Chapter 9 Plate Tectonics

Plates

Section 9.1 Continental Drift In 1915 a German scientist named Alfred Wegener proposed his hypothesis of continental drift, which simply states that the continents had once been joined in a supercontinent and then moved to their present positions. Pangea was the name given to the supercontinent.

Wegener’s Evidence for Continental Drift The continents look like they would fit together like puzzle pieces. Fossils of the same organisms were found on separate coastlines of continents. Matching types of rocks and mountains found on opposite continents. He found evidence of ancient climates. He found where glaciers had been in the tropics, and he found tropical plant fossils at the poles. He could never find or describe what makes the plates move, so it was rejected.

Section 9.2 Sea-Floor Spreading Other studies followed Wegener, helped by the invention of SONAR. Sonar uses sound waves to calculate the distance to an object, which allowed underwater mapping. Scientists found deep-ocean trenches, mid- ocean ridges, rift valleys, etc.

Map of the Oceans

Sea-Floor Spreading After looking at the new underwater maps, Geologist Harry Hess came up with the hypothesis of sea-floor spreading. Sea-floor spreading – new ocean floor forms along Earth’s mid-ocean ridges and slowly moves outward across ocean basins. We now know that new ocean crust is formed when magma rises at mid-ocean ridge rift valleys, and old ocean floor is subducted at deep-ocean trenches.

Sea-floor spreading Diagrams

Evidence of Sea-Floor Spreading Paleomagnetism – as molten rocks harden their iron content moves toward the magnetic pole.

Evidence of Sea-Floor Spreading Earthquake Patterns – convinced scientists about subduction zones.

Evidence of Sea-Floor Spreading The age of the ocean floor – the newest rocks or oceanic crust were found next to the mid-ocean ridges, and the oldest oceanic crust was found next to continents or at subduction zones.

Section 9.3 Theory of Plate Tectonics The Earth’s lithosphere (crust and upper part of mantle) is broken into plates that move slow relative to each other, driven by CONVECTION CURRENTS in the mantle.

Types of Plate Boundaries Divergent boundary – found where plates move apart. Mid-ocean ridges are an example.

Types of Plate Boundaries Convergent boundaries – where two plates move together. Examples include mountain building and subduction zones.

Types of Plate Boundaries Transform Fault Boundary – also know as Strike-Slip, occurs where two plates slide and grind past each other. Ex. San Andreas

Section 9.4 Mechanisms of Plate Motion Convection currents – when heated material rises and colder material falls. These currents can happen in air, water, and partially melted and molten rock. Warm material is less dense than cold material, so it rises while cold sinks. Convection currents are driven by heat, and they are the driving forces in many of Earth’s systems, including Plate Tectonics.

Convection Currents in the Mantle The driving force behind Plate Tectonics

Plate Motion Mechanisms Slab-pull – gravity pulls dense oceanic lithosphere down into the deep mantle at subduction zones.

Plate Motion Mechanisms Ridge-push – the newly formed oceanic crust slides down the asthenosphere that is elevated near mid-ocean ridges.

Plate Motion Mechanisms Mantle Plumes – upwelling of hot mantle rock found at hot spots, such as Hawaii and Yellowstone National Park.

Facts you need to know The Mid-Atlantic Ridge is the largest mid- ocean ridge in the world. It is also the largest feature on Earth. The only place it comes above the surface is the island of Iceland. Divergent – plates move apart Convergent – plates collide Transform – plates slide past each other.