The Rock Docs: Everything you wanted to know
about rocks- We asked.
Dr. Timothy La Tour, Geology Professor, GA State University
What are the most interesting or
unusual facts you have learned about rocks
and minerals?
I think the most interesting thing about minerals is how they are
different from other solid substances that are not crystalline,
such as glass. For example, light passing through minerals is
polarized into two vibration directions. You can block out one of
the directions at a time by using a polarizing filter or even
some polaroid sunglasses. Glass does not do that to light.
Some minerals are able to generate small electric currents by
heating them (pyroelectric) or tapping them (pizoelectric).
Conversely, if you drive electricity through a pizoelectric
cryatal (such as quartz), the mineral vibrates at a regular
frequency. That's why quartz and a small
battery are used in a wristwatch -- the regular vibration of the
quartz can be used to keep time.
Electric currents always produce magnetic fields (That's why the
car radio gets static when you go under power lines.). The
magnetic field can be "photographed" with the right
film. Some very common minerals such as quartz are pizoelectric
and produce magnetic "auras." that can be
"photographed." This gives quartz (and many other
crystals) a sort of mystical quality. I think that underlies the
"New Age" fascination with crystals.
How did you get started in
geology?
It was by accident. When I first went to college at Louisiana
State University I was pretty immature. I wasn't really
interested in academics, so the courses I was taking were all
"boring." Fall of my second year I took a geology
course to fulfill a science requirement, and I discovered
that it captured my interest. But the second geology course was
even better. My teacher was Dr. Keith Rigby from Brigham Young
University who was a visiting professor at LSU. He taught us
about earth history -- those billions of years during which the
earth evolved. That absolutely hooked me, and I have taught my
students all my career that earth history is I still the ultimate
reason for studying geology. Even today, I often think about what
it would be like to watch the earth undergoing small changes day
by day, year by year, century by century, millennium by
millennium. If only we could have videotaped the whole thing!
By the way, I am the advisor for geology undergraduate students
here, and I have encountered very few students who came to the
university to major in geology. Virtually all geology majors
across the country first discovered geology in college, just as I
did. That is one reason we take
our freshman geology courses very seriously at GSU.
Are geologists discovering
new types of rocks today? If so, what are the
new classifications?
I don't think we are discovering any new types of rocks on the
earth's surface anymore. But many geologists work on trying to
reproduce the conditions (pressure and temperature) that exist in
the earth's mantle. We have never actually seen any rocks below
earth's crust, i.e. in the mantle, but we use indirect ways of
studying them. Also, in some places parts of the earth's upper
mantle have been pushed up onto the continents. Soapstone Ridge
in Atlanta is an example of this.
Some geologists (geophysicists) use seismic methods (sound waves)
and gravity measurements to give us ideas about what rocks might
look like in the mantle. Other geologists (experimental
petrologists) then try to artificially produce those rocks in the
lab. So there are undoubtedly
rocks making up the vast majority of the earth that we will
probably never see directly. If we ever do, we will be
discovering new rocks.
One great place that geologists are discovering new types of
rocks is on other planets. The rocks returned from the moon had
some things in common with earth rocks, but there are small
consistent differences. For example, the most common igneous rock
on earth is basalt (the entire oceanic crust is made of basalt).
Much of the moon's surface is also basalt, but it is
not the same as terrestrial basalt. The soft landers on Mars were
able to chemically analyse Martial soil and small rock fragments.
The results of the analyses tell us that most of the Martial
surface is basalt (We can see the basalt boulders lying around on
the Martian surface, also). We haven't landed on Venus, and we
haven't even photographed its surface with visible light. But we
have "photographed" Venus's surface with radar, and
what we see looks like basalt volcanoes. Almost surely, Venus is
covered with basalt, but that basalt will almost surely be
different from basalt on earth, Mars, and the moon.
Geologists are continually discovering new minerals, however.
These geologists are called mineralogists. A rough guess would be
that upwards of 50 new minerals are discovered every year.
How do meteorites first
form?
The vast majority of meteorites come from the Asteroid Belt which
lies between the orbits of Mars and Jupiter. There are many
different kinds of meteorites, and these probably all form in
different ways.
Meteorites are divided generally into "stones" and
"metals." Stones are more common than metals and have
minerals that we find on earth, so they resemble earth rocks. If
you walked past a stony meteorite on your way to school, you
probably would not even notice it. They also have a chemical
composition that resemble some of earth's upper mantle.
