Circulation of the Solid Earth: Plate Tectonics
Learning Objectives
- Know the most important feature of the Earth’s interior is that it is differentiated into a series of layers. These layers differ in both their chemical composition and in their physical properties (solid, plastic, liquid)
- Know about seismic waves and their characteristics
- Understand how the magnetic striping pattern forms, and know why these stripes are symmetrical about the crests of the mid-ocean ridges
- Know how plate tectonics relates to ocean history and the formation of ocean basins (Sea-floor Spreading)
- Understand the concept of plate tectonics and mantle convection
- Know the three plate tectonic boundaries, the forces at work there, and the types of earthquakes and volcanism likely to occur along those boundaries.
- Know how the convection currents beneath the plates move the crustal plates in different directions
- Know the source of heat driving the convection currents is radioactivity deep in the Earth’s mantle
- Know how mountains grow
Review Questions
1.)Why was the theory of continental drift not immediately embraced by the scientific community in the 1920s?
Wegener’s theory of continental drift was not well-received by the geophysicists of his day. The British scientist Sir Harold Jeffreys presented calculations in 1925 demonstrating that the continents could not possibly plow through the rigid sea floor, as the theory seemed to require. Other scientists were unconvinced because Wegener could not propose a physical mechanism for driving the motion of the continents. Indeed, many of Wegener’s own calculations and proposed mechanisms were found to be in error and untenable.
2.)What are the bases for the two major divisions of Earth’s interior--one that distinguishes crust, mantle, and core and the other that distinguishes lithosphere and asthenosphere?
Physical properties define the lithosphere and asthenosphere. The plates consist of an outer layer of the Earth, the lithosphere, which is cool enough to behave as a more or less rigid shell. Occasionally the hot asthenosphere of the Earth finds a weak place in the lithosphere to rise buoyantly as a plume, or hotspot. Only the lithosphere has the strength and the brittle behavior to fracture in an earthquake.
Chemical properties define the crust, mantle, and core. The general structure of the Earth as revealed by seismic imaging is a layered planet composed of a crust, a mantle (consisting of an upper mantle and a lower mantle), an outer core, and inner core. These distinctions are defined on the basis of contrasts in seismic wave velocities, which are affected by chemical composition.
3.)Compare and contrast P and S seismic waves.
Body waves are categorized as either P waves or S waves on the basis of their mode of propagation through Earth. P waves, or primary waves, result from the compression of material in Earth’s interior. The material is alternately compressed and, as the wave travels away, stretched. Thus, a P wave travels as a series of compressions and expansions in the overall direction of wave movement, similar to the way sound travels or to the response of a spring or a Slinky. S waves, which are also called secondary or shear waves, are transmitted as displacements perpendicular to the overall direction of wave travel, like the movement of a wave on a string.
4.)Why are earthquakes focused along plate margins?
Earthquakes are the result of plate motion: The plates move relative to each other at average speeds of a few centimeters per year. As a result of friction between the plates, there are alternating periods of stasis (during which stresses build) and periods of movement (when they are released) both at the plate boundary and near the surface. (Seismic and satellite measurements indicate that at greater depths or farther from the plate boundary, the motions are more continuous.) After a period of stasis, pent-up energy is released suddenly as the plates jump past each other, causing earthquakes. As predicted, the distribution of earthquakes at Earth’s surface follows plate boundaries quite closely
5.)What are the sources of heat in the Earth’s interior?
There are two sources of heat in the Earth’s interior:
- radioactivity decay
- residual heat from Earth’s formation
6.)What is magnetic polarity? What role did it play in the generation of ideas regarding sea-floor spreading?
The magnetic polarity is the geographic orientation of the North and South Poles. Magnetic stripes develop as new crust is added to the ocean floor at mid-ocean ridges and cools, becoming magnetized according to the magnetic field that exists at the time. As this material moves away from the axis, new sea floor is created, and its magnetization may be reversed if Earth’s magnetization has reversed polarity in the intervening time.
7.)What are the three types of plate boundaries, and what surface features are characteristic of each?
There are three types of plate boundaries (or margins): divergent, convergent, and transform. At divergent margins, lithospheric plates are moving away from each other. At convergent margins, plates are moving toward each other. At transform margins, plates are slipping past each other. Each boundary type is represented differently at Earth’s surface. In other words, each type of plate margin is reflected in distinctive surface features: mid-ocean ridges, deep-sea trenches, and transform faults, respectively.
