Environmental Geology
LECTURE 5 - Plate Tectonics
Lecture Outline
Reading: Chapter about plate tectonics in text. Pages 42-59.
I. Evidence for plate tectonics
Much evidence suggests that the earth's surface is best modeled as a set of rigidly behaving plates that move slowly with respect to each other. Here, we'll review one line of evidence. You are additionally responsible for the more extensive list of evidence shown in the previous web page lecture notes.
VLBI Concept - Signal from a given quasar is received at slightly different times by radio telescopes that are at different locations on the earth's surface. The time delay can be used to determine the distance between the radio telescopes to a precision of roughly 1 part per billion. Since radio telescopes tend to be spaced every 1000 km to 10,000 km, the precision is to the nearest centimeter. Most plates move several centimeter every year, thus, repeating measurements between the same two radio telescopes every few months yields the relative motion between radio telescopes that are on different plates.
Example - VLBI observations since 1983 at two sites on either side of the Atlantic, which is widening because new seafloor is being created along the Mid-Atlantic ridge, show that the Atlantic is widening at a rate of about 17 millimeters per year. This agrees to the geologically predicted rate to within a few percent.
Penultimate Point - Within the past 3-4 years, space-based measurements of plate velocities, particularly using the new high-technology Global Positioning System satellites that are launched and maintained by DOD, have exploded. Continuous measurements of plate motions and a host of other earth- and climate-related processes are now being made at over 100 sites on all of the plates. Thus, as we speak, the first models of real-time plate motions are being constructed (some in my office)! Thus far, these models give plate velocities that agree to within a few percent of velocities that are predicted by conventional geologic models.
Final point - YOUR GOVERNMENT has fostered and paid for all of this research - it has in essence spawned an enormous new industry (GPS) that has huge commercial applications and is ushering in a new age in earth and atmospheric science research. Some government investment in R&D is thus an effective use of tax dollars - it spurs both economic development and provides the technology for advances in basic and applied sciences. If you didn't believe that investment in your future wasn't worthwhile, you wouldn't be spending the money to go to college! We should thus support similar kinds of investments by our government in the future of our society, including support for educational and research programs.
Even before we had direct measurements of plate motions, the evidence for the theory of plate tectonics was overwhelming enough to have caused a fundamental shift in our view of the earth. Many lines of evidence exist - each is mentioned in the web page and most of these are described in the textbook.
How fast do plates move?
Slide 1 - Plate Velocities - Typical velocities are from 1 mm/yr to 20 centimeters per year (8 inches). Sounds slow, but if a plate continues to move even 10 cm/yr for 10 million years, it will have moved 1000 km (600) miles in that time span. Thus, movement of plates seems slow to us, but in the course of geological time, causes significant re-arrangement of the earth's surface.
II. Types of Plate Boundaries
As the global distribution of earthquakes demonstrates to us, deformation of the earth's crust tends to be concentrated along narrow boundaries between plates. Most major features on earth's surface such as mountains, volcanoes, and ocean basins thus have originated due to interactions of plates along their narrow boundaries. Here are some of the important details about the different types of plate boundaries....
IIa. Seafloor spreading centers
The most common form of divergent plate boundaries are seafloor spreading centers, where new seafloor is created continually as two plates diverge from each other. Seafloor spreading centers have the following interesting characteristics:
Why does the earth's mantle melt and form new seafloor only at spreading centers?
At a given depth beneath the surface, the earth's mantle has a nearly uniform temperature. Thus, if the mantle is hot enough to melt beneath a spreading center, why isn't it melting and rising through the surface nearly everywhere?
Structure of a Spreading Center
As plates move apart, a void is created at plate boundary. Mantle in this region experiences increasingly lower pressure as overlying crust thins. A decrease in the pressure being exerted on a material also decreases the temperature at which it will melt (the melting point of any material material depends on both the pressure and temperature environment of the material).
