Earth System Science - Lab 4
ìEarth Structure and Rock Cycles"
This is our fourth "rendez-vous" and we are finally
touching ground. From the outskirts of the Universe to elementary particles, we
have finally managed to land on this pebble in the sky, the center of our
interest for the remainder of this course. But you should know me by now and
realize I canít just go straight at it. Iíve decided to take you on a little
tour inside Earth before we roam its surface. Happy "touring"!!!
Objectives: The primary goals of these exercises are to
1) illustrate
some of the internal properties of the Earth
2) test
our skills at system conceptualization by practicing on a very simple rock
cycle and using temporal scaling, and
3) understand
the dynamic nature of the Earthís interior and how the processes of Plate
Tectonics can affect the evolution of well yes, life!
What is our position in the universal scheme of things (in other words, where is this lab?leading to)? After this exercise, and in conjunction with lecture material, you should have a better sense of the physical structure of Earth and how lengthy temporal and spatial scales can affect what we see today and what will come to be tomorrow (in geological terms, tomorrow is measured in hundreds of thousands to millions of years). How does this fit in the course of our learning? Once we have defined some of the long term events that shape the Earth, we will move to events that occur on shorter time frames (earth energy balance, atmospheric and ocean circulation).
Introduction:
In lecture, we have described the evolution of the
Earth. How it went from elements in a nebular dust to a celestial body with
specific composition and physical properties. Weíve learned that the interior
of the Earth is not uniform in either chemical composition or physical
properties. We know now that the Earthís internal mass is in fact arranged in concentric
shells or layers. The Earth is a big Onion! Well, not really. Actually, I
prefer to say itís a bit like a Peach (See Figure 1 below), with the central
"pit" (we call it the Core) comprised of dense material (iron, nickel and maybe
some sulfur), itself surrounded by lighter "flesh" (what we call the Mantle
composed mostly of magnesium, iron, silicon, and oxygen), all of it encased in
a thin veneered even lighter "skin" which we call the Crust (composed mostly of
aluminum, silicon, and oxygen)
Figure
1. A sliced view
of the "Peach".
The internal structure of the Earth reveals layers of different
composition and physical properties. Note 1) that the vast majority of Earth
material resides in both the Mantle and the Core, 2) that the crust is thicker under
the continents than under the ocean, 3) that layers may show more than one
state (e.g. solid and liquid), which itself leads to 4) that boundaries
between zones of different physical properties (e.g. lithosphere, astenosphere,
mesosphere) do not coincide with compositional boundaries. Figure from The
Blue Planet, 2nd Edition (Skinner et. Al., 1999).
By now, you
know I dream things up on how to have YOU develop your own experience of this planet.
And of course the more and more we go, the more and more weíll need to include
and relate diverse processes. But I was struck with an ailment by my birth
star, I always feel an urgent need to start everything I do from the beginning.
I envision that ìstarting at the beginning" is a good way to see it all! I
guess some of you may not share this "philosophy" and may want to get over with
it quiiiiiiiick (I wish I could have a cartoon-like zooming sound on that one).
Anyway, you may read the end of the lab, you may start in the middle, you may
even do it singing Guantanamera backwards. Itís your prerogative. Mine
is still to start at the beginning. And to do that Iíd like for you to test and
demonstrate that the density of Earth is indeed what we measured in class.
Part I. Internal Structure of the Earth
You may
remember that the mass of the Earth is now well known thanks to studies of the
Earthís orbit that have been improving since the 18th century. That
mass is approximately 6 billion billion billion grams, which is written 6.0 1027
g in scientific notation (see Table 1 below). As far as shape, humans have
known that the Earth is round for quite some time now. In the third Century
B.C., a Greek librarian, astronomer and philosopher named Erastosthenes was to
the first to accurately calculate the circumference of Earth. Although
Pythagoras had realized the Earth was spherical by the sixth Century B.C.,
Erastosthenes was the first one to estimate its size down to less than 10% of
the true value. A few centuries later, most people (I should say, instructed
people) in the Mediterranean Basin knew the Earthís approximate size. Why is it
then that people in the West forgot about these findings and reverted to a
planar model of Earth (meaning "flat as a pancake" is just probably another lesson
in history that you need to see something more than once before you actually
learn it.
From the
previous labs, you know both the circumference and radius of the Earth. Now
letís pose our initial hypothesis from the last lecture:
"Does the density of surface rocks equal that of the Earth?"
Imagine you
are a scientist on a mission to test this hypothesis. You have no prior
knowledge of composition or structure of the surface or internal Earth but you
have your wits and some ancient knowledge about Earth.
Q1) Please cite the things that you DO know
up to this point.
