PLATE TECTONICS & TECTONIC RECYCLING
Click on this image to enlarge it (this will take a long time to load on a dial-up
This page is dedicated to the ongoing process of plate tectonics ... the large-scale
movement of giant slabs of the Earth's crust. Through this movement, the Earth's
surface is recycled. While a complete resurfacing of the planet takes a long
time (hundreds of millions of years), it is accomplished nonetheless by the
presence of a hot inner core and movement of the mantle in huge convection cells.
It is into this realm of the inner Earth and its effect on the upper crust that
you are now directed to go.
Scientists who date ancient rock find that the continental rock is much older,
on the order of billions of years, while the oldest ocean basalts are less than
200 million years. An beautiful image of the differences in the ages of oceanic
crust is found atop this paragraph, but this smaller version is hard to read
and difficult to decipher the subtle changes on the ocean floor. Click on the
image to see a full version of the ages of ocean floor rock. WHY DOES THIS AGE
DIFFERENCE EXIST? Furthermore, when scientists looked at ancient rock formations
and fossils on the coast of west Africa and compared them to rock formations
and fossils on the east coast of South America, they found an identical match.
HOW DO YOU EXPLAIN THESE MATCHES? Thirdly, scientists found fossils of ancient
marine life on top of huge mountains. It is relatively easy to measure the rate
of erosion of mountains by wind and rain, and these rates indicate that earth
should have no mountains except volcanoes, and volcanic mountains would not
have marine fossils at their tops because the rocks there come from melts. How
did those fossils get up there ... and I mean way up there. I was excited when
I found fossil snails on top of the limestone ridge in the heart of the Black
Hills, but similar marine fossils are near the summit of the Himalayas too!
HOW DO MOUNTAINS FORM IF THEY ARE NOT VOLCANIC?
Alfred Wegener, in the early 1900's proposed that the continents across the
Atlantic Ocean were at one time connected. As evidence for this proposal, he
cited the identical nature of the rock formations at opposite shores of the
Atlantic Ocean, as well as identical fossil records. He reasoned that the Atlantic
Ocean previously did not exist, but rather began to appear as a break between
the land masses, and that these masses moved in opposite directions. His ideas
were not accepted, and it was not until the 1960's that scientists finally established
the theory of Plate Tectonics as an ongoing process. Today the theory is now
a fact. Here is what we know, based on presently observable evidence!
earth is made of layers, formed during a rather fast process called the "Iron
Catastrophe." The heaviest materials fell into the center, while the
lightest materials migrated toward the surface. From here, gravity and radioactive
decay of core materials produced a deep interior which is significantly hotter
than the surface. Remember, gravity is a relentless force which attracts everything
in a sphere toward the center. The more mass, the stronger the force of gravity,
and the greater the interior pressure due to gravity. This internal pressure
is similar to what might happen if I were to squeeze my flat palms of my hands
on your head. Beside the headache from the pressure, you would also sense heat
from the squeezing. So too, the internal gravitational pressure generates heat
deep in the Earth's core. Since heat rises, there must be a mechanism for the
core heat of the Earth to escape to space. What is so fascinating is how this
heat leaves the core.
This diagrams below show the heat rising from the core. It simply "boils"
out from the inner core through the liquid outer core. Much like water boils
in a pot, the Earth's interior is "boiling" except it is not water
that is doing the boiling but the rock.
is the vertical movement of liquid or gas. Due to differences in density of
a hot and cool liquid or gas, the hotter the sample the less dense it becomes,
and the cooler the sample the more dense it becomes. This is evident in the
image to your right. If you place a pot of water on the stove and watch it for
a long time, nothing ever seems to happen due to the Law of "Watched Pots
Never Boil." Only when you look away will the pot begin to boil, and by
then you have missed the initiation of the event. By placing a beaker of water
over a Bunsen Burner, you avoid the boiling pot law and can witness this event.
The water closest to the heat source absorbs heat. As a result, this water becomes
less dense and moves upward toward the surface of the beaker. When the water
reaches the surface, it interfaces with the cooler air and the most energetic
water molecules escape in the form of steam. The resulting remainder of the
liquid water is cooled by this evaporation and since cooler water is more dense,
it sinks back down into the beaker. As a result, there is a vertical current
of warm water rising and cool water sinking. This is called a Convection Cell.
If you place peas in the boiling water, you can witness their vertical movement
carried along by the convection cell.
Convection granules on the photosphere of the Sun. The hottest material is
in the bulging white gas, and the cooler material is in the sinking brown gas.