8.)What are the driving forces for plate movement?
Figure 7-23 from main textbook
F1:Mantle drag or friction between the convecting asthenosphere and the overlying rigid lithosphere
F2:The gravitational “push” generated by the high topography of a mid-ocean ridge on the rest of the oceanic plate
F3:The increasing density of the oceanic lithosphere as it cools, which pulls the opposite end of the plate into a subduction zone
F4:The elastic resistance of the oceanic plate to being bent into a subduction zone
F5:The tendency for the overriding plate to be drawn toward a subduction zone as the subducting slab bends (which otherwise would move the trench away from the overriding plate)
F6: Friction between the subducting slab and the overlying lithosphere
F7: A tendency for the oceanic plate to sink as it cools and becomes denser
The overall motion of a given plate is the result of the balance of all these forces.
Analysis of plate motions today argues for a predominant role played by the push of the ridges and the pull of the subduction zone, but in the past, mantle drag may have played a more important role in the rifting apart of supercontinents.
Critical Thinking Problems
1.)Duplicate Figure 7.10 (use the color version for better clarity) and answer the following questions.
- Draw a line from the tip of Florida horizontally across the Atlantic to northwest Africa, a distance of about 6400 km. Now, graph the age of the sea floor (on the y-axis) against the distance from the ridge axis (on the x-axis). From this graph, determine the spreading rate for each geologic interval represented, averaging the two values determined for eastward and westward spreading. Graph these values (y-axis) as a function of time (in million years, on the x-axis).
Oldest sea floor = 200 Ma
Distance from mid oceanic ridge = 6400/2 = 3200 km
Average spreading range: 3200/200 = 16 km/Ma
For determining spreading rate for each geologic interval we need to define a scale for this map.
Measured horizontally, distance from the tip of Florida to Northwest Africa = 6400 km
Horizontal distance from the tip of Florida to Northwest Africa in this map:
Assumption:
For simplicity we consider symmetric spreading rate from the edge. Also the enlarged map doesn’t have good resolution, so we are going to consider only 7 stripes.
By measuring from the edge to Florida:
AB = 85 mm
LocationAge
Location 1 = 5 mm(144-208 m.y.)
Location 2 = 4 mm(117-144 m.y.)
Location 3 = 11 mm( 84-117 m.y.)
Location 4 = 5 mm(66-84 m.y.)
Location 5 = 4 mm(58-66 m.y.)
Location 6 = 5.5 mm(37-58 m.y.)
Location 7 = 8 mm (consider from the edge) (0-37 m.y.)
For this map: 4600 km/ 85 mm 1mm ~ 54 km
Location / Distance (mm) / Distance (km) / Age (m.y.) / Year (m.y.) / Spreading rate (km /m.y.)1 / 5 / 270 / 144-208 / 64 / 4.22
2 / 4 / 216 / 117-144 / 27 / 3.38
3 / 11 / 594 / 84-117 / 33 / 9.28
4 / 5 / 270 / 66-84 / 18 / 4.22
5 / 4 / 216 / 58-66 / 8 / 3.38
6 / 5.5 / 297 / 37-58 / 21 / 4.64
7 / 8 / 432 / 0-37 / 37 / 6.75
- How has the Atlantic spreading rate varied over the last 200 million years?
The Atlantic spreading rate first decreased from 208 m.y. ago to 142 m.y. and then it increased until 91 m.y. ago. From that time until now it has been increasing.
Now, duplicate your map four more times. Trim the Atlantic margins at the boundaries between the regions of sea floor of various ages (as identified in the legend of the figure). For example, for the first map, cut out that part of the Atlantic that is younger than 37 million years, and then try to match the two halves. On the next sheet cut out all the sea floor younger than 66 million years, and so on. Then compare your reconstructions of the relative positions of North and South America, Europe, and Africa to the paleographic maps in Figure 7-12.
This last part of the question can be done as a lab activity and is a good way to get students to understand plate tectonics by “moving” the plates together themselves. This hands-on task may help students visualize the movement of the plates over time.
Resource Guide (Websites)
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