Wegener’s Continental Drift • The proposition first published in 1912. • Continents can shift positions around the globe. • Continents were once joined together to form one landmass. • Supercontinent PANGAEA
Evidence for Wegener’s Continental Drift • Jigsaw Puzzle • Edges of Continents fit together like puzzle • Rock Layers • Similarities of rock layers and structures on opposing shores of Atlantic • Fossil Correlation • Glossopteris plant leaf fossils found in Southern Africa, Australia, South America, India, and Antarctica • Mesosaurus dinosaur fossils found in both South America and Southern Africa
Evidence for Wegener’s Continental DriftMatching Mountain Ranges and Glaciers
Plate Tectonics Theory • Widely accepted theory for plate movement in scientific community. • Earth’s lithosphere is broken up into rigid sections called plates. • The movement and interaction of these plates cause major changes in Earth’s surface. • Plates can move horizontally and vertically. • Plates move due to floating on the asthenosphere. • Continents move as the plates they are attached to move.
Mantle Convection • Scientists generally agree that mantle convection is the basic driving force for plate movement. • Convective flow is the motion of matter resulting from changes in temperature. • Plate tectonics is a density driven process. The molten rock is less dense than the surrounding rock.
Types of Plate Boundaries Plate Tectonics 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.
Divergent Plate Boundaries – Spreading Centers • plates move apart • magma rises from below to fill gap and push plates away from each other • occurs at the mid-ocean ridges and at continental rifts • creates new basaltic ocean crust (dense) at the ridge • the oldest basaltic ocean crust is at the edge of the continents – see diagram
Spreading CenterDivergent Boundary Youngest Oceanic Crust Oldest Oceanic Crust
Mid-Ocean Ridges • The mid-ocean ridge is a mountain range at the bottom of the ocean that is composed mainly of volcanoes and lava flows. See map p. 5 ESRT • The rocks created at the mid-ocean ridge have magnetic minerals that are aligned with Earth’s magnetic field. • Earth’s magnetic field reverses polarity on a cycle of thousands of years.
Evidence for Plate Tectonics Testing Plate Tectonics The discovery of strips of alternating polarity, which lie as mirror images across the ocean ridges, is among the strongest evidence of seafloor spreading.
Transform Fault Boundaries 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 are roughly parallel to the direction of plate movement. • They aid the movement of oceanic crustal material.
Convergent Boundaries 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.
Convergent Boundaries 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.
Convergent Boundaries 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.
Passive Continental Margin • 5 main features • Coast – edge of continent • Continental Shelf – wide shallow extension of continent • Continental Slope – steepest gradient drop toward ocean floor • Continental Rise – collection of sediment from turbidity currents • Abyssal plain – ocean floor, flattest places on Earth
Active Continental MarginSubduction Zone • Subduction is the sinking of the denser crust out of the two plates that are colliding. See map p. 5 ESRT • Ocean trenches are the deepest parts of the oceans. • created at subduction boundaries by bending down of the subducting plate and warping of the crust
Active Continental MarginSubduction Zone • Continental Volcanic Arcs form on the edge of the continental plate due a combination of factors: • Friction between plates • Subducting slab • Overriding slab • Increased temperature at depth • Lower density magma rises through denser solid rock
Volcanoes • created at subduction boundaries due to the melting of the subducting plate at great depth • the rock becomes magma and rises because it is now less dense than the surrounding rock • continental volcanic arcs form on the edge of a continent that the plate is subducting beneath • Examples: Andes in South America, Cascades in United States • volcanic island arcs form on a plate boundary where an oceanic plate is colliding with another oceanic plate and the denser (older) plate is subducting below the less dense (younger) plate • Example: Mariana, Aleutian, and Tonga Islands
Hot Spots • are areas where intraplatevolcanism occurs. • where a mass of hotter than normal mantle material called a mantle plume rises toward the surface • Examples: Hawaiian Islands, Yellowstone, Iceland, Canary Islands • See Tectonic Plates p. 5 ESRT