INTRO to PLATE TECTONICS

For the next lectures, we will be discussing the most important theory in modern earth sciences: plate tectonics . It is difficult to overestimate the importance of plate tectonics to the study of the Solid Earth, as it has revamped nearly every earth science discipline, ranging from seismology to climatology and from geomagnetism to palaeontology.

The theory of plate tectonics was developed in the 1960s to explain lots of accumulated observations about Earth, some of which were known at Wegener's time (the list above and others), and some of which were new. It is important to note that it was these new observations which were the key to the development and acceptance of plate tectonics. The most important (in my opinion) of these new data was information on the structure and age of the seafloor, primarily in the Atlantic Ocean. These data showed that, rather than being billions of years old, the Atlantic Ocean floor was actually less than 200 million years old and that the closer the rocks were to the Mid-Atlantic Ridge, the younger the rocks were. These observations led Harry Hess and Robert Dietz, among others, to believe that the seafloor was formed at the Mid-Atlantic Ridge, then spread away to the west and east, with new ocean floor continuing to be formed at the Mid-Atlantic Ridge. Hess and Dietz called this idea seafloor spreading, and plate tectonics helps explain the data which went into developing the seafloor spreading idea. There is more evidence, which we will discuss next time, but for now it will suffice to say that plate tectonics explains the observations that continental drift did, and new ones as well.

Before we look at the central idea of plate tectonics, there are two words you need to know:

Lithosphere: The lithosphere (derived from Greek, it means ``rock sphere'') consists of the crust and the uppermost mantle directly beneath the crust. It is the cold, strong layer we walk around on, and is generally around 50-100 km thick.

Asthenosphere: The asthenosphere (asthenes is Greek for ``weak'', so asthenosphere is the ``weak sphere'') is the region of the mantle directly below the lithosphere. In contrast with the lithosphere, the asthenosphere is hot and relatively fluid; it will flow over long periods of time. In some sense, the lithosphere ``slides'' on the asthenosphere.

Central Idea: The central idea in plate tectonics is that the lithosphere is not a single uniform layer; it is broken into several discrete, rigid, moving plates. Some of these plates are mostly continental and others are mostly oceanic.

Moving Plates: The plates are not stationary; they move. A reasonable rule of thumb is that plates move roughly as quickly as your fingernails grow (more or less). To explain why the plates move, you need another vocabulary word: convection. Imagine a pot of soup on the stove. When you turn on the burner, the bottom of the pot is heated, and the soup just above the bottom of the pot warms as well. When it warms, it becomes less dense than the soup above it and because it is less dense, it rises toward the surface of the soup. The soup at the surface is still cold, and therefore more dense than the soup at the bottom, and the surface soup sinks toward the bottom of the pot. Once the cold soup hits the bottom, it begins to warm; at the same time, the warm soup has hit the surface and begins to cool. The process continues, and eventually, the soup is in constant motion with hot soup rising and cooler soup sinking. This process is called convection, and it happens inside Earth as well; for plate tectonics, the relevant convection is convection inside the mantle. The base of the mantle is hot, and so the mantle rock there rises toward the top of the mantle. At the same time, the cooler material at the top of the mantle sinks toward the outer core. This process sets up convection in the mantle (as a sidelight, one of the hottest - no pun intended - debates in earth science is whether or not mantle convection involves the whole mantle or just parts of it...). Mantle convection is related to the motion of plates, and earth scientists think they have a reasonable handle on the general issues, but the details are still a matter of debate. We will discuss that more in a couple of lectures, but for now suffice it to say that plate motions and mantle convection are believed to be intimately related to each other.

Plate Boundaries

There are three main types of plate boundaries. Each is characterized by specific kinds of geologic phenomena, and each can be found in different parts of the world.

Divergent Boundaries

Areas where plates move away from each other and new lithosphere forms to fill in the gap between the plates. The Mid-Atlantic Ridge is a good example of a divergent boundary. At divergent boundaries, volcanism, earthquakes, and seafloor spreading occur.

Convergent Boundaries

Regions where plates come together. There are two major types: subduction zones and continental collision zones .

Subduction zones are places along where oceanic and continental plates meet. Oceanic lithosphere is denser than continental lithosphere (and denser than the underlying asthenosphere), and the oceanic plate therefore sinks downward under the continental plate. This process, called subduction, destroys lithosphere and makes earthquakes and explosive volcanism. Deep-earthquakes occur along the subducted slab (called Benioff Seismic Zone). Subduction also happens when two oceanic plates meet and one of them subducts under the other plate.

When two continental plates collide, both plates are too light to sink into the earth; instead of subduction, a continental collision zones develops. The process of collision causes the two plates to buckle and fold, and high mountain ranges and great earthquakes happen. The best example of this process is the collision of India and Eurasia. This collision has bent the Indian plate and raised the Himalaya mountains. In addition, it has compressed Eurasia and raised the Tibetan plateau, and caused great faults to form throughout Central and East Asia.

Transform Boundaries

At these boundaries, plates simply slide past one another, without creating or destroying lithosphere. Earthquakes are common along transform faults. A good example of a transform boundaries are theNorth Anatolian Fault Zone, and San Andreas Fault in US,California.

How do we know that plates move?

Evidence for Plate Tectonics and Sea-floor Spreading

The lithosphere thickens from a few kilometers near the ridge to over 200 km beneath some continental regions. The increasing thickness of the lithosphere correlates well with increasing depth of the ocean and decreasing heat flow as the lithosphere ages. The lithospheric plate model predicts that the amount of heat coming out of the top surface decreases exponentially with age. As the oceanic lithosphere cools, it contracts or shrinks, so that the seafloor depth should be only a function of its age and should decrease exponentially away from spreading centers. The ocean should become deeper with distance from a hot mid-ocean ridge.