On the other hand, the metals are very rare, and they are the
ones we think of when we think of meteorites. They are very heavy
and consist of metallic iron and nickel. They have compositions
that probably are like the earth's core. If fact, we use
meteorites to help us guess what the deep inside of the earth is
made of, because we will never actually see it.
We think that the Asteroid Belt came from a planet that had
partially formed before it "blew up." What broke it up,
we don't know -- maybe gravitational tugging by Jupiter. But it
probably had already formed a core and mantle (like the earth),
and the pieces are what we know as the
metals and stones. Meteorites are among the oldest things in the
solar system. They have been dated at 4.6 billion years old. On
earth, no rocks are older than about 4.0 billion years. That is
because the earth is constantly recycling rocks, whereas the
doomed planet between Mars and
Jupiter never had that chance. We say the earth is 4.6 billion
years old, however, because that is the age of the asteroid belt.
Where is the biggest
crater on the earth caused by a meteorite?
The biggest one we know of is the Chicxulub crater on the
northern edge of the Yucatan penninsula of Mexico, about 110
miles (180 km) km across. (That's as far as from Atlanta to
Chattanooga., TN) The meteorite that made this crater was about
10 km across (6 miles) and hit about 65 million years ago. This
impact must have drastically changed the climate and was
probably responsible for the extinction of the dinosaurs.
Unfortunately we can't see this crater directly. It is filled in
and covered by seawater in the Gulf of Mexico and recent
sediments. It has been studied using geophysical techniques and
has been drilled, so quite a bit of information is being
accumulated. No big pieces of the actual
meteorite have yet been found, but there are small traces of it
all over the world at the boundary between the Cretaceous and
Tertiary Periods (K/T boundary).
There were surely even bigger impacts in earth's history,
especially early on. But the earth's surface evolves through the
process of plate tectonics. Also, weathering causes the land to
be worn down, obscuring ancient craters.
Have you ever found a
meteore yourself? If so, what did it feel like?
No, I never have. Most meteorites are found by ordinary people,
or by researchers who search the Greenland and Antarctical ice
caps. I guess I am always sort of looking for them, but so far no
luck (same with UFOs).
I think that if I did find one, I would feel humble to be
handling something from another world. Most of us are earthbound
forever. We will never leave this planet during our lifetimes,
and we will never encounter any extraterrestrial entity, be it
living or rock. Finding a meteorite
would put us in rare company, indeed, and so I encourage you to
look.
If you find a rock that you think might be a meteorite, you
should take it to a meteorite expert. An ordinary geologist is
normally unqualified to make a good judgement because he or she
has seen so few meteorites. However, you can call your local
university such as GSU or Ga. Tech to get a name or two of some
meteorite authorities that you can contact. Alternatively, you
could call the Smithsonian Institution in Washington, DC. They
will generally have someone look at it if you send it to them.
Sometimes the rock must be cut before a determination can be
made, so you should tell them whether you are willing to have
them cut it.
Is there any way to tell
if a diamond is real or fake just by looking at it?
Not just by "looking" at it unless you are a trained
and experienced gemologist. Even then, you may make a mistake.
But scientists "look" at things many different ways.
Checking the hardness of your mystery gemstone can usually tell
you if it is diamond. Diamond has a hardness of 10, the greatest
of any natural material. Quartz's hardness is only 7. Common
glass is about 5 or 5.5.
The old method of scratching a window pane by the young lady who
has just received her engagement ring will not do. Many gemstones
are harder than glass, and even common quartz will scratch glass
easily.
Qualified gemologists can use a combination of tests to identify
diamonds. They can test the stone's density, hardness, and
reaction to light. With careful examination, many gemologists can
tell you which part of the world the diamond came from and
sometimes even which mine produced the stone. This is also true
for other gems such as rubies, sapphires, and especially
emeralds.
Although diamonds are transparent or "clear" they have
"fire" or exhibit a "play on colors." In
other words, they sparkle in different colors. This happens
because of the way diamonds are cut, combined with the property
of strong "dispersion." Dispersion causes some
wavelengths
(colors) of light to emerge from the diamond whereas other
wavelengths are reflected back into the diamond. Changing the
angle the light is hitting the faces (called facets on gemstones)
will change the color effect, so the way the diamonds are cut
determines how the dispersion effect will look.
Many other cheaper materials, mostly artificially manufactured,
also have strong dispersion and can mimic diamonds. Gemologists
can recognize these, however.
