Petrology is the study of rocks (Petros = rock, ology = class that probably has a
big lab component).
Igneous rocks form when molten
(liquid) rock cools and crystallizes to a solid, like water freezing on
a pond. Sometimes that happens on the surface of the earth, as in
the case of lava flows and other volcanic eruptions. Sometimes
that happens deep underground, requiring hudreds of thousands to
millions of years for the magma to solidify.
Metamorphic rocks form when heat
and/or pressure cause rocks to recrystallize and change. Remember
(about the fellow who wakes up one morning to find himself transformed
into a giant bug), or how caterpillers form cacoons and transform into
butterflies? Well, so it is with rocks!
My petrology course focuses
on the major processes in forming igneous and metamorphic rocks.
We have several fieldtrips in the class, including a 5-day long camping
trip in the Adirondack mountains of New York where there are some spectacular rocks!!!
Autumn 2004 - we started at the Barton garnet deposit
Panning for garnet gems. The
Barton family are truly excellent citizens! They are a credit to
mining and the Adirondack region. They do a lot of work with
school groups, run a tight and clean ship, and produce great
product. Great people!
Garnet porphyroblasts with
hornblende rims in metamorphosed metaolivine gabbro.
Zach posing for scale with garnets
Wollastonite is used in a variety
of everyday products, including a sort of microscopic re-bar in
plastics! How massive deposits of wollastonite form is the puzzle
students must solve here!
Josh showing good hand lens
technique. Head held high to allow more light - lens just at
eyelash distance from his eye - eye that's not looking through the loop
is open and relaxed - no facial scrunching. Good job!
Andy was such a professional and a
great guy. It's both very happy and quite sad for professors when
good students graduate. I miss a lot of them.
We camp at night to keep costs
down. It's tricky finding campgrounds that are open after Labor
Day, but we seem to manage.
I used to run this fieldtrip in
the late spring, but the snow caused some problems. Moving it to
fall creates different problems, but was overall a good idea.
Charyn always cooked up something
wonderfully aromatic and different. I'm afraid I usually just
throw together spaghetti, burritos, or other very quick and simple food
when I cook for the group. This year will be different,
though! I have big plans!
Anorthosite - a special igneous
rock characteristic of the Adirondacks - warrants close
inspection. These students are not kissing the rock, rather
they're looking at the mineral crystals with high powered magnifying
lenses identical the the jewlers loops you might have seen in diamond
It's always good to see thoughtful
discussion on the outcrop. Here, students discuss the different
types of anorthosite found in the outcrop and the relative timing of
the injection of different magmas.
For me, this will always be the Outcrop
first group I took up here didn't like this outcrop at all because I
asked them to precisely estimate the proportions of different minerals
in the igneous rock. It was a fiasco! Since then, I've
taken two other groups and no one's had problems doing it.
Look ma! No tears!
This group did a great job on this outcrop!
In addition to getting right up to
the rock, it's also important to step back and try to see the big
picture. That's a lesson I learned from one of the smartest guys
I've ever met (Lans Taylor - extraordinarily nice guy and so smart it
Zach and Josh trying to work out
the rock type. The debate here was in the identity of the dark
colored mineral - biotite, hornblende, pyroxene, or olivine? The
Adirondacks have some unusual rocks, so it's a good question!
I guess I must have interupted
them in their work to warrant scowls like this! It's good to see
people so focused on their work!
The Lion Mountain iron deposit was
mined long, long ago, but is now closed. It's a good example,
though, of granitic gneiss hosted iron ores of the Adirondacks.
After studying the iron ores in
the pit, we stop to visit the giant pile of sand made from ground up
ore. It's an amazing pile!!
Three students climbed to the top
to illustrate how big this pile of sand really is.
Patroling campsite for litter -
leave the site cleaner
than you find it!
The fayalite granite is a neat
stop. Fayalite is a mineral belonging to the olivine mineral
family. Olivine is typical of very iron-magnesium rich rocks more
characteristic of the ocean floors. Granite, on the other hand,
is iron and magnesium-poor and so only very, very, very rarely contains
olivine. There are some chemical reasons for this, but I can't
tell you on this website or else my students will know the answer
before they get to the outcrop. One big part of fieldtrips with
me is learning to make careful observations. The other big part
of fieldtrips with me, though, is thinking about rocks in terms of
process while you're on the outcrop.
This outcrop is half red and half
gray. What causes this difference?
Mylonite is a layered rock that
forms by crushing of the mineral grains when the rock is sheared.
The two main zones of the Adirondack region are separated by a mylonite
zone, suggesting one of the zones slid down along a shear zone from a
Andy points out the direction of
the lineation in the mylonite.
Proud of their oriented sample,
Josh and Andy pose by their treasure. An oriented sample is one
which the geologist carefully records the orientation of before
bagging. This allows the geologist to orient the sample in the
exact same direction, tilt, etc. back in the lab. Some mineral
textures tell us the direction a rock was smeared/squished/etc.
We need to be able to analyze those mineral textures knowing their
Cooling off in Cranberry Lake
after a day of rock study. The temperature was actually quite
cool, but a dip in a mountain lake is tough to pass up!
It's good to wake up early so you
can see the sun rise.
The world is an incredibly
beautiful place, isn't it?
Migmatite is a rock that forms
when rocks are buried so deeply and heated so much that they start to
melt. The liquid migrates into swirly zones like the ones Dana's
A basalt dike cuts across a
meionitic marble in the northwest Adirondacks. Rocks in the
northwest Adirondack lowlands are mostly metamorphosed sedimentary
rocks. The basalt formed much, much later when molten magma
injected into fractures. Such basalt dikes are commonly
associated with rifting, although I don't know if this particular dike
formed during the Neoproterozoic rifting, or the late Triassic rifting
The Train Wreck - a very famous
outcrop in the Adirondack region. The dark blocks of garnet were
once a cotinuous, albeit brittle bed of rock. The surrounding
gray swirly stuff is marble. In metamorphic temperatures and
pressures, the marble flows like toothpaste, but the brittle garnet
fractures into pieces, which are smeared out to to geologic forces.
Glacial striations formed when the
giant continental glacier of the last ice age scraped across these
rocks 20,000 years ago. The scratches tell the direction the ice
The Steer's Head outcrop - studing
chemical reaction rims that form when rocks of different compositions
are in contact with one another during metamorphism. Students
here must first identify the minerals present and then deduce what
chemical reactions took place. Again, I can't tell you the answer
right here in case future students are reading this!
Close-ups of the reaction rims
Talking about the Balmat zinc
deposit with Bill DeLorraine - a geologist working at the mine.
Bill's incredibly sharp - the folks at HudBay Minerals, Inc. are lucky
to have such a smart and experienced guy working for them. Here,
the class poses by the three dimensional mine model that shows the ways
the orebodies twist and turn deep underground. We were not able
to tour underground, though, because the mine was closed down.
The Balmat mine, being a smaller mine, must open and close depending on
the price of zinc metal. Opening and closing a mine's opperations
is an expensive endeavor, so it's not a decision made lightly.
They keep the mine pumped dry, though, during the time they're closed
down, which dramatically reduces the start up costs.
Hunting for marble outcrops in the
Some strange features in the
marble. The locations of these features is a secret - they are
pretty incredible and should not be disturbed, as you well know if you
recognize what they are!