Continental Drift Take a look at a globe sometime and observe the remarkable fit between South America and Africa. Could they have, in fact, been connected? During the 19th and early 20th centuries, several geologists explored the idea of moving continents by observing the possible “fit” between Africa and South America. In 1912 Alfred Wegener, a lecturer in astronomy and meteorology, hypothesized that the earth’s continents were all together at one time, forming the supercontinent Pangaea, and then they broke apart, drifting through the ocean floor to produce the present-day continental configurations. This is known as the Continental Drift Hypothesis. Wegener supported his Continental Drift Hypothesis with fossil evidence and lithological correlation. For example, Wegener correlated several land type fossils with other land type fossils on different continents. Additionally, Wegener pointed out remarkable lithologic characteristics that matched with those of other continents. Continental drift was not immediately accepted by Wegener’s peers when he published his findings in 1915. Critics of continental drift indicated that his evidence was not substantial, and Wegener could not explain how continents move. In fact, critics bitterly attacked Wegener, destroying his teaching career and credibility as a scientist. During the early 1900’s, two primary viewpoints or doctrines were currently accepted, thus dismissing continental drift. Permanentists believed that continents and basins are virtually unchanged and continents remained stationary. Contractionists explained earth features using the Shrinking Earth Hypothesis, which suggested that gradual contraction of the solid earth allowed the ocean floor to become dry land and dry land to become the ocean floor. The Birth of Plate Tectonics Alfred Wegener died in 1935, found frozen in the Artic. However, some scientists stayed on board supporting Wegener’s Continental Drift Hypothesis. In 1947, technology allowed the mapping of the Atlantic Ocean floor and led to the discovery of a linear mountainous terrain with abundant volcanic and earthquake activity. It was learned that this mountain range was produced by volcanism in which lava pushes up between the mountains, spreading apart the ocean floor. This is known as sea floor spreading or referred to as a spreading ridge. This particular ridge is called the mid-Atlantic ridge. Magma rises and moves in between the ocean plates, producing volcanism and earthquakes. Lava extrudes on both sides of the ridge, spreading the plates apart. This is known as a spreading center. Paleomagnetism By the 1960’s, scientists discovered a series of linear magnetic anomalies or magnetic stripes located on each side of the mid-Atlantic ridge. In other words, the magnetic stripes on one side of the ridge matched magnetic stripes on the other side of the ridge. These stripes possessed alternating magnetic orientations which reflected the earth’s magnetic polarity at the time of ocean floor formation. These are known as magnetic reversals. The data and observation of these magnetic reversals show that the sea floor is spreading to produce stripes on both sides of the ridge, giving a mirror pattern. The discovery of these geometric magnetic patterns is referred to as paleomagnetism. Paleomagnetism demonstrates that continents are on the move and provides the pivotal evidence necessary to explain the theory of plate tectonics. Wouldn’t Alfred Wegener have been proud? A B C In each diagram A,B, and C, the earth’s crust is slowly spreading. This is reflected by the magnetic reversal stripes producing the same pattern on either side of the spreading center. This is known as paleomagnetism. Which magnetic stripe is the oldest? Why? Plate Tectonics By the late 1960’s, scientists had joined together to create the plate tectonic model. The plate tectonic model is used to describe various geologic features, geological rock environments, and the pattern of volcanism as well as earthquake activity. According to the plate tectonic model, the surface of the Earth consists of a series of relatively thin but rigid plates which are in constant motion. The surface layer of each plate is composed of oceanic crust, continental crust, or a combination of both. The lower part consists of the rigid upper layer of the Earth's mantle. The rigid plates pass gradually downward into the plastic (soft) layer of the mantle, the astenosphere. The plates may be up to 70 km thick if composed of oceanic crust or 150 km thick if incorporating continental crust. Plates move at different velocities; the African plate moves about 25 mm per year whereas the Australian plate moves about 60 mm per year. Volcanic and earthquake activities primarily occur along plate boundaries. Here, plates may move away from each other, slam into each other, or grind past one another. There are three types of plate boundaries: divergent, convergent, and transform plate interactions. Divergent plate boundary The divergent boundary represents two plates moving away or separating from each other, hence the term divergent. At this type of boundary, new oceanic crust is formed in the gap between two diverging plates as magma rises and fills the gap. The divergent boundary is characterized by high-rising ridges at the boundary while plate material thins away from the divergent boundary. Presently, most divergent boundaries occur along the central zone of the world's major ocean basins. The process by which the plates move apart is referred to as sea floor spreading. The Mid-Atlantic Ridge and East Pacific Rise provide good examples of this type of plate margin. The divergent boundary is represented by two plates moving apart or away from each other. Magma rises, filling the gap and pushing the plates apart. Earthquakes and volcanic activity are common along the divergent boundary. Convergent plate boundaries The convergent boundary is represented by two plates moving toward each other, hence the term convergence. As the two plates converge, one plate typically slides beneath the other. The sliding plate descends below the overriding plate and is assimilated into the Earth’s upper mantle. The descending plate moves along the subduction zone before entering the mantle. In other words, it can be stated that one plate subducts beneath another. The point of subduction is typically marked by