1. Geology of Australia and New Zealand, HWS/UC 2007 2. Plate Tectonics

2. Continental drift, sea floor spreading, evidence for Plate Tectonics The magnetic field of the earth reverses from time to time, in a random pattern. Magmas that cool during times when the magnetic field is “normal” or “reversed” become magnetized by the prevailing field of the time. These magnetized rocks retain their magnetization and either add to or subtract from the present day field producing positive or negative magnetic anomalies. The pattern is like a tape recording of the earth’s field through time, spread out in space, symmetrically on either side of ocean ridges. The best explanation is spreading of the sea floor, a key element in plate tectonics (and continental drift)

3. Earth structure, Plate features Note the difference in thickness between oceanic and continental crust (and lithosphere). This produces the characteristic topography of ocean basins and continental platforms, as well the high elevations of the mountain ranges. Also note how relatively thin is the rigid outer part of the earth, the lithosphere. Most of the earth is relatively plastic, i.e. is capable of flowing over geologic periods of time.

4. Divergent Margins (the plates are moving away from each other) Iceland is on a divergent plate boundary. The rocks that make up most of Iceland are basalts, which are the result of melting of upper mantle rocks, followed by rising and freezing of that magma along the divergent plate boundary, where it forms new oceanic (basaltic) crust. The crust typically doesn’t accumulate to thicknesses of more than about 10km because of the balance between magma generation and sea-floor spreading. Iceland is unusual because it overlies a “hot spot” in the mantle where magma generation rates are unusually high, supplying extra magma to make a thicker ocean crust. This thicker than average ocean crust is why Iceland sticks up above sea level.

5. Convergent margins (the plates are moving toward each other) The top diagram shows subduction of oceanic lithosphere (with ocean crust on top) beneath oceanic lithosphere. The gray beneath the upper plate of ocean crust on the left side is cold upper mantle that is part of the lithosphere. You can imagine a similar thickness of upper mantle being part of the lithosphere of the subducting plate (on the right hand side). Release of water and melting of ocean crust from the subducting slab rises to the surface and is deposited within and on top of the upper slab to make thick crust and an island arc, like Japan. Unlike a spreading center, the magma is emplaced again and again in the same location, building up thick (and eventually continental) crust. The bottom diagram shows subduction at a continental margin (as in the Andes Mountains). The lithosphere beneath the continent and the continental crust is actually thicker than shown here. Note that the magma is also emplaced at the same location for an extended period of time, producing thick continental crust.

6. Transform Margins Plates slide past each other at transform faults. These often produce shallow, and destructive, earthquakes. Often there is an element of convergence or divergence (i.e. the plates do not slide perfectly parallel to one another) at a transform boundary, which produces differential uplift across the fault.

7. Hot Spots-what are they and how are they important? Plate movement across hot spots is a classic demonstration of relative movement of the plate and some part of the underlying mantle (where the magma is produced). The sinking of older parts of the chains of islands associated with hot spots gives rise to atolls and guyots (see lecture on carbonate reefs).

8. The convective origin of plate motions Think about the “stew-pot model” of plate tectonics while looking at this diagram for plate tectonic motions. The cold lithosphere at the top (which includes the crust) is the product of magmatism. Continental crust differs from oceanic crust in important ways (see Lecture 4), but both begin as a result of magmatic activity.