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Archive for March, 2010

Ron is posting a sequence of pictures of rocks on his desk (“Deskcrops”) and today’s struck a chord.   I first went to Gore Mt. on a mineralogy trip in 1997 on one of those typical northern New England / New York spring days.   We had sun, rain, snow, and cloudy conditions during at least one point on the trip (several of them while actually in the upper mine!).   I was wearing my green rain jacket, which to this day still has garnet shards in one of the upper pockets due to all of the small garnets I picked up while in the mine 🙂

My return to Gore Mt occurred as part of a trip at Vassar in 2007 under slightly more favorable weather conditions (sunny, but cool).   I have no idea whether I took pictures during the first trip–if I did, I can’t find the prints / slides–but during the second trip I ran around trying to look all the different garnet textures present.   So, I’ll post a few pictures from the trip & try to explain what’s going on.   (Apologies for some of the picture quality–this was the first semester with my new camera & I was still figuring some things out.)

upper mine at Gore Mt, Oct 2007

garnet with narrow plagioclase (white) then wider hornblende (black) reaction rim (lens cap 72 mm)

As Ron mentioned, the garnets have several different types of rims surrounding them.   In some cases, you can see differences for the same garnet:

garnet with both a plagioclase (white) rim & a hornblende (black rim)

The variety of rims are most likely due to variances in the bulk composition of the rock as you move around the outcrop.   In rocks that are more hornblende-rich, all of the plagioclase would have been used to form garnet leaving hornblende rims around the garnet.   In regions where the rock was more plagioclase-rich, garnet growth would have consumed a large percentage of the present hornblende and left a plagioclase rim around the garnet.

Besides the variation in rims (both in composition and width), the other thing that fascinated me on my second trip was the variance in garnet sizes.

large garnets (bottom) vs. small garnets (top) in the upper mine

I believe the size variation is due to the presence / relative absence of fluids in the system during garnet growth.   In situations where diffusion is relatively quick (high temperature and/or high amounts of fluid present), large garnet porphyroblasts are more likely.   When the diffusion is slower (lower temperatures and/or low amounts of fluid present), the various components that are needed to grow garnet can not diffuse as far and thus add onto the closest garnet they can find resulting in the growth of lots of little garnets.   So, some of the rock had a larger amount of fluid available (large garnets), while other regions were drier.   The assumptions I’m making is that all of the garnets grew at about the same time in the rock (which is probably fair since there aren’t any major faults) and that we had lots of nucleation sites (probably due to rapid overstepping of the garnet-in reaction).   In the larger garnet region, many of the nucleation sites simply didn’t get enough components to keep growth and ended up dissolving back away or merging into another larger garnet.

side wall in the upper mine--all of the reddish circular objects are garnet

At Gore Mt, the garnets are simply everywhere, which is why the mine has been used a prime source of non-jewelry grade garnets.   The garnets have been used for a variety of purposes including sandpaper (garnet has a hardness between 7.5 & 8.5 depending on composition, so its harder than quartz sandpaper) and other abrasives.   Maybe my favorite application is that you can buy slabs for kitchen counters:

slab of the garnet countertop material

Gore Mt is open to the public, but you will be taken to the lower mine on the general tour.   Geology groups should contact the mine directly to arrange to access the upper mine (which has better garnets & exposure in my opinion).

lower mine

Final parting shot is of the floor of the upper mine, but both look like this.   When the sunlight hits the garnets, the floor of the mine looks blood red.   Very, very cool.

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Two-tone effect, Mount St. Helens summit, March 30, 1980, image by Robert Krimmel, USGS

A magnitude 4.2 earthquake 30 years ago this past Saturday marked the reawakening of Mount St. Helens after 123 years of inactivity and set the stage for the most destructive eruption in U.S. history.
March 31, 1980, both craters enlarged as explosions continued. A change in wind direction brought ash to the Kelso-Longview area by noon. To date none of the ash from these explosions has come from new magma, but rather pulverized bits of older rocks that make up the summit. The frequency of earthquakes decreased but the number of larger earthquakes has increased, so the total energy release remained about the same. Explosions and earthquakes triggered two avalanches of snow and rock near the Goat Rocks dome. Cowlitz County Commisioners declared a state of emergency in an attempt to obtain assistance from the Washington National Guard in staffing roadblocks.

On that fateful day May 18, 1980, Mount St. Helens Volcano in Washington exploded violently after 2 months of intense earthquake activity and intermittent, relatively weak eruptions, causing the worst volcanic disaster in the recorded history the United States. The cataclysmic eruption and related events May 18 rank among the most significant geologic events in the United States during the 20th century.

