Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Mountains and basins Isostasy • Bouyancy -- the principle that an object immersed in a fluid experiences an upward force equal to the weight of the fluid displaced • If the object is less dense than the fluid, then the object will float in bouyant equilibrium 1 • Lithospheric plates float in the underlying asthenosphere – Thick and less dense continental crust stands higher isostatically; continental surface is several hundred meters above sealevel – Thin and more dense basaltic ocean crust stands lower; top of ocean crust (the sea floor) lies several kilometers below sealevel • Orogenic (mountain-building) processes tend to thicken the crust, causing it to reach a new equilibrium, with the surface at a higher level – Heat -- thermal expansion inflates crust – Thrust faulting -- multiplication of slabs of crustal rock – Intrusion and extrusion (volcanism) of magma of granitic composition 2 Erosion • Erosion of mountain belts thins the crust, disturbing isostatic equilibrium • Plate rises to new equilibrium point, lower than the original; this process of erosion and isostatic uplift can continue for hundreds of millions of years 3 4 5 Collage tectonics • Mountain belts found to contain suspect terranes, with distinctive: – Fossil contents – Paleomagnetic signatures • Direction of magnetic pole • Inclination of paleomagnetic field – Rock types • Modern oceans found to contain areas of anomalously thick crust, which might accrete to a continental margin rather than be subducted – Hot spot basalt plateaus – Extinct volcanic arcs – Fragments of rifted continental crust 6 7 8 Continental collision • Passive margins of continents are sites of great thickness of sediments (10-15 km) • Closure of ocean by subduction can deform and uplift the passive margin strata, thrusting them onto the adjacent continent to form a mountain belt 9 Sedimentary basins • If the continental crust is stretched (extensional strain), it tends to thin by faulting near the surface and by ductile flow at depth • Thinner crust stands lower isostatically; with sufficient stretching it will subside below sealevel and provide space for accumulation of sediments 10 11