Lithospheric Plates

Let's Go Back to those Plates  What is a plate ? What does a tectonic plate consist of ? We think of plates as the outer shell of the Earth divided up into a number of thin rigid plates.  Lithospheric Plates Plate velocities vary from 1-10 cm/yr  A large fraction of all earthquakes, volcanoes, and mountains occur at plate boundaries  How are plates created ?  Lithospheric Plates Plates are made by cooling of magma into rock  How thick of a rock section is created by cooling ? (How thick are plates typically ?)  Lord Kelvin's estimate for the age of the earth assumed this process, but only achieved cooling of the top 100 km of the earth!  Lithospheric Plates What is the lithosphere ?  The interior of the Earth which is made up of plates is called the lithosphere.  Lithospheric rocks are cool and behave rigidly  These rocks do not deform significantly in geologic time  Most geophysicists opposed the hypothesis of plate tectonics in the early 1900's, even as late as the 1960's. Why is this ?  Lithospheric Plates Most geophysicists opposed the hypothesis of plate tectonics in the early 1900's, even as late as the 1960's. Why is this ?  Surface observations and seismic waves showed that the Earth's interior was solid. How would solid material accomodate movement in this solid body ?  No clear driving mechanism for plate movement.  Lithospheric Plates Most geophysicists opposed the hypothesis of plate tectonics in the early 1900's, even as late as the 1960's. Why is this ?  Surface observations and seismic waves showed that the Earth's interior was solid. How would solid material accomodate movement in this solid body ?  No clear driving mechanism for plate movement.  Lithospheric Plates Later studies of gravity surrounding mountain ranges and rebound observed in Scandinavia (Haskell, 1935) showed that deep mantle rocks shift or move in some sort of semi-solid fashion.  Much later the study of solid state creep suggested that mantle material deforms and has a “viscosity” which allows movement of rigid plates at its surface. (Viscosities ~1021 – 23 Pas)  Lithospheric Plates The lithosphere can be defined thermally by an isotherm at o the base of the lithosphere which should be around 1350 C.  Mantle rocks below this isotherm are cool and behave rigidly  Rocks above this isotherm are hotter and may deform  Lithospheric Plates  Lithospheric plates are rigid and behave elastically. Plate transmit stress over long distances providing important implications for plate driving mechanisms.  Rigid plates can bend when subject to a load such as volcanism or seamounts (e.g. Hawaii).  Lithospheric Plates  ? Does the entire lithospheric plate transmit stress in the same way The lithosphere is found to have an elastic portion which is roughly the upper ½ of the plate.  Stresses relax in the lower portion of the plate where creep processes dominate, but still behaves coherently with the plate.  Heat Conduction of Lithospheric Plates When you subject a plate to heat or cold, will it be in steady state immediately ? Describe this process and what this is called ?  The one-dimensional, time-dependent heat conduction equation best describes this process.  Read through pages 178-185 in your text for next time.  Early Case for Continental Drift • Puzzle-piece fit of coastlines of Africa and South America has long been known Early Case for Continental Drift •In early 1900s, Alfred Wegner noted South America, Africa, India, Antarctica, and Australia have almost identical rocks and fossils – Glossopteris (plant), Lystrosaurus and Cynognathus (animals) fossils found on all five continents – Mesosaurus (reptile) fossils found in Brazil and South Africa only Glaciers • Most of the Earth's ice is found in Antarctic continental glacier. • Where are some other continental glaciers ? • Glacial striations on a rock from stones grinding at the base of a heavy ice sheet leave these shiny linear marks on the bedrock below. Glacial Characteristics • Glaciers flow downhill as a solid mass that creates channels, and walls made of ground up rock debris known as a merraine. Erosional Landscapes • Erosional landforms produced by valley glaciers include: – U-shaped valleys – Hanging valleys • Smaller tributary glacial valleys left stranded above more quickly eroded central valleys Early Case for Continental Drift • Wegner reassembled continents into the supercontinent Pangaea • Late Paleozoic glaciation patterns on southern continents best explained by their reconstruction into (Pangaea) Gondwanaland Early Case for Continental Drift • Coal beds of North America and Europe indicate Laurasia super continent • Continental Drift hypothesis initially rejected – Wegener could not come up with viable driving force – continents should not be able to “plow through” sea floor rocks The Earth's Magnetic Field Can Give Us Clues Paleomagnetism and Continental Drift Revived • Studies of rock magnetism allowed determination of magnetic pole locations (close to geographic poles) • Paleomagnetism uses mineral magnetic alignment and dip angle to determine the distance to the magnetic pole when rocks formed – Steeper dip angles indicate rocks formed closer to the magnetic poles • Rocks with increasing age point to pole locations increasingly far from present magnetic pole positions Paleomagnetism and Continental Drift Revived • Apparent polar wander curves for different continents suggested plate movement ! • Wegner was right! Earthquakes don't occur randomly – but in patterns Friction and motion at plate boundaries seem to produce earthquak