Lab 6: Plate Tectonics: Introduction to Relative Plate Motions. Geology 202: Earth s Interior

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1 Introduction: Lab 6: Plate Tectonics: Introduction to Relative Plate Motions Geology 202: Earth s Interior In plate tectonics, the mantle and the crust of the earth are divided into layers, called the lithosphere and the asthenosphere. The crust and upper mantle make up the lithosphere, which is rigid and resistant to deformation. On average, the lithosphere is about 100 kilometers thick (but it can be thicker under the deep roots of continents as low as0kmthick where it first is formed at ridges). The layer below the lithosphere is called the asthenosphere and it is made up of easily deformable material that is a few hundred kilometers thick. With plate tectonics theory, the outer surface of Earth is made up of rigid plates of lithosphere which "float" on the weaker asthenosphere. Since the plates are rigid, this theory predicts that deformation on Earth s surface, in the form of earthquakes, takes place within the lithosphere along the boundaries of plates. On Earth there are exactly three types of plate boundaries. New crust forms along ridges or rises. These are places where plates move apart, or spreading centers. Where two plates collide and one plate overrides another in a subduction zone, the boundary between the two plates is defined by a trench. In these scenarios, typically a volcanic arc forms within the overriding plate. In other instances of plate convergence neither plate overrides the other, and this collisional boundary forms a mountain range. Along transform boundaries, plates move past each other in a direction which is parallel to the plate boundary. i) Using your understanding of geography and the detailed map of ocean floor bathymetry in Room 301, what sort of plate boundary (or relative plate motion) would you find at the following places: a) San Andreas Fault b) Marianas Trench c) East Pacific Rise d) Red Sea e) Himalayas f) Aleutian Trench g) Kermedec Islands h) Hawaii Islands i) Gulf of California Since Earth is a sphere, ridges are commonly offset by transform faults. Fr acture zones have many characteristics of transform faults and they follow the same trends as the faults which offset ridges. They are distinct topographic features which look like faults because they divide lithosphere of different ages. These represent areas which were once transform faults, but there is no relative motion across the fracture zone boundary. ii) Would you expect the ocean depth to vary on either side of a fracture zone? Why or why not?

2 -2- At subduction zones it is important to determine which plate will override the other, which is referred to as its polarity (not to be confused with magnetic polarity). The polarity of a subduction zone depends on the type of crust, oceanic or continental, of the two plates involved in the collision. iii) Using your understanding of of the differences in density between between oceanic and continental crust, which would you expect to override the other? An example of variation in subduction zone polarity is found along the Pacific/Australian plate boundary, which cuts through New Zealand. The plate motion of the Pacific plate relative to the Australian plate (assuming that the Australian plate is fixed) is directed at the angle of 225 relative to North. iv) Using the sketch of the plate boundary along New Zealand, for regions where you would expect to find subduction zones, draw a saw tooth pattern with teeth pointing toward the overriding plate. v) Where would neither plate subduct along this plate boundary? Mark this area with a dashed line. In the case where there is a collision between two plates of oceanic lithosphere, the plate with the oldest crust subducts beneath the other plate. In the case where there is a collision between two plates of continental crust, neither plate subducts. The plate push together, forming mountains. A good example of this is the Himalayan mountains. In this region, the Indian Plate is colliding with the Asian plate, and since both plates are made up of continental crust neither one overrides the other, and it is in these regions where the continental crust attains its greatest thickness, on the order of 70 km. The two plates push together forming the mountain chain.

3 -3- Part One: Plate Geometry Reconstructions Using ideas previously mentioned, now we will consider how various plate boundaries can evolve over time. Let s imagine that we made an observation where we found three east-west, inactive fracture zones all located on a plate (Figure 1). If we could generate isochrons (using magnetic lineations), we could infer the time which specific parts of the plate formed. vi) The isochrons show ages which are younger in which direction? What geological process can you think would form such afeature? What is missing? vii) Can you imagine what process was responsible for cutting of the side of Plate A? viii) What do these fracture zones represent? What does it mean that they lie in an east-west direction? Now, try "rolling" back time to determine what processes were responsible for creating the pattern of Figure 1. Using tracing paper, sketch the location of the ridge, plate A, and the missing plate B at times of 33 Ma (millions of years before present), 53 Ma, 60 Ma and 70 Ma. Make sure to label the locations of all ridge, trench, transform fault and fracture zone sections.

4 -4- Below is a representation of the current configuration of two plates, A and B. Assume plate A will remain stationary (its motion is said to be fixed). The magnitude and direction of the relative velocity of plate B relative to plate A, V BA is indicated by the arrow (in terms of mm/yr). Sketch the configuration of the plates 10 Ma from the present (10 million years from now). Do the same for 20 Ma.

5 -5- Part Two: Plate Velocity Diagrams Earth s surface is made up of many plates. We can use velocity diagrams to determine relative velocities of many different plate pairs. To see how this works, first consider the case of three plates, A, B, and C. If you know the velocity of plate B relative to plate A, V BA,and the velocity of plate C relative to plate B, V CB,you can calculate the velocity of plate C relative to plate A, V CA,using the following relationship: V CA = V CB + V BA Using velocity diagrams, we can also determine the relative motion between three plates. The AB boundary is a transform with movement that is 3 mm/yr (this is called a right lateral transform fault). The BC boundary is a trench along which convergence is occurring obliquely at an angle of 45 degrees east of north. On the velocity diagram below, plot the vectors representing the motions of plate B and C (holding plate A fixed). ix) Using the velocity diagram, decide whether the boundary between plates A and C is a ridge, trench or transform. x) Find V CA.

6 -6- xi) Using the example and velocity diagram below, decide whether the boundary between plates A and C is a ridge, trench ortransform. xii) Find V CA.