EARTH STUDIES: PLATE TECTONICS (see Chapter 16)
Structure of the Earth
Inner Core – solid (Fe/Ni), 5500

, 1220 km radius
Outer Core – liquid (Fe/Ni), 3000

, next 2200 km
Lower Mantle – semi-liquid (silicate rock Fe/Mg), next 2250 km
Upper Mantle – solid (silicate rock Fe/Mg), next 650 km
Lithosphere – top 60 km of upper mantle, more rigid
Asthenosphere – next 200 km below lithosphere, less rigid
Crust – uppermost 5 to 60 km, “floats” on the lithosphere
Oceanic crust – 5 km thick (dense Fe/Mg silicate)
Continental crust - 60 km thick (lighter silicates)

Continental Drift
The idea that the continents might actually be moving has been around a long time. In fact, Wegener proposed an entire theory of continental drift in 1912.
He suggested that the Earth’s continental crusts consisted of numerous “plates” of various sizes which were continuing to move over the Earth’s surface, and that about 200 million years ago, most of these had in fact formed, at least temporarily, a single gigantic land mass (which came to be called “Pangaea”).
This “supercontinent” had since split up and slowly drifted apart. (Prior to this, presumably, a number of smaller continental plates had come together to form “Pangaea” in the first place.)

Considerable and diverse evidence was offered to support
Wegener’s theory: geological evidence (based on rock strata); climatological evidence (related to glaciation patterns); and paleontological evidence (based on fossils of long extinct animals/plants). Still, Wegener’s theory was not taken very seriously because it included no mechanism to explain how such huge land masses might actually be caused to move.
Plate Tectonics
In the 1950s and 1960s, detailed mapping of the ocean floors revealed huge underwater mountain ranges (called mid-ocean
ridges). Further studies of these provided conclusive evidence that these ridges are in fact long fissures or cracks in the oceanic crust, and that new oceanic crust is being created by the volcanic upflow of molten magma which quickly hardens upon contact with the cold water. The result is a phenomenon known as
seafloor spreading.
Interestingly, confirmation of this phenomenon involved the discovery that alternating bands of spreading oceanic crust exhibit reversed magnetic polarity, which coincides with the known fact that the Earth’s magnetic field naturally reverses approximately every 500,000 years.
The spreading apart of these plates along the mid-ocean ridges provided proof of the existence of divergent plate boundaries.
In other locations on Earth, plates can come together, in a sort of slow motion collision. When this happens, one plate (usually the thinner or weaker one) is forced downward under the other in a process called subduction. The thicker heavier plate is raised slightly in the process, with results that can be quite dramatic

over time. These subduction boundaries where plates come together are called convergent plate boundaries. A good example of this exists along the west coast of South America, where the eastward drifting oceanic crust is being subducted under the westward drifting continental crust, forming the Andes
Mountains in the process. An even more spectacular example is the collision of the northward drifting Indian plate with the
Asian plate, which is continuing to thrust upward creating the highest mountain range on Earth—the Himalayas.
A third type of plate boundary can exist as well, one where two plates move in parallel (either in opposite directions or in the same direction but at different speeds), with one plate essentially sliding along the edge of the other. These boundaries are called
transform boundaries. The best known example of this is the
“San Andreas Fault” in California, where the northward moving
Pacific plate is scraping along the southward drifting North
American plate.
Regardless of which type of boundary is at work, the result is increased volcanic and seismic activity, i.e. active volcanoes and severe earthquakes.
Mantle Convection
The underlying driving force behind plate tectonics (and hence continental drift) is believed to be mantle convection, a large scale process which can explain both divergent plate boundaries
(seafloor spreading) and convergent plate boundaries (with subduction). Figure 16.6 (p. 605) in the textbook illustrates the idea very well.

Essentially the idea is that the interior temperature of the Earth is not perfectly even at each depth, and therefore that there can occur “hot spots” and “cool spots” within the mantle which can then set up natural convection cycles, and bring molten magna up to the surface in some locations. The resulting divergent plate boundaries (with their accompanying spreading of the crust) would inevitably produce convergent plate boundaries
(with subduction) elsewhere. If this theory is correct, it could certainly explain how, for example, South America and Africa
(originally together as part of Pangaea) could have drifted 8,000 km apart over the past 200 million years.
If the above example still seems far-fetched, you can easily calculate that a separation of about 8,000 km (8.0 × 10 6 m) in roughly 200 million years (2.0 × 10 8 years) requires a continental
“drift speed” of only
8 .
0

10
6
2 .
0

10
8

0 .
04
m/year, or 4 cm/year!
Earthquakes
Earthquakes can occur at any of the aforementioned boundaries.
Essentially, an earthquake results when the two sides of the boundary which have been held together by friction suddenly
“let go”.
Theoretically, the most violent earthquakes should be expected along convergent plate boundaries, or thrust faults, where compressive stress can lead to a sudden upward thrust of one rock mass (the hanging wall) over another (the foot wall).

Divergent plate boundaries, or normal faults, can also produce earthquakes (though usually less severe), in which increasing tension leads to a sudden “pulling apart” and causes the foot wall to suddenly drop below the hanging wall.
Finally, an earthquake along a transform boundary, or strike-
slip fault (like the San Andreas Fault) is characterized by a sudden, and sometimes very significant, horizontal shift of the rock masses. Along some parts of the San Andreas, the two plates have shifted by as much as 10 m in the past 200 years.
These three types of earthquakes are illustrated in Figure 16.8
(p. 609) of the text.