Seafloor Spreading

With Wegener's death, interest in his ideas waned but were never completely forgotten. In 1928, British geologist Arthur Holmes proposed a mechanism that could possibly move the continents. He suggested that if the mantle behaved as a fluid, convection currents within the mantle could drag the solid crust along with them as the mantle material slowly circulated. Holmes admitted that he had no evidence to back his hypothesis and that it was fabricated to fit the continental drift model.

During World War II, geophysicists convinced the Allied Command that warships should be equipped with geophysical equipment, particularly sonar, used for submarine detection. The increase in data accumulation rates provided the first real picture of what the shape of the ocean basins was really like. It wasn't until after the war was over that oceanographers were able to start to put the picture together.

The major discovery that emerged from these data was that all oceans have a mid-ocean rdige that has a valley running down the axis of the ridge. In 1962, Princeton University's Harry Hess published a paper in which he speculated that these mid-ocean ridges were where Holmes' mantle convection cells diverged, allowing magma to flow on to the ocean floor. In short, new oceanic lithosphere was created at the mid-ocen ridges and pushed older lithosphere. Eventually the oceanic lithosphere pushed against continental lithosphere causing the continent to move.

How can this be proven? Even Hess, in the introduction to his paper, says, "I shall consider this paper an essay in geopoetry. In order not to travel any farther into the realm of fantasy than is absolutely necessary I shall hold as closely as possible to a uniformitarian approach..."

See Diagram

Paleomagnetism's Great Debate: Does Earth's magnetic field flip?

Background

Paleomagnetism is the study of Earth's ancient magnetic field. Volumes of experimental data indicate that Earth has always had a magnetic field that was oriented parallel to its rotational axis. Paleomagnetism works because magnetic minerals become oriented parallel to Earth's magnetic field under certain conditions. It's liking taking a compass and locking it into position. Everyone knows that the only thing a compass does is point to the North Magnetic Pole.

All of my research in the Andes and Trinidad deals with magnetostratigraphy, a branch of paleomagnetism. An understanding of the basic ideas of paleomagnetism was crucial to our acceptance of the plate tectonics model. Here's why:

Thermal Remenant Magnetization The mineral magnetite (Fe3O4) is a magnetic igneous mineral but at high temperatures (>565º C) it is not magnetic. When it cools to 565º C, the Curie temperature, it acquires magnetism that is parallel to Earth's magnetic field. We call this Thermal Remanent Magnetization (TRM). When igneous rocks cool below the Curie temperature, the magnetite preserves the orientation of Earth's magnetic field on that day.

Detrital Remanent Magnetization Once the igneous rock becomes exposed on Earth's surface, it starts to weather and erode. Magnetite crystals are disloged and are transported downstream. They maintaine their magnetism but not their orientation toward the North Magnetic Pole as they tumble through the current. Eventually, they reach quiet water and begin to settle out. The tiniest grains, those < 17m in diameter, become oriented parallel to the magnetic field as they settle to the bottom. For this reason, the orientation of the magnetic field on the day that those grains were deposited is preserved in fine-grained sedimentary rocks. We refer to this as Detrital Remanent Magnetization (DRM).

Declination vs. Inclination Regardless of whether TRM or DRM is used, the magnetic mineral grains are locked into pointing toward the North Magnetic Pole on the day that the remanent magnetization was acquired. This direction has two components, the declination and the inclination.

Declination is simply the compass direction. Initially, declination is always toward the North Magnetic Pole. Most people are familiar with declination because we grow up knowing that a compass always points North.

We don't grow up understanding the other component, inclination. To understand this, we need to see how the magnetic field is organized.

As seen in the diagram on the right, Earth's magnetic field lines ascend vertically out of the South Magnetic Pole and wrap around to descend vertically at the North Magnetic Pole. Because of this phenomenon, the angle of inclination varies with latitude. At the Equaror, for instance, the field lines are parallel to the surface so the inclination at the Equator is 0º. If we apply this information, we can determine at what latitude a rock was situated on the day it acquired its remanent magnetization. (http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magearth.html ). This means that simply by measuring the inclination of a compass needle, one can calculate the latitude.

