Lecture Outlines Physical Geology, 15/e Plummer, Carlson & Hammersley
Plate Tectonics: The Unifying Theory Physical Geology 15/e, Chapter 19
Plate Tectonics Plate Tectonics Earth s surface is composed of a few large, thick plates that move slowly and change in size Plate boundaries plates move away, toward, or past each other through intense geologic activity Developed from the continental drift and seafloor spreading hypotheses
The Early Case for Continental Drift Puzzle-piece fit of coastlines of Africa and South America has long been known Alfred Wegener noted South America, Africa, India, Antarctica, and Australia have almost identical late Paleozoic rocks and fossils in early 1900s Glossopteris (plant), Lystrosaurus and Cynognathus (animals) fossils found on all five continents Mesosaurus (reptile) fossils found in Brazil and South Africa only
The Early Case for Continental Drift Pangaea supercontinent proposed by Wegener Laurasia - northern supercontinent containing North America and Asia (excluding India) Gondwanaland - southern supercontinent containing South America, Africa, India, Antarctica, and Australia Late Paleozoic Glaciation patterns were evident on the southern continents (Gondwanaland) Coal beds deposited in the northern continents from swampy, probably warm environments (Laurasia)
The Early Case for Continental Drift Paleoclimate belts suggested polar wandering as potential evidence for continental drift. Polar Wandering the apparent movement of the poles The evidence for this can be explained in three ways: 1).The continents remained motionless and the poles actually moved (polar wandering) 2). The poles did not move and the continents moved (continental drift) 3). Both occurred
The Early Case for Continental Drift Skepticism About Continental Drift Land Bridges could explain the distribution of land-dwelling reptiles on scattered continents Winds or Oceans currents could explain distribution of fossil plants on separate continents Polar Wandering could be explained by moving poles rather than moving continents Mechanism Wegener s proposed mechanism was not accepted by most geologists in the northern hemisphere His idea that the continents plowed through the oceans propelled by centrifugal forces from the Earth s rotation and gravitational forces that cause the tides were proven to be insufficient
The Revival of Continental Drift Evidence from Paleomagnetism Paleomagnetism the study of ancient magnetic fields The mineral magnetite becomes magnetized in cooling lava once its temperature drops below the Curie Point Uses mineral magnetic properties to determine direction and 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
The Revival of Continental Drift Evidence From Paleomagnetism Apparent polar wander curves for different continents suggest real movement relative to one another Permian rocks in every continent show a different pole position which seems highly unlikely By reconstructing their locations to form Pangaea, the polar wandering paths are nearly identical indicating that the continents were once joined together.
The Revival of Continental Drift Geologic Evidence for Continental Drift Fitting of Continents Redefined the edge of each continent as the middle of the continental slope greatly improved the fit Isotopic ages, glacial striations, rock types, structure and sequence match History of Continental Positions Rock matches show when the continents were together Paleomagnetic data indicate the direction and rate of movement Pangaea split apart 200 million years ago but the continents have been in motion for much longer (2-4 billion years)
Seafloor Spreading Seafloor spreading the concept that the sea floor is moving like a conveyor belt away from the crest of the mid-oceanic ridge until it disappears by plunging beneath a continent or island arc. Proposed in 1962 by Harry Hess Hess s Driving Force Deep Mantle Convection circulation pattern driven by rising of hot material (hot mantle rock) and/or the sinking of cold material (oceanic crust)
Seafloor Spreading Explanations The Mid-Oceanic Ridge Hot mantle rocks rises beneath Decompression melting occurs Circulation pattern diverges moving rock away from the ridge Rift valley forms due to tensional forces at the ridge crest Oceanic Trenches Rock has cooled and become denser Crust Sinks beneath a continent or island arc back into the mantle Age of the Sea Floor Overall young age for sea floor rocks (everywhere <200 million years) is explained by this model
Plates and Plate Motion Tectonic plates composed of the relatively rigid lithosphere Lithospheric thickness and age of seafloor increase with distance from midoceanic ridge Float upon ductile asthenosphere Interact at their boundaries Divergent boundaries plates move apart Convergent boundaries plates move together Transform boundaries plates slide past one another
