Chapter 9 Plate Boundaries

9.2 Plate Tectonics Earth’s Major Roles  According to the plate tectonics theory, the uppermost mantle, along with the overlying crust, behaves as a strong, rigid layer. This layer is known as the lithosphere. • A plate is one of numerous rigid sections of the lithosphere that move as a unit over the material of the asthenosphere.

Plate Tectonics Theory Lithosphere is divided into plates, which move and continually change shape and size Under the lithosphere, there is a layer of mantle that is super soft (like melted plastic) that the plates float on top of This layer is called the asthenosphere 7 major plates (pages 256-257, Figure 8) The grinding movements of the plates generate earthquakes, create volcanoes and deform masses of rock into mountains

Plate Movements Plates are continuously moving There are two types of plates: Continental Plates and Oceanic plates Each plate can do different motions on different sides These movements create landscapes and mountain ranges They can also cause dangerous events

9.2 Plate Tectonics Types of Plate Boundaries  Divergent boundaries (also called spreading centers) are the place where two plates move apart.  Convergent boundaries form where two plates move together.  Transform fault boundaries are margins where two plates grind past each other without the production or destruction of the lithosphere.

Three Types of Plate Boundaries

9.3 Actions at Plate Boundaries Divergent Boundaries 9.3 Actions at Plate Boundaries • Oceanic ridges are continuous elevated zones on the floor of all major ocean basins. The rifts at the crest of ridges represent divergent plate boundaries. • Rift valleys are deep faulted structures found along the axes of divergent plate boundaries. They can develop on the seafloor or on land. • Seafloor spreading produces new oceanic lithosphere.

Spreading Center

9.3 Actions at Plate Boundaries Divergent Boundaries 9.3 Actions at Plate Boundaries  Continental Rifts • When spreading centers develop within a continent, the landmass may split into two or more smaller segments, forming a rift.

East African Rift Valley

9.3 Actions at Plate Boundaries Convergent Boundaries 9.3 Actions at Plate Boundaries  A subduction zone occurs when one oceanic plate is forced down into the mantle beneath a second plate.  Oceanic-Continental • Denser oceanic slab sinks into the asthenosphere. • Pockets of magma develop and rise. • Continental volcanic arcs form in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent. • Examples include the Andes, Cascades, and the Sierra Nevadas.

Oceanic-Continental Convergent Boundary

9.3 Actions at Plate Boundaries Convergent Boundaries 9.3 Actions at Plate Boundaries  Oceanic-Oceanic • Two oceanic slabs converge and one descends beneath the other. • This kind of boundary often forms volcanoes on the ocean floor. • Volcanic island arcs form as volcanoes emerge from the sea. • Examples include the Aleutian, Mariana, and Tonga islands.

Oceanic-Oceanic Convergent Boundary

9.3 Actions at Plate Boundaries Convergent Boundaries 9.3 Actions at Plate Boundaries  Continental-Continental • When subducting plates contain continental material, two continents collide. • This kind of boundary can produce new mountain ranges, such as the Himalayas.

Continental-Continental Convergent Boundary

Collision of India and Asia

9.3 Actions at Plate Boundaries Transform Fault Boundaries 9.3 Actions at Plate Boundaries  At a transform fault boundary, plates grind past each other without destroying the lithosphere.  Transform faults • Most join two segments of a mid-ocean ridge. • At the time of formation, they roughly parallel the direction of plate movement.

Transform Fault Boundary

Causes of Plate Motion According to the Plate Tectonics Theory there is an uneven distribution of heat in the mantle which is caused by heat from the core. Hot mantle material will rise from near the core and sink as it reaches near the lithosphere This motion of hot rising and cold sinking is known as a convection cell

Plate Motions Associated with Convections Cells 1. Slab Pull When ocean lithosphere is being subducted beneath another plate, it drags the rest of the lithosphere behind it down as well. This slab pull is the downward arm of the convection cell Found at convergent boundaries

Plate Motions Associated with Convections Cells 2. Ridge Push When oceanic crust is being made at divergent plate boundaries, it is pushing the oceanic lithosphere away from the upward arm of the convection cell This motion is found at divergent plate boundaries

Plate Motions Associated with Convections Cells 3. Mantle Plume The mantle material rising in the upward arm of the convection cell can happen in a single place in the middle of a plate and create a hot spot/island Example: Hawaii

Ring of Fire An area where a large number of earthquakes and volcanic eruptions occur The Ring of Fire surrounds the Pacific plate

Mid-Atlantic Ridge A mid-ocean ridge, a divergent tectonic plate boundary, located along the floor of the Atlantic Ocean Part of the longest mountain range in the world It separates the Eurasian and African Plate from the North American and South American Plate