Plate Tectonics and Earth Processes

• Plate tectonics is a theory describing the movements of Earth's plates, with the lithosphere divided into plates that float on the asthenosphere.
• These plates move in various directions, spreading, colliding, and grinding against each other.
• Plates are made up of stiff and rigid blocks of lithosphere.
• Plate tectonics helps explain how Earth works as a system, connecting the rock and carbon cycles, mountain formation, ore deposits, and the locations of continents, oceans, mountains, volcanoes, and earthquakes.

Plates

• The major plates consist of seven plates, all greater than 20 million km²: Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American.
• Minor plates range from 1 to 20 million km², and microplates are less than 1 million km².

Driving Forces of Plate Tectonics

• The driving force behind plate tectonics is convection in the mantle.
• Hot material rises from near the Earth's core, while colder mantle rock sinks.
• Convection drives plate tectonics through a combination of pushing and spreading at mid-ocean ridges and pulling and sinking at subduction zones.

Forces Moving the Lithosphere

• Ridge push occurs at mid-ocean ridges, where young lithosphere sits atop a topographic high, and gravity induces it to slide down, pushing against thicker lithosphere.
• Slab pull happens at subduction zones, where cold, dense slabs sink into the mantle and pull the rest of the lithosphere into the oceanic trench.
• Friction from viscous mantle rock borders the lithosphere and drags on the top, bottom, and leading edge of descending lithosphere in the subduction zone, as well as elsewhere at the base of the plate.

Continental Drift Theory

• Plate tectonics concept combines continental drift with seafloor spreading.
• Continental drift suggests continents move freely over the Earth's surface, changing their positions relative to one another.

Supporting Data

• In the early 1900s, Alfred Wegener noted similar late Paleozoic rocks and fossils in South America, Africa, India, Antarctica, and Australia.
• The plant Glossopteris is found in Pennsylvanian and Permian age rock across all 5 continents.
• Mesosaurus, a freshwater reptile, are found solely in Permian-age rocks in Brazil and South Africa.
• Lystrosaurus and Cynognathus fossil remains are found in Triassic-age rocks on all five continents.

Pangaea

• Wegener reassembled the continents into the supercontinet of Pangae.
• Pangaea separated into Laurasia, the northern supercontinent, and Gondwanaland, the southern supercontinent.
• Paleomagnetic evidence shows matched rock contacts and radioisotope ages between South America and Africa.
• Rocks in Brazil and Gabon are similar in type, structure, sequence, fossils, ages, and metamorphism.

Seafloor Spreading

• Harry Hess proposed seafloor movement.
• Hess theorized the the seafloor moves like a conveyer belt from the mid-oceanic ridge, down the ridge's flanks, across the basin, plunging beneath a continent or island arc (subduction).
• Seafloor spreading is driven by deep mantle convection where hot material rises and cold material sinks; hot material has lower density, and cold material has higher density.

Evidence of Plate Movement and Magnetic Anomalies

• Magnetometer surveys show magnetic anomalies arranged in bands parallel to the rift valley of mid-oceanic ridges.
• Alternating positive and negative anomalies form a stripe-like pattern parallel ridge crest.

Plate Boundaries

• Subduction zones, or convergent margins, where plates collide.
• Divergent margins, where two plates spread apart, occur at seafloor spreading ridges or continental rift zones (ex East Africa Rift).
• Transform margins mark slip-sliding plates (ex. California's San Andreas Fault, where the North America and Pacific plates grind past each other).

Convergent Boundaries:

• Where two plates collide
• Subduction zones occur when one or both plates consist of oceanic crust, where the denser plate is subducted and melted.
• Island arcs and trenches occur when both plates are oceanic crust, with back-arc basins and submarine volcanoes often present.
• Oceanic crust meeting continental crust creates mountain ranges on the continent.
• Continental crusts collide to create large mountain ranges, like the Himalayas.

Divergent Boundaries:

• Where two plates move apart.
• Created space can fill with new crustal material from molten magma.
• Divergent boundaries can form within continents, becoming ocean basins.
• This process creates rifts and rift valleys on land.
• The most active divergent boundaries are between oceanic plates, known as mid-ocean ridges.

Transform Boundaries:

• Where plates slide past each other
• They are occur underwater or on land, crust is neither destroyed nor created.
• Friction prevents smooth gliding, causing stress to build up and the energy released causes earthquakes.

Explore the relationship between the lithosphere and asthenosphere, the primary driving force behind tectonic plate movement, and the role of friction in subduction zones. Understand the forces at mid-ocean ridges and the connection between plate tectonics and Earth processes.