CHAPTER 12 Focus Points

• Breakup of Pangaea: The supercontinent Pangaea split into Laurasia and Gondwana about 200 million years ago. The opening of the Atlantic Ocean led to the modern configuration of the contents.

• Continental drift: Alfred Wegener was the first to formally propose the theory of continental drift, which states that continents move.

• Plate tectonics theory: Plate tectonics theory addresses why and how lithospheric plates move.

• Oceanographic evidence: Many forms of evidence from the ocean basins, including seafloor bathymetry, rock sample ages, and patterns of magnetization, led to the development and acceptance of the theory of plate tectonics.

• Moving plates: Plate tectonics theory states that there are 14 major lithospheric plates that move independently of one another across the surface of the planet.

• Plate boundaries: Earthquake activity reveals plate boundary locations.

• How plates move: Ridge push, mantle drag, and slab pull move the lithospheric plates.

• Plate velocity: The fastest relative plate movement is 18 cm (7 in) per year.

• Unifying theory: Plate tectonics is a unifying theory that helps to explain many seemingly separate physical phenomena.

• Plate boundaries: There are three types of plate boundaries: divergent plate boundaries, convergent plate boundaries, and transform plate boundaries.

• Plate boundary landforms: Landforms created by plate movements include deep-sea trenches, volcanic island arcs, continental volcanic arcs, and nonvolcanic mountain ranges.

• Rifting: Continental rifting splits a single continental landmass, creating a volcanic rift valley. Rifting can lead to new plate margins and a new ocean basin.

• Plate recycling: Subduction recycles oceanic lithosphere into the mantle.

• Buoyant continental crust: Continental lithosphere does not subduct. Because of this, Earth’s oldest crust is continental crust.

• Accreted terranes: Much of western North America is the result of the accumulation of accreted terranes.

• Hot spots: Hot spots are created where stationary mantle plumes melt through the crust.

• Folding and faulting: Earth’s crust can be folded and faulted in response to tectonic stresses. Folding produces anticlines and synclines, and faulting produces earthquakes.

• Orogenesis: Many mountain ranges form in orogenic belts along plate boundaries.

• The Tibetan Plateau and people: About 20% of the human population relies on rivers that originate from glaciers and snowmelt on the Tibetan Plateau. Climate change threatens the food and water security of these populations.