Plate Tectonics A Unifying Theory Mid-Atlantic Ridge, Iceland

Theory of Plate Tectonics The upper mechanical layer of Earth (lithosphere) is divided into rigid plates that move away from, toward, and along each other Most (!) deformation of Earth’s crust occurs at plate boundaries

Convergent Convergent Ocean-continent (e.g., Andes, Cascades) Ocean-continent (e.g., Andes, Cascades) Ocean-ocean (Japan, Indonesia, Alaskan Islands) Ocean-ocean (Japan, Indonesia, Alaskan Islands) Continent-continent (Himalayas, Alps) Continent-continent (Himalayas, Alps) Oceanic crust subducts (cold, dense), continental crust is usually too bouyant (less dense) & does not subduct Oceanic crust subducts (cold, dense), continental crust is usually too bouyant (less dense) & does not subduct Divergent Divergent New oceanic crust created (e.g., African rift zone, Red Sea) New oceanic crust created (e.g., African rift zone, Red Sea) Transform Transform Strike-Slip fault (e.g., San Andreas Fault) Strike-Slip fault (e.g., San Andreas Fault) Between mid-ocean ridge spreading centers Between mid-ocean ridge spreading centers Often difficult to recognize Often difficult to recognize 3 Main Boundary Types

Ocean-Continent Collision Andes Mountains

Ocean-Ocean Collision Japan

Continent-Continent Collision Himalayas

The Himalayas— Roof of the World During the Early Cretaceous, During the Early Cretaceous, India broke away from Gondwana and began moving north India broke away from Gondwana and began moving north oceanic lithosphere was consumed at a subduction zone along the southern margin of Asia oceanic lithosphere was consumed at a subduction zone along the southern margin of Asia

India’s Northern Trek

Divergent Boundary East African rift zone Red Sea Mid-Atlantic Ridge

Transform Boundaries 1.Areas in-between mid- ocean ridge spreading centers 2.Areas such as the San Andreas Fault San Andreas Fault Mid-Atlantic Ridge

San Andreas Fault Transform Boundaries

San Andreas Fault Fence split by 1906 earthquake along the San Andreas Fault. Point Reyes moved northwest up to 20 feet. Photo by L. Stone – Point Reyes National Seashore – June 2005

Plate Tectonics... Recent Recent Unifying theme of solid earth sciences Unifying theme of solid earth sciences Framework on which we hang (test!) observations about Earth’s geology and geophysics Framework on which we hang (test!) observations about Earth’s geology and geophysics Is a THEORY Is a THEORY What makes the plates?

3 main layers defined by composition: Crust - Outer Mantle - Middle Core - Center Earth’s Layers

Why Do the Plates Move?

Got Heat? Loss of original heat of formation (geothermal / core is cooling) Radioactive decay of elements in Earth’s materials The Sun - external; not important to plate tectonics Earth - 3 Heat Sources:

Interior of Earth has sluggish convection in some regions Interior of Earth has sluggish convection in some regions Heat from core rises, creates convection cells in the mantle Heat from core rises, creates convection cells in the mantle NOT LIQUID! Given an infinite amount of time, solids flow Convection: Driving Force of Plate Tectonics

Rising hot material at mid-ocean ridges and mid- ocean volcanic islands Rising hot material at mid-ocean ridges and mid- ocean volcanic islands Descending cooler material at trenches Descending cooler material at trenches Lithospheric plates “carried” with the convection cells Lithospheric plates “carried” with the convection cells

More than Convection? In addition to thermal convection cells, some geologists think that movement may be aided by In addition to thermal convection cells, some geologists think that movement may be aided by “slab-pull” - slab is cold and dense and pulls the plate “slab-pull” - slab is cold and dense and pulls the plate “ridge-push” - rising magma pushes the ridges up and gravity pushes the ocean floor toward the trench “ridge-push” - rising magma pushes the ridges up and gravity pushes the ocean floor toward the trench

