TODAY IS 15 SEPTEMBER 2007 -- Transform faults

Its not your fault; its not my fault! Transform faults are part of our planet's geology. We need to understand them for our own survival, as they are the source of innumerable, destructive earthquakes. Transform faults are long and relatively continuous where they cut across continents, but tend to appear as short discontinuous segments offsetting sections of spreading ridges on the ocean floor. The transform fault is simply a fault connecting two other kinds of active plate boundaries, but that is a deceptively simple definition. Follow the link about paper models of transform faults. Do that now, please, and print out the model. (The web address is http://web.mala.bc.ca/earle/transform-model/ if you have trouble with the link.) Directions for printing the model are contained in that website. Then, return here:

Welcome back. Now, assemble your model from the directions given. Your model is very similar to small portions of the Mid-Atlantic Ridge between South America and Africa.

Begin with the model in its closed position. The two points marked A and B should be juxtaposed (what does "juxtaposed" mean?), and the gaps at the two spreading ridges should be closed tightly. Now open the model. Note how new plate material is "created" at each spreading ridge and an equal area is added to each of plate A (the shaded plate) and plate B (unshaded). Also, notice that after spreading has occurred, the two ridges are no farther apart than before spreading. Verify this by measuring distance DD' with the model closed and distance EE' with the model open.

The model illustrates the fact that the shape of an oceanic ridge does not change with time.

Now turn your attention to the fault itself. With the model closed, imagine yourself standing on point A, looking across the fault to the juxtaposed point A'. Slowly open the model, watching how point A' moves as seen from point A. If you are standing at point A, you will see point A' move to your right.

Turn the model around and repeat the process. If you are standing at point A', you will see point A moving to your right. Because of this independence of where you happen to stand, this particular transform fault is said to display right-lateral motion. That is, the motion is side-to-side and the other side of the fault always moves to your right.

Transform faults may also be left-lateral, if the ridges are offset in the opposite sense. To see this, turn the model page over to the other side and trace the positions of points A and A' on the reverse side of the paper. Holding the closed model so that you are looking at its reverse side, open it and note the motions of points A and A'. If you are standing at point A, you will see point A' move to your left.

The combination of ridge segments and transform faults forms a rectilinear zigzag pattern for oceanic plate boundaries that may be seen clearly in Figure 3-3 (on the website). It is still not clear just how this zigzag pattern is formed initially, but because the pattern does not generally change shape with time, it must have come into existence at about the same time as the ridges themselves. The process by which this happens is still not fully understood.

Where transform faults cut across continents, however, they tend to be long and relatively continuous, with few, if any, spreading segments. The best known and most studied example of a continental transform fault is California's San Andreas.

Now look at the satellite photo below:

It shows land and ocean floor to the west of the San Andreas Fault. This area is part of the Pacific Plate and is moving to the northwest, parallel to the fault. To the east of the fault is the North American Plate. Where the San Andreas Fault crosses the North American continent it is long and unbroken, but where it goes out to sea, it is cut into shorter segments separated by spreading ridges. Some of the ridge segments themselves are quite short, as in the Gulf of California.

Note that the ridge and fault geometry is similar to that of your paper model, for which right-lateral motion is expected along the fault. This is in fact what is observed along the length of the San Andreas. It is a classic transform fault, where the Pacific Plate is sliding past the North American Plate, carrying Los Angeles and Baja California along with it.

Along the fault, the rocky edges of the plates grind against one another. In a few places, the slippage occurs smoothly. Here, any structure such as a fence or road that crosses the fault is offset at a rate of up to six centimeters (2-1/2 inches) per year. But in other places, the fault is jammed and does not move steadily. As the plates continue their inexorable motion, the forces exerted on the pinned fault build up with each passing year. Finally the rock can stand no more and it breaks, unleashing the pent-up energy as strong vibrations of the ground: an earthquake.

Today is 28 September 2007 – We are studying Dynamic Earth Processes

Dynamic Earth Processes is a course of study identified by California as Standard 3.

“Earth sciences use concepts, principles, and theories from the physical sciences and mathematics and often draw on facts and information from the biological sciences. To understand Earth’s magnetic field and magnetic patterns of the sea floor, students will need to recall, or in some cases learn, the basics of magnetism. To understand circulation in the atmosphere, hydrosphere, and lithosphere, students should know about convection, density and buoyancy, and the Coriolis effect. Earthquake epicenters are located by using geometry. To understand the formation of igneous and sedimentary minerals, students must master concepts related to crystallization and solution chemistry. “

“Because students in grades nine through twelve may take earth science before they study chemistry or physics, some background information from the physical sciences needs to be introduced in sufficient detail. From standards presented earlier, students should know about plate tectonics as a driving force that shapes Earth’s surface. They should know that evidence supporting plate tectonics includes the shape of the continents, the global distribution of fossils and rock types, and the location of earthquakes and volcanoes. They should also understand that plates float on a hot, though mostly solid, slowly convecting mantle. They should be familiar with basic characteristics of volcanoes and earthquakes and the resulting changes in features of Earth’s surface from volcanic and earthquake activity.”