By the way, whereas strong dispersion is good in gemstones, it
plays havoc in most other applications. For instance, dispersion
is responsible for what is called "chromatic
aberration" in telescopes and cameras. The glass or plastic
lenses all have at least some dispersion that causes the images
to have blurry color fringes instead of sharp edges. This is
especially true in cheap telescopes and cameras. Unfortunately,
the same processing used to make glass suitable for high-power
lenses also increases the dispersion. Some natural crystals have
very weak dispersion. These make poor gemstones, but good lenses.
The extremely long lenses that you
see cameramen use at sporting events are mostly made of the
mineral fluorite which has extremely weak dispersion. The
fluorite is manufactured just for this application and is very
expensive.
How much of the world is
actually covered by rocks?
All of it, or none of it, really, depending on how you look at
it. In fact, you could think of the whole earth as one big rock,
so it isn't actually "covered" by rocks any more than
your history book is "covered" by paper -- it is all
paper. Now, having said that, we could note that the
book is surrounded by its cover of cardboard which is a different
kind of paper than the insides. Same with the earth -- the earth
is surrounded by a crust which is different rock from the rest of
the earth inside. The earth's crust is less than 1% of the
earth's volume.
Where the crustal rocks are the heavy variety (such as basalt),
they settle low and get covered by seawater to make up the
earth's oceans. Where the crustal rocks are of the lighter
variety (such as granite and sandstone), they don't sink very low
and so they stand above sealevel. In
this way we distinguish between oceanic crust (mostly basalt) and
continental crust (granite and lots of sedimentary rocks such as
sandstone and limestone.) The continents are made of
continental-type rock.
For the most part, the oceanic crust is covered by the
hydrosphere (oceans), and the continental crust is covered by the
earth's atmosphere. But the edges of the continents are also
covered by the oceans right now because sealevel is pretty high
worldwide. During the last ice age (about
10,000 years ago), the oceans were not as deep, and much more of
the continent edges were above sealevel. Conversely, about 75
million years ago the oceans were so deep that the seashore in
Georgia reached inland as far as Columbus, Macon, and Augusta.
Now for some numbers:
Percent of earth's surface that is continental crust 41%
Percent of earth's surface that is oceanic crust 59%
Percent of earth's surface below present sealevel 71%
Percent of earth's surface above present sealevel 29% Average
depth of the oceans 4.8 km
Average elevation of earth above sea level 800 m
As a geologist, if you see
a specific land formation, can you tell what
types of rocks and minerals are under it?
Not always, but often. For one thing, different types of rocks
weather differently. Soft rocks such as shale or schist weather
easily. Areas with lots of resistant rocks such as quartzite
weather only very reluctantly. Here's an example from Georgia:
Brasstown Bald, the highest
point in Georgia, is made mostly of mica-rich schist. The
surrounding rocks on the Richard Russell Thrust Sheet are rich in
quartz and feldspar, much more resistant to weathering than mica
schist. So why is Brasstown Bald Mountain not Brasstown Hole? It
should weather down much lower than the Richard Russell rocks.
But it hasn't. Why not?
Well, I have thought a lot about it and explored around that
area, and I think I have the explanation. Hiking up Brasstown
Bald you can find numerous chunks of white and yellow quartz
lying around, some a foot across or more. The higher you go, the
more there is. These are the remnants of massive veins of quartz
that have cut numerous paths through the schist, forming an
extremely resistant framework for the mountain. It is as if the
quartz vein network acts as a sort of internal skeleton for the
mountain, supporting it and allowing the schist to survive. (How
tall would you be if someone took out your skeleton?)
By the way, another example would be Bell Mountain in Towns
County (I think) near Hiawassee, GA. It is topped by a huge vein
of white quartz that has been quarried and is visible from many
miles away. Next time you are on U.S. 76 near Hiawassee look for
it.
Lots of time we can tell something about what minerals are below
ground by examining the soil above the rock and by studying the
streams flowing through the area. One thing that mining companies
and prospectors do is to analyse chemically the soil and the
stream water. If the stream water has dissolved gold in it, it is
reasonable to conclude that the source of the
gold is uphill from that location (water flows downhill, after
all). They can also analyse the soil for its chemical
composition, and since soil comes from weathering of rock, the
soil composition can hint at the composition of the rocks below.
