Also called: transform fault, transform - Connects convergent or divergent plate boundaries - Fracture zone: a transform fault that connects two divergent boundaries - As a result, magnetic anomalies are offset to each other across fracture zones Differences in pate velocity may be related to the amount of continental crust carried by a plate - Plate velocities along same ridge also vary from place to place The islands and submarine seamounts have been dated by the K/Ar method and found to increase in age with distance from Kilauea Tuzo Wilson proposed that hot-spot volcanoes develop over a long-lived magma source originating deep within the mantle. Hot-spot volcanoes are probably a consequence of mantle plumes. As a plate drifts over the hotspot, a chain of extinct volcanoes (a “hot-spot track) forms, allowing a determination of the absolute velocity of the lithospheric plate. Over time, a hot-spot volcanic island erodes and subsides, eventually sinking below sea level to form a seamount. Guyots are flat-topped seamounts. For at least the last 5 million years, a new island has emerged every million years Mantle convection - Jason Morgan suggested that the ascending currents coming from deep in the mantle are the driving force of mantle convection and, therefore, of plate tectonics - The rate at which heat reaches the Earth’s surface can only be accounted for if convection (unlike conduction) in the mantle brings heat from the deep interior… but it is impossible to draw a global arrangement of convection cells than can explain the complex geometry of plate boundaries Slab Pull vs Ridge Push - At first, it was thought that rising magma at a spreading center creates new lithosphere, and in the process it pushes the plates sideways - Alternatively, it has been suggested that plate motion is driven by the attitude of the ridges and the subduction of older oceanic lithosphere. Hence, the upward movement of the asthenosphere beneath the mid-ocean ridge and the intrusion of magma are a consequence of seafloor spreading rather than the cause. - So, slab pull rather than ridge push?
Earthquakes in the vicinity of subduction zones are mostly extensional, as if something was pulling the plate. The spreading rate and the length of subduction zones are correlated, supporting the hypothesis that subduction is responsible for spreading. Convection keeps the asthenosphere hot and weak by bringing up heat from the deep mantle and the core. As a result, the lithospheric plates can slide on the plastic mantle (the asthenosphere) as they are pulled to subduction zones. Gemstones, when cut and facetted for jewelry are called gems Minerals: occur in varying shapes, sizes, and colours Crystals: occur as a single, continuous piece of crystalline solid, typically bounded by flat surfaces called crystal faces that grow naturally as the mineral forms Quartz – SiO 2 - Euhedral: with well-formed crystal faces - Anhedral: without well-formed crystal faces -
- Fall '09
- Plate Tectonics, Magnetic Field, rocks