How Do We Know That Plates Move? Marine magnetic anomalies alternating positive and negative magnetic anomalies that form a stripelike pattern parallel to the mid-oceanic ridges The Vine-Matthews Hypothesis New basaltic magma continually extrudes at the ridge crest and cools to record the earth s magnetism including magnetic field reversals Matches pattern of reversals seen in continental rocks which allows us to measure the rate of movement and to predict the age of the sea floor
How Do We Know That Plates Move? Marine Magnetic Anomalies Measuring the Rate of Motion Compare to known magnetic reversals from lava flows on land Rate of plate motion equals distance from ridge divided by age of rocks Predicting Seafloor Age Seafloor age increases with distance from mid-oceanic ridge Symmetric age pattern reflects plate motion away from ridge
How Do We Know That Plates Move? Another Test: Fracture Zones and Transform Faults Mid-oceanic ridges offset along fracture zones transform fault fracture zone segment between offset ridge crests relative motion along fault is result of seafloor spreading from adjacent ridges Measuring Plate Motion Directly Plate motion can be measured using satellites, radar, lasers and global positioning systems
Divergent Plate Boundaries Divergent plate boundaries plates move away from each other can occur in the middle of the ocean or within a continent marked by rifting, basaltic volcanism, and eventual ridge uplift eventually creates a new ocean basin
Transform Plate Boundaries Transform plate boundaries plates slide horizontally past one another Transform faults may connect: two offset segments of mid-oceanic ridge a mid-oceanic ridge and a trench two trenches transform offsets of midoceanic ridges allow series of straight-line segments to approximate curved boundaries required by spheroidal Earth
Convergent Plate Boundaries Convergent plate boundaries plates move toward one another Ocean-ocean plate convergence marked by ocean trench, Benioff zone, and volcanic island arc Ocean-continent plate convergence marked by ocean trench, Benioff zone, volcanic arc, and mountain belt Continent-continent plate convergence marked by mountain belts and thrust faults
Do Plate Boundaries Move? Plate boundaries can move over time Mid-oceanic ridge crests can migrate toward or away from subduction zones or abruptly jump to new positions Convergent boundaries can migrate if subduction angle steepens or overlying plate has a trenchward motion of its own Transform boundaries can shift as slivers of plate shear off
Can Plates Change Size? Plates can change size over time. North American plate is increasing in size New sea floor is being added on the trailing edge as the Atlantic sea floor spreads Most of the plate is not being subducted The Nazca Plate is getting smaller Leading edge is being subducted under South America Trailing edge is adding sea floor but at a slower rate The Attractiveness of Plate Tectonics Explains distribution and composition of volcanoes, earthquakes and mountain belts. Explains the major features of the sea floor
What Causes Plate Motions? Any proposed mechanism must explain why: mid-oceanic ridges are hot and elevated, while trenches are cold and deep ridge crests have tensional cracks the leading edges of some plates are subducting sea floor, while others are continents (which cannot subduct) Mantle convection may be the cause or an effect of circulation set up by ridge-push and/or slab-pull
What Causes Plate Motions? Ridge Push as new plate moves away from the divergent boundary it cools and thickens and subsides Slab Pull cold lithosphere sinking at a steep angle through the hot mantle should pull the surface part of the plate away from the ridge crest. Trench Suction if subducting plates fall into the mantle at angles steeper than their dip then trenches and the overlying plates are pulled horizontally seaward toward the subducting plate.
What Causes Plate Motions? Mantle Plumes and Hot Spots Mantle Plume - narrow columns of hot mantle rock that rise through the mantle Stationary with respect to moving plates Large mantle plumes may spread out and tear apart the overlying plate forming a Hot Spot at the Earth s surface (examples include Hawaii, Yellowstone and Iceland) flood basalt eruptions rifting apart of continental land masses New divergent boundaries may form
What Causes Plate Motions? Mantle Plumes and Hot Spots Mantle plume hot spots in the interior of a plate produce volcanic chains Orientation of the volcanic chain shows direction of plate motion over time Age of volcanic rocks can be used to determine rate of plate movement Hawaiian Islands are a good example
A Final Note Objections to Plate Tectonics Some seafloor objects did not seem compatible with a moving sea floor. The geology of many continental regions did not seem to fit the theory Refinements of the theory made these feature more compatible with theory The evidence for Plate Tectonics is very convincing and has changed the whole concept of Earth dynamics in the last fifty years.
End of Chapter 19