3. Plate tectonics operating over geologic time has changed the patterns of land, sea, and mountains on Earth’s surface. As the basis for understanding this concept:

a. Students know features of the ocean floor (magnetic patterns, age, and sea-floor topography) provide evidence of plate tectonics.

Much of the evidence for continental drift came from the seafloor rather than from the continents themselves. The longest topographic feature in the world is the midoceanic ridge system—a chain of volcanoes and rift valleys about 40,000 miles long that rings the planet like the seams of a giant baseball. A portion of this system is the Mid-Atlantic Ridge, which runs parallel to the coasts of Europe and Africa and of North and South America and is located halfway between them. The ridge system is made from the youngest rock on the ocean floor, and the floor gets progressively older, symmetrically, on both sides of the ridge. No portion of the ocean floor is more than about 200 million years old. Sediment is thin on and near the ridge. Sediment found away from the ridge thickens and contains progressively older fossils, a phenomenon that also occurs symmetrically.

Mapping the magnetic field anywhere across the ridge system produces a striking pattern of high and low fields in almost perfect symmetrical stripes. A brilliant piece of scientific detective work inferred that these “zebra stripes” arose because lava had erupted and cooled, locking into the rocks a residual magnetic field whose direction matched that of Earth’s field when cooling took place. The magnetic field near the rocks is the sum of the residual field and Earth’s present-day field. Near the lavas that cooled during times of normal polarity, the residual field points along Earth’s field; therefore, the total field is high. Near the lavas that cooled during times of reversed polarity, the residual field points counter to Earth’s field; therefore, the total field is low.

The “stripes” provide strong support for the idea of seafloor spreading because the lava in these stripes can be dated independently and because regions of reversed polarity correspond with times of known geomagnetic field reversals. This theory states that new seafloor is created by volcanic eruptions at the midoceanic ridge and that this erupted material continuously spreads out convectively and opens and creates the ocean basin. At some continental margins deep ocean trenches mark the places where the oldest ocean floor sinks back into the mantle to complete the convective cycle. Continental drift and seafloor spreading form the modern theory of plate tectonics.

3. b. Students know the principal structures that form at the three different kinds of plate boundaries.

There are three different types of plate boundaries, classified according to their relative motions: divergent boundaries; convergent boundaries; and transform, or parallel slip, boundaries. Divergent boundaries occur where plates are spreading apart. Young divergence is characterized by thin or thinning crust and rift valleys; if divergence goes on long enough, midocean ridges eventually develop, such as the Mid-Atlantic Ridge and the East Pacific Rise.

Convergent boundaries occur where plates are moving toward each other. At a convergent boundary, material that is dense enough, such as oceanic crust, may sink back into the mantle and produce a deep ocean trench. This process is known as subduction.

Transform boundaries occur where two plates, or fractured portions of a plate, attempt to slide by each other. These boundaries are known as “faults” and are the source of much earthquake activity.

Today is 12 October 2009 -- We want to place the age of the earth into a context that students understand

Pretend that you are on a road trip across the United States.  Assume that the age of the earth can be represented by the distance you travel.  For example, at the start of the trip, the earth is a molten glob; by the end of the trip we have reached the present day.  What will we see along the way?  To prepare for our trip, do the following:

1.  Get a blank map of the United States.

2.  On the map, find San Francisco, CA, and New York City, NY.

3.  Draw a straight line between these points. 

4.  The distance between SF and NYC is 2640 miles.  Let this represent the age of the earth, about 4.5 billion years.  If the line on the map is 12 inches, for example, then every inch represents 240 miles AND  375 million years.

5.  Teacher reads from book "Basis of Human Evolution" and identifies points on the map along the line representing milestones in the history of the planet.  (Refer to "ICS Page.")  

6.  As teacher reads, student is to scale the time in terms of miles traveled and mark locations (the "where") the milestones in history (the "when") occurred. 

After doing this, students see that the breakup of the super continent Pangea, 225 million years ago, occurred within 159 miles of the present on our time/distance line which is about 0.66 inch on the map.  (How was this distance calculated?)  

What if we were interested in the events only from 225 million years ago?  We could draw a longer line on the map, or we could "change the scale."  If we change the scale, we let a line on the map represent a different time span.  Try this:

1.  Find Victoria, British Columbia, and Jacksonville, FL, on the map.

2.  Draw a straight line between these points.

3.  The distance between Victoria and Jacksonville is about 3134 miles and represents 250 million years.

4.  Plot the center points of the Permian (250M), the Triassic (200M), Jurassic (135M) and the Cretaceous (65M) years before present on the map.

5.  Measure the length of the line, in cm.

6.  Identify features of the land masses associated with each of these eras.

7.  Write a paragraph about each era, comparing the changes.

8.  Write about how the changes occurred, according to the text.

Students need to know that the time, in terms of distance, can be found by simple ratios or proportions.  Thus:  250M/225M = 22cm/x.  Solving, x = 19.8 cm.