Ash: Rise of the Republic (15 page)

"Well then," Burns began, "I trust everyone
was able to finish their pressing conversations?" His eyes
traversed the room, as if expecting a response. "Splendid, we can
begin then. Now, your Captain has informed me that he has foolishly
spent all his time training you to kill and has completely ignored
his responsibility to ensure you a sound academic education. I know
most of you were wards of the state, so you must have had some
rudimentary schooling. And, since you each volunteered for the
rangers rather than enroll in the only institute of higher learning
in the country that is both free to all and still in operation, I
must assume that you are satisfied with the paltry level of
knowledge that was beaten into you in your primary schools?" There
were a few nods from his audience, "Of course you are. I will
maintain, however, that you are all idiots. Uneducated
ignoramuses." This elicited sour looks. A few of the rangers
muttered to each other angrily. Captain McLelland smiled to
himself, his expression hidden to those behind him. He had heard
this lecture a dozen times.

"Oh I don't blame you for your anger, I know
that's not something one would normally say in polite conversation,
especially to a bunch of highly trained killers, but it is, sadly,
the truth. I am confident that I can prove it if you will allow
me.

"For example, if you were to take a shovel
outside the Campus walls, find a level spot, and start digging,
your hole would be, on average, ten feet deep by the time you
reached what thirty years ago would have been ground level. That's
a little over three meters. The surface area of the US in
pre-pillar days was close to ten million square kilometers. Now,
let's assume, for arithmatic simplicity, that the whole thing is
covered to the same average depth." He moved slowly over to the
blackboard as he spoke.

"I'll write this out for you so you can
follow it. Ten million kilometers square times three meters high is
thirty million cubic kilometers. Thirty million cubic kilometers of
material. Over thirty years that works out to eighty thousand cubic
kilometers a day. I will pause to let that soak in.” He paused.

“Now, obviously that figure has an awfully
large margin of error – there is such a thing as topographic
relief, after all, so we can’t assume all ten million square
kilometers are covered to the same depth. Further complicating the
matter is the fact that we honestly don’t know the status of the
rest of the world. There might very well be ash on the ground in
Russia or India as well, but there has been no communication. For
the purpose of this argument, we will ignore such far off lands and
focus instead on what we can lay our hands on. We could, I suppose,
endeavor to generate a more precise volume, but we geophysicists
don’t usually have much need for exact figures. What matters is the
order of magnitude. Regardless of what the precise volume is, we
can all agree that there is a lot of ash on the ground.

“Now, tell me someone, during the
comprehensive schooling provided by the state school, did any of
the sad lumps that called themselves teachers happen to tell you
where all of that came from?" He paused again, peering out at the
room. "Don't be timid. You there, in the pirate costume."

Unphased by the jab at his appearance,
Pirate lowered his hand. With a triumphant glance at Lee, he
responded proudly, a hint of bile in his voice, "Everyone knows, a
volcano erupted up north. One the scientists had been watching for
a while. Yellow-something. It was bigger than they expected."

"Who agrees with young Mr. Sparrow here?" A
scattering of hands were raised. "I rest my case. You see, the thin
layer of education that was spread on you by the state has shown
itself to be woefully insufficient. You are wrong. No, I suppose
that's unfair. Yellowstone did blow. You are, to a small degree,
correct, but there is a portion of your statement which is, at
best, a naïve approximation of the truth. Can anyone think why that
might be?" He peered around the room again. The rangers looked
unsure now.

Lee spoke in a soft voice from the row of
seats near the front that she shared with her sister, "Because
there's too much ash to have come from a volcano?"

The old professor raised his eyebrows in
mild surprise. "Ah, I see a lack of education does not preclude
intelligence at least. Too much ash, very good young lady.

