Plate Tectonics The theory that the Earth’s outermost layer is fragmented into a dozen or more large and small plates that move relative to one another as they ride on top of hotter, more mobile material. What Are Tectonic Plates? • A plate is a large, rigid slab of solid rock. – Plates are formed from the lithosphere: the crust and the upper part of the mantle. • The plates “float” on the slowly flowing asthenosphere: the lower part of the mantle. • The plates include both the land and ocean floor. • The Mohoriovicic discontinuity or Moho is the boundary between the crust and the mantle. What Drives Plate Tectonics? • The slow movement of hot, softened mantle lies below rigid plates. • The hot, softened rock in the mantle moves in a circular manner in a convection flow – the heated, molten rock rises to the surface, spreads, and begins to cool, and then sinks back down to be reheated and rises again. Earth’s Plates Movement of the Plates Over Time Permian Triassic 248 million years ago 206 million years ago Jurassic Cretaceous 206 million years ago 65 million years ago Present Day Progression of Indian Land Mass Plate Boundaries Different Types of Boundaries Different Types of Boundaries • Convergent boundaries come together – Places where crust is destroyed as one plate dives under another • Divergent boundaries spread apart – Places where new crust is generated as the plates pull away from each other – New crust is created from magma pushing up from the mantle • Transform boundaries slide against each other – Places where crust is neither produced nor destroyed as the plates slide horizontally past each other Oceanic-Continental Convergence • • • • • • The oceanic plate subducts under the continental plate because it has lower density. The oceanic Nazca Plate is being subducted under the continental part of the South American Plate. The South American Plate is being lifted up, creating the Andes mountains. Strong, destructive earthquakes and rapid uplift of mountain ranges are common in this region. These earthquakes are often accompanied by uplift of the land by as much as a few meters. Mount Saint Helens is along the subduction zone of the Juan de Fuca plate (an oceanic plate) and the North American plate (a continental plate). Oceanic – Continental Convergence Oceanic-Oceanic Convergence • • • • • When two oceanic plates converge, one is usually subducted under the other. An older oceanic plate is colder, therefore more dense and less buoyant, and will subduct under a younger, hotter, less dense, and more buoyant oceanic plate. In the process, a trench is formed. – The deepest trenches in the oceans are along oceanicoceanic subduction zones (i.e., the Marianas Trench in the Pacific, which is deeper than Mt. Everest is high). Subduction in oceanic-oceanic plate convergence can result in the formation of volcanoes. Examples of oceanic-oceanic convergence are the arcuate chains of islands in the southwest Pacific, Japan, and the Aleutian Islands. Oceanic – Oceanic Convergence Continental-Continental Convergence Continental – Continental Convergence • When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. • Instead, the crust tends to buckle and be pushed upward or sideways. The collision between the Indian and Eurasian plates has pushed up the Himalayas and the Tibetan Plateau. Continental-Continental Convergence • 50 million years ago, the Indian Plate collided into the Eurasian Plate. • After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalaya and the Tibetan Plateau to their present heights. • The Himalaya form the highest continental mountains in the world. Divergence • Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle. • The Mid-Atlantic Ridge is a divergent boundary. • Sea-floor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the ocean that exists today. Divergence • Iceland is splitting along the Mid-Atlantic Ridge between the North American and Eurasian Plates, as North America moves westward relative to Eurasia. • In East Africa, spreading processes have already torn Saudi Arabia away from the rest of the African continent, forming the Red Sea. • The actively splitting African Plate and the Arabian Plate meet in what geologists call a triple junction, where the Red Sea meets the Gulf of Aden. Transf orm • The zone between two plates that slide past one another is called a transform-fault boundary, or transform boundary. • These large faults connect two spreading centers or connect trenches. • Most transform faults are found on the ocean floor. Transform • The San Andreas Fault is one of the few transform faults exposed on land. – It connects the East Pacific Rise, a divergent boundary to the south, with the Juan de Fuca Ridge, a divergent boundary to the north. – Most earthquakes in California are caused by the accumulation and release of strain as the two plates slide past each other. Consequences of Plate Tectonics • Earthquakes and volcanic activity are linked to plate tectonic processes. • The Ring of Fire is the most seismically and volcanically active zone in the world. Consequences of Plate Tectonics The San Andreas Fault – a transform fault Aerial view of the area around Thingvellir, Iceland, showing a fissure zone (in shadow) that is an on-land exposure of the Mid-Atlantic Ridge. Right of the fissure, the North American Plate is pulling westward away from the Eurasian Plate (left of fissure). Consequences of Plate Tectonics The Aleutian Islands, an island arc The 1980 eruption of Mount Saint Helens Consequences of Plate Tectonics Helicopter view (in February 1994) of the active lava lake within the summit crater of 'Erta 'Ale (Ethiopia), one of the active volcanoes in the East African Rift Zone. The convergence of the Nazca and South American Plates has deformed and pushed up limestone strata to form towering peaks of the Andes, as seen here in the Pachapaqui mining area in Peru. References • Understanding Plate Motions. USGS. http://pubs.usgs.gov/publications/text/unde rstanding.html • Plate Movement. OptIPuter Outreach. http://education.sdsc.edu/optiputer/teacher s/platemovement.html • Plate Tectonics. The Way the Earth Works. LHS GEMS, 2002.
