Plate Tectonics: Boundaries!

Review Main objection to Wegener's proposal Wegener - continental drift hypothesis 4 lines of evidence Continental puzzle Fossil records Matching mountain ranges Ancient climate Main objection to Wegener's proposal Inability to provide a mechanism

Earth’s internal structure Earth’s internal layers can be defined by Chemical composition Physical properties Layers defined by composition Crust Continental Oceanic Mantle 82% of the volume Core

Density The physical property that ties most things on earth together? What type of processes does density effect? Formation of the earth Anything that works through convection currents Plate movements Wind and ocean currents Weather Erosion + Weathering

Earth’s internal structure Four main layers of Earth are based on physical properties and hence mechanical strength Lithosphere Asthenosphere Mesosphere Core

Earth’s internal structure Asthenosphere Exists beneath the lithosphere Hotter and weaker than lithosphere Allows for motion of lithosphere Plate boundaries All major interactions among plates occur along their boundaries

Plate Boundaries Types of plate boundaries Divergent plate boundaries (constructive margins) Convergent plate boundaries (destructive margins) Transform fault plate boundaries

Divergent plate boundaries Divergent plate boundaries (constructive margins) Two plates move apart Mantle material upwells to create new seafloor Ocean ridges and seafloor spreading Oceanic ridges develop along well-developed boundaries Along ridges, seafloor spreading creates new seafloor

Divergent boundaries They are located mainly along oceanic ridges Figure 15.10

Divergent boundaries Some are found on land The East African rift Within a continent, continental rifts form at spreading centers

Convergent plate boundaries Convergent plate boundaries (destructive margins) Plates collide An ocean trench forms and lithosphere is subducted into the mantle

An oceanic-continental convergent plate boundary Oceanic-continental convergence Denser oceanic slab sinks into the asthenosphere Pockets of magma develop and rise Continental volcanic arcs form Examples include the Andes, Cascades, and the Sierra Nevadan system

An oceanic-oceanic convergent plate boundary Two oceanic slabs converge and one descends beneath the other Often forms volcanoes on the ocean floor Volcanic island arcs forms as volcanoes emerge from the sea Examples include the Aleutian, Mariana, and Tonga islands

A continental-continental convergent plate boundary When subducting plates contain continental material, two continents collide Can produce new mountain ranges Figure 15.14 C

The collision of India and Asia produced the Himalayas Before After Figure 15.15 A

Transform fault boundaries Plates slide past one another No new crust is created or destroyed Most join two segments of a mid-ocean ridge Aid the movement of oceanic crustal material