🌎Plate Tectonics Unit 10 – Plate Tectonics and Landforms

Key Concepts and Terminology

• Lithosphere consists of Earth's crust and uppermost mantle, broken into rigid plates that move over the asthenosphere
• Asthenosphere is a semi-molten layer beneath the lithosphere that allows for plate movement due to convection currents
• Plate boundaries are areas where two or more plates meet and interact, causing various geological features and events (divergent, convergent, transform)
• Subduction occurs when one plate sinks beneath another at a convergent boundary, often forming deep ocean trenches and volcanic arcs
• Seafloor spreading is the process by which new oceanic crust is formed at divergent boundaries as magma rises and solidifies
• Hot spots are stationary regions of intense heat in the mantle that can cause volcanic activity and form island chains (Hawaiian Islands)
• Orogeny refers to the formation of mountain ranges, typically resulting from plate collisions or subduction
• Isostasy is the principle of buoyancy that explains how the lithosphere floats on the asthenosphere, maintaining equilibrium

Plate Tectonic Theory

• Plate tectonic theory explains the large-scale motion of Earth's lithosphere and its effects on geological processes and landforms
• Earth's surface is divided into several major and minor plates that move relative to one another over the asthenosphere
• Plate movement is driven by convection currents in the mantle, ridge push, and slab pull forces
• Interactions at plate boundaries cause various geological phenomena, including earthquakes, volcanic activity, mountain building, and seafloor spreading
• Plate tectonics provides a unifying framework for understanding Earth's geological history and the distribution of landforms and resources
• Evidence supporting plate tectonic theory includes seafloor spreading, magnetic anomalies, age of oceanic crust, and fossil records
• Plate tectonics has been occurring for billions of years, shaping Earth's surface and influencing climate and the evolution of life

Types of Plate Boundaries

• Divergent boundaries occur where two plates move apart, allowing magma to rise and create new oceanic crust (Mid-Atlantic Ridge)
  • Seafloor spreading occurs at divergent boundaries, forming new oceanic crust and widening ocean basins
  • Shallow earthquakes and basaltic volcanism are common at divergent boundaries
• Convergent boundaries occur where two plates collide, resulting in subduction, mountain building, or continent-continent collision
  • Oceanic-continental convergence leads to subduction of the denser oceanic plate, forming deep ocean trenches and volcanic arcs (Andes Mountains)
  • Oceanic-oceanic convergence results in the subduction of one plate beneath the other, creating island arcs and back-arc basins (Mariana Islands)
  • Continental-continental convergence causes the formation of high mountain ranges and plateaus (Himalayas)
• Transform boundaries occur where two plates slide past each other horizontally, causing frequent earthquakes but little to no volcanism (San Andreas Fault)
• Plate boundary zones are broad regions where the effects of plate interactions are distributed over a wide area, often characterized by complex deformation and seismicity

Earth's Interior Structure

• Earth's interior is divided into layers based on chemical composition and physical properties
• The crust is the outermost layer, composed of solid rock and varying in thickness between oceanic and continental regions
  • Oceanic crust is thinner (~5-10 km), denser, and younger than continental crust
  • Continental crust is thicker (~30-50 km), less dense, and older than oceanic crust
• The mantle is the layer beneath the crust, extending to a depth of about 2,900 km and composed primarily of silicate rocks
  • The upper mantle includes the asthenosphere, which is a semi-molten layer that allows for plate movement
  • The lower mantle is solid but can deform plastically over long time scales
• The outer core is a liquid layer composed primarily of iron and nickel, extending from about 2,900 km to 5,100 km depth
• The inner core is a solid layer at the center of Earth, composed primarily of iron and nickel, with a radius of about 1,220 km
• Seismic waves provide evidence for Earth's interior structure, as they behave differently in each layer (P-waves, S-waves)

Plate Movement Mechanisms

• Convection currents in the mantle are the primary driving force behind plate movement
  • Hot, less dense material rises from the lower mantle, while cooler, denser material sinks back down
  • These convection currents create a slow, circular flow that drags the overlying lithospheric plates
• Ridge push is a force generated by the gravitational potential energy of the elevated mid-ocean ridges, causing plates to slide away from the ridge
• Slab pull is a force generated by the weight of the cold, dense subducting plate as it sinks into the mantle at convergent boundaries
• Mantle plumes are localized upwellings of hot material from the deep mantle that can cause volcanic activity and contribute to plate movement (Yellowstone)
• Basal drag is a resistive force between the lithosphere and asthenosphere that can slow plate motion
• Plate movement rates vary but typically range from a few millimeters to several centimeters per year, with the fastest rates observed at mid-ocean ridges

