Continental Drift PLATE TECTONICS E.B. Taylor (1910) and Alfred Wegener (1915) published on Continental Drift. Continental Drift Wegener s evidence 1. Fit of the Continents 2. Fossil Evidence 3. Rock Type and Structural Similarities 4. Paleoclimatic Evidence Continental Drift- Wegener s Evidence In the 1960's, it was recognized that the fit of the continents could be even further improved by fitting the continents at the edge of the continental slope the actual extent of the continental crust. Continental Drift Continental Drift Wegener s Evidence, fossils Wegener s evidence Fossils - Mesosaurs 1
Continental Drift Wegener s Evidence Rocks and Geologic Structures Same rock types Same order Same faults and folds Same age 300my old orogeny Continental Drift- Wegener s Evidence Climates Evidence of glaciation Unlikely places Better explained by fit Continental Drift Wegener s Evidence Climate Coal Deposits Should be in warm temperate areas or equatorial areas Better explained by fit Continental Drift Wegener lacked a mechanism for continental movement through oceanic crust. Plate Tectonics Evidence from exploration of the ocean floor Global oceanic ridge system and guyots Paleomagnetic stripes Polar wandering Deep ocean trenches associated with earthquakes All seafloor less than 180my old Sediments thin where expected to be thick 2
Plate Tectonics The Ocean Floor Harry Hess (1906-1969) Surveyed ocean floor Discovered guyots Hypothesized seafloor spreading Plate Tectonics the ocean floor The mid-ocean ridge system is a nearly continuous volcanic ridge found in all oceans. Sea Floor Spreading There is nearly continuous volcanic activity somewhere along the mid-ocean ridge system. Undersea volcano photographed from Japanese aircraft The Ocean Floor - guyots Sea Floor Spreading A process known as seafloor spreading occurs where magma from the mantle wells up into the divergent boundary - forming new basaltic seafloor. Spreading rates average ~5 cm/year. 3
Sea Floor Spreading The mid-ocean ridge has an elevated position on the seafloor because it is formed from relatively hot igneous rocks. Plate Tectonics- Sea Floor Spreading As the seafloor moves away from the ridge, it cools and contracts thus the seafloor generally is at a greater depth as you move away from the midocean ridge. Sea Floor Spreading Plate Tectonics Sea Floor Spreading Fred Vine and D.H. Mathews Interpreted magnetic stripes Earth has experienced periods of reverse polarity As magma solidifies it is magnetized according to the polarity at the time Stripes of magnetic polarity are formed at midoceanic ridges Magnetic reversals Magnetic Stripes The figure shows that the Earth s magnetic field is currently oriented so that magnetic lines of force are entering the Earth near the north pole. It has been recognized that the Earth s magnetic field has reversed polarity many times in the past - known as magnetic reversals. In the figure, regions of normal polarity are indicated in white where magnetic north is coincident with the geographic north. 4
Magnetic Stripes Stripes are parallel to midoceanic ridge Stripes are mirror images across ridge Dates of stripes indicate sea floor spreading Evidence: Polar Wandering Keith Runcorn PLATE TECTONICS Sea-floor spreading provided the missing mechanism for Continental Drift Plate Tectonics If the sea floor is spreading, and the earth is not expanding, where does the excess crust go? Subduction zones Subduction Zones Studies of crust around deep sea trenches showed zones of earthquakes beneath the crust called Wadati-Benioff Zones 5
Heat and heat flow studies around trenches revealed a cold slab Plate Tectonics Subduction Zones Earthquake epicenters revealed plate boundaries. The surface expression of a subduction zone is a deep-ocean trench - these trenches maybe thousands of km long, 50-100 km wide, and 8-12 km deep. Plate tectonics is a theory about how the surface of the Earth evolves due to strong internal forces. The surface of the Earth is composed of rigid plates that are mobile and move relative to one another. Plate tectonics is a unifying theory in Geology different geologic phenomena such as mountain building, earthquakes, volcanoes, and the distribution of fossils and organisms can be explained through plate tectonics. Earth s Major Plates The Earth s surface is composed of a strong, rigid layer known as the lithosphere. The lithosphere is broken into pieces known as tectonic plates. Lithospheric plates are thinnest in the oceans (<100 km thick) and may be more than 250 km thick on the continents. There are 7 major plates and over a dozen smaller plates. 6
The lithospheric plates overlie a weaker region of the mantle known as the asthenosphere. Note that the plates generally include a continent or a portion of a continent AND a portion of the ocean floor. Each plate moves as a coherent unit relative to others The rocks in the asthenosphere are near their melting point and are relatively weak and ductile. The asthenosphere allows the plates to move above it. Plates move slowly but continuously - generally on the order of a ~5 cm/year. Plate Tectonics- Plate Boundaries Plate Boundaries are where earthquakes, volcanoes, and crustal deformation take place. There are three general types of boundaries: Divergent Plates move away from each other Convergent- Plates move toward each other Transform Plates move past each other Divergent Boundaries Most divergent boundaries are located along mid-ocean ridges. Divergent plate boundaries are known as constructive margins because they are the site where new oceanic crust (lithosphere) is generated. The process known as seafloor spreading occurs where magma from the mantle wells up into the divergent boundary - forming new basaltic seafloor. Spreading rates average ~5 cm/year. Some divergent boundaries occur under continental crust This figure illustrates the different major types of plate boundaries. We just considered two types of divergent boundaries: mid-ocean ridges and continental rifts. Now we will consider the different types of convergent boundaries. The East African Rift represents a modern example of a continental rift. If this rift is successful, eastern Africa may split off from the rest of the continent and a new ocean basin may form between the two Africas. 7
