Plate tectonics

Plate TectonicsIntroduction to Continental DriftAlfred Wegener and the Continental Drift Hypothesis

• Alfred Wegener first proposed his continental drift hypothesis in 1915 in his publication The Origin of Continents and Oceans.

• The hypothesis stated that a supercontinent called Pangaea began breaking apart approximately 200 million years ago.

• Wegener suggested that the continents "drifted" to their current positions over millions of years.

Evidence for Continental Drift

Wegener supported his hypothesis with several lines of evidence:

• Fit of the Continents: The coastlines of continents, particularly South America and Africa, appear to fit together like puzzle pieces.

• Fossil Evidence: Identical fossils of ancient organisms have been found on continents now separated by vast oceans. Examples include:

  • Remains of the freshwater reptile Mesosaurus found in both Brazil and Africa.

  • Fossils of the Triassic land reptile Lystrosaurus found in Africa, Antarctica, and India.

  • Fossils of the fern Glossopteris found on all the southern continents, indicating they were once joined.

• Rock Type and Structural Similarities: Similar rock types and geological structures (like mountain ranges) are found on continents that are now separated.

• Paleoclimatic Evidence: Evidence of past climates, such as glacial deposits in tropical regions or coal deposits (formed in warm, swampy environments) in colder regions, suggests that continents have moved to different climatic zones.

Objections to Continental Drift

Despite the evidence, Wegener's hypothesis faced significant opposition from the scientific community due to:

• Lack of a Mechanism: Wegener could not provide a convincing explanation for how the continents moved. He incorrectly suggested continents broke through the ocean crust like ice breakers.

• Scientific Skepticism: The established scientific consensus at the time did not readily accept such a radical idea without a plausible mechanism.

Paleomagnetism and Renewed InterestMagnetized Minerals in Rocks

• Renewed interest in continental drift emerged in the mid-20th century, largely due to studies of rock magnetism (paleomagnetism).

• Magnetized minerals within rocks record the direction of Earth's magnetic poles at the time the rocks formed.

• This magnetic orientation allows scientists to determine the latitude at which the rocks originally formed.

Polar Wandering

• Apparent Polar Wandering: By analyzing the magnetic orientation of rocks of different ages on various continents, scientists observed that the apparent position of the magnetic poles seemed to have "wandered" over time.

• Interpretation: This "wandering" was not due to the poles moving, but rather because the continents themselves had moved relative to the magnetic poles.

• Evidence from Different Continents: Polar-wandering paths calculated for North America and Europe showed similar trends but were offset. This discrepancy could be resolved by placing the continents next to each other, further supporting the idea of continental movement. For instance, the apparent polar path for Eurasia and North America, when plotted separately, shows a divergence that aligns when the continents are reconstructed to their Pangaean positions.

The Scientific Revolution: Seafloor Spreading and Plate TectonicsTechnological Advancements and Seafloor Mapping

• During the 1950s and 1960s, technological advancements allowed for extensive mapping of the ocean floor.

Seafloor Spreading Hypothesis

• Proposed by Harry Hess in the early 1960s, the seafloor spreading hypothesis suggested that new oceanic crust is formed at mid-ocean ridges and then spreads outwards.

Geomagnetic Reversals

• Earth's Magnetic Field Reversals: Earth's magnetic field periodically reverses its polarity, meaning the north magnetic pole becomes the south magnetic pole and vice versa.

• Dating Reversals: The timing of these geomagnetic reversals could be determined by studying lava flows on land.

• Recording in Ocean Crust: Geomagnetic reversals are recorded in the newly forming oceanic crust at mid-ocean ridges. As magma erupts and solidifies, the magnetic minerals align with the Earth's magnetic field at that time.

• Magnetic Stripes: In 1963, Vine and Matthews linked the discovery of symmetrical "magnetic stripes" on either side of mid-ocean ridges to Hess's concept of seafloor spreading. These stripes represent periods of normal and reversed magnetic polarity.

The Theory of Plate Tectonics

• The convergence of evidence from continental drift, paleomagnetism, and seafloor spreading led to the development of the theory of plate tectonics, which became the new paradigm in Earth science.

• Plate Tectonics: This theory describes the Earth's outer rigid layer, the lithosphere, as being broken into several large and small plates that move relative to each other.

Plate Tectonics: The New ParadigmEarth's Major Plates

• The lithosphere, consisting of the uppermost mantle and the overlying crust, is divided into approximately seven major lithospheric plates.