We can also use geophysical methods to "look" at the
rocks below the surface. Gravity measurements can tell us whether
the rocks just below our feet are dense (heavy) or light. The
higher the gravity, the heavier the rocks below. Many metallic
ore deposits have been discovered this way. If we get lower
gravity than we expect, it might be because there is a cave
below our feet. A cave is filled mostly with air (and some
water), and these are much lighter than rock. We can also use
seismic (sound) to help us guess what kind of rocks are below.
Sometimes we even use magnetism and electricity. We usually don't
think of rocks as magnetic or as conductors of electricity, but
they are weakly magnetic and can conduct electricity slightly. By
knowing which rocks behave which way, we can detect these rocks
below the surface by running electrical or magnetic surveys.
Finally, some minerals are radioactive. Uranium, Throium,
Plutonium, and some other heavy elements are radioactive, that
is, they spontaneously turn into other elements. When this
happens, energy is released. The energy can be detected by
special instruments, such as a Geiger counter. Prospecting for
Uranium is usually done this way, sometimes from an
airplane or helicopter.
We can be grateful that searching for useful rocks and minerals
is largely done through indirect methods. These methods are
non-destructive, non-polluting, and non-disruptive. Most of the
time the searching can be done without anyone ever noticing. This
is especially important when
prospecting is done in environmentally sensitive areas, such as
in the nation's great Wilderness Areas and National Parks. Even
if we never intend to exploit the minerals in those areas, it is
important to know if they are there in case we need them in a
national emergency.
What dinosaurs were native
to this area?
Well, we weren't exactly the dinosaur capital of the world, and I
don't think Michael Crichton had Georgia in mind when he wrote
"Jurassic Park." But some of the big beasts did call
Georgia home.
The dinosaurs ruled the world during the Mesozoic Era, from about
245 million years ago to about 65 my ago. The Mesozoic Era is
divided into three periods: Triassic, Jurassic, and Cretaceous.
Almost the only Triassic and Jurassic rocks in Georgia are near
the Atlantic Ocean, and they are covered by the more recent rocks
of Georgia's coastal plain. So finding a Triassic or Jurassic
rock requires drilling through hundreds of feet of sediment, most
of which accumulated after the dinosaurs' demise. Chances of
hitting a dinosaur fossil with a tiny drill are very small, and
if you did hit one, whatever you could pull up wouldn't be very
impressive.
But the Cretaceous Period is another story. We actually have a
pretty respectable bunch of fossils of dinosaurs that lived
during the Cretaceous Period (about 145 my to 65 my ago). The
fossils all come from the Blufftown Formation of the Coastal
Plain. The Blufftown Fm is Upper
Cretaceous in age, probably no older than about 83 my and no
younger than about 75 my. Rocks that were deposited during this
time interval make up what we call Campanian Stage, specifically
the early-to-middle Campanian.
The place where all the fossils have been found is in the far
western part of the state, near the Chattahoochee River along a
little stream known as Hannahattchee Creek. This creek is in
western Stewart County to the northwest of the little town of
Lumpkin and not far from Providence Canyon. (Stewart County is
among the poorest counties in Georgia (perhaps the
poorest), and I think it's only fair that they should have
something that no other county has.)
There are only three groups of dinosaur fossils found in Georgia:
family Hadrosauridae, family Ornithomimdae, and genus
Albertosaurus. In (sort-of) plain English, they are Hadrosaurs,
Onithomimids, and Albertosaurs. The most abundant fossils are
from the Hadrosaurs and include limb bones, teeth, and vertebrae.
These creatures are commonly known as the "duck
billed dinosaurs" and were 15-35 feet long. They were
herbivores (they ate plants) and had lots and lots of teeth (up
to 700 at one time!). They ran in herds and nested together like
birds.
The next most common fossils are the remains of Albertosaurs.
These were large carnivorous creatures similar to Tyrannus Rex.
They reached about 30 feet in length as adults, but almost all
the Albertosaurus fossils found in Georgia belonged to juveniles.
The least abundant fossils are those of the Ornithomimids. The
name means "bird mimic," and these creatures resembled
the present-day large ground birds such as the ostrich or emu.
(But perhaps surprisingly, these are not the dinosaurs that we
think led eventually to birds.) They had no teeth and probably
ate both meat and plants -- whatever they could find or catch.
During the Cretaceous Period a large seaway ran north-south right
through the middle of North America. In fact, from about the
Rocky Mountains in the west to the Appalachians in the east, the
continent was under a shallow sea. It is in this sea that the
Blufftown Fm was deposited.