"Three billion cubic meters of volcanic ash
over thirty years is a lot. That's many times more than any volcano
in recorded history, or any we know of in the rock record. We don't
have time to go into too much detail, but suffice it to say that
one run-of-the-mill volcano could not have produced all of this
material in just thirty years. So then, the question remains
unanswered: where did it all come from? Would anyone like to know
the answer?" A few rangers nodded. "So would I. The fact is we
don't know much. What I can tell you is that Yellowstone is not
some run-of-the-mill volcano.” He squinted at Pirate again. “No, my
young swashbuckler, it was not a volcano which erupted. A more
accurate statement would be that a caldera exploded, or perhaps we
could even call it a supervolcano, although I’ve never liked either
term. Allow me to explain:

"Yellowstone, as your Captain and his lovely
wife will remember, was at one point one of our more spectacular
national parks. One of the more popular tourist attractions in this
park was a huge geyser nicknamed 'Old Faithful'. This was basically
a column of superheated water that would periodically erupt from
the ground. Around the middle of the last century, a few geologists
decided to define exactly what the heat source was for this
phenomenon. A seismic investigation revealed a large body of magma
beneath the park. This was strange. There are plenty of places
where magma accumulates within the crust, but they are almost all
associated with subduction zones." The professor broke off to peer
out at the rangers again. Confusion was evident on their faces.
With a sigh, he continued, "Ok, maybe I had better take a step
back. We'll touch a bit on plate tectonics first." He paused to
gather his thoughts, then moved surprisingly quickly back to the
board and drew three concentric circles.

"I'll have to oversimplify to an extreme
here, we don't have time to go into detail." He pointed to his
drawing. "This is a cross section of the earth. The small circle in
the center we will call the core. It is very hot. The thick section
in the middle we will call the mantle, it is also very hot, though
not as hot as the core, at least on average. The thin outer circle
here we'll call the crust. This is what we all live on, it's the
continents as well as the ocean floor. Now, as I said, the core is
very hot. It consists almost entirely of molten iron and nickel.
The center is solid, but we won't go into that. The portion of the
mantle that is adjacent to this very hot core grows very hot
itself. The portion of the mantle which is furthest from the heat
of the core is much cooler. The temperature difference between the
two regions drives something called convection.

"Think of it in terms of a pot of boiling
water." he drew a crude pot on the board. "The water at the bottom
of the pot, closest to the heat source, gets very hot, the water at
the top, exposed to the air, cools off. The hot water rises, and
the cold water settles; they displace each other. Hot water cools
off when it reaches the surface and the cold water heats up when it
gets to the bottom of the pot, and the cycle repeats itself until
the heat source is removed.

"Going back to our simplified cross section
of the earth, the core is the heat source, the mantle the water,
and the crust the air above the pot. What you end up with is very
similar to the boiling water, though on a much larger and slower
scale. The huge temperature differential spurs a convection cycle.
What this means is that hot material from lower in the mantle is
constantly rising toward the surface, and cooler material nearer
the surface is constantly sinking to take its place." As he spoke
he drew a number of circles and arrows representing the convection
cycles. “It’s tempting to think of this convecting material as
molten rock, but that is inaccurate. The rock is not molten, it is
merely under enough pressure and heat to display fluid properties.
I won’t go into more detail about that, just take it to mean that
it moves very slowly.

"Now, drastically oversimplifying again in
the interest of brevity, we will say that the crust is made of two
different materials, we will call them granite and basalt. The
continents are made of 'granite', and the ocean floor is made up of
'basalt'. This material sits on top of the mantle like a broken
eggshell. The continents and ocean floor are separated into dozens
of discrete pieces called 'plates' which move relative to each
other. The mechanism which drives this movement is extremely
complicated, so once again we will oversimplify its explanation."
He stopped briefly to point at the convection circles he had
drawn.

"The easiest way to look at it is to
consider the plates as 'riding' the convection. The hot mantle
material is welling up in some places and sinking in others.
Friction drags the crustal plates along with the convecting mantle
material. Now, remember, we said that the plates are made up of two
different materials. These materials have different densities. The
oceanic 'basalt' is significantly denser than continental
'granite'. When the convection of the mantle forces these two
materials together, the denser 'basalt' plate follows the path of
least resistance and sinks underneath the less dense 'granite'
continent. This process is known as 'subduction'. When one plate is
subducted under another, suddenly there is less crust to go around.
That portion of the oceanic plate which is beneath the continental
plate is no longer at the surface and so must be replaced by new
material elsewhere, otherwise the surface area of the Earth would
be constantly diminishing. This new material is mostly produced in
regions known as mid-ocean ridges." He pointed at the upwelling
side of the convection diagrams on the board.