Q2) Please cite the things you need to find
out to test (accept or reject) this hypothesis
Q3) Well, OK. You can measure seismic waves (those
generated by earthquakes) and you and your colleagues have quite a few monitors
around the Earth waiting to pick up signals. What kind of waves can you detect?
Q4) What can each of these waves tell you about
the internal structure of the Earth?
Q5) What do these waves actually tell you?
Why?
Now, Iíd like
you to actually calculate what youíve put in words. I have given you some information
(clues) and ask you to fill in the blanks in Table 1 below
|
Layer |
Depth (km) |
Average Density |
Mass (g) |
Volume (m3) |
% Total Mass |
% Total Volume |
|
Whole Earth |
6370 |
|
5.95E+27 |
|
100.0% |
100.0% |
|
Crust |
0-70 |
|
|
8.66E+18 |
0.4% |
|
|
Mantle |
70-2900 |
|
4.06E+27 |
|
|
83.0% |
|
Core |
2900-6370 |
|
|
|
|
|
Table 1. Characteristics of the Earthís layers.
Please enter
the average density youíve calculated previously for each layer and press the "Draw"
button to have a visual image of how the density of Earth changes from its
surface to its inner core.
Q6) Since you ARE good scientist, you ALWAYS double
check your calculations. How would you double check your calculations of
densities?
Q7) Write down the equation to double check
you calculations of densities in Table 1 (at first write only symbols for quantities,
no numbers. Make sure you identify your symbols. Then solve the equation and
make sure IT MAKES SENSE. (You loose all the point of this question if your
answer doesnít make sense with your logic!)
The data and graphs you generated are actually an
oversimplification of the internal structure of the Earth. To have a more
appropriate picture (but still an approximation), please click here.
Q8) As a good scientist, now you want to explain
why you observed strong changes in densities within the interior of the Earth.
Q9) Please, also say why you also observed
changes in state for certain layers (or sub-layers) within the interior of the
Earth.
Part II. Fluxes
and Reservoirs
Youíve learned in lecture that the thin and light
outermost layer called of Earth (the Crust) can be divided in two types: Continental
and Oceanic Crust. Continental crust is heterogeneous consisting of many
different rock types in different parts of the world. But if one could blend
continental crust into a well stirred mixture, one would end up with a rock
type we call granite. On the other hand, oceanic crust is very
homogeneous and is made primarily of a rock type called basalt. Both
these crustal materials have slightly different densities (basalt is slightly
denser than granite) which explains why oceanic crust is thinner and "rides
lower" than continental crust. Weíve learned that in the isostasy
segment of the lecture. What we donít really know, however, is how these
originated. To talk about the origin of crustal material (whether oceanic or
continental) we need to give a quick review of plate tectonics.
The ocean basins host the longest and most
impressive continuous mountain chain at the surface of the Earth. We canít
enjoy good skiing or trekking on its slopes because the vast majority of it is
under water. The crust we associate with ocean basin is actually formed within
these mountain belts where hot molten mantle material moves towards the surface
(see Figure 2 below). As the hot material rises to the surface it fills gaps in
the long cracks present in the middle of the mountain ridge and in so doing
tends to push away the crust on each side of the crack. As the environmental
temperature surrounding the magma drops this one cools and crystallizes into
new oceanic crust that is further displaced away from the crack by new incoming
hot material.
Figure 2. Driving mechanism for plate motion.
Convection in the astenosphere brings new material up into the "cracks" of the
mid-oceanic ridge and at the same time drags plates away from the center point
of separation. Figure from This Dynamic Earth, The Story of Plate Tectonics
(USGS, http://pubs.usgs.gov/publications/text/dynamic.html)
As mantle material partially melts and solidifies
again there seem to be an enrichment of specific elements (light ones) in the
newly formed crustal material and retention of remaining elements (heavier
ones) in the originating mantle material. This process may not end with the
formation of oceanic crust but may also be responsible for the formation of
continental crust as well. This is a perfect example of a system in which we
deal with reservoirs, fluxes, residence time, etc. letís play with it and see
if we can actually confirm that our hypothesis is acceptable.
So what is our hypothesis now:
ìThe origin of continental crust lies in the transformation of oceanic crust."
Letís test this proposition. Letís see how we can do this. Letís first define our whole system. We have a two reservoirs a) continental crust and b) oceanic crust. I am willing to share with you that the average thickness of continental crust is approximately 40 km whereas that of oceanic crust is approximately 8 km. Moreover, continents cover only ~30% of the total surface area of Earth whereas the remainder of the surface is covered by oceans. Knowing that, you can start building two boxes each representing either continental or the oceanic crust. Please look at Figure 3 below and identify each reservoir and its dimensions.
Figure
3
Q10) What reservoir is box "A"?
Q11) What reservoir is box "B"?
Q12) What are the dimensions of "a" and "b"?