There are no dinosaurs known to have been marine (ocean
dwelling), but the dinosaur fossils found here in Georgia are in
the Blufftown Fm which is a marine unit. So the dinosaurs
presumably did not actually live in the place they are now found,
but rather they were washed into the sea after they were already
dead. Their bodies would have been carried downstream in
rivers (either intact or in pieces), eventually to be dumped into
the shallow sea near the shore. Almost everything we know about
dinosaur fossils in Georgia comes from the research of Dr. Dave
Schwimmer of Columbus College. He has spent many hours and days
along Hannahattchee Creek.
As you know the dinosaurs are extinct (at least we think so!).
Dinosaur fossils are found in the upper parts of sedimentary
rocks of Cretaceous age, but not in the Tertiary-age sediments
just above. So the boundary between the Cretaceous and Tertiary
periods, the so-called K/T boundary, marks the demise of these
great creatures.
Geologists argue all the time about what killed the dinosaurs.
Some think they were slowly dying out over a long time, and they
just eventually reached their end. Others think they were killed
suddenly by a catastrophic event -- in this case a massive
meteorite impact. The truth
might lie somewhere in between, because there is evidence for
both arguments. Imagine that the dinosaurs were slowly
disappearing when the meteorite hit -- that was the last straw.
Now, here is a point that most people don't fully appreciate: It
is not a coincidence that the dinosaurs died at the end of one
period (Cretaceous) and the beginning of another period
(Tertiary). The geologic time scale (calendar) is based on the
fossil record, not some arbitrary scale. (For example, our year
is based on the observed fact that about 365 days go by
during one revolution of the earth around the sun -- so our year
is based on something real. On the other hand, the length of an
hour is arbitrary; it is not based on anything real.)
In fact, so many things died out at the same time that the
dinosaurs did, this marks a big boundary in the geologic time
scale, the boundary between the Mesozoic Era and Cenozoic Era. So
it is a very important point in geologic history -- over 75% of
the species were apparently wiped out.
But for us, this was a good thing. When the dinosaurs ruled,
mammals were around but were little more than small rodents. When
the dinosaurs died, mammals were able to become dominant. We
eventually evolved from some of these, so we owe the big
meteorite impact a debt of gratitude!
By the way, why did some people suspect that a meteorite killed
the dinosaurs? Well, it begins with a father-son team, physicist
Louis Alverez and his son Walter. Walter, a geologist, was
discussing some ideas with his famous father, wondering how one
could determine how much time had elapsed between the Cretaceous
and Tertiary periods. His father mentioned that we know there is
always "cosmic dust" falling to the earth at a pretty
consistent rate, and suggested that if you could find in the
rocks the actual boundary between the 2 periods you could measure
how much cosmic dust was there and figure out how many years had
gone by between the two
periods. Walter went to the Cretaceous-Tertiary boundary near the
town of Gubbio, Italy, and collected a thin clay layer between
rocks of the two periods. When they analysed it for cosmic dust,
instead they found meteorite dust. (They are different -- in
particular, the element Irridium
is a dead giveaway for meteorites. The Gubbio rocks had a lot of
Irridium, a so-called Ir anomaly). Many other places in the world
have the same Ir anomaly, so we think that the big meteorite
smashed into dust when it hit, and spread around the globe,
eventually to settle to earth. The dust that it formed remained
in the high atmosphere, blocking out sunlight and
altering climates all over earth. Animals and plants died out in
response (some people have wondered how anything at all
survived).
Can you recommend any
books or web sites?
This may be the toughest question of all. I have been generally
disappointed in what geology books are written for bright kids. I
have seen some, and my main complaint is that they are too
simple. I think kids are mostly smarter than we old fogies think,
especially nowadays. As a
kid, I was considered pretty smart, but looking back I think that
mostly I was interested in things. If you are interested in
something, you think a lot about it, read about it, ask about it,
etc. Pretty soon, even though are a kid, you know more about that
that subject than most adults, and you
get the reputation of being smart. But mostly, you are curious,
and that is more important than what your IQ is. It is amazing
how powerful a combination of an average IQ and an inquisitive
mind can be. Watch the movie "Contact" and you will see
that intense interest in something while a kid can lead to
worderful things down the road. Dr. Carl Sagan, the author of
Contact, once described how excited he got when he found out that
he could get paid for doing astronomy.