• These plates are in constant motion, changing in shape and size over geological time.

• The Pacific plate is the largest.

• Some plates comprise an entire continent along with a significant portion of the adjacent seafloor.

Plate Motion

• Plates move relative to each other at slow, continuous rates, typically around 5 centimeters (2 inches) per year.

• Cooler, denser oceanic lithosphere sinks into the mantle at subduction zones.

Plate Boundaries

Interactions among plates occur at their boundaries, which are classified into three main types:

• Divergent Plate Boundaries (Constructive Margins):

  • Occur where plates move apart.

  • Most are located along the crests of oceanic ridges (e.g., Mid-Atlantic Ridge).

  • Oceanic Ridges and Seafloor Spreading: At these boundaries, new oceanic crust is generated as magma rises from the mantle and solidifies. The rate of spreading influences the topography of the ridge system.

  • Continental Rifting: Divergent forces can also split continental landmasses, creating rift valleys (e.g., East African Rift Valleys).

• Convergent Plate Boundaries (Destructive Margins):

  • Occur where plates collide.

  • Older portions of oceanic plates are returned to the mantle at these boundaries.

  • Ocean Trenches: The surface expression of subduction is often an ocean trench.

  • Types of Convergence:

    • Oceanic-Continental Convergence: A denser oceanic plate subducts beneath a continental plate. This generates magma, leading to the formation of continental volcanic arcs (e.g., the Andes).

    • Oceanic-Oceanic Convergence: When two oceanic plates converge, one subducts beneath the other. This can form volcanoes on the ocean floor that may emerge as volcanic island arcs (e.g., Japan, Aleutian Islands).

    • Continental-Continental Convergence: When two continental plates collide, neither subducts significantly due to their buoyancy. The immense pressure causes crustal thickening and the formation of large mountain ranges (e.g., the Himalayas).

• Transform Fault Boundaries (Conservative Margins):

  • Occur where plates slide past each other horizontally.

  • Lithosphere is neither created nor destroyed.

  • Transform Faults: Most commonly connect segments of mid-ocean ridges, forming fracture zones. Some transform faults cut through continental crust, posing significant seismic hazards (e.g., the San Andreas Fault).

The Wilson Cycle

• The Wilson Cycle describes the opening and closing of ocean basins over geological time, driven by plate tectonic processes, from initial continental rifting to eventual continental collision.

Measuring Plate MotionHot Spots and Mantle Plumes

• Hot Spots: Areas of volcanic activity not necessarily located at plate boundaries, often caused by rising plumes of hot mantle material.

• Mantle Plumes: Long-lived columns of upwelling mantle material that can originate deep within the Earth.

• Tracking Plate Movement: As lithospheric plates move over relatively stationary mantle plumes, chains of volcanoes can form (e.g., the Hawaiian Islands). The age and location of these volcanic chains can be used to determine the direction and rate of plate motion.

Measuring Plate Velocities from Space

• Modern geodetic techniques allow for precise measurements of plate motion:

  • Very Long Baseline Interferometry (VLBI)

  • Global Positioning System (GPS)

• These methods establish exact locations on opposite sides of plate boundaries and measure their relative movements, providing data on both the direction and rate of plate motion.

What Drives Plate Motions?Convective Flow in the Mantle

• The basic driving force behind plate tectonics is widely accepted to be convective flow within the Earth's mantle.

Forces Driving Plate Motion

Several forces are thought to contribute to plate movement:

• Slab-Pull: The gravitational pull of dense, subducting oceanic lithosphere sinking into the mantle. This is generally considered the dominant driving force.

• Ridge-Push (Gravitational Sliding): The force generated by the elevated elevation of oceanic ridges, causing the lithosphere to slide away from the ridge.

• Mantle Flow (Viscous Drag): The drag exerted by convection currents in the asthenosphereon the overlying lithospheric plates.

• Slab Bending: Forces associated with the bending of the oceanic plate as it begins to subduct.

• Gravity Spreading: A form of ridge-push where the weight of the elevated ridge causes the lithosphere to spread outwards.

Models of Plate-Mantle Convection

• Models of mantle convection must be consistent with the physical and chemical properties of the mantle.

• Different models exist, including those that propose layering at specific depths (e.g., 660 km) or whole-mantle convection.

Importance of Plate Tectonics

The theory of plate tectonics is fundamental to understanding:

• Earth's major surface processes.

• The geological distribution of earthquakes, volcanoes, and mountain ranges.

• The distribution of ancient organisms and mineral deposits.