√ 02 Geology
M: Good day, everyone. Today, we’re going to be looking at some
of the effects the moon has on our lives. Can anyone tell me
one such effect?
W1: Well, ummm, I’ve heard that full moons make people do crazy
M: Ha ha. Yes, I’ve heard that, too. Some statistics support that idea,
but other studies refute it. In any case, you can debate that more
in your psychology classes. But this is geography, so what physical
effect does the moon have on our planet?
W1: Oh, OK then. Well, how about the tides? The moon’s gravitational pull causes the tides, right?
M: Right you are. That’s the information I was looking for, thank you.
So, we know the moon causes tidal movement in the oceans, but
can the moon cause rain? Do the moon and sun create tidal effects
in the atmosphere as well as the oceans? In the past century, an
air tide, or rather a kind of shifting of the atmosphere has been
recognized. That, specifically, is what we’re going to discuss today.
As always, you are more than welcome to ask any questions you
have as we go along.
The possibility of gravitational tides in the Earth’s atmosphere
was first suggested by Sir Isaac Newton. Newton is most famous
for what discovery? Anyone?
W2: Wasn’t he the gravity guy?
M: The gravity guy? Yes, I think you could say that. Newton was
the first person to describe the force of gravity. Now, he came
up with this theory on atmospheric tides while developing an
explanation for ocean tides. Since 1918, scientists have been
measuring air tides in the Northern Hemisphere, and although the
changes in air pressure are small, their effects are not. Studies have
shown that more cloudiness, rainfall, and storms are generated
during certain lunar phases, such as the full moon.
In fact, even before Newton set down his theories on tides, people
were aware that lunar phases corresponded with the rise and
fall of the ocean tides. More recently, we have found that the moon
is even able to cause deformations in the solid crust of the Earth.
So, much in the same way the moon affects tidal movement in
certain ways, it also pulls on the Earth’s crust as well, causing it to
move, too. Yes. Amazing, isn’t it? The ground we walk on every day
isn’t necessarily as solid as it seems! Yes, there in the blue sweater?
W1: The textbook mentions that the moon can cause an atmospheric
tide, and that it can create changes in air pressure. Is this an air
tide, and is it true that these changes in air pressure can cause
M: Ah, as I was saying earlier, studies have shown an increase in
storms corresponding to certain lunar phases. While we cannot
yet explain this in full, it does appear that the moon has an
influence on the weather. Whether they directly cause hurricanes
or not, well, we’ll have to wait for further research to determine
that with any certainty, I’m afraid.
Let’s look more at what happens during a full moon, though.
Researchers at the University of Arizona discovered that at the
time of a full moon, the temperature of the lower four miles of
the Earth’s atmosphere increases by a few hundredths of a degree.
Now, a few hundredths of a degree may not sound like much to
you, and you probably couldn’t feel the difference yourselves, but
it is significant. These researchers suggest that the moon warms
our atmosphere in two ways. First, the moon’s surface is heated
by the sun and radiates thermal energy at the Earth. This energy
is much less intense than the energy we get directly from the sun,
but it is supplemented by a second phenomenon. The moon of
course mirrors, or reflects, sunlight at the dark, or night, side of
the Earth. However, the mirror effect is relatively slight because it
reflects just 10 percent of the light of the sun. Nonetheless, as you
all have experienced, I’m sure, that 10 percent is quite significant
when compared to the zero percent reflected during a new moon.
T he difference is night and day, so to speak, if you’ll forgive the
pun. Anyhow, what I want you to take away from today’s lecture
is the fact that the moon can affect our weather. While we still have
much to learn, we may well be justified in blaming the moon for
a rainy day!
W: Today, I’ll begin with the basics about minerals. It’s important
that you supplement this information by reading chapter 3 in
your textbook because I’m sticking only to the bare bones here.
All right...it’s essential to remember that both chemical composition
and crystal structure together define a mineral. Some students
find that surprising. They think that crystals are pure --- are just one
element. That may be true for some crystals, but not all. Minerals
range in composition from pure elements and simple salts to very
complex silicates with thousands of known forms. So to define
a mineral, we have to figure its composition. What all is in it?
Now, here is a useful tip that may save you a point or two on the
next exam. Organic compounds are usually excluded from the
category mineral. Got that? If it’s organic, don’t classify it as a
mineral. In fact, there are five main criteria for calling something a
mineral. Let’s go through those criteria. First of all, it must be in a
solid state, not liquid, gas, or plasma. Minerals are solid. Second, it
must be naturally occurring. In other words, it can’t be man-made.
Third, it has to be inorganic. Like I said, if it’s organic, it’s not a
mineral. So, third --- oh, sorry --- we’re on number four now. Fourth,
for something to be a mineral, it needs to have a fixed composition,
which means the chemical composition is the same everywhere it
is found and every time it is found. Mineral X found in my backyard
is going to have the same composition as Mineral X found in
Australia. Finally, our fifth criterion is that a mineral must be
either an element or a compound; so it cannot be a mixture of
a chemical compound and an element. Don’t worry if that last
one seems a bit vague at the moment. We’ll talk a lot more
about that over the next couple of classes.
Sometimes we get certain cases that satisfy all but one criterion.
That’s close, but not a mineral. These things are usually classified as
mineraloids. Pearls would be a good example. Pearls are solid. They
occur naturally. They have a fixed composition, and they’re a
compound. The only criterion they don’t meet is the “inorganic
test.” Pearls are actually a mixture of organic and inorganic
substances. So, because they have that extra organic stuff mixed
in, we can’t classify them as minerals. Pearls should be called
Now, here’s another interesting case. Two or more minerals may
have the same chemical composition, but differ in crystal structure.
These are known as polymorphs. A good example of a polymorph
pair is pyrite and marcasite, which are both iron sulfide. Let’s create
a simple analogy to help you grasp that concept in case you’re
confused. Let’s say Michelangelo has one large piece of marble.
He splits it in two. One piece, he carves into the shape of a horse,
and the other piece into the shape of a woman. They are exactly
the same in chemical composition, but nobody would really claim
they’re the same after he’s finished. Think of pyrite and marcasite
as two of nature’s sculptures, both made of iron sulfide!
All right, let’s see if you’ve been listening (laughs). Here’s my
question. How about frozen H2O...or ice in layman’s terms? Is
it a mineral? Anybody? Yes, Sam?
M: Well, I’m not positive about this, but...in liquid state, it’s just a
chemical compound, right? But as ice it becomes a mineral.
W: We’ve got the five criteria for minerals, right? Tell me about each
one in terms of ice and we can check.
M: OK. Ice is a solid with crystalline structure, and it’s not a humanmade
substance. Ice isn’t alive and never has been; it’s...how did
you put it?...exactly the same everywhere you find it and every time
you find it, or whatever; it’s a pure compound although it might
have other elements suspended in it. Did I cover everything?
W: Well done, Sam. I’m glad somebody was listening (laughs). You’re
absolutely right. Ice is a mineral.
M: Kind of strange though. Before this class, I never would have
thought of ice as a mineral.
W: I agree that it’s odd to think of it as a mineral. That’s because
most of the minerals around us seem like metals or rocks. Most
people forget that minerals come in many states of matter and
forms. That’s why we have those five criteria for determining
whether or not a substance is a mineral. Also, we need to keep
in mind that both chemical composition and crystal structure
together define a mineral.