For general books, I would visit www.amazon.com and look under
juvenile first. There are many interesting books on dinosaurs
(see below) and maybe minerals, too. (But most of the mineral
books concentrate on "museum quality" specimens -- not
normal looking minerals.) Then I could skip the high school books
in favor of some of the college books on general geology. (I
think that high school books are too focused on exercises and
hung up on the "scientific method" which can be
amazingly boring!) The college books are very readable (except
for all the new words you run into), and are pretty well
organized. You can look up the new words in the back of the book.
There are lots of good photographs and diagrams. And you can be
sure that the information is not over simplified for the sake of
simplicity. I can recommend "Essentials of Geology" by
Lutgens and Tarbuck. I think that with some help from your
teacher or parents there is nothing in there that you can't
understand. I use it in my first-year college course.
For dinosaur reading, I suggest just about anything by Dr. Jack
Horner. Jack is probably the best known dinosaur expert and was
the technical advisor for the movie "Jurassic Park."
But be warned, Jack doesn't care what killed the dinosaurs -- he
is interested in how they lived, not died.
He wrote a book for kids (with a videotape) about a young
dinosaur called "Maia," I think. If you can find that
in the library I recommend it. A word about Jack Horner -- he was
a student in geology at the University of Montana a year or so
before I arrived there to do my Masters degree. He used to visit
the U. of M. geology department, and of course I got to know him
casually. He loved dinosaurs, but he couldn't manage to finish
college. He seemed to be a "slow learner," but of
course he didn't learn slowly about dinosaurs. Anyway, it turns
out that he was dyslexic or had some sort of learning disability
like that. I don't think that he has ever
gotten a regular college degree, but he has an honorary Ph.D.
from University of Montana, and probably from other places as
well. Today Dr. Horner is the curator at the Museum of the
Rockies at Montana State University in Bozeman, MT.
Jack Horner has done more than anyone else to show us how the
dinosaurs lived. He was the first to suggest that dinosaurs'
tails stuck out (for balance) rather than hung down, that many
ran in herds, and that they nurtured their young like birds and
mammals today. He was the first to
find a nest of dinosaur eggs. His latest effort at debunking
dinosaur mythology is that T. Rex was a scavenger rather than a
predator. He wrote a book about it -- "The real T Rex"
or something like that. I also found a web site that discusses it
--
www.ucmp.berkeley.edu/trex/specialtrex2.html
Jack thinks that the predators were the smaller animals, such as
the raptors shown in the movie "Jurassic Park."
Predators have to be smart, cunning, dexterous, and fast. In the
movie they also seem to outsmart most of the human beings. (Note
also in the movie that the velociraptors are shown as warm
blooded. Can you find in the movie where that is shown?) He also
thinks that today's birds (warm blooded, of course) are the
direct decendents of certain dinosaurs, but not all
paleontologists agree.
There are dozens of web sites for other geology subjects, for
example
earthquakes, volcanoes, meteorites, Martian geology. A good place
to start
is the U.S. Geological Survey site, the NASA site, and the Park
Service site.
www.usgs.gov
www.nasa.gov
www.nps.gov
Many geology departments in universities have links to good
geology sites,including our own. Just click on "other
links" on our homepage.
www.gsu.edu/geology
We have found conflicting
answers on the question of the "biggest crater caused by a
meteorite". Can you explain the differences?
I just so happens I can clear this up. I did my Ph.D. fieldwork
just outside Sudbury, and I have spent many weeks there. It is
primarily a mining town, if you can call a place with 100,000
people a town. The city of Sudbury developed when International
Nickel Co (INCO) opened its first nickel mine there early in this
century. Since then, dozens more mines were opened, both by
INCO and Falconbridge, to exploit the nickel (and copper) that is
found there. The nickel comes in the form of sulphide minerals,
in which the nickel is combined with sulphur. The earliest
processing of the nickel ore was done in open hearth smelters.
Now for the geology: The nickel deposits come from a large
elliptical ring (mashed circle) that forms high hills. This ring
and what is inside it form a kind of oval bowl that is called the
Sudbury basin. The Sudbury basin is about 50 miles across, and
all the mines are dotted along this ring. Inside the ring is a
set of sedimentary rocks that make up several formations that
together are called the Sudbury Series. The Sudbury Series rocks
are not found anywhere except inside the Sudbury basin. The
lowermost formation of the Sudbury Series is the Onaping
Formation. The Sudbury basin is what we see, but the Sudbury
structure itself extends deep below ground. For years, most
geologists thought that the Sudbury structure was formed by a
giant volcano. But starting about 20 years ago, people began to
suspect that it might have been formed by meteorite. There were
two main reasons for suspecting a meteorite impact origin:
(1) Deep in the mines the geologists found zones in the rocks
that were shock melted. In other words, something hit the rocks
so hard that enough heat was formed to partly melt the rocks.
