The Lithosphere and the Tectonic System Objectives: Understand the structure of the planet Earth Review the geologic timescale as a point of reference for the history of the Earth Examine the major relief features of the Earth Compare and contrast lithospheric plates and the implication of plate movements and boundaries The Structure of the Earth Layered structure: Earth has a radius of 6400 km Core radius 3500 km Composed of iron and nickel Inner core solid Outer core liquid Temperature 3000º - 5000ºC Mantle Ultramafic rock 2900 km thick Temperatures range from 2800º near the core to 1800º near the crust Outer layer of the mantle is brittle
Crust 8 to 40 kms deep Contact between crust and upper mantle = Mohorvicic discontinuity (or Moho) It marks a change in density Close to the surface below ocean floors (mafic) (8 km) Deeper under land (felsic) (40 km) The solid upper part of the mantle plus the crust is often referred to as the lithosphere It is discontinuous (i.e. there are gaps) Beneath it lies the asthenosphere; the layer of soft, plastic rock in the mantle. Lithosphere moves over the asthenosphere The lithosphere is fragmented into litthospheric plates ( or tectonic plates)
Geological Time Scale Eon Era Period Epoch Date (yrs. Millions BP) Phanerozoic Cenozoic Geologic Quaternary Time Holocene Scale Pleistocene Tertiary Pliocene Miocene Oligocene Eocene Paleocene Mesozoic Cretaceous End of Mesozoic 65 Jurassic Triassic Paleozoic Permian End of Paleozoic 245 Carboniferous Devonian Silurian Ordovician Cambrian Proterozoic Precambrian End of Precambrian 543 Archean (87% of Hadean geologic time scale) The major relief features Continents Young, dynamic alpine belts Old, stable continental shields. Oceans Extensive, smooth abyssal plains Long, narrow midoceanic ridges. Relief Features of the Continents Alpine chains - include mountain arcs and island arcs Continental shields - low lying older areas of igneous and metamorphic rocks exposed shields covered shields Ancient mountain roots - remains of mountain belts in shields
Relief Features of the Ocean Basins Mid ocean ridge and ocean basin floor mid ocean ridge with AXIAL RIFT at the centre ocean basin floor are deep plains (ABYSSAL PLAINS) some include small hills (ABYSSAL HILLS) Continental Margins where ocean and continental lithospheres meet CONTINENTAL RISE - steep slope CONTINENTAL SHELF - gentle slope Fig 13.6 Shallow continental shelves are found beneath the ocean next to continental shields Deep oceanic trenches are found adjacent to alpine belts. Continental Shelves Geosat Satellite Altimeter
Plate Tectonics and Continental Drift 1915: Alfred Wegener proposed that landmasses were once united (PANGAEA) Split into LAURASIA and GONDWANALAND During this time, land masses have formed and moved (continental drift) Lithospheric plates There are 7 major plates and many smaller ones Proposed that convection cells within the earth s interior drag the crust along Plate tectonic movement
Plate Boundaries Divergence (Seafloor Spreading) Convergence (Collision) Lateral (Transform) Plate Divergence New crust is created and forced outwards from a RIFT Upwelling magma creates new crust and younger rocks. Most occur at midoceanic ridges (SEAFLOOR SPREADING) (e.g. Mid-Atlantic Ridge) Mid-Atlantic Ridge at Iceland 1.8 cms per year South Atlantic 4.0 cms per year
Plate Convergence Ocean - land convergence = subduction zone Continental crust is lighter than oceanic crust (oceanic crust is subducted) Produces large earthquakes, volcanism, mountain ranges e.g. Nazca plate subducted under South America plate (7.4 cms per year) Oceanic-oceanic Plate Convergence Produces deep ocean trenches and many volcanoes eg. collision of North America and Pacific Plates (Aleutian Islands)
Continental-continental Plate Convergence Produces thick land mass, earthquakes, but very little volcanism e.g. Eurasian (S) and Australian-Indian (N) Plate Lateral (Transform) Plate Contact Two plates are sliding past one another at different rates Is accompanied by frequent earthquakes eg. San Andreas Fault