GEY 101 - Introductory Geology: Exploring Planet Earth
Plate Tectonics

Alfred Wegener first developed the theory of Continental drift (1915), the idea that the continents have gradually moved across the earth's surface. Many early scientists noticed that the edges of the continents can be fit together like a puzzle if the Atlantic ocean is removed. The same rock sequences and mountain ranges are found in pieces on opposite sides of oceans. Several fossils are only found on parts of South America, India, Antarctica, and Africa that would fit together. However, nobody could come up with a convincing theory that would cause the continents to scoot around the earth's surface.

In the 1950's, geologists began studying the earth's magnetic field. They discovered that in older rocks, the magnetic pole was not in its present location. This apparent polar wandering is actually a function of continental drift. It also became apparent that the earth's magnetic field reverses periodically, and that these reversals are recorded in rocks. Surveys of the ocean's floor showed that it consisted of stripes of normal and reversed magnetism.

Plate tectonics: The new paradigm

• Earth’s major plates are associated with Earth's strong, rigid outer layer, which is known as the lithosphere. The lithosphere consists of uppermost mantle and overlying crust it overlies a weaker region in the mantle called the asthenosphere.
• There are seven major lithospheric plates. These plates are in motion and continually changing in shape and size. The largest plate is the Pacific plate. Several plates include an entire continent plus a large area of seafloor
• Plates move relative to each other at a very slow but continuous rate. Sea floors spread at rates of 1 cm/yr to 12 cm/yr and average 5 cm/yr (2 inches/yr). The cooler, denser slabs of oceanic lithosphere descend into the mantle
• All major interactions among individual plates occur along their boundaries
  • Types of plate boundaries
    • Divergent plate boundaries (constructive margins)
    • Convergent plate boundaries (destructive margins)
    • Transform fault boundaries (conservative margins)
• Each plate is bounded by a combination of the three types of boundaries. New plate boundaries can be created in response to changes in the forces acting on these rigid slabs

• Divergent Boundaries --plates move away from each other
  • oceanic spreading centers
    • create new oceanic crust
    • surface features - mid-ocean ridges
    • volcanism:
      • basalt volcanoes
      • lava flows
    • earthquakes:
      • shallow to intermediate earthquakes;
      • small to medium magnitude
      • modern examples: mid-Atlantic ridge, Iceland
    • continental rifts
      • break-up continent, earliest stages of ocean formation
      • surface features rift valley and volcanoes
      • volcanoes:
        • basalt volcanoes
        • lava flows
      • earthquakes:
      • shallow to intermediate earthquakes;
      • small to medium magnitude
      • modern example: Rio Grande Rift, East African Rift, and the Rhine Valley in northern Europe
    • Topographic differences are controlled by spreading rates
      • At slow spreading rates (1-5 centimeters per year), a prominent rift valley develops along the ridge crest that is wide (30 to 50 km) and deep (1500-3000 meters)
      • At intermediate spreading rates (5-9 centimeters per year), rift valleys that develop are shallow with subdued topography
      • At spreading rates greater than 9 centimeters per year no median rift valley develops and these areas are usually narrow and extensively faulted
• Convergent Boundaries -- plates move toward each other
  • subduction zones - requires involvement of at least one ocean plate; Average angle at which oceanic lithosphere descends into the mantle is about 45°
    • destroy ocean crust
    • surface features:
      • trench
      • island arc or volcanic arc
    • volcanoes:
      explosive composite volcanoes
    • earthquakes:
      • Benioff zone
      • small to huge earthquakes
      • shallow to very deep (700 km)
    • modern examples: Japan, Philippines, Aleutians, Andes, S. Mexico, Mt. St. Helens
  • Types of convergent boundaries
    • Oceanic-continental convergence
      Denser oceanic slab sinks into the asthenosphere
    • Oceanic-continental convergence
      • As the plate descends, partial melting of mantle rock generates magmas having a basaltic or, occasionally andesitic composition
      • Mountains produced in part by volcanic activity associated with subduction of oceanic lithosphere are called continental volcanic arcs (Andes and Cascades)
    • Oceanic-oceanic convergence
      • When two oceanic slabs converge, one descends beneath the other
      • Often forms volcanoes on the ocean floor
      • If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands)
    • Continental-continental convergence
      • Continued subduction can bring two continents together
      • Less dense, buoyant continental lithosphere does not subduct
      • Result is a collision between two continental blocks
      • Process produces mountains (Himalayas, Alps, Appalachians)
• Transform Boundaries -- plates slide side-by-side
  • neither create nor destroy crust
  • surface expression: fault which is often hard to identify, subltle landscape variations
  • Most join two segments of a mid-ocean ridge as parts of prominent linear breaks in the oceanic crust known as fracture zones
  • A few (the San Andreas fault and the Alpine fault of New Zealand) cut through continental crust

Testing the plate tectonics model

• Plate tectonics and earthquakes
  • Plate tectonics model accounts for the global distribution of earthquakes
  • Absence of deep-focus earthquakes along the oceanic ridge is consistent with plate tectonics theory
  • Deep-focus earthquakes are closely associated with subduction zones
  • The pattern of earthquakes along a trench provides a method for tracking the plate's descent
  • Deep-focus earthquakes occur along convergent boundaries
• Evidence from ocean drilling
  • Some of the most convincing evidence confirming seafloor spreading has come from drilling directly into ocean-floor sediment
  • Age of deepest sediments
  • Thickness of ocean-floor sediments verifies seafloor spreading
• Hot spots
  • Caused by rising plumes of mantle material
  • Volcanoes can form over them (Hawaiian Island chain)
  • Most mantle plumes are long-lived structures and at least some originate at great depth, perhaps at the mantle-core boundary
  • The Hawaiian Islands have formed over a stationary hot spot

The driving mechanism

• No one driving mechanism accounts for all major facets of plate tectonics
• Researchers agree that convective flow in the rocky 2,900 kilometer-thick mantle is the basic driving force of plate tectonics
• Several mechanisms generate forces that contribute to plate motion
  • Slab-pull
  • Ridge-push
  • Mantle Plumes
• Models of plate-mantle convection
  • Any model describing mantle convection must explain why basalts that erupt along the oceanic ridge
  • Layering at 660 kilometers
  • Whole-mantle convection
  • Deep-layer model

Importance of plate tectonics

• Theory provides a unified explanation of Earth’s major surface processes
• Within the framework of plate tectonics, geologists have found explanations for the geologic distribution of earthquakes, volcanoes, and mountains
• Plate tectonics also provides explanations for past distributions of plants and animals

Additional Study Guide

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