PLATE TECTONICS & TECTONIC RECYCLING



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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!

The 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.

 

What process causes the plates to move?

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.

Convection 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 to move.

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.

The 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.

I included 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

 

Just 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.

Here 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."

 

 

 

 

 

 

 

In 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 down.

 

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.

 


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