USGS

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Lavafall

…Just thought I would share this amazing picture.  Magma pours down a snow-capped mountain in Iceland after a recent volcanic eruption.

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This article was written over a year ago, however I recently stumbled upon it and thought it was quite interesting.   Most of the small earthquakes that occur in central U.S. are actually aftershocks of the bigger earthquakes that occured in the New Madrid seismic zone that hit the Midwest back in 1811 and 1812.  In the middle of a continent, aftershocks tend to go on much longer.  The reason for this is because aftershocks occur after an earthquake because the fault changed forces in the Earth.  The aftershocks go on until the fault recovers and this takes much longer in the middle of the continent.  The scientists compared this movement to the San Andreas fault where the two sides move past each other at about 1.5 inches per year, and this motion will swamp the small changes caused by past large earthquakes which will suppress aftershocks after about 10 years.  In the New Madrid fault system, the two sides move more than 100 times more slowly, and it takes hundreds of years to swamp the small changes caused by large earthquakes.

http://www.sciencedaily.com/releases/2009/11/091104132652.htm

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This paper talks about the structural geology project being carried out in British Columbia.  The principal objectives of the program were  to  provide  guidance  on  the  structural  and  geological  controls  on  the  distribution  of  gold mineralisation and to produce a predictive structural model to support targeting efforts. 

In strike-slip vein systems (both sinistral and dextral) sub-vertical high grade ore shoots tend to be best developed. These will be oriented normal  to  the along  strike  slip direction, with dilational zones preferentially formed within along strike jogs (changes in strike) in the vein system (Figure 2 and Figure 3).

 In this figure the geometry of dilatant zones in different structural-kinematic settings

Please read the rest of this publication at

http://sonaresources.com/_resources/pdf/downloads/Structural_Geology_Elizabeth_JPS_CL_rev04_FINAL.pdf

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Yes, that is the glaciers real name.

Hundreds of people were evacuated Sunday March 21st amid flood concerns after a long-dormant volcano erupted beneath a glacier in south Iceland.

It was the first time since 1821 that the volcano under the Eyjafjallajokull glacier has erupted. (Eyjafjallajokull is about 100 miles (160 kilometers) east of the capital, Reykjavik.)

The fissure measured about 1,640 to 3,281 feet (500 to 1,000 meters).

Despite the remote location of the eruption, if the fissure “develops further towards the glacier, the melting floodwater … will create dangerous floods in a populated area in south Iceland,” said Gudrun Johannesdottir, a project manager for Iceland’s Joint Rescue and Coordination Center.

CNN

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As mentioned previously, I went on a post-conference field trip that involved a transect from the Blue Ridge in VA to the Valley & Fold belt in West Virginia.   Callan originally live-blogged both days (end of day 2) and is now working his way through several synthesis posts, so I’m going to leave the majority of the actual trip description to him.

Several vans were involved in the trip, but mine had several discussions about what exactly a charnockite was and how it formed.   Callan has pictures in hand sample, but I found a photomicrograph from approximately equivalent Blue Ridge rocks to the ones we saw:

In the labeled image: q = quartz, f = feldspar (mainly plagioclase), and p = orthopyroxene.   The rocks are anhydrous, which explains the lack of biotite, hornblende or other OH-bearing mafic minerals.

The majority of pyroxenes contain iron and/or magnesium, silica and oxygen.   Clinopyroxenes (augite, diopside, hedenbergite) include calcium as well and have a lower amount of symmetry than the orthopyroxenes (enstatite, ferrosilite, hedenbergite).   In thin section, both clinopyroxenes (cpx) and orthopyroxenes (opx) have cleavage at about 90 degrees and tend to be more equant than elongate in shape.   Cpx, however, has inclined extinction (as you rotate the stage in crossed-polars, the mineral goes black when the cleavage does not line up N-S or E-W with the cross-hairs) and tends to have higher orders of interference colors (2nd-3rd order on a Michel-Levy diagram).   Opx has parallel extinction (black when cleavage N-S or E-W) and only mid to upper 1st order interference colors.  Of the two, normally cpx is present in more mafic (iron / magneisum / calcium rich samples) and opx in rocks that are less mafic.   Most modern magmas contain at least minor amounts of water, which would lead to the formation of biotite or hornblende as the magma crystallizes more intermediate and then felsic rocks (silica, sodium, and potassium rich rocks).   This normally produces a more “standard” granite composition of quartz + plagioclase + k-feldspar +/- biotite +/- hornblende.   In this case, the lack of water removes the potential for biotite or hornblende forms a charnockite or “orthopyroxene granite.”

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