plate tectonics volcanoes EEn Explain how plate tectonics, and volcanoes impact the lithosphere. I. Plate Tectonics Chapter 9, Section 2 A. Actions at Plate Boundaries Chapter 9, Section 3 B. Mechanisms of Plate Motion Chapter 9, Section 5 C. Plate Tectonics and Igneous Activity Chapter 10, Section 3

I. Plate Tectonics pages plates The lithosphere (the uppermost mantle, along w/the overlying crust, that behaves as a strong, rigid layer) is divided into segments called plates, which move and continually change shape and size. Most of the seven major plates shown on p include an entire continent plus a large area of seafloor. Compare these plate boundaries to figure 9 (map showing earthquake distribution) on p 226.

Types of Plate Boundaries Divergent Divergent boundaries (also called spreading centers) occur when two plates move apart…upwelling of material from the mantle creates new seafloor. Convergent Convergent boundaries form where two plates move together…oceanic lithosphere plunges beneath an overriding plate, descending into the mantle. Transform Transform fault boundaries are margins where 2 plates grind past each other. The San Andreas fault in California is an example.

A. Actions at Plate Boundaries pages Divergent Boundaries—constructive plate margins a.Ocean Ridges and Seafloor Spreading a.Ocean Ridges and Seafloor Spreading Most divergent plate boundaries are located along the crests of oceanic ridges. This system of ocean ridges is the longest physical feature on Earth’s surface, winding through all major ocean basins like the seam of a baseball. Seafloor spreading is the process by which plate tectonics produces new oceanic lithosphere.

A. Actions, 1. Divergent Boundaries b.Continental Rifts b.Continental Rifts When spreading centers develop on a continent, landmasses split. The active East African rift valley (deep faulted structure) is an example. Mount Kilimanjaro shows the kind of volcanic activity that accompanies continental rifting. When continents split in two, the rift becomes a narrow sea. The Red Sea formed when the Arabian Peninsula rifted from Africa.

A. Actions at Plate Boundaries 2.Convergent Boundaries—destructive plate margins subduction zones subduction zones—where oceanic crust is being forced down into the mantle beneath a second plate trench ocean trench—the surface feature produced by the descending plate

A. Actions, 2. Convergent Boundaries a.Oceanic-Continental asthenosphere a.Oceanic-Continental When the leading edge of a continental plate converges w/an oceanic plate, the less dense continental plate remains floating, and the more dense oceanic slab sinks into the asthenosphere— a weaker region in the mantle under the lithosphere. 1)continental volcanic arcs 1)continental volcanic arcs—when the descending plate melts, being less dense than the rocks of the mantle, it rises to reach the surface and causes volcanic eruptions. (Andes Mountains, S Am.)

A. Actions, 2. Convergent Boundaries b.Oceanic-Oceanic b.Oceanic-Oceanic When two oceanic slabs converge, one descends below the other. 1)volcanic island arc 1)volcanic island arc—arc-shaped chain of small volcanic islands (Aleutian Islands off the shore of Alaska, next to the Aleutian trench) c.Continental-Continental c.Continental-Continental The collision between two continents causes the formation of complex mountains, composed of deformed and metamorphosed sedimentary rocks. (Himalayas in S. Asia, Alps, Appalachians)

A. Actions at Plate Boundaries 3.Transform Fault Boundaries Plates grind past each other without destroying the lithosphere. Most transform faults are located within the ocean basin. Along the San Andreas fault, the Pacific plate is moving toward the northwest, past the North American plate. The part of California west of the fault zone will become an island off the west coast of the United States and Canada, eventually reaching Alaska.

B. Mechanisms of Plate Motion 1.Mantle Convection 1.Mantle Convection: The unequal distribution of heat within Earth causes the thermal convection in the mantle that drives plate motion. During convection, warm, less dense material rises and cooler, denser material sinks. Hot mantle plumes of rock (hot spots and volcanoes on Earth’s surface) are the upward flowing arms in mantle convection. The slabs of cold oceanic lithosphere descending into the mantle provide the downward arm of convective flow.

1. Mantle Convection a.Slab-Pull a.Slab-Pull: Old, cool, and dense oceanic crust sinks into the asthenosphere pulls the trailing lithosphere along. b.Ridge-Push b.Ridge-Push (gravity) causes oceanic lithosphere to slide down the sides of the oceanic ridge.