plagioclase.   In this case, though, the cordierite came with a calling card: pleochroic halos around radioactive inclusions.

NEK-97-13 in PPL.   Field of view is 1.2 mm.

For this rock, the radioactive mineral present is monazite (a similar halo would form around zircon, xenotime, or allanite).   The cordierite is wrapped by fibrous sillimanite (there are also large prismatic sillimanite grains in this thin section, just not in this field of view).   Biotite (the yellowish-brown more boxy mineral) is present as inclusions in some of the cordierite.

Pleochroic halos form as radioactive elements in one mineral decay and release alpha (or beta) particles.   If the original mineral is small enough, the alpha particles travel into the surrounding mineral and the particles themselves cause the crystal lattice of the host mineral to be deformed.   The deformation continues over time as more & more alpha particles are released and leads to the formation of an approximately circular halo around the included mineral.   In cordierite, the ring of deformed crystal lattice around the inclusion is yellow in color.   If the grain was plagioclase or quartz, the yellow halo would be absent, since these minerals don’t react in the same way to lattice deformation.   (I looked on the internet a bit for an appropriate diagram, but couldn’t find anything–its at this point I normally would just draw on the chalkboard for my students!)

The cordierite itself adds a very interesting part of the overall story, since the mineral is stable at relatively high temperatures, but low pressures in metamorphic rocks.   As it turns out, the texture of this sample was almost more interesting than the presence of cordierite within it, but we’ll get to that in the weeks to come.

(Oh, and for those budding gemologists out there, iolite is the jewelry form of cordierite.)

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EQ&V Friday #3

The following image was taken from GoogleEarth and I purposely took off the scale & N arrow.   For the image:

  1. The island in the middle is volcanic for two separate reasons–what are they?  (hint: one is a type of plate boundary)
  2. Label all of the pieces of land that are present above water

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Structure Wednesday #2

(Most of the UPJ students should remember this site from Field Methods last May.)

For this image, indicate two things:

  1. This fold is comprised basically of two lithologies–which one is the competent layer?
  2. If we were to return to the fold today to determine the orientation of the fold axis, how many strikes & dips would you want to take?  give an approximate location for each measurement.

Fold on the north side of 199 just before Kingston-Rhinecliff Bridge on the east side of the Hudson River.   The road sign (on the right side of the image) is about 4 ft tall.

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