Q13) What are the dimensions of "c" and "d"?
Q14) Which box is greater than the other and by how much?
Q15) If you
know that oceanic basalt contains only 0.1% potassium (K) and that continental
granite contains 1% potassium (K), how
many more units of K are there in a volume of continental crust vs. the
same volume of oceanic crust?
Q16) Based
on what you just found and one of your previous answers, how many amounts of
oceanic crust do we need to utilize to produce all the K found in the
continental crust?ÝÝ
Q17) Last
piece of information that I give you: The average age of the ocean crust is
approximately 100 million years (that is the average time it takes for ocean
crust to come up at the ridges and disappear in trenches. See Figure 2).
Knowing this, how much time does it take to produce all the K found in the
continental crust?
Q18) How does that time compare to the age of the Earth (meaning, is one greater, and by how much?)
Q19) Hence,
will all the work youíve done can you accept or should you reject your initial
hypothesis (look up to remember what it was)?
While this calculation
is certainly not very realistic and overly simplistic, it does tell us
something about the origin and rate of formation of the continental crust.
Part III. Plate
Tectonics
Most of the input of new mantle material to the Earthís surface seems to occur at plate boundaries we call ridges (See Figure 2). However, less common but still important outpouring of lava occurs in the interior ofplates, thousands of kilometers away from the plate edges. In 1963, J. Tuzo Wilson, a Canadian geophysicist, came up with an ingenious idea that became known as the "hotspot" theory. Wilson noted that in certain locations around the world, such as Hawaii, volcanism has been active for very long periods of time. This could only happen, he reasoned, if crustal plates were to drift slowly over deep-seated relatively small, long-lasting, and exceptionally hot regions - called hotspots or mantle plumes -. Mantles plumes are placed where molten rock originates deep below the astenosphere. As this molten rock rises its melts its way through the lithosphere, spilling out as lava on the top of the plate. With time (million of years) large quantities of lava are added to the pile, creating a volcanic cone that towers above the ocean floor (and eventually above sea level forming thus an island). Ultimately, the motion of the plate, as a result of sea-floor spreading, transports the newly formed island beyond the mantle plume cutting off its supply of lava. At this stage the island stops growing and erosion begins to wear down its rocks. As one island volcano becomes extinct, another develops over the hotspot, and the cycle is repeated (Figure 4). The growth of volcanic islands by mantle-plume injection (hot-spot activity) thus results in the formation of linear chains of islands such as the one observed in the Hawaiian Islands
Figure 4. Illustration of the movement of the Pacific Plate over the fixed
Hawaiian "Hot Spot," illustrating the formation of the Hawaiian
Ridge-Emperor Seamount Chain. The volcanoes of the Hawaiian chain get
progressively older and become more eroded the farther they travel beyond the
hotspot. Figure from This
Dynamic Earth, The Story of Plate Tectonics (USGS, http://pubs.usgs.gov/publications/text/dynamic.html)
There
is actually another neat environment to study plate motion in conjunction to
hot spot island formation. This environment is a chain of small islands that
are produced from a hot-spot center deep below the ridge in the middle of the
South Atlantic Ocean. The latest of these island is called Ascension (see
Figure to the right) and as it "sits" presently right in the Middle of the
South Atlantic Ocean and it also tells us something quite amazing about how
life forms can adapt to perform astounding exploits. It is the story of sea
turtles and 100 million years of their evolution. Millions of years ago the
shapes of the continent(s) were not what they are today and landmasses and
oceans basins were organized in a different fashion. Please consult the
following web site to see the evolution and distribution of landmasses in the
last 200 million years: OSDN Plate
Tectonic. Click on animation
to have a dynamic vision of the change that occurred during that time. Now, in the
main page please write 200 in the field for "Age to be reconstructed [My]:"
to obtain a snapshot of how the Earth looked 200 million years in the past. Do
the same for 150, 120, 100, and 60 millions years
Q20) What
can you say about the opening of the Atlantic Ocean. Describe it in several
steps, indicating differences you may note between North and South Atlantic?
Now, look at Figure 5 and Figure 6.
Based on the age scale and the distance away from the spreading center (Mid-Atlantic
Ridge), please calculate the speed of separation of the plates based on:
Q21) The North Atlantic sea floor (Figure 5)?
Q22) The South Atlantic sea floor (Figure 6)?
Q23) What
can you say about these two measurements?
Q24) Can
you explain why the North Atlantic has blue regions in its outer Eastern and
Western regions but not the South Atlantic?