Thus, beneath spreading centers, even though the mantle is the same temperature as elsewhere, the pressure exerted on that material is lower because there is less rock above. Some of the mantle minerals thus melt. The minerals with the lowest melting points melt first and this melt squeezes through the cracks between the still-solid minerals and migrates upwards to a magma chamber. The magma in this chamber then feeds to the surface, where it contacts seawater, solidifies, and forms new seafloor (basalt). Some of the magma solidifies beneath the surface and forms gabbro.
IIb. Transcurrent boundaries and transform faults
The most common transcurrent boundary is an oceanic transform fault, which is a long straight fault that connects two segments of a spreading center.
If we consider how transform faults work, we can understand a puzzling feature of earthquakes along spreading centers, namely, that earthquakes occur only along sections of transform faults between spreading segments. Why is this?
OVERHEAD - How does a transform fault work?
Transform faults are the active part of oceanic fracture zones. Transforms transfer motion between the spreading segments that they offset and are thus seismically active between the spreading segments. As seafloor spreading continues, the parts of the actively slipping transform that move beyond the spreading segments on either end of the transform become inactive and are preserved as fracture zones. This is why earthquakes occur only between the spreading segments. Fracture zones are "fossil" transform faults.
The San Andreas fault is an example of a transcurrent plate boundary within a continent. Contrary to popular belief, the part of California that lies west of the San Andreas isn't going to fall into the ocean. Instead, western California will migrate slowly to the northwest along the San Andreas until Los Angeles becomes a suburb of San Francisco in roughly ten million years!
IIc. Convergent plate boundaries - Subduction zones and mountain ranges
As two plates move toward each other, something must give way. If a dense oceanic plate collides with a buoyant continent, the usual result is that the oceanic plate to subduct back into the earth's mantle beneath the more buoyant continent. When two buoyant continents collide, the usual result is for both to crumple, thrusting some regions up into the atmosphere (mountains) and casting some regions off to the side (lateral escape).
Here, we focus on subduction zones. Two types of evidence tell us that subduction zones mark places where a plate is subducting into the mantle. First, the distribution of earthquakes with depth along a line crossing from that subduction zones mark places where a plate is subducting into the mantle. First, the distribution of earthquakes with depth along a line crossing from the overriding to the subducting plate defines a slab that is plunging at an angle into the mantle.
OVERHEAD - Earthquakes and Subduction Zones
Definition: Benioff Zone - the inclined zone of earthquakes that marks the subduction of a plate into the mantle.
Recalling that earthquakes reveal places where the earth's crust is displaced across a fault, the Benioff zone reveals that a plate is moving downward with respect to the surrounding mantle. Subduction zone earthquakes typically rupture the fault that separates the upper face of the slab from the lower part of the overlying plate. The largest known earthquakes are almost all subduction zone events, and studies of the seismic waves released by these earthquakes indicate that they represent a downward thrusting of the subducting slab.
OVERHEAD - Volcanoes and Subduction Zones
The second line of evidence that a plate is penetrating into the mantle is the extraordinary correlation between volcanic arcs and subduction zones. Geochemical analyses of magma erupted from volcanoes located above Benioff zones indicate that material has moved upward from deep beneath the continent - something has therefore changed in the mantle beneath the continent in order to cause melting of the mantle in these regions.
OVERHEAD - Temperatures of subducting slabs and surrounding mantle
OVERHEAD - Does the subducting slab melt?
No, the mantle in the wedge above the subducting slab melts. This occurs because at about 100 km depth, where the temperature of the subducting slab finally gets high enough to cause the minerals in the subducting slab to dehydrate or lose their water molecules. The water that is shed by the subducting slab rises into the overlying mantle material, thereby changing its composition. As the overhead demonstrates, adding water to dry mantle material significantly lowers its melting point, thereby enabling the mantle above the subducting slab to melt (partially) and ascend to the base of the overlying continent. Some of the melted material works it way to the surface, where a series of volcanoes forms, and some of the material ponds and solidifies into batholiths, which are eventually exposed (via erosion) at the surface. The Sierra Nevada mountains in eastern California are an example of a batholith that formed within the lower reaches of a continent above a subducting plate.