Plate Tectonics: The Unifying Theory
Theory of Plate Tectonics describes the movement of plates and the forces acting between them. It also explains the distribution of many large-scale geologic features - mountain chains, structures on the seafloor, volcanoes, and earthquakes
From Controversial Hypothesis to Respectable Theory
Continental Drift (Alfred Wegner - 1912)
Jigsaw-puzzle fit of North America, Europe, Africa, and South America coasts
Similarity of rocks and geologic structures on both sizes of Atlantic Ocean
Fossils - reptile fossils only found in Africa and South America.
Pangaea - Supercontinent that broke up into our present continents as the Atlantic Ocean formed
Thermal Convection and Seafloor Spreading
A plausible driving force that would split Pangaea and move continents apart. Thermal convection in mantle proposed in 1928
Exploration of seafloor after WWII. Discovery of rift at crest of Mid-Atlantic Ridge.
Seafloor Spreading (1964) - seafloor separates at rifts in mid-ocean ridges and that new seafloor forms by upwelling of hot mantle material, followed by lateral spreading.
Acceptance of Plate Tectonics
Identify the separate lithospheric plates (1967)
Oldest oceanic crust is 175 my. Older oceanic rocks have been recycled back into the mantle.
Continents are too buoyant to be subducted. Complex history of accretion by plate collisions (oldest crust is at center of continents, gets younger towards the margin). Oldest continental crust is almost 4 b.y.
Mosaic of Plates - lithosphere broken into a dozen or so large rigid plates and half a dozen or so smaller platers. Plate move thousand of kilometers as distinct, rigid units with little deformation except at the boundaries.
Divergent Boundary (ridges or rifted continents)
Mid-ocean ridge - active basaltic volcanism, shallow-focus earthquakes, and normal faulting caused by tension.
Continental rifts - long, downfaulted rift valleys, basaltic volcanic activity, and shallow-focus earthquakes (East Africa, Red Sea, Gulf of California)
Convergent Boundary (deep-sea trench, mountain range, and magmatic belts)
Deep-sea trench (where overridden plate is subducted), adjacent mountain range of folded and faulted rocks, and magmatic rocks. Shallow- and Deep-focus earthquakes
Island Arc - two oceanic plates (western Pacific), earthquakes and volcanoes.
Nazca and South American plates - oceanic under continental plate. Earthquakes, volcanoes and compression
Himalayas - Eurasian plate is overriding Indian plate creating a double thickness of continental crust and the highest mountain range in the world.
Transform Boundary
Plates slide past each other producing earthquakes (San Andreas). Transform faults occur where the continuity of a divergent boundary is broken and offset.
Combination of plate boundaries - North America bounded by Mid-Atlantic Ridge, San Andreas transform fault, and Oregon to Aleutians subduction zones.
Rates of Plate Motion - 1 to 15 cm/yr
Magnetic Anomaly Patterns - Seafloor as a Magnetic Tape Recorder. Research ships measure the local magnetic field create by magnetized rocks beneath the sea.
Long, narrow bands of positive and negative anomalies showed an almost perfect symmetry around the crest of mid-ocean ridges
Positve and negative bands correspond to bands of rock that were magnetized during ancient episodes of normal and reversed magnetism of Earth's field.
Magma flowing up the crack at mid-ocean ridge solidifies and becomes magnetized in the direction of the Earth's field. As the seafloor splits, approximately half of the newly magnetized material moves to each side of the ridge.
Seafloor imprints the history of the opening of oceans
Inferring Seafloor Ages and Spreading Velocity - Velocity of plate is distance between two points divided by the difference in age (determined from magnetic reversal patterns). Mid-Atlantic ridge - 2 cm/yr; East Pacific rise - 12 cm/yr.
Deep-Sea Drilling - cores of sediments and oceanic crust.
Age of sediments determined from foraminifera fossils. Age of oldest sediments increases with distance from the ridges.
Isochrons - bands or lines of equal age
Seafloor becomes progressive older on both sides of the ridge.
More widely spaced isochrons of the eastern Pacific signify faster spreading rates than in the Atlantic.
Fast moving plates are connected to subducting slabs. Negative buoyancy of slab pulls plate after it.
Slow moving plates contain large continents. Drag of embedded continents
Geometry of Plate Motion
Transform boundaries reveal the direction of plate movement
Isochrons reveal the positions of plates in earlier times. Bring together isochrons of the same age on either side of the ridge to show position of continents at that time.
Hot spot tracks (string of extinct volcanoes) record the motion of an individual plate (e.g., Pacific plate over Hawaiian hotspot).
Satellite Measurement of Plate Motions
Orbiting satellite bounces radar beam off the surface of the ocean. Altimetry mapping can show trenches, ridges, and transform boundaries
Global Position System (GPS) - 24 satellites with atomic clocks. Time difference between radio signals from five satellites can locate a receiver to within one centimeter or so.
Plate Reconstructions
Gondwanaland - present day southern continents (Africa, s. America, Australia, and Antarctica) formed a single giant continent near the south pole about 450 Ma.
Pangaea Forms - supercontinent "all lands" that forms when paleo-Tethys ocean closes and Europe, North America, Siberia, and China crash into Gondwanaland around 240 Ma. This forms Appalachian mountains in eastern U.S.
Breakup of Pangaea - supercontinent began to break up about 200 Ma with opening of North Atlantic and closing of Tethys ocean as Eurasia rotates clockwise towards Gondwanaland. South Atlantic opens about 150 Ma.
End of the Age of Dinosaurs - North and South Atlantic oceans open. Gondwanaland breaks up into individual present continents. Tethys ocean continues to close as Eurasia rotates clockwise and Africa and India move northward.
Present Day - Atlantic ocean fully open. Tethys almost gone. Mediterranean sea is all that is left. Himilayan mountains form as India crashes into Asia.
50 m.y. in the future - Mediterranean sea completely closed forming major mountain range. Convergent boundaries form along east coast of North America and South America as Atlantic ocean basin starts to close.