Seafloor spreading and the odyssey of the Green Turtle
There is an amazing
story that needs to be told. Almost anyone seems to be interested by sea
turtles, maybe because of their looks, a strange mixture of armored reptile and
quiet "water glider" maybe because of their tears (which they constantly
produce to extract salts from their bloodstream), or simply maybe because they
have been around for hundreds of millions of years. Whatever the reason, there
is one that is quite compelling and that helps us understand the strong
influence on geology on the evolution of species. Biological evolution is not a
concept we can always grasp that easily. How can you actually explain natural
selection? The survival of the fittest? How does that really work? It is indeed
a bit obscure for anyone to truly make out natural selection unless we have a
direct indication that reproduction (the ultimate transfer of oneís gene pool
to descendant generations) is favored in one individual relative to another
one. Let me talk to you about a specific case that not only speaks for itself
on behalf of natural selection (and thus evolution) but amazingly illustrates
the process of plate tectonics. Part of this discussion comes from a seminal
paper by Archie Carr and Patrick Coleman published in 1974 in the international
journal Nature ("Seafloor spreading theory and the odyssey of the Green
Turtle" Nature, Vol. 249, p. 128-130).
A subpopulation of green
turtles (Chelonia mydas) lives on the coast of Brazil but breeds and
nests 2000 km away on Ascension Island in the Central equatorial Atlantic (see
Figure above). Although green sea turtles live most of their lives in the
ocean, adult females must return to land in order to lay their eggs. Biologists
believe that nesting female turtles return to the same beach where they were
born. This beach is referred to as a natal beach. Nesting on offshore
and oceanic islands are likely to have good surf-built beaches, that are
favored by turtles to nest, and they almost always offer relative freedom from
nest-predators which plague mainland nesting beaches. It thus makes a lot of
sense, if you are a turtle, to try your luck at swimming out to a relatively
distant offshore island to reproduce and lay your eggs. However, the Ascension
colony has apparently made a choice in the face of huge difficulties which
seems to make impossible demands on the process of natural selection. Imagine I
asked YOU to swim 2000 km so you can meet your significant other?Well, if I
gave you, and your descendants 100 million years of history to adapt to the
swim by small increments, then maybe, maybe these descendants of yours could
manage the feat.
It seems that about 100
million years ago, marine turtles of Chelonia types that inhabited the seas between "North America" and
Gondwanaland traveled between residence-pasture and breeding grounds along the
shore of South America. Islands on the mid-oceanic ridge were used as breeding
grounds as they conveniently were formed just a few hundreds of kilometers away
from the continental shores. As seafloor spreading started to "push" the
continental masses of South America and Africa apart, the usual ridge island
used as breeding grounds was pushed westward along the ìconveyor belt" of the
moving plate. The island, however, did not only move westward but also down as
the oceanic crust of the ridge was pushed towards the deep oceanic basin. As
time went on, the sporadic creation of volcanic islands thus replaced previous
ones as these "sunk" below sea level. Please click here
to see an animation of the formation and "drifting" of the Ascension Island
chain away from the "hot-spot" activity. Since males accompany the females
during the migration and mate with them off the shores of the nesting beaches,
you can realize that any specimen that had a propensity for bulkier musculature
and increased fat reserves may have become more successful at reproducing thus
making a strong case for selection. Theory has it that as turtles were looking
for their "disappeared" natal beach they kept on swimming. Fortunately they
kept on reaching a new target, the newly formed volcanic island. Eventually,
this one too was bound to be submerged and turtles needed to keep on swimming
to reach a new goal. And the story continues today.
Q25) Based on this animation and what you know so far, which island is the oldest?
Q26) Which island is the youngest?
Q27) Green turtle reaches sexual maturity at approximately 10 years of age (that means it can start reproducing and thus going to Ascension to lay eggs). If each turtle in the Brazilian subpopulation reproduces once every 4 years for 20 years, what additional distance will it have to swim at the end of its life than during its first "reproduction voyage"?
Q28) Does it make a difference in relation to the trip they have to swim?
Q29) If you assume spreading rate of the South Atlantic Seafloor has remained constant since South America and Africa started separating, how long was the "voyage" for a green turtleís ancestor 60 million years ago?
Q30) How can you explain that turtles have "learned" or adapted to swim such long distances?
The Challenge Corner
Any question answered in this section can only add to your points but not take anything from you (you can actually make more than a 100%!). This is intended for the "adventurous" who desire to test their skills at tougher challenges. You miss?Never mind (at least youíve tried and youíve probably learned something). You got it?There you go, youíre on your way to be a pro!
Good luck and remember, that the first and only rule here is
have
fun!
Q31) Now,
what would happen to the turtles if suddenly nuclear tests would take place on
Ascension Island destroying and removing it from the surface of the Earth (or
just destroying enough of it that it now lies below sea level)? Would
there be able to keep on swimming across the Atlantic to the shores of Africa?
Why? What does that tell you about temporal and spatial scaling of evolutionary
processes (biological ones)?