2.2 Plate tectonics and geologic processes

Plate tectonics shapes Earth's surface, driving the movement of continents and formation of oceans. This theory explains how Earth's crust is divided into plates that slide and collide, causing earthquakes, volcanoes, and mountain ranges.

Evidence for plate tectonics includes the fit of continents, matching rock formations, and seafloor spreading. Modern tech like GPS confirms plate motion, showing how our planet's surface constantly changes over millions of years.

Plate Tectonics Theory and Evidence

Foundations and Historical Development

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• Plate tectonics unifying theory in geology explains large-scale motions of Earth's lithosphere divided into several plates moving relative to one another
• Evolved from continental drift concept proposed by Alfred Wegener in 1912 suggested continents moved across Earth's surface
• Continental drift initially met with skepticism due to lack of mechanism explaining movement

Supporting Evidence

• Fit of continents particularly noticeable between South America and Africa
• Matching rock formations and fossils across continents (Glossopteris flora found in South America, Africa, India, and Antarctica)
• Paleomagnetism study of Earth's magnetic field preserved in rocks reveals past positions of continents
• Seafloor spreading process of creating new oceanic crust at mid-ocean ridges
• Distribution of earthquakes and volcanoes concentrated along plate boundaries

Modern Confirmations

• Discovery of mid-ocean ridges and magnetic striping patterns on ocean floor provided crucial evidence for seafloor spreading
• Magnetic striping results from periodic reversals of Earth's magnetic field recorded in newly formed oceanic crust
• Modern technologies (GPS and satellite measurements) provide direct evidence of plate motion
• GPS measurements confirm plates move at rates of a few centimeters per year

Plate Boundaries and Features

Divergent Boundaries

• Occur where plates move apart creating new crust
• Associated with mid-ocean ridges underwater mountain ranges formed by upwelling magma
• Rift valleys form where divergent boundaries occur on continents (East African Rift)
• Characterized by shallow earthquakes and basaltic volcanism
• Examples include Mid-Atlantic Ridge and East Pacific Rise

Convergent Boundaries

• Involve plates moving towards each other resulting in either subduction or collision
• Subduction zones where one plate sinks beneath another characterized by:
  • Deep ocean trenches (Mariana Trench)
  • Volcanic arcs (Aleutian Islands)
  • Formation of accretionary wedges sediment scraped off subducting plate
• Collision zones where two continental plates meet result in extensive mountain building:
  • Himalayas formed by collision of Indian and Eurasian plates
  • Alps resulted from African and Eurasian plate collision

Transform Boundaries

• Plates slide past each other horizontally creating strike-slip faults
• Associated with shallow earthquakes but typically no volcanism
• San Andreas Fault in California prime example of transform boundary
• Alpine Fault in New Zealand another significant transform boundary

Complex Interactions

• Triple junctions regions where three plate boundaries meet
• Can result in unique geologic features (Afar Triple Junction)
• Some areas experience diffuse plate boundaries broad zones of deformation (Basin and Range Province in western North America)

Mechanisms of Plate Motion

Primary Driving Forces

• Convection in Earth's mantle caused by heat from radioactive decay and residual heat from Earth's formation
• Slab pull gravitational force exerted by subducting plates considered dominant mechanism for plate motion
• Ridge push force exerted by elevation difference between mid-ocean ridges and older cooler oceanic crust
• Mantle plumes rising columns of hot material from deep mantle influence plate motion

Secondary Factors

• Tidal forces from Moon and Sun may play minor role in plate motion
• Earth's rotation (Coriolis effect) influences direction of plate movement
• Gravitational potential energy differences due to topography contribute to plate motion

Resulting Geologic Processes

• Crustal deformation folding and faulting of rocks
• Metamorphism alteration of rocks due to heat and pressure
• Magmatism formation and movement of magma within Earth's crust
• Rock cycle continuous transformation of rocks between igneous sedimentary and metamorphic types

Wilson Cycle

• Formation and destruction of ocean basins over hundreds of millions of years
• Stages include:
  • Rifting of continents
  • Seafloor spreading
  • Subduction of oceanic crust
  • Continental collision
  • Eventual rifting and cycle restart

Formation of Earthquakes, Volcanoes, and Mountains

Earthquake Distribution and Mechanisms

• Primarily occur along plate boundaries due to sudden release of accumulated stress in rocks
• Majority occur in subduction zones particularly around Pacific Ring of Fire
• Large magnitude events common in subduction zones due to extensive contact area between plates
• Intraplate earthquakes occur within stable continental interiors (New Madrid Seismic Zone)
• Earthquake depth varies by tectonic setting:
  • Shallow at divergent and transform boundaries
  • Deep at subduction zones (up to 700 km)

Volcanic Formation and Distribution

• Form through various tectonic processes:
  • Subduction creating volcanic arcs (Cascade Range)
  • Divergent boundaries producing shield volcanoes at mid-ocean ridges (Mauna Loa)
  • Hotspots forming volcanic chains (Hawaiian Islands)
• Global distribution closely correlates with plate boundaries
• Notable exceptions intraplate hotspot volcanoes (Yellowstone)
• Different magma compositions result in varied eruption styles:
  • Basaltic lava flows at divergent boundaries
  • Explosive andesitic to rhyolitic eruptions at subduction zones

Mountain Range Formation

• Three primary mechanisms:
  • Continental collision (Himalayas)
  • Subduction of oceanic crust beneath continental crust (Andes)
  • Rifting processes (East African Rift Valley)
• Orogenies mountain-building episodes directly linked to plate tectonic cycle
• Significant impacts on global climate and biodiversity:
  • Weathering of newly exposed rocks affects atmospheric CO2 levels
  • Creation of new habitats and barriers influencing species distribution