a deep ocean trench on the surface of the earth. A major consequence of plate subductiion is marked by deeply focused earthquakes along the subduction zone. Once the subducting plate reaches depths ranging between 90-150 km, it begins to melt forming magma. The less dense magma begins to rise (like a balloon in a deep swimming pool) and reaches the Earth’s surface, forming a volcanic arc or volcanic chain above the earth’s surface on the non-descending plate. There are three types of convergent plate interactions: ocean to continent, ocean to ocean, and continent to continent convergence. A B C Convergent boundaries showing plate subduction. There are three primary convergent boundaries: (A) ocean to continent, (B) ocean to ocean, and (C) continent to continent. Each convergent boundary produces various types of geologic features: (A): volcanic chain on the continent (B): volcanic arc rising above the ocean (C): high-rising mountains on the continent Transform Plate Boundaries The transform boundary is represented by areas where two plates are grinding or sliding past one another. In the area of the grinding or fracture zone, faults typically occur and are known as transform faults. Most transform faults are located on the ocean floor where they primarily offset spreading ridges (mid-ocean ridges). However, some transform boundaries are located on continental crust, creating transform faults that traverse long distances and result in “sliding” between sections of continental crust. The most famous example is the San Andreas Fault Zone (SAF). The SAF begins in the Gulf of California, traverses through southern and central California (approximately 70 miles west of Bakersfield), and exits north of San Francisco where it changes into offset spreading ridges on the ocean floor. Another example of a transform boundary on land is the Alpine Fault of New Zealand. Typically, one can expect recurring earthquake activity along the transform fault zone. Earthquakes are usually shallow due to the lack of subduction processes. Volcanic activity is not present because of the nature of plate motion. Below is a diagram illustrating the occurrence of a transform boundary and the location of the SAF: Transform Faults A Diagram (A) shows typical transform faults located on the ocean floor. Note that oceanic ridges are offset due to the “sliding” motion of transform boundaries. B Diagram (B) illustrates the geographical location of the San Andreas Fault zone. Note the arrows indicating the transform motion along the plate boundary. Will the Los Angeles Dodgers be city rivals to the San Francisco Giants someday? How do plates move? How does convection work? Convection describes the process in which either hot liquids or gasses rise from a heat source and cool when away from the heat source. The cooling material sinks and is again heated by the heat source, rising and then cooling. The repeated process creates circular motions of energy or convective cells. Below, a diagram illustrates convection in boiling water. Boiling Water (A) Hot water molecules rise to the surface. (B) Water molecules cool and begin to sink. (C) Convection cells are created. B C A Scientists hypothesize that convection processes deep within the earth drive the motion behind moving continents. The heat source is believed to be generated from leftover heat during Earth’s formation, radioactive decay of elements, and the effects of the geothermal gradient. Applying convective motions in the Earth’s interior, heat at the Earth’s lower mantle creates rising plumes of magma which move toward the surface. Once the plumes reach the upper mantle, they cool, causing a sinking effect. As the plume sinks, it “drags” the plate, which causes the plate to move. This is known as mantle convection. Is mantle convection uniform in the mantle? Most scientists consider convection within the mantle analogous to the characteristics of a lava lamp. Below are illustrations of mantle convection in an ideal earth and a lava lamp version. A B Diagram (A) shows mantle convection in which magma rises from the core (heat source) and sinks as it nears the upper mantle. Convection here illustrates a series of homogenous-type convective cells. Diagram (B) shows a more realistic view of mantle convection. Here magma plumes rise individually (as magma blobs) at different rates but still maintain the process of convection. This type of convection is observed in a typical lava lamp. NOTES: PART A – Plate Tectonic Definitions Below are various plate tectonic terms that must be mastered in order to perform successfully on lab tests as well as lecture tests: Continental Drift Hypothesis Permanentists Contractionists paleomagnetism divergent boundary convergent boundary subduction zone ocean trench transform boundary San Andreas Fault Zone convection mantle convection lava lamp PART B- Identification of plate boundaries In each space provided, draw a diagram that illustrates each plate boundary. In your diagram, label all pertinent characteristics, color the various characteristics, and identify at least two geographical regions that represent your plate boundary. Use text information from this lab and other text book sources. Divergent Boundary Convergent Boundary (O-O), (O-C), (C-C) Transform Boundary PART C- Plotting Earthquake Locations and Identifying Plate Boundaries Using longitude and latitude data (next page), plot the various locations of earthquakes. Note the location and depth of each earthquake. After the earthquakes have been plotted, determine what type of plate tectonic boundary is present. PART D – Critical Plate Tectonic Thinking Questions 1. What evidence do scientists observe or look for when proposing the convection hypothesis for the movement of continents? 2. Investigate what the Ring of Fire represents (not from the Johnny Cash song or a particularly distressing episode of diarrhea). What tectonic boundaries are involved? Where is the Ring of Fire located? 3. How is plate tectonic theory used to support the believed age of the earth? 4. Using the steps in the scientific method, how did scientists formulate the theory of plate tectonics? In other words, what observations led to hypotheses, experiments, and, finally, a theory? 5. What evidence do you think scientists use to determine the motion of the North American and Pacific plates along the San Andreas Fault?