Major Tectonic Plates

• The Earth's lithosphere is divided into several major tectonic plates and numerous smaller plates
• The seven major plates are the African, Antarctic, Eurasian, Indo-Australian, North American, Pacific, and South American plates
• The Pacific Plate is the largest and fastest-moving plate, characterized by its oceanic crust and numerous subduction zones along its margins (Ring of Fire)
• The North American Plate and Eurasian Plate are primarily continental plates, with the Mid-Atlantic Ridge forming their divergent boundary
• The Indo-Australian Plate is a composite plate that includes both oceanic and continental crust, with subduction zones along its northern and eastern margins
• The African Plate is a largely continental plate, with divergent boundaries in the Red Sea and East African Rift
• The Antarctic Plate is a mostly oceanic plate that includes the continent of Antarctica, surrounded by divergent boundaries
• Smaller plates, such as the Nazca, Cocos, and Philippine plates, also play important roles in regional tectonics and seismicity

Landforms at Plate Boundaries

• Divergent boundaries create various landforms, including mid-ocean ridges, rift valleys, and submarine volcanoes
  • Mid-ocean ridges are elevated, linear features where new oceanic crust is formed (Mid-Atlantic Ridge)
  • Rift valleys are elongated depressions that form as plates pull apart, often associated with continental rifting (East African Rift)
• Convergent boundaries produce landforms such as subduction zones, volcanic arcs, and mountain ranges
  • Subduction zones are characterized by deep ocean trenches, where the subducting plate descends into the mantle (Mariana Trench)
  • Volcanic arcs are chains of volcanoes that form parallel to subduction zones, resulting from the melting of the subducting plate (Aleutian Islands)
  • Mountain ranges form through the collision and uplift of continental crust (Himalayas) or the accretion of volcanic arcs and sediments (Andes)
• Transform boundaries are associated with linear features such as strike-slip faults and offset ridges
  • Strike-slip faults are characterized by horizontal displacement of the crust, with little vertical movement (San Andreas Fault)
  • Offset ridges occur where transform faults intersect mid-ocean ridges, causing lateral displacement of the ridge segments
• Hot spots create landforms such as volcanic islands, seamounts, and plateaus
  • Volcanic islands form as magma from the mantle plume erupts onto the seafloor and builds up over time (Hawaiian Islands)
  • Seamounts are underwater volcanoes that have not reached the surface, often forming chains or clusters (Emperor Seamounts)
  • Plateaus are broad, elevated regions of the seafloor formed by extensive volcanic activity (Ontong Java Plateau)

Geological Processes and Events

• Earthquakes are sudden releases of energy in the Earth's crust, caused by the movement of tectonic plates
  • Earthquake magnitude is measured using the moment magnitude scale, which quantifies the energy released
  • Seismic waves generated by earthquakes provide valuable information about Earth's interior structure and plate boundaries
• Volcanic eruptions occur when magma from the Earth's interior reaches the surface, often associated with plate boundaries or hot spots
  • Volcanoes can be classified based on their shape, composition, and eruptive style (shield, stratovolcano, cinder cone)
  • Volcanic activity can have significant impacts on the environment, climate, and human populations
• Tsunamis are large, powerful waves generated by sudden displacements of water, often caused by underwater earthquakes or landslides
  • Tsunami waves can travel across entire ocean basins, causing destruction and loss of life in coastal areas
• Orogeny is the process of mountain building, typically occurring at convergent plate boundaries over millions of years
  • Orogeny involves the deformation, uplift, and metamorphism of rock layers, often resulting in the formation of fold and thrust belts
• Seafloor spreading is the process by which new oceanic crust is formed at divergent plate boundaries, driving the movement of tectonic plates
  • Magnetic anomalies in the oceanic crust provide evidence for seafloor spreading and the reversal of Earth's magnetic field over time
• Plate reconstructions use various lines of evidence to reconstruct the positions of continents and oceans in the past
  • Evidence for plate reconstructions includes the fit of continental margins, distribution of fossils and rock types, and magnetic anomaly patterns
  • Plate reconstructions help scientists understand the evolution of Earth's surface, climate, and life over geological time scales