When these zones were mapped out, they formed a geometric pattern
that resembles the shatter pattern in rocks that are hit by a
strong blow. So geologists concluded that a huge explosion must
have occurred there.
(2) Geologists calculated the amount of energy necessary to
produce the shock melting and concluded that a volcano couldn't
have done it. This is because the energy would have built up over
a long time and released all at once. The earth's crust can
accumulate only a limited amount of energy before it gives way,
so the energy had to come from some extraterrial source. Although
there are a few holdouts, almost all geologists familiar with the
Sudbury structure like the meteorite idea.
I already mentioned the Onaping Formation inside the Sudbury
structure. This formation seems to be what geologists call a
fallback breccia. Breccia is a rock made up of very angular
fragments of older rock. We know from experiments that impacts
produce a lot of broken pieces or rock of all sizes that are
thrown up a short distance in the air and fall right back down
into the
crater. These fragments are hot (from the energy of the impact),
and have a peculiar look to them after they cool. The Onaping
Formation looks exactly like a fallback breccia. By the way,
although some meteorites are rich in nickel, the great nickel
deposits of the Sudbury area are not extraterrestrial. The
meteorite (bolide) contributed the energy for melting and
concentration of the nickel that was already scattered in the
rocks. Another impact feature found here is shatter cones.These
are funny fractures in rocks that look a bit like horse tails.
They are pointed at one end and flare at the other end; they form
only in rocks that are shocked. The size of the shatter cones is
a measure of how strong the shock was. You can see these in
freshly fractured rocks where a new highway is being built. The
shatter cones of Sudbury are up to a couple of feet long (this is
very long!) So the evidence seems to point to a huge meteorite
impact for the
formation of the Sudbury structure. The Sudbury basin itself is
not the crater that would have formed -- the crater would have
been much larger. But the problem is that there is no crater! You
see, the Sudbury structure formed about 2 billion (2,000 million)
years ago.That's a long time. Erosion has eliminated the crater
(assuming there was one). We could think of the The Sudbury
structure as the point of impact of the bolide (the thing that
hit), where the greatest
energy was expended. This is why we find the shock melted rocks
here. It might be OK to say that the effects of the impact extend
across a distance of 200 km -- that is probably about right. For
example, we find the shatter cones all around and many miles from
the Sudbury basin; we might want to argue that anywhere shatter
cones are found is part of the impact site. We also might argue
that at one time the Sudbury crater was 200 km across, but we
don't know that.I think it is still correct to argue that the
Chicxulub crater of the Yucatan is the largest known crater in
the world.
In response to this question, I went searching for web sites. I
discovered that INCO has a web site:www.inco.com and so does
Sudbury www.cyberbeach.net/~seajay/sudbury.html>
Go back to the Mining Tunnels!
Dr. Bill Witherspoon, Geology Expert, Fernbank
Museum
What are the most
interesting or unusual facts you have learned about rocks
and minerals?
Rocks can tell us a lot about the environments in which they
formed. Reconstructing the "big picture", both about
past life on earth and about the forces that made mountains and
other earth features, is what I find most interesting about
geology. For example, in one geology course, I was taken to an
outcrop of limestone and led to visualize the tidal flat that was
there some
400 million years before, evidenced by things like mudcracks, old
ripped up pieces of tidal flat mud, burrows of the worms that
lived there, and so on. It kind of gave me a chill to feel that I
was seeing into the past with my own eyes and observations.
How did you get started in
geology?
I was a rock collector beginning in third grade. I picked up
rocks at every opportunity on the trip my family took out west
that year (1962). People we met over the years were kind enough
to give me some nice specimens to add to my collection. I kind of
forgot about rock collecting through high school and
undergraduate college, but in 1974 I visited an exhibit at a
natural history museum in London about plate tectonics - the
concept that moving plates explain mountains, volcanoes, and
other earth features. By the time I went to graduate school I was
hooked, and I completed a PhD in 1981.
Are geologists discovering
new types of rocks today? If so, what are the
new classifications?
I'm not aware of new types of rocks since the Apollo astronauts
returned from the moon. Even the rocks they found there were not
too different from rocks we know from places like the Adirondacks
in New York state. A specific mixture of minerals has to occur in
bulk (over a large enough region to appear on a geologic map) at
the earth's surface to be classified as a rock. So we've pretty
much seen and classified the combinations of minerals that occur
at the surface.
