Archive for January, 2010

The epicenter of the Haitian earthquake this past saturday (Jan. 14th) lies just off of the island of Hispaniola home to the boundary of the North American and Carribean tectonic plates. The Enriquillo fault as it is known is a slip-strike fault wherte portions of the earth’s crust are sliding past each other.

Tim Dixon a geophysicist at the University of Miami stated that the Caribbean plate was moving east and that the two parts of the island are moving apart at a fairly quick rate, geologically speaking, the north one centimeter and the south one centimeter per year. For comparison, the plates at the San Andreas fault in California are moving at a rate of about 3.5 centimeters (1.4 inches) a year, Dixon says. “Given that you’ve had a one centimeter a year movement for all that time,” the 7.0 quake represented a tremendous release of energy. The historical records back up the findings. “It turns out that there are records of big earthquakes on the southern coast going back to the mid 1600s,” says Dixon.

references: Elizabeth Weise Haiti’s geology points to big quakes (USA TODAY)


Read Full Post »

Answer Tuesday #2

So, what was Structure Wednesday #1 depicting?   A sinistral shear fracture from Piseco Lake Shear Zone (PLSZ).*   I have a number of pictures from this field trip, so a few more may pop up over the next few weeks.

(oh, since I forgot earlier–the lens cap in the picture is for a 72 mm lens. )

EQ&V Friday #2 is a picture of the ocean-ocean convergent boundary between the Atlantic and South Sandwich plates.   The South Sandwich Islands are located on the microplate and are a chain of volcanic islands.   The last eruption was of Montagu Island in 2007.

(image from GeoMapApp)


  • Piaschyk, D., Valentino, D., Solar, G., and Chiareenzelli, J.R., 2005, Variations in L- and S-tectonite on the northern boundary of the Piseco Lake Shear Zone, Adirondack Mountains, New York: Guidebook – New York State Geological Association, 77, pp. 75-91.
  • Valentino, D., Solar, G., Chiarenzelli, J., Gates, A., and Freyer, P., 2004, L- versus S-tectonite fabric variations within the southern Adirondack shear zone system: progressive deformation associated with a sinistral conjugate to a Grenville syntaxis: Guidebook – New York State Geological Association, 76, pp. 191-220.

Read Full Post »

Today’s photomicrographs are also from my bachelor’s area east of the Victory Pluton in Vermont.   Though a number of the garnets preserved an older fabric as inclusions, quite a few of the garnets have been partially (or completely) replaced by chlorite.   The chlorite was quite late in the metamorphic history of the rock and is either randomly oriented or as pseudomorphs of garnet in all of the thin sections I examined.   The two pictures are of the exact same field of view (1.2 mm wide), but one is in plane light (PPL) and one in crossed polars (XPL).   The garnet (clear in PPL and completely black in XPL) is fairly easy to distinguish from the chlorite (yellowish-green to a darker green in PPL and blues & purples in XPL).

PPL of ERG-32b:

XPL of the same view:

Garnet is anhydrous and is stable at higher pressures and temperatures then chlorite.   Chlorite is a hydrous mineral and commonly grows during retrograde metamorphism, while the rock is being brought back up towards the surface of the Earth.   If a rock that is garnet-bearing begins to come back up towards the surface of the Earth, the pressure & temperature will drop and garnet will stop being stable.   If water is also present at this point, the reaction garnet + water -> chlorite (very simplified!) may occur.

Read Full Post »

EQ&V Friday #2

This week’s image was also taken from GeoMapApp.   Since we don’t have class on Monday (and therefore no quiz!), you may simply send me an email with the answer to the following questions by Monday, 18. January 2010, at 9 am for the extra credit.

1) what type of plate boundary is shown above (be specific!)?

2) where geographically is this located?

3) what are the names of the plates that are interacting?

Read Full Post »

Structure Wednesday #1

For the following picture, determine 1) what is the name of the structure and 2) what is the sense of the offset.

(Picture was taken in 2007 on a Structural Geology field trip while I was teaching at Vassar.)

Read Full Post »

Answer Tuesday #1

(Normally, there will be both an answer to post for Structural and EQ&V, but we’ve only got one this week.)

EQ&V mystery area #1:

Where was the trench off the east coast of Japan.   As for the elevation change, the white line on the map above indicates the elevation profile I produced with GeoMapApp:

with the highest region being the volcanoes on Japan and the deepest region the trench that lies to the east of the country.   Over 7000 m of elevation change in just over 500 km distance isn’t bad 🙂

Read Full Post »

Today’s photomicrograph comes courtesy of finding the envelope of pictures I took for my bachelor’s research in the Northeast Kingdom of Vermont.   The majority of my samples were garnet-biotite-muscovite schists part of the contact aureole around the Victory Pluton.   These samples didn’t add much to the general understanding of the tectonic history of the region, but I’ll get into some of the samples that actually were more interesting in future weeks.   For now:

ERG-28a.   PPL.   Field of view is 6.8 mm (?).

The thin section has a fairly typical texture for this area, where the aligned inclusions within the garnet preserve an older foliation then the matrix of the sample.

Read Full Post »

« Newer Posts - Older Posts »