To summarize, paleomagnetism provides us with two pieces of information: 1) the direction (declination) to the North Magnetic Pole on the day remanent magnetization was acquired and 2) the latitude (inclination) at which remanent magnetization was acquired. A third piece of information is also possible. From the data, the latitude and longitude of the North Magnetic Pole can also be determined. This is referred to as the Virtual Geomagnetic Pole (VGP).

Two great debates emerged in paleomagnetism in the 1920's. Both dealt with the behavior of the North Magnetic Pole. When geologists sample rocks for paleomagnetic study, they sample them at different stratigraphic levels so that the magnetic field from different times can be compared with each other. When this was first tried the results were different than had been expected.

Polar Wandering When the results from different stratigraphic levels were compared with each other, the declinations and inclinations were progressively different. This suggested that the magnetic pole was not fixed and tended to wander through time. Furthermore, when the VGP from rocks of the same age on different continents were plotted, they plotted in different places suggesting that not only was the pole wandering but its location varied depending on one's point of view. There were more than a few skeptics about the science of paleomagnetism. Adherents to Continental Drift claimed that it was the continents that were in motion, not the pole.

Flipping magnetic fields Another early observation was that the North Magnetic Pole sometimes plotted in the northern hemisphere and sometimes plotted in the southern hemisphere. One group of paleomagnetists, mainly in North America and Europe, concluded that Earth's magnetic field must occasionaly reverse its polarity, that is, the North Magnetic Pole, which today is near the North Rotational Pole, sometimes relocates near the South Rotational Pole. An opposing group, led mainly by Japanese investigators, discovered a variety of magnetiv mineral that spontaneously flips its own local field. They claimed that the apparent magnetic reversals were due to high concentrations of self-reversing minerals.

The battle lines were drawn: The Field Flippers vs. The Self-Reversers.

The debate continued without resolution for several decades. Paleomagnetists continued to collect data to try to resolve the problem.Three groups made particularly important contributions.

Vine and Matthews (1963)--Midocean Ridge Survey

In 1962 H.M.S. Owen made a detailed magnetic survey across a portion of the Carlsberg mid-ocean ridge in the Indian Ocean. The results were summarized by Fred Vine and Drummond Matthews of Cambridge University, UK. They discovered that the rocks on the ocean floor produced fluctuating magnetic intensities as they crossed the ridge and that this patter appeared to be symmetrical with the axis of the ridge as the line of symmetry. They speculated that if Hess's ideas about seafloor spreading were correct and if the magnetic field does occasionally reverse itself, then these observations might be the proof.

A schematic diagram of Vine and Matthews (1963) work shows symmetrical magnetic patterns about the midocean ridge axis. In reality, the results were not nearly as detailed as shown here.

Cox, Doell, and Dalyrymple (1963)--Continental Magnetostratigraphy

Reasoning that if Earth's magnetic field did reverse itself, its effects should be seen in the DRM of rocks deposited on the continents, Allan Cox, of Stanford University, Richard Doel of the USCS, and Brent Dalyrymple of the University of California-Berkeley, investigated very young sediments exposed along Jaramillo Creek in New Mexico's Valles Caldera. Their results showed remarkably similar magnetic polarity pattern to those seen on the seafloor. Skeptics pointed out that continental sedimentation is seldom uniform. In other words, a single flood might deposit several meters of sediment while several centuries of drought might be recorded by only a few centimeters of strata. The sediment accumulation rate could vary from centimeters to tens of meters per thousand years. If the sediment accumulation rate was not constant, how could anyone be sure that the magnetic pattern could be calibrated with time?

