The Way of Science

3. Plate Tectonics

For the new model, your text and the film should allow you to develop a solid basic picture of how the Earth's surface was actually shaped. There are a number of Web sites with animations to explain the Plate Tectonics. You might start with the USGS site, http://pubs.usgs.gov/publications/text/dynamic.html. You will need to know certain terms

(below), and you must be able to describe in a logical and step-wise fashion how tectonic activities produce what we observe on Earth. Here is your guide: define/describe plates; include differences between oceanic and continental crust, and the three directions of relative plate movement. Where do these movements occur? Define/describe mid-ocean ridge-rift systems, deep ocean trenches, subduction, San Andreas fault, "hot spots" (Hawaii), convection currents in the mantle; causes of earthquakes and volcanic activity; how both volcanic mountain ranges (Andes, e.g.) and non-volcanic ranges (Himalayas) are formed.

Wegener first published his lateral motion hypothesis in 1912; it wasn't until the mid-sixties that the geologic community abandoned the "shrinking apple" model and fully embraced what is basically Wegener's idea. That's over 50 years of resistance; you will return to why it took so long after we consider the new evidence for sea-floor spreading that blossomed in the 'fifties and early 'sixties.

4. Evidence: Paleomagnetism and sea-floor spreading

In order to understand the evidence from the late fifties and sixties, some background on magnetism is necessary. There is a brief and very basic description in your text, under "electromagnetism." What you need right now is the concept of magnetic lines of force that extend in three dimensions to connect the two poles of any magnet. If you have not seen the demonstration of these "lines" before, perhaps your instructor will demonstrate them in class using a bar magnet and iron filings.

The relevance to geology, of course, is that the Earth is a giant magnet. The cause of this magnetism is circulation of the molten iron in the core, and thus the Earth is effectively a gigantic dynamo. In Figure 5, one can see a very diagrammatic representation of the Earth and its magnetic field. For simplicity's sake, the diagram shows the axial (rotational) poles and the magnetic poles in the same spot. This identity of location is not always in place, since both wander. However, they do remain rather close to each other. They are about 11.5 degrees apart right now. Note that the lines converge and "dive" at the poles. "Enrich your life": These lines of force trap high energy particles emitted by the sun, and funnel them toward the polar regions. As these energetic particles ionize the nitrogen and oxygen of the atmosphere, those molecules glow in shades of pink and green. In the Arctic, these are called "northern lights" (aurora borealis); in the Antarctic, it is the glow of the aurora australis that is seen.

Suppose we had a very sensitive compass, what useful information might we obtain from these lines?

Figure 5. The Earth's magnetic field and the inclination/declination of a compass needle in response to the field. At the Equator, the North-pointing end parallels the surface. Moving north, declination (down-pointing) increases. Moving south, inclination increases.

• Obviously, the north-south axis could be determined for any location on Earth. Remember, though, that every way is south if you are standing at the north pole.
  
  
• We could determine which pole is north, which is south.
  
  
• The latitude could be determined. This one requires some additional explanation, since it is both very important for our evidence, and may not be immediately apparent to most of you how one can determine latitude with a compass. See Figure 5. Your instructor may demonstrate this in class using a magnetic model of Earth.

If one stands at the equator, a compass needle would be parallel to the Earth's surface. The north-pointing tip would not point toward the center of the Earth, and we would say that there is thus zero degrees of inclination. As one moves northward, the north-pointing tip increases its "dip" (its declination), until it points essentially straight down at the north pole. If the compass is moved from the equator to the southern hemisphere, the inclination, or angle of upward deflection, would increase (for the north-pointing tip). Aha! With a little trigonometry, we now have a way of determining latitude from compass data.

These three sets of data (direction of poles; which pole is which; how far north or south the observer is) allow one to determine a fixed position on the Earth's surface. If one now had a way of 1) doing these for a piece of rock of any age, and 2)"dating" the rock (i.e., determining when it formed), one would have the necessary tools for paleomagnetic studies. The next bit of necessary background is understanding how a record of the Earth's magnetic field is trapped in certain kinds of rocks. This phenomenon is variously called paleomagnetism, natural remanent magnetism (NRM), or "fossil" magnetism.

There are several iron-containing minerals (like magnetite and hematite) that act as natural magnets. If tiny grains or crystals of these minerals can float freely in some medium, they will align with the lines of force. If the medium then solidifies, they are trapped in that configuration, and constitute a permanent record of the magnetic field at that time. There are many ways for such records to form in the geology of an area; basalt, for example, is liquid when it exits from volcanoes or rifts, and it is also frequently rich in these tiny magnets. As the basalt hardens, it traps the grains. Sedimentary rocks, formed from sand and /or gravel settling and compacting, can also maintain such records.