4_OCE 3008_Chapter 2 Pt2

Announcements

• No class on Monday (MLK Holiday)

• Midterm #1 scheduled for Tuesday, Feb 04, covering Chapters 1-4.

Chapter 2 - Plate Tectonics and the Ocean Floor

Key Concepts

• Summarize the evidence supporting continental drift and plate tectonics theory.

• Discuss the origin and characteristics of features at different types of plate boundaries.

• Explain how plate tectonics accounts for geologic processes and features in the ocean.

• Understand historical and future changes in the configuration of land and oceans on Earth.

Application of Plate Tectonic Theory - Wilson Cycle

• Wilson Cycle: A predictive model showing the life cycle of ocean basin formation, growth, and destruction, consisting of:

  1. Embryonic - Heat source and uplift.

  2. Juvenile - Spreading and downdropping.

  3. Mature - Fully formed basin with mid-ocean ridge.

  4. Declining - Formation of subduction zones; shrinking basin.

  5. Terminal - Progressive narrowing of ocean.

  6. Suturing - Ocean disappears; continents collide.

• Plate tectonics provides a comprehensive framework describing the movement of the Earth’s outermost portion and the creation of landforms and seafloor features.

Three Main Types of Plate Boundaries

• Divergent Boundary:

  • Mid-ocean ridges formed by mantle upwelling and creation of new lithosphere.

• Convergent Boundary:

  • Plates collide, leading to one plate subducting beneath another.

  • Three types defined by plate compositions.

• Transform Boundary:

  • Two plates slide past each other, typically offsetting segments of ocean ridges, can connect different boundary types.

Divergent Boundaries

• Features:

  • Plates move apart along mid-ocean ridges, creating new ocean floor.

  • Rift valleys form as land downdrops.

  • Earthquakes typically shallow and less energy is released with faster spreading rates.

• Characteristics:

  • High geological activity, including volcanism.

  • Most divergent boundaries occur at the crest of mid-ocean ridges.

Formation of an Ocean Basin

• Heat source causes upwarping, leading to new oceanic crust as land splits apart, creating linear rift valleys.

• As rifting continues, low areas form that eventually flood, giving rise to ocean basins with mid-ocean ridges.

East African Rift Valley

• Series of linear rift valleys and volcanic peaks in NE Africa.

• Notable features:

  • Down-dropped lakes and young seas (e.g., Red Sea).

  • Expect future development into large ocean basins like the Indian Ocean.

Divergent Boundaries - Types of Spreading Centers

• Oceanic Ridge:

  • Slow-spreading (~2.5 cm/year): Steep, rugged slopes, significant seismic activity.

• Oceanic Rise:

  • Fast-spreading (up to 16.5 cm/year): Gentle slopes and less seismic activity.

• Ultra-slow Spreading (< 2 cm/year): Deep rift valleys with scattered volcanoes.

Convergent Boundaries

• Features:

  • Plates converge resulting in oceanic crust destruction, forming ocean trenches and volcanic arcs above subduction zones.

• Three Types:

  1. Oceanic-Continental Convergence: Subduction leads to continental volcanic arcs (e.g., Peru-Chile trench).

  2. Oceanic-Oceanic Convergence: Creates volcanic island arcs (e.g. Mariana Trench).

  3. Continental-Continental Convergence: Results in tall mountain ranges (e.g. Himalayas).

Future Predictions from Plate Tectonics

• If motion remains consistent, expect:

  • Atlantic Ocean to enlarge and Pacific Ocean to shrink due to subduction zones.

  • Linear seas from East African rift valleys expanding.

  • Continuous uplift of the Himalayas from Indian plate movement into Asia.

  • Further separation of North and South America, a potential land bridge with Antarctica.

Characteristics of Plate Boundaries

• Divergent Boundaries:

  • New sea floors created: Example includes the Mid-Atlantic Ridge.

• Convergent Boundaries:

  • Plates collide, creating subduction features, e.g., Peru-Chile Trench; volcanic arcs formed.

• Transform Boundaries:

  • Plates slide past each other, such as the San Andreas Fault leading to faulting and earth quakes.

Intraplate Features: Hotspots and Mantle Plumes

• Hotspots define intraplate volcanism due to mantle plumes, resulting in volcanic activity away from boundaries, as seen in Hawaii and Yellowstone.

• Seamounts and Tablemounts (guyots) referred to in the context of water erosion and subsidence of volcanic islands.

Great Barrier Reef and Paleogeography

• Great Barrier Reef: Formed as the Indian-Australian plate moves northward.

• Paleogeography: Historical changes of continental shapes and positions, noting the assembly and break up of Pangaea 240 million years ago, with subsequent continental shifts noted.

Impact of Terranes

• Terranes: Fragments of crustal material that may accrete to larger continents, illustrated with the movement of California terranes into Alaska in 50 million years.