Opdyke (1964)--Deep Marine Magnetostratigraphy

Neil Opdyke, of Columbia University, realized that the best way to resolve the dilemma facing Cox, Doell, and Dalyrymple was to get samples from a place where he could be reasonably sure that the sediment accumulation rate was constant. The best place for that was in the middle of the ocean, far from the effects of sediment coming off of the continents. He selected a site in the South Atlantic that was roughly equidistant from Africa, South America, and Antarctica. The sediment accumulation rate there would be measured in millimeters per thousand years rather than meters or tens of meters. Fortunately, the Deep Sea Drilling Project had collected a core from this area on a recent voyage. The core was stored at Columbia so Opdyke obtained the youngest part of the core and tested it for polarity reversals. Once again, the apparent polarity reversal pattern looked remarkably like those collected from the other areas discussed above but this time there was reasonable assurance that the sediment accumulation rate had been constant.

At the Annual Meeting of the American Geophysical Union, all of these investigators presented their data. Because the similarity of the reversal patterns could only be explained by reversals of Earth's magnetic field and not by spontaneous self-reversal, paleomagnetists finally came to consensus that Earth's magnetic field does occasionally reverse its polarity. The great debate was over. The Field Flippers won!

But the truly astounding conclusion that fell out of the paleomagnetic debate was Seafloor Spreading Must Be Real!!! This conclusion had to be acknowledged because seafloor spreading was the only mechanism which could explain the symmetry of the magnetic stripe patterns around the midocean ridges from the three great oceans on the planet. Plate Tectonics was born!

The Great Synthesis (1963-1968)

Once the general seafloor spreading model was accepted, a fluorish of activity took place. Earth's Rosetta Stone had been found! One of the first problems that was resolved was the fate of the ocean floor. If new lithosphere was being generated at the midocean ridges, then Earth would have to be expanding unless tere was some mechanism for recycling oceanic lithosphere.

Wadati-Benioff Zones

Seismologists already had the answer and were waiting for a model to hang it on. Japanese seismologist Kiyoo Wadati, in 1927, and American Hugo Benioff, in 1954, independently plotted the foci of earthquakes (the place where the rock first breaks) near deep ocean trenches . They found that the earthquakes defined a plane dipping downward from the trench to a depth of about 700 km below the surface. Because of lingering post-World War II sentiments, these became known as Benioff Zones. In the 1990's, the Wadati's pioneering work was finally given full recognition and we now refer to the dipping earthquake planes adjacent to deep ocean trenches as Wadati-Benioff Zones.

Once the seafloor spreading model was accepted, it became clear that the Waditi-Benioff Zones are where oceanic lithosphere is recycled. One slab of oceanic lithosphere is pushed beneath another in a process called subduction.

Plate Boundaries

Canadian geologist J. Tuzo Wilson first described global tectonics in terms of the motion of rigid "plates". He also defined the three basic types of plate boundaries. All are defined by the high frequency of earthquakes that occur in the boundary zones.These boundaries are:

1. Compressional where two plates converge on each other head-on. One plate subducts beneath the other. This occurs at deep ocean trenches.
2. Tensional where two plates move apart (diverge) from each other. New oceanic lithosphere is formed. This occurs at midocean ridges.
3. Shear, transform, or strike-slip where two plates slide past each other. California's San Andreas Fault is an excellent example of this.

When plates converge or diverge at an oblique angle, there are components of either tension and shear or compression and shear. These motions are called, respectively, transtensional and transpressional movements.

The Mosaic of Plates

Plate tectonics is the motion of more than a dozen rigid, lithospheric plates riding on top of a plastic, convecting asthenosphere. Change in the direction of motion of any one plate, necessarily has some effect on every other plate. The features present at a given boundary are depent on two primary factors:

1. The type of motion at the boundary.
2. The types of lithosphere (continental or oceanic) that are involved at the boundary.

We will examen the variety of possibilities below.

Divergent Boundaries

Divergent boundaries are often characterized in three ways which really constitute the evolution from the breaking up of a continent to the formation of a full-fledged ocean basin.

