Plate Tectonics

Early Ideas About Continental Drift

        Several scientists and thinkers have concluded that the continents were not always in their current locations:

• Sir Francis Bacon (1620) - Noted similarity of the shapes of shorelines of eastern South America and western Africa. Did not propose that the continents were once joined.
• Antonio Pellegrini (1858) - Suggested that continents were all linked together during the Pennsylvanian and later spilt apart, based on the similarities between plant fossils found in Pennsylvanian-aged coal beds of North America and Europe. Separation of continents attributed to biblical deluge.
• Edward Suess (late 1800s) - Noted similarities between Paleozoic plant fossils of India, Australia, Africa, S. America and Antarctica and evidence for glaciation on these landmasses. Proposed these landmasses were all once joined together in a supercontinent (Gondwanaland or Gondwana) by land bridges. Land bridges later sank beneath the ocean.
• Frank Taylor (1910) - Explained formation of mountain ranges as the result of the lateral movement of continents. Envisioned present-day continents as part of larger landmasses that were in polar locations. Huge tidal forces resulting from the Earth's capture of the moon caused the Earth's rotation to slow and broke apart the polar continents. Continents resulted from polar landmass fragments migrating toward the equator. Proposed that the Mid-Atlantic Ridge (discovered by H.M.S. Challenger expeditions 1872-1876) marked the site along which an ancient continent broke apart to form the modern Atlantic Ocean.

        Wegener proposed that the continents were once joined in one supercontinent (Pangaea), which began breaking up 200 million years ago. Produced maps showing movement of continents to their current locations. The hypothesis was refined by Alexander du Toit (1937) who proposed that Pangaea initially split into two large land masses: Gondwanaland in the Southern Hemisphere and Laurasia (N. America, Greenland, Europe, and most of Asia) in the Northern Hemisphere, based on similar ages for glacial deposits on Gondwana landmasses and coal deposits on Laurasia landmasses.

        All evidence listed above was attacked by different geologists as being inconclusive. Major evidence of continental drift were in the Southern Hemisphere. Most geologists lived in the Northern Hemisphere. Review of hypothesis was conducted in an international symposium held by the American Association of Petroleum Geologists in 1928. Idea was largely abandoned by 1929 because of the lack of a reasonable driving mechanism for continental motion (Wegener believed that continents plowed their way through oceanic crust under tidal energy).

• Fit of Continents - Continental slopes of S. America and Africa at 2,000 meters depth match in shape like pieces of a jigsaw puzzle.
• Rock and Structural Similarities - Mountain chains that end abruptly at the edge of a continent can be traced to continue on another  continent on the other side of an ocean. Rock types and ages correlate across continents, as well as faults.
• Paleoclimates - Rock deposits restricted to certain climates  (glacial till, coal, evaporites) have been found in areas having very
• different climates today. Glacial deposits of Paleozoic age covered sections of S. America, Africa, India, Antarctica, and Australia (Gondwanaland). Other rock deposits (coal and evaporites) show matches across the Northern Hemisphere continents (Laurasia) and indicate a tropical climate (location near equator).
• Fossil Evidence - Most compelling of the older evidence for continental drift. Uniform plant fossils of distinct type (Glossopteris floral) have been found in equivalent-aged rock on the Gondwanaland continents only. Seeds were too large to have been wind-carried across oceans and present-day climates are too varied. Restricted animal fossils: Mesosaurus (a freshwater reptile) and 2 land-dwelling reptiles, also point to linkages of S. America, Africa, and Antarctica.

        New techniques developed in the 1950s caused a resurgence of interest in continental drift:

• Paleomagnetism (Natural Remnant Magnetism) - Iron-bearing rocks contain minerals like magnetite which act as fossil compasses. The Earth's magnetic field orientation is "frozen in" these iron-bearing minerals when igneous rocks are cooled below the Curie Point (about 580 degrees C for magnetite). Later movement of the rock can be determined by the orientation of the current magnetic field compared to the rock's paleomagnetism. The declination (angle away from true north) of the paleomagnetism gives the longitude that the rock formed at, and the inclination (tilt from horizontal) gives the latitude.
• Polar Wandering - Study of paleomagnetism of rocks showed that either (1) the magnetic poles have moved dramatically in the Earth's past, or (2) that the continents have changed position relative to stationary magnetic poles, or (3) that both continents and magnetic poles have moved. The only way to reconcile the apparent paleomagnetic inconsistency and produce only one N and S pole (rather than different poles for each continent) is explanation (2). When the continents are fitted together, the paleomagnetic data points to one magnetic pole each in the Northern and Southern Hemispheres.