Convergent margins are also known as destructive margins since oceanic crust is destroyed or consumed. Most convergent margins are associated with a subduction zone The map shows the world s oceanic trenches. Note that the Pacific is Nearly encircled in deep-ocean trenches. Convergent Boundaries: Oceanic-Continental Convergence Oceanic-continental convergence occurs when leading edge of one plate is composed of continental rocks (granitic) and the other is oceanic (basaltic). The denser oceanic plate dives beneath (subducts) the lower-density continental plate. Lower density granitic rocks tend to float in the asthenosphere. Dewatering of the subducted slab causes melting in the wedge of the asthenosphere above it. The magma that is produced is buoyant and rises through the mantle toward the Earth s surface. The magma that is produced in the asthenosphere is basaltic in composition. As the magma rises, it must penetrate through the thick continental (granitic) rocks. As it assimilates the continental rocks, the composition of the magma changes from mafic to intermediate. The magma results in volcanic activity along a line parallel to the subduction zone known as a continental volcanic arc. Examples of continental volcanic arcs include the Cascade volcanoes such as Mt. Ranier and Mt. St. Helens and the volcanoes of the Andes mountains along the west coast of South America. Mt. St. Helens Convergent Boundaries: Oceanic-Oceanic Convergence Oceanic-oceanic convergence occurs when the leading edge of both plates consists of oceanic crust. These plate boundaries have many of the same features as in oceaniccontinental convergence. In oceanic-oceanic convergence, the line of volcanoes forms a string of islands parallel to the subduction zone known as a volcanic island arc. Examples of island arc systems include the Aleutian Islands, Tonga, Indonesia, and Japan. Ocean to Ocean Convergence NOTE: Oceanic crust subducts other oceanic crust Volcanoes form islands Composition of magma is mafic to intermediate 8
Continent-continent convergence usually begins as oceaniccontinental convergence (ex. Andes). As the oceanic crust is subducted, a continental block on the subducting plate may approach the continent. The Himalayan mountains were formed by the collision of the Indian subcontinent into the Asian mainland. These figures show the convergence of India into Asia over the last 71 million years. Continent to Continent Collision NOTE: Uplifted continental crust No volcanoes Convergent Boundaries: Continental-Continental Convergence Continental-continental convergence defines a plate margin where the leading edge of both plates contains continental crust. This type of plate boundary is associated with mountain-building. Folding and faulting Transform Fault Boundaries Transform plate boundaries are where plates slide past one another. Most transform boundaries are associated with mid-ocean ridges where they form linear breaks in the ridge system. The active transform boundary exists between the two offset ridge segments Transform Faulting 9
Evidence Supporting Plate Tectonics Evidence: Patterm of volcanoes, trenches, ridges, earthquakes. Overall pattern of volcanic and earthquake activity Polar Wandering Evidence from Deep Sea Drilling Evidence from Hot Spots Evidence: Ocean Drilling An active program of sampling and drilling in the seafloor has provided considerable evidence in support of plate tectonics. The figure shows the age of the seafloor. The pattern is as is predicted by the theory of plate tectonics. The seafloor is very young at the mid-ocean ridges and gets progressively older as a function of distance from the ridge. Evidence: Ocean Drilling The figure shows the thickness of sediment on the seafloor throughout the ocean basins. The seafloor at the mid-ocean ridges is young and has essentially no sedimentary cover. Generally, the sedimentary cover increases with distance from the mid-ocean ridge. Evidence: Hot Spots Mapping of the seafloor indicates that there are linear chains of volcanic islands structures known as seamounts. The Hawaiian Islands are part of a chain of islands and seamounts extending to the Aleutian trench. The Big Island of Hawaii is the only island in the chain with active volcanism. Radiometric dating of the volcanic rocks of this chain indicate that they get progressively older as a function of distance from the Island of Hawaii. 10
The origin of the volcanic islands and seamounts of this and other chains is from an anomalously hot portion of the mantle known as a hot spot that remains relatively stationary. As the Pacific plate moves over the hot spot, volcanoes are formed from magmas generated by the hot spot. As the plate continues to move, the volcano moves off of the hot spot and becomes extinct but is replaced by a new one directly above the hot spot. These observations are consistent with the theory of plate tectonics and support it. The Breakup of Pangaea Now that we understand plate tectonics, we can use geologic data to reconstruct Pangaea and model the movement of the continents during the last 200 million years. By about 150 m.y. ago, the N.Atlantic began to open. ~130 m.y. ago, the S. Atlantic was opening and India began its journey north toward Asia. ~44 m.y. ago, India began to collide with Asia forming the Himalayan Mtns. Continental Drift animation What Drives Plate Motions? We have described plate motions but have not really defined the forces that drive the plates to move. This is an active area of research and there is a diversity of opinions. Most geologists agree on the following points about the driving forces for plate motion: 1. The Earth s mantle is convecting - hotter rocks rise buoyantly and cooler denser rocks sink. This motion helps drive plate motion. 2. Mantle convection and plate tectonics are part of the same system. 3. Density differences due to the unequal distribution of heat within the Earth s mantle ultimately drive the mantle convection cells and plate motion. 11