The picture is from the Swedish Institute
for Solar Physics, and they are doing some fascinating research on the Sun
... but that is a topic covered elsewhere (The
Sun). Convection cells in the Earth are what causes the plates of crust
Within the Earth, as well as within the Sun (any any other planetary or stellar
object whose interior is hot enough for melting to exist) these convection cells
are bring heated material to the surface and bringing the cooler material back
into the interior depths. On the top of the beaker of water, you will see surges
of hot rising water. These same surges are visible on the photosphere of the
Sun. The "granules" are as big as entire Earth continents, but they
operate under the same convection principle as the boiling water, except it
is hot gases that are convecting. Within the Earth, it is both the liquid core
and the heated mantle rock that convect vertically.
The convection of hot liquid core material boils to the mantle boundary, carrying
the heat from the solid inner core. At this boundary, the hot liquid iron encounters
a more solid material known collectively as the mantle. This diagram (below
and left) divides the mantle into the Tectosphere and the Aesthenosphere. Both
regions are characterized by a rock which acts like an ice glacier. The rock
moves, ever so slowly, but is moves. The heat is trapped in the rock, and this
heated rock rises toward the surface. At the surface, the heated rock releases
its heat, and thus becomes cooler. Since the cool rock is more dense than hot
rock, it sinks back in toward the core, only to be reheated and begin its journey
anew. The entire process of rising and sinking takes about 500 million years.
One might think that the Earth's core would eventually cool and our planet become
geologically dead, but there is sufficient mass for gravity to continually generate
enough internal pressure and keep the core hot ... at least 7000oC. From this
image and process, you have now been made aware that the Earth is recycling
its rock on a global scale, always bringing new, hot rock to the surface and
taking cool, dense rock back to the interior.
This images to your upper right and immediate left show what happens when the
hot rock reaches the surface. As mentioned above, the hot plastic mantle encounters
a thin, brittle crust at the surface. The crust of the earth is made of two
kinds of rock. One kind is called Continental Rock (granitic lithosphere) ,
and is composed of silicate rock called Granite, with an average density of
2.7 g/cm3, and a thickness of 40-60 km. This rock has a lot of feldspar, quartz,
muscovite, and biotite, and is brightly colored and often with larger crystals.
The other kind is called Oceanic Rock (basaltic floor), and is composed of silicate
rock called Basalt, with an average density of 2.9 g/cm3, and a thickness of
8-10 km. Basalt has a lot of hornblende and biotite, and is darkly colored,
and often with smaller crystals. To learn about these minerals, go to the Mineralogy
page, where all of the minerals above are described.
Now, picture what would happen when a pair of rising masses of mantle move
adjacent to each other. One "convection cell" would move to the left,
and the other to the right, as this diagram shows. The process of moving away
from each other causes the rock to drag on the overlying, brittle crust. The
result is a crack in the crust and a widening gap between slabs. The large mass
on the left moves more toward the left, while the slab on the right moves toward
the right some more. The gap between the continents becomes filled with lava.
The lava comes from melted mantle rock which rises in liquid form and hardens
on the surface. Melted earth under the surface is called Magma, and melted earth
on the surface is called Lava. As the continents move farther and father away
from each other, dragged by the hot, plastic mantle convection cells, the gap
widens and widens, always being filled with molten earth. If every case, this
molten rock is Basalt ... dark and relatively dense. The greater density causes
it to solidify and sink deeper onto the plastic mantle beneath it. This results
in a vast plain of basalt which sinks, while the less dense continental granite
floats up higher. If we were to remove all of the water on the planet, the western
hemisphere would look like the picture below:
The blue areas are low-lying basalt plains, while the greens and browns are
high floating granite mesas and mountains. Even if there were no water, the
Earth surface would look like this. The oceans are not deep because the heavy
water is lying on top of it, but because the rock at the ocean bottom is dense
and heavy. The oceans form there because water always moves to it slowest point.
You will learn later that Venus has a surface just like this picture, but no
oceans are present ... they boiled away a long time ago. As convection cells
of hot mantle rise up all over the Earth, the crust is broken up and pieces
are moving away or toward each other everywhere you go. It is as if the Earth
was a giant egg whose shell was cracked everywhere. The next image shows the
location of the moving plates.
To see a full picture of the oceanless earth, click on Full
Earth Topographical Map. This image is pretty big, but if your computer
is fast, the look is worth it.
earth's crust is divided into large plates, floating upon the mantle below,
and these plates are in constant motion relative to each other. The image at
the left shows the major plates of the Earth. While smaller ones exist, these
large plates serve to give a student enough of a picture of the cracked crust
of the earth. There are several ways in which these plates might interact with
each other: a) Spreading centers - where new oceans are being created as magma
wells up from deep reservoirs and splits the overlying brittle crust; b) Subduction
zones - where a granite plate is sliding over a sinking basalt plate whose deepest
sunken portions are being re-melted; c) Collision zones - where one plate directly
collides with another thrusting up great mountain formations; d) Transform faults
- where one plate slides alongside another. We will look at these interactions
in a moment.