Asthenosphereic convection creates tension on an overlying block of continental lithosphere. The pulling apart results in a rift valley or graben that has volcanism associated with it. Earthquakes are frequent along the boundaries of the graben. The best example of this today is the East African Rift Zone. Other older rift basins are present on most of the continents. These are often rifts that failed to mature because successful rifting moved elsewhere. We call these failed rift basins aulacogens. Very often aulacogens are a failed third arm of rifting while the other two arms were successful.We call areas where three tectonic arms come together triple junctions. The best example of an active triple junction is East Africa's Afar Triangle. Examples of failed rifts are,Europe's Rhine Valley, the Rio Grande where it flows north to south from Colorado to the Mexican border, New England's Connecticut River Valley, New Brunswick and Nova Scotia's Bay of Fundy.

The Landsat image to the right ( http://www.landsat.org/index.html) shows the Rio Grande rift in central New Mexico. The dark areas are reservoirs. The white area in the bottom center is White Sands National Monument. The other white areas are clouds.

The portion of the East African Rift System, in the Landsat immage to the left ( http://www.landsat.org/index.html), in central Ethiopia is situated inan extremely arid region. Several natural lakes are present in the lowest areas. The green area is cultivated land along the Omo River.

Thingvallavatn is Iceland's largest lake. It is situated in the rift valley along the mid-Atlantic Ridge which is exposed as it crosses the island.. The photo was taken from the North American Plate. The hills are on the Eurasian Plate. (Photo by Jim Reynolds)

IntermediatePhase of Continental Rifting

Rifting is said to have enter an intermediate phase once new oceanic lithosphere begins to form through the creation of a new midocean ridge. Earthquake activity is confined to the midocean ridge but the new ocean basin is so small that the quakes can usually be felt on both shores. Three places on Earth exhibit this tectonic style: 1) the Red Sea and 2) the Gulf of Aden, and 3) Mexico's Gulf of California.

Mature Phase

Finally, after millions of years of seafloor spreading, a mature ocean basin develops with a spreading ridge at its center. Earthquakes are generally restricted to the midocean ridge. The continental margins have rare earthquakes as sediment accumulated on the continental shelf settles. We call these continental marginal areas passive margins because of the exceedingly rare tectonic activity. Because of the lack of tectonic activity, shoreline processes, specifically currents, waves, and tides, become the dominant sculptors of coastline features. The east coasts of North America and South America and the west coasts of Europe and Africa are all examples of passive margins on a mature ocean basin.

Convergent Boundaries

There are also three general varieties of convergent plate boundaries. These variations are associted with the type of lithospheric plates that are interacting with each other. The three possible types are: 1) Ocean-Ocean, 2) Ocean-Continent, and 3) Continent-Continent.

Ocean-Ocean Convergence

If two oceanic plates converge, one of them, usually the older, denser one, will be forced to subduct beneath the other. Ocean floor sediments carried down the subduction zone, as well as the rest of the oceanic lithosphere, will be subjected to increasingly higher pressures and temperatures. It is in this zone that most metamorphic rocks form.

Also carried down into the subduction zone is a lot of seawater. When that water comes in contact with the hot asthenosphere, usually about 100 km below the surface, it causes the asthenosphere to partially melt. The molten component is not as dense as the mantle rock and it flows much faster. As a consequence, it rises through fractures in the rock into the crust. Most of this molten material, called magma, cools deep within the crust to form plutonic igneous rocks. Some of it, however, finds its way to the surface to be erupted as volcanic igneous rocks. Most of these rocks will be of basaltic composition.

When a plane interesects the surface of a sphere, an arc is formed. For this reason, volcanic island chains tend to have an arcuate shape and are referred to as island arc volcanoes. Examples of island arc volcanism are seen in Alaska's Aleutian Islands, the West Indies, the Philippines, Indonesia, and other island chains in the Pacific Ocean.

This map of Alaska's Aleutian Islands displays the origin of the term "Island Arc".

(http://www.inforain.org)

To get a better feeling for the 3-dimensional view, go here and play with Steps 2, 3, and 4.