Alfred Wegener 1912 Continental Drift Observations Fit of Continents Geology Paleontology Climate belts Pangaea 200 Ma Breakup 180 Ma Rigid bodies moving through yielding seafloor – could not provide mechanism of movement

Jigsaw-Puzzle Fit of Continents Matching mountain ranges Matching mountain ranges Matching glacial evidence Matching glacial evidence

Matching Fossils

Arthur Holmes (Early 1920’s) Arthur Holmes (Early 1920’s) Interior of Earth has sluggish convection (transport of heat from core) Interior of Earth has sluggish convection (transport of heat from core) New ocean crust injected into ocean floor (where?) New ocean crust injected into ocean floor (where?) Mechanism for Plate Movement!

Harry Hess and Seafloor Spreading Crust moves with convection currents Crust moves with convection currents New ocean crust at MOR’s New ocean crust at MOR’s Ocean crust dragged down at trenches; mountains form here Ocean crust dragged down at trenches; mountains form here Continental crust too light; remains at surface Continental crust too light; remains at surface Earthquakes occur where crust descends Earthquakes occur where crust descends “It explains everything….”

Fit of continents - new material pushes them apart Topography of ocean floors - hot ridges, trenches Volcanism at ridge axes - hot mantle material Seismic zones near margins - descending plates Seafloor Spreading - Observations

Earth has magnetic field Earth has magnetic field Similar to a giant dipole magnet Similar to a giant dipole magnet magnetic poles essentially coincide with the geographic poles magnetic poles essentially coincide with the geographic poles may result from different rotation of outer core and mantle may result from different rotation of outer core and mantle Magnetism – The Final Piece

Earth’s present magnetic field is called normal Earth’s present magnetic field is called normal magnetic north near the north geographic pole magnetic north near the north geographic pole magnetic south near the south geographic pole magnetic south near the south geographic pole At various times in the past, Earth’s magnetic field has completely reversed At various times in the past, Earth’s magnetic field has completely reversed magnetic south near the north geographic pole magnetic south near the north geographic pole magnetic north near the south geographic pole magnetic north near the south geographic pole Magnetic Reversals

When magma cools, takes on signature of Earth’s prevailing magnetic field magnetic iron-bearing minerals align with Earth’s magnetic field

What does this have to do with seafloor spreading? Mid-Atlantic Ridge

Evidence for Hess’s Hypothesis

Magnetic stripes and isotopic clocks An observed magnetic profile (blue) for the ocean floor across the East Pacific Rise is matched quite well by a calculated profile (red) based on the Earth's magnetic reversals for the past 4 million years and an assumed constant rate of movement of ocean floor away from a hypothetical spreading center (bottom). The remarkable similarity of these two profiles provided one of the clinching arguments in support of the seafloor spreading hypothesis.

Seafloor spreading theory indicates that Seafloor spreading theory indicates that oceanic crust is geologically young oceanic crust is geologically young forms during spreading forms during spreading destroyed during subduction destroyed during subduction Radiometric dating confirms young age Radiometric dating confirms young age youngest oceanic crust occurs at mid-ocean ridges youngest oceanic crust occurs at mid-ocean ridges and the oldest oceanic crust is less than 180 million years old and the oldest oceanic crust is less than 180 million years old oldest continental crust is 3.96 billion yeas old oldest continental crust is 3.96 billion yeas old Oceanic Crust Is Young

Age of Ocean Basins

Intra-Plate Volcanism Provides absolute rates and motions Fixed “hot spot” in the mantle; deep, long- lived magma chamber If hot spot is fixed then plates are moving

Hawaiian Ridge-Emperor Seamounts chain - 6,000-km-long Bend at 43 Ma indicates motion of Pacific Plate abruptly changed from north to west (??related to collision of India and Eurasia??)

Accumulation of Observations - Evidence  Patterns of continents  Paleontology  Geology  Patterns of sea floor ages  Patterns of seafloor depth  Patterns of volcanoes  Patterns of earthquakes Plate Tectonics as the Unifying Concept of Earth Science