LOOKING AT THIS GLOBAL MAP, CAN YOU FIND LOCATIONS WHERE EACH TYPE OF PLATE
INTERACTION IS OCCURRING?
Huge Convection Cells of heat drive the plate motions, and a 500 million year
period is required to bring hot mantle from the core boundary to the surface,
where it subsequently cools and sinks back down, only to be reheated again.
The liquid iron core moves at rates of kilometers per year, but the plastic
mantle rock moves at rates of centimeters per year. Mantle rock convects toward
the surface, but very slowly. This 500 million year amount of time coincides
nicely with the length of time between repeated periods of supercontinent formation
and break up. Roughly 8 such events have occurred since the earth's crust finally
cooled around 4.1 billion years ago. The best example of this 500 million cycle
is found in the Atlantic Ocean. Running along the entire north-south length
of the ocean is the mid-Atlantic Ridge, a place where the basaltic plant is
cracked and separating in an east-west direction at a rate of 4 cm/year. Millions
of years ago, Europe and Africa were connected to North and South America. Slow
spreading of the continents, driven by deep convection cells, pushed America
away from Europe and created the Atlantic Ocean. Eventually, the basalt bottom
pushing against each continent will cool and break off from the granite continent
and subduct. Europe and America will slowly slide over slabs of basalt toward
each other again, until they collide and the Atlantic Ocean will be gone.
this illustration of the plates because it shows a few more details than the
image above, but it is not as colorful or vibrant, so I relegated it to this
position of lower status :)
Pangea - Laurasia - North America
250 million years ago, all of the earth's land masses were connected in one
large plate called Pangea. Pangea split into Gondwanaland and Laurasia, and
eventually into the continents we see today. This image to your left shows the
suspected location of Pangea relative to the poles. It is interesting to note
that geologists and astronomers worked together to solve the riddle of the Pangea
break-up. With all of the continents locked together in one large mass, the
Earth continents were unevenly distributed and this caused a larger-than -normal
wobble in Earth's rotation. The original break-up apparently was quite a bit
faster than the present rates, made so by the strain of the masses on the rotation
of the planet.
Below, you can find a nice set of drawings which depict the break-up of Pangea
into the present-day distribution of continents.
is the suspected sequence of Pangea's separation, and the dates. The plate motions
are continuing today ... India crashing into Asia, Africa narrowing the Mediterranean
Sea, and New York and London moving away from each other. We are living on constantly
shifting plates. Carol King was right all along when she sang, "I feel
the Earth move under my feet."
the future, 100 million years from now, the Earth surface will look quite different
from what it is today. Australia will no longer be "down under," people
will walk across the Strait of Gibraltar, and the Queen Elizabeth will use a
lot of fuel to sail from America to England. You might think that the present
measured rate of Atlantic Ocean widening of 4 cm/yr is insignificant, but to
the engineers of the Queen Elizabeth it is very important. Since this giant
ocean liner gets only 6 inches to the gallon of fuel, the widening of the oceans
will represent a significant increase in fuel costs ... passed on of course
to the tourists. Perhaps that is why the Supersonic Transport jets were shut
Recently, I found a FANTASTIC site that clearly shows the motions of the tectonic
plates in shorter time sequences, and in particular, shows the locations of
these plates relative to the geographic poles of the planet. The site is at
the University of
Wisconsin at Green Bay (I mean, can it be at a better place that Titletown,
USA?). I am including a critical image from the collection:
The image above is the best estimation of the location of Pangea
at a date of 420 million years ago. Notice the large amount of landmass at the
southern pole. This particular drawing over-emphasizes the amount of continental
crust down there, but the point remains that a large percentage of the total
continental crust was in the southern latitudes. Scientists at Cal
Tech published a report that stated that this assemblage would have put
a great deal of strain on the continents due to the Earth's rotational speed.
It is reasoned that the continental split of Pangea from this position would
have happened at a hugely greater rate of movement per year because the unequal
dispersion of the conntinent mass would have caused an extra wobble in the Earth's
spin. Once the continents had broken and move apart rapidly toward the equatorial
regions, the rate of continent movement would have slowed to its present-day
observed rates. This faster motion in the past and slower present motion is
in conflict with the Doctrine of Uniformitarianism.
Please move forward to Plate
Interactions to discover what happens when plates meet, part ways, of pass
by in the night. Once again, you could return to the Home
Page or the Syllabus
but why would you want to do that when you are gripped with anticipation on
what these plates do.
| Home | Course
Assignments | Teacher Bio
| Course Units
| Syllabus | Links