        Detailed mapping of the ocean floors began in the 1950s using echo-sounding techniques. Allowed topography of the
ocean floor to be determined. Studies revealed:

1. Mid-ocean ridge and rift valley with associated volcanism.
2. Deep ocean trenches with associated earthquake activity.
3. Thinness of oceanic crust compared to continental crust.
4. Lack of compressive deformation in oceanic crust.
5. All dredged sediment samples were relatively young in age.
6. Sediment thickness increases away from the oceanic ridge.
7. Presence of guyots (submerged, flat-topped volcanoes) extending away from oceanic ridges.

        Magnetic mapping of the ocean floors began in the early 1960s and is now complete for most of the ocean floor. Discovered that parallel, symmetric stripes of normal and reversed polarity rocks lie on either side of the oceanic ridges. Reversed polarity rocks can also be found on the continents, which show that reversals of the Earth's magnetic field (N becomes S and vice versa) have occurred many times in the geologic past. Reversals may be caused by sunspot activity at times of decreased magnetic field intensity. Historical pattern of the Earth's magnetism has been determined for the past several million years by combining the techniques of radiocarbon dating with paleomagnetism. Ocean floor magnetic surveys show youngest crust is located adjacent to the spreading ridges and that the age of the crust increases with distance away from the ridge.

Sea-Floor Spreading

Theory that the ocean floors are spreading apart and moving away from the oceanic ridges (propelled by thermal convection cells in the mantle) and that ocean crust is created at oceanic ridges and destroyed by subduction in deep ocean trenches. Proposed by Harry Hess in 1962 to explain the topographic features of the ocean floor and supported or explained by:
 

• Paleomagnetism - Theory supported when Vine and Matthews (1963) explained the pattern of alternating stripes of normal and reversed polarity rocks on either side of the oceanic ridges using sea-floor spreading. Also explains increasing age of the oceanic crust with increasing distance from the ridge. Rates of sea-floor spreading varies from almost 2 cm/yr in the North Atlantic to more than 18 cm/yr in some sections of the Pacific oceanic ridge. Sea floor spreading provides a driving force for continental movement (continents ride on plates which are driven by the Earth's internal energy).
• Deep-Sea Drilling Project - Sea Floor Spreading has been confirmed by the Deep-Sea Drilling Project (DSDP began in 1968) which has obtained many sediment cores from the ocean floor has shown that the ocean basins are very young features. This indicates that ocean crust is continually being created at the oceanic ridges, and old ocean crust is destroyed by subduction. Evidence includes:

1. No rocks older than 180 million years have been found.
2. Ocean sediments are absent at the ridge and get progressively thicker away from the ridge.
3. Ocean sediments get progressively older away from the ridge.
4. Rate of sea-floor spreading calculated from fossils is identical to that estimated from paleomagnetic data.

        Combines the concepts of continental drift and sea-floor spreading together into a unified theory of the Earth's dynamics. Explains many apparently unrelated geologic events and features. Elements of theory are:

• World System of Plates - The Earth's lithosphere is broken up into 6-7 major plates and about 14 minor ones. Oceanic plates are 50-100 km thick. Continental plates are 100-250 km thick. Tectonic plates can include both continental and oceanic areas. Six (or 7) major plates are:

• Plate Boundaries - Tectonic plates interact in various ways as they move across the asthenosphere, producing volcanoes, earthquakes and mountain systems. Types of boundaries are:

• Divergent - Plates move away from one another, creating a tensional environment. Characterized by shallow-focus earthquakes and volcanism. Release of pressure causes partial melting of mantle peridotite and produces basaltic magma. Magma rises to surface and forms new oceanic crust. Occur in oceanic crust (oceanic ridges) and in continental crust (rift valleys). Continental rift valleys may eventually flood to form a new ocean basin.
• Convergent - Plates move toward one another, creating a compressional environment. Characterized by deformation, volcanism, metamorphism, mountain building, seismicity, and important mineral deposits. Three possible kinds of convergent boundaries:
1. Oceanic-Oceanic Boundary - One plate is subducted, initiating andesitic ocean floor volcanism on the other. Can eventually form an island arc volcanic island chain with an adjacent deep ocean trench. Characterized by a progression from shallow to deep focus earthquakes from the trench toward the island arc (Benioff zone). May also form a back-arc basin if subduction rate is faster than forward motion of overriding plate.
2. Oceanic-Continental Boundary - Oceanic plate is dense and subducts under the Lighter continental plate. Produces deep ocean trench at the edge of the continent. About half the oceanic sediment descends with the subducting plate; the other half is piled up against the continent. Subducting plate and sediments partially melt, producing andesitic or granitic magma. Produces volcanic mountain chains on continents called volcanic arcs and batholiths. Part of the oceanic plate can be broken off and thrust up onto the continent during subduction (obduction). Obduction can expose very deep rocks (oceanic crust, sea floor sediment, and mantle material) at the surface. Characterized by shallow to intermediate focus earthquakes with rare deep focus earthquakes.
3. Continental-Continental Boundary - Continental crust cannot subduct, so continental rocks are piled up, folded, and fractured into very high complex mountain systems. Characterized by shallow-focus earthquakes, rare intermediate-focus earthquakes. and practically no volcanism.
• Transform - Plates move laterally past one another. Largely shear stress with lithosphere being neither created nor destroyed. Characterized by faults that parallel the direction of plate movement, shallow-focus earthquakes, intensely shattered rock, and no volcanic activity. Shearing motion can produce both compressional stress and tensional stress where a fault bends. Transform faults occur on land, connect segments of the oceanic ridge, and provide the mechanism by which crust can be carried to subduction zones.

        Plates move in different directions, change direction over time, and move at different rates. The Pacific and Cocos plates are the fastest moving and the Arabian and southern African plates are slowest. The rates and directions of lithospheric plate movement can be calculated in several ways:

1. Date Ocean Sediments - Determine age of sediments at a point and divide by distance from ridge. Gives average rate of movement, but no direction. Least accurate method.
2. Magnetic Reversals - Date magnetic reversals and divide by distances from ridge. Gives both average rate of movement and relative motion during the past. Wider stripes indicate faster plate motion.
3. Satellite Laser Ranging Techniques - Laser beams from a station on one plate are bounced off a satellite and returned to a station on another plate. With divergent plate movement, the laser beam takes more time to reach the receiving station. Time difference is used to calculate the rate of movement and relative motion of the plates.
4. Quasar Ranging Technique - Difference in arrival times of signal from a quasar to receiving stations on different plates. Calculated rates and relative directions of plate motion correlate well with those determined from magnetic reversals.
5. Hot Spots - Hot spots provide a fixed reference point that allows the absolute (rather than relative) direction of motion to be determined.

        The forces that drive the motion of plates are assumed to be associated with the Earth's internal heat and involve
flow of material in the asthenosphere. Various mechanisms have been proposed:
 

• Convection Cells - Thought to be primary driving force for plate motion. Unequal heat distribution in the mantle may produce

convection cells below the lithosphere. Hot material rises (correlates to spreading center), spreads laterally, cools and sinks deeper into the mantle to be reheated. Two convection cell models:
 
1. Shallow Convection Cell Model - Convection cells are restricted to the asthenosphere. Difficult to explain the source of heat for convection and the reason convection is restricted to the asthenosphere.
2. Deep Convection Cell Model - Entire mantle is involved in convection. Outer mantle is source of heat. Problem explaining how convection involved both the asthenosphere and lower mantle and how heat is transferred from the outer core to the mantle.
• Mantle Plumes (not in book) - Hot spots, or plumes, of hot rising mantle material are known to exist around the world. Hot spots occur primarily at spreading centers (black smokers), although a few occur in the centers of oceanic plates and result in the formation of volcanic island chains. Hot plume upwarps overlying lithosphere which cracks and moves laterally away from the plume. Downward flow of the mantle must occur somewhere to balance the upward flow in the plumes.
• Push-Pull Model - Lithospheric plates are pushed apart at hot spreading centers. Cold lithospheric plates are dense and tend to sink into the mantle, pulling the rest of the plate with it. Each part of the model can operate independently and are gravity driven.
• Expanding Earth (not in book) - Model holds that the Earth has expanded through its history, so that overall new crust is being created at spreading centers. Has few supporters. Would require a 50% increase in the volume of the Earth over the last 200 million years.