New minerals are discovered occasionally, most commonly, I think,
variations on very fine clay minerals that have to be studied
using technologies like x-ray diffraction and electron
microscopy.
How do mereorites first
form?
Most meteorites are thought to be left over from the original
material that accreted (clumped) together to form our solar
system, out of the remains of an extinguished star, just as our
earth was formed originally. Most of these resided in the
asteroid belt between Mars and Jupiter until they became
deflected into earth's orbit. A few meteorites are parts of other
planets such as Mars that were flung into space during meteorite
impacts on those planets.
Where is the biggest
crater on the earth caused by a meteorite?
At Sudbury, Ontario is the largest preserved on earth, at about
200 km in diameter. By contrast, the moon has a crater 2200 km in
diameter! Earth's erosional forces have erased most of the
earliest and largest craters. Sudbury is where much of the
world's cobalt and nickel comes from - not from the meteorite
itself but from the magma that apparently flowed to the surface
after
the earth was "wounded" by a huge meteorite, about 1.7
billion years ago.
Have you ever found a
meteore yourself? If so, what did it feel like?
I found one that someone had neglected in a closet once! It felt
very heavy and had a "thumbprinted" appearance. I don't
expect ever to find a meteorite in the field as they are
extremely rare.
Is there any way to tell
if a diamond is real or fake just by looking at it?
I know that a jeweler can do this. First, they can tell glass
from diamond because diamond has a more brilliant luster. I
believe they also look for tiny impurities in a diamond, that
would distinguish it from a synthetic imitation such as a cubic
zirconia. Of course, a diamond is far harder than any other
mineral and therefore can be distinguished with a scratch test.
It will also
burn, being pure carbon, but I don't recommend this test.
How much of the world is
actually covered by rocks?
All the world has rock, soil, or loose sediment at or very near
the surface, and usually hard rock is only a few feet down. I
would make a wild guess that actual solid rock at the surface
makes up less than 10 percent of the total - more if you are in a
dry environment such as the Western US, and much less in a humid
place like Georgia.
As a geologist, if you see
a specific land formation, can you tell what
types of rocks and minerals are under it?
Geologists predict what is in the subsurface by looking at bodies
of rock that are found at the surface and inferring where their
extensions might be in the subsurface. For example, if a layer of
sandstone is found at 1000 foot elevation wherever it is exposed
in a given county, chances are that a well drilled into a 2000
foot high mountain in the same county will encounter that
layer at 1000 feet. Geophysical information, similar on a large
scale to the ultrasound used in hospitals to see a fetus in the
womb, also helps us infer what is below the surface.
Concerning fossils in
Georgia, what dinosaurs were native to this area?
The following is copied from Martha Brown's writeup from
http://www.mindspring.com/~gamineral/ga_dino.htm
At this time only three dinosaurs are known from the Cretaceous
age deposits. [the only dinosaur-bearing deposits in Georgia].
The three dinosaurs known are:
Hadrosauridae - Full size, approximately 25 feet long. [You may
know hadrosaurs as "duckbills"]
Ornithromimdae - Of unknown species [I'm not sure what these are]
Gorgosaurus- A theropod found only in juvenile size. They weighed
about 800 pounds and never reached full size. [T. Rex is another
example of a theropod]
[Ms. Brown is a knowledgeable amateur fossil and mineral
collector who often assists us at the Fernbank Museum geology
open house, third Saturday of each month. Dr. David Schwimmer, on
whom she reports, is the recognized expert on Georgia dinosaurs.
Ms. Nancy Huebner, geologist at Fernbank, is also very
knowledgeable and has worked with Dr. Schwimmer in the field.]
Could you recommend any
additional books or web sites that we could suggest
for children our age?
Yahooligans is a great kid's web site for all kinds of things,
with links to many sites appropriate for children. Try
http://www.yahooligans.com/Science_and_Oddities/The_Earth/Geology/.
As for books, I like Janice Van Cleave's series on all kinds of
sciences. I know there are other titles. Zany Brainy might be a
good place to look for them, or the store at Fernbank Museum,
though I haven't checked either of these places lately.
Georgia Geoscience On-line at
http://dekalb.dc.peachnet.edu/~pgore/gore.htm is not especially
geared for kids, but is a good starting point for your questions
on Georgia.