"Remember there is hot mantle material
welling up from the core, and that the crustal plates are riding
that convection at the surface? Well those plates are necessarily
moving away from the upwelling. The portion of a plate which is
being subducted lies on the down-welling portion of these
convection cycles. At the opposite edge of the plate, hot mantle
material is constantly welling up, melting, erupting at the
surface, and cooling into new crustal rock. This material replaces
the crust which is lost to subducton at the other margin of the
plate. Immense mountain ranges can be formed at these rifts. The
speed of the spreading plates dictates how high these mountains
grow. The most extensive mountain ranges on Earth lie along these
mid-ocean ridges, but you will never be able to climb them. This is
because, as the name suggests, they lie deep beneath the ocean. As
a caveat, there are a few of these zones in the middle of
continents, known as rift valleys, but they are distinct enough in
their mechanism that we will ignore them for this lecture.

"For the purposes of our discussion, we need
to consider the various kinds of volcanism. We've touched on the
slow, seeping eruptions at spreading rifts, but what about classic
volcanoes? Big, smoky, conical mountains, spewing fiery lava and
exploding with ash and toxic gas? Volcanism such as this is the
product of subduction. When oceanic crust is subducted, it's very
wet. It's wet not only in terms of water being stuck in pores
between the rocks, but it is also included in the crystalline
structure of the rock, at the molecular level. As it sinks below
the continental crust, it heats up. At a certain depth, the
pressure and heat are great enough that the molecular water is
released and rises into the overlying continental rock.

“Now, this continental rock is essentially
dry, but it is under a considerable amount of heat and pressure.
Under normal, dry conditions, that rock is basically stable, but
when the water is introduced, the physical properties of the rock
are altered enough to spark a process known as partial melting.
Essentially, the added water lowers the melting points of the
various mineral compounds in the rock. The lighter minerals and
chemicals within the rock suddenly find that, with the introduction
of the water, they are now at or above their new melting points. A
fraction of the rock thus melts away. It is this partial melt which
forms magma and gas.

“This mixture builds up slowly, deep within
the rocks at the margins of the continent until the pressure is
great enough that it erupts violently. Every subduction zone
produces this type of volcanism. It is characterized by explosive,
spectacular eruptions driven by the presence of volatile
components, highly viscous magma, and immense quantities of ash and
projectiles of molten rock commonly known as bombs. It is important
to remember that the material which erupts is produced entirely
within the continental rock itself and therefore its composition is
directly related to the composition of the source rock. It is not
molten mantle material erupting at the surface, but rather the
'granite' of the continent which is made to melt by the
introduction of a catalyst. Once again, I must assure you that this
is an extreme oversimplification. The actual processes are much
more complicated.

"Besides the two mechanisms for volcanism we
have discussed, one gentle yet constant, the other profoundly
violent yet sporadic, there is another. As we discussed, these
first two types are essentially the product of convection within
the mantle and the interactions of the crustal plates which are
driven by this convection.

"Convection drives the third type as well,
but on a smaller scale. This last type we will call 'hot-spot'
volcanism. Now, it is tempting to think of mantle convection in
terms of these pretty circles I've drawn on the board here, but in
reality the pattern of rising heat and sinking cool is complicated
and chaotic. The physics which govern this are beyond the scope of
this lecture, so let's turn instead back to our culinary metaphor.
If you watch the surface of a pot of boiling water, you will notice
a number of dimples on its surface. These dimples are the tops of
columns of hot water rising to the surface. The water in the center
of the dimple is hotter than the water which is rolling to the
edges. You will also notice that these dimples vary in size and
position. Variations in the heat source cause this lack of
uniformity. Convection cycles within the mantle are similarly
chaotic. Besides the huge rolling plumes on which the crustal
plates ride, there are also smaller, localized plumes, driven by
variations in the heat produced at the mantle/core boundary. Some
of these small, hot plumes are geologically fleeting, but others
are persistent. The causes of these heat variations are not well
understood, but they can be readily identified by the presence of
so called hot spots.