Ocean-Continent Convergence

When an oceanic plate converges with a continental plate at an acute angle, the thinner, denser oceanic plate always subducts beneath the continent. The results are similar to Ocean-Ocean convergence but we call the resulting volcanoes a Continental Arc. The composition of the lavas varies from basalts to andesites to rhyolites. This type of continental margin is often referred to as an Andean margin because South America's Andes Mountains are the best example. Other areas that exhibit this type of convergence are Central America, North America's Cascade Mountains, Italy, and Japan.

The arrows on the Landsat image (http://www.landsat.org/index.html) show the nine volcanoes considered to be active in the 100 km stretch between Guatemala City (G) and Quezaltenango (Q). Cities show as a fuzzy blue-gray. Fuego volcano (F) erupts frequently while Pacaya (P) and Santiaguito (S) are in almost continuous eruption. Two other arrows point to the sites of huge ancient eruptions. The large lake is the Pleistocene Atitlán Caldera (A). I discovered and documented the Miocene Santa Rosa de Lima Caldera (R) to the east of Guatemala City (Reynolds, 1987). The Pacific Ocean is at the bottom of the image.

"Flat" Subduction

The Andean margin is not really the simple case that it is made out to be above. The map to the right shows the location of all active volcanoes in the Andes as red dots. Note that there are two distinct gaps in the volcanism: 1) northern Chile/Argentina and 2) Peru. In the 1970's it was discovered that the Wadati-Benioff zones in these areas descended at a very shallow, almost horizontal, angle. Because of this, the descending slab has still not reached the "magic" 100 km depth needed to generate magma. A growing number of geologists now believe that "flat" subduction of the Farallon Plate beneath the North American Plate throughout the Mesozoic was partly responsible for the complicated geology presented in the western United States.

Continent-Continent Convergence

Eventually, after millions of years of Ocean-Continent convergence, another continent comes into the picture and the two bump into each other. One continent begins to subduct beneath the other. Continental lithosphere, however, is less dense than oceanic lithosphere. This limits the depth to which the lithosphere can subduct. The result is that compression fols and faults the doubly thick continental lithosphere. The buoyant mass rises isostatically to form the highest mountains in the world. The Himalyas, Iran and Iraq's Zagros Mountains and the Caucausus Mountains of Armenia, Azerbaijan, and Georgia are all modern examples of Continent-Continent collision. The Appalachian Mountains represent the late Paleozoic collision between Africa and North America. We can imagine that the Appalachians were at one time as high as the Himalayas are today.

Transform Fault Boundaries

When two plates collide at an oblique angle their directions of motion tend to reorient so that they slide past each other rather than subduct. Earthquakes from these faults tend to be generated near the surface and often result in severe damage to nearby population centers. Perhaps the best example of a transform fault boundary is California's San Andreas Fault. Other examples are the Motagua Fault that separates the Caribbean and North American plates in Guatemala and the Alpine Fault that cuts across New Zealand. Another important strike-slip fault of local interest is the Brevard Fault. It is more properly termed the Brevard Fault Zone because it is constituted of a series of parallel faults. This fault zone assumes different names as it runs up the east side of the Appalachians but it essentially runs from Georgia to Newfoundland. The Brevard Fault Zone is comparable to the San Andreas fault but is of Paleozoic age and is no longer considered active. The Brevard College campus is situated within the most intensely deformed part of the fault zone.

There are two types of strike-slip faults determined by their relative motion: right-lateral and left-lateral. To determine which is which, imagine that you and a friend are standing on opposite sides of a strike-slip fault looking at each other. Simulate the motion by both taking a step to the right. Which direction did your friend move relative to you? If you said, "to my left" you are correct. That means the imaginary fault between you is a left-lateral strike-slip fault. look at the directional arrows on the San Andreas Fault in the image to the right. Is the San Andreas a right-lateral or left-lateral strike slip fault?

Combinations of Plate Boundaries

Different types of plate boundaries merge into each other as the boundaries bend.