Magnetic Bands bordering Oceanic Ridges:

Marine magnetic anomalies at sea are arranged in bands that lie parallel to the rift valley of the mid-oceanic ridge. Alternating positive (normal polarity) and negative (reversed) anomalies form a stripe-like symmetric pattern to the ridge crest. According to the Vine-Matthews hypothesis, as well as seafloor spreading, there is continual tensional opening of cracks within the rift valley on the mid-oceanic ridge crest. These tensional cracks are filled by basaltic magma which cools to form dikes. Cooling magma in the dikes records the earth’s magnetism at the time of the magnetic minerals crystallize. When the earth’s magnetic field has a normal polarity, cooling dikes are normally magnetized. Dikes that cool when the earth’s magnetic field is reversed are reversely magnetized. So each dike preserves a record of the polarity that prevailed during the time the magma cooled. In this way a system of normal and reversed magnetized dikes forms parallel to the rift valley. These dikes are cause of the marine magnetic anomalies.

There are two important points about this hypothesis of magnetic anomaly origin. The first is that it allows us to measure the rate of plate motion (rate of seafloor spreading) and second is predicts the age of the sea-floor . Because magnetic reversals have already been dated, anomalies caused by these reversals can be used to discover how fast the sea floor has moved. For instance, a piece of the sea floor representing the reversal that occurred 4.5 million years ago may be found 45 kilometers away from the rift valley of the ridge crest. The piece of seafloor has traveled 45 kilometers since it formed 45 million years ago. Dividing the distance the sea floor has moved by its age gives 10 km /million years, or 1 cm/year for the rate of sea floor motion here. In other words, on each side of the ridge crest, the sea floor is moving away from the ridge crest at a rate of 1 centimeter per year. Most sections of the sea floor have magnetic anomalies. By matching the measured anomaly pattern with known pattern, the age of the sea floor in the region can be predicted. Therefore, a study of magnetism of ancient rocks ( paleomagnetism) can give a moderately good inclination of the relative positions of continents.

Earthquake Epicenters:

Another test of plate motion has been made by studying the seismicity of fracture zones. The mid-ocean ridge was once continuous across a fracture zone but has been offset by strike-slip motion along the fracture zone. The portion of a fracture zone between two offset sections of ridge crest is called transform fault .

Aseismic Ridges and Hotspots:

In addition to great ocean ridges where seafloor spreading is taking place with along the seismic activity, there are various long volcanic ridges which are aseismic or at least their seismicity are related to volcanic activity. It thought that the heat for the volcanism comes directly from the deep mantle, and that, as the moving asthenosphere carries the overlying lithosphere over this stable hot spot (providing the driving force for the plate motion), new volcanoes develop. There are three of the Pacific aseismic ridges (Hawaiian-Emperor, Tuamotu-Line, and Austral-Cook-Gilbert-Marshall) have had parallel movement and even show the same change in trend in their westerly portions. This suggests plate movement over three hotspots and a change in direction of the motion.

Heat Flow from Ocean floors:

The rate of heat flow is easier to measure from the ocean bottom than from the continents. The probes driven into the seafloor with thermistors have indicated that heat flow is greater the crests of oceanic ridges than from flanks. This suggests that hot mantle rock is moving up into the ridge crests, which is in accordance with the other evidence for spreading ocean floors.

Fit of Opposite Oceanic Margins:

There is a remarkable fit of the two sides of the Atlantic Ocean that suggests the continents have drifted. Indeed, the fit of mountain ranges of Europe with ranges of Northern America is impressive. And, South America and Africa have late-Paleozoic mountains would well mach if the continents were joined.

Permo-Carboniferous Glaciation:

Glaciation is known to have been widespread during the Permian in South America, South Africa, India, Australia, and Antarctica (of Pangea continent). The glaciation in low latitudes was all of the high-mountain type, it seems amazing that so much of the glacial pavements and other evidence would have remained. This used by A. Wegener to support continental drifting. The direction of ice movement suggests that much of the ice was coming onto present-land margins. Thus, ice appears to have moved onto Argentina and South Africa from the east and onto southern Australia from the south.

Evaporite Distribution in History:

At present, salt, gypsum, and other saline deposits that constitute evaporites are being deposited near 30°N and 30°S latitudes in the areas of low rainfall. If the plates have moved north away from the South Pole, there should have been a shift of the dry belts. Maps show that evaporite belts shifted progressively to the south, as would be expected from the north motion of the plates. This movement indicates a polar shift, but it could be related to the sliding plates or to both. Furthermore, the Jurassic opening of the Atlantic would have modified the zones of low rainfall in the center of the continents.

Paleontologic (Fossil) Evidence of Continental Drifting:

The similarity of certain fossils found on the continents (on both sides of the Atlantic) is difficult to explain unless the continents were once connected. Floating and swimming organisms could migrate in the ocean from one shore to another, but land-dwelling animals, such as reptiles, and insects, and certain land plants could not migrate. Fossils of the seed fern have been found in rocks of the same age from South Africa, Australia, India and Antarctica. An example is a mammal-like reptile, Lystrosaurus. Its fossils are found in South Africa, South America, Asia, and Antarctica.

(Note: I recognize that this is a large amount of reading. Don't feel like you have to do all of it at once; you have at least two more lectures - really 5 days+ to read it in. Take it slowly, as this is important stuff.)

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Nilgün Okay

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Last modified on Tues. Dec. 4 15:35:47 PST 2001