Plate Tectonics — a Unifying Theory Chapter 3

Classified boundaries based on OBSERVATIONS of PATTERNS: – Topographic expression – Seismicity - distribution and depth of earthquakes – Volcanology - distribution of volcanoes – Age of the sea floor terra.rice.edu/plateboundary/ Discovering Plate Boundaries

Boundary Observations: Ocean floor boundaries with positive topography Shallow earthquakes Volcanism, basalt (low silica) Symmetric age patterns; young age ? Anomalies Divergent - Plates Moving Apart

Boundary Observations: Deep ocean trench near high positive relief Shallow, intermediate, and deep earthquakes Volcanism, andesite (intermediate silica) Asymmetric age patterns; often older ages ? Anomalies Convergent - Plates Moving Together

Boundary Observations: High positive relief; continental crust Shallow and intermediate earthquakes Volcanism, andesite (intermediate silica) No age information ? Anomalies Convergent - Plates Moving Together

Boundary Observations: No topographic markers Shallow earthquakes No volcanism Asymmetric age patterns ? Anomalies Transform - Plates Moving Past Each Other

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

Continent-Continent Collision

Transform Boundaries

Convergent –Ocean-continent –Ocean-ocean –Continent-continent –Oceanic crust subducts – cold, dense Divergent –New oceanic crust created Transform –Often difficult to recognize 3 Main Boundary Types

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

Earth: three main layers defined by composition: Crust - Outer Mantle - Middle Core - Center

Composition - How Do We Know? Best Guess! Whole Earth Meteorites - Fe, Ni (same age as Earth) Information from velocities of seismic waves indicate material Crust (5-40 Km) Samples (mountain building helps!) Mantle (5/40 to 2885 Km) Kimberlite pipes - intrusive igneous rock from the mantle Lava / volcanic rock Mountain building Core (2885 to 6371 Km) Inference –Earth’s mean density = 5.5 g/cm 3 –Crust 2.5 to 3 g/cm 3 ; mantle 3.3 g/cm 3 to 5.5 g/cm 3 –Density of core at least 10 to 11 g/cm 3 (iron and nickel)

Crust Our deepest hole: 9 Kilometers ….. we have a long way to go! Two types of crust: –Continental 30% of crust Granites and Diorites - rich in silicates and feldspars (lighter materials) 40 Km thick Oldest is 4.0 billion years (90% solar system age; missing ~600 m.y.) –Oceanic crust Basalt - Mg, Fe (heavier materials) 5-10 Km thick 200 Ma oldest; 100 Ma average

Mantle MOHO - Mohorovic Discontinuity Core mantle boundary - change in mineralogy Density - getting heavier g/cm 3 Probably material such as peridotite (lots of heavy olivine - Fe, Mg) Samples from kimberlites, xenoliths in volcanic eruptions, basalt composition; lab experiments

Core Outer core –Molten, near solid point (does not transmit certain seismic waves) –Density of pure iron or nickel/iron –Includes ~ half of diameter of Earth –2x density of mantle Inner core –Solid (higher pressure than outer core) –Density of pure iron or nickel/iron –~ Size of moon

Crust Mantle Core (Composition) Earth: three main layers defined by mechanical properties - strength: Lithosphere Asthenosphere Mesosphere

Lithosphere –PLATES in Plate Tectonics –Upper 100 km –Crust and upper mantle –Rigid Asthenosphere –100 km to ~700 Km –Upper mantle –Near melting point; little strength; ductile - NOT A LIQUID! –Plates moving on this –Magma generation Mesosphere –Extends to core –Also hot; strong due to pressure

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 Heat from core rises, creates convection cells in the mantle NOT LIQUID Convection: Driving Force of Plate Tectonics

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

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

1912 Continental Drift Observations Fit of Continents Geology Paleontology Climate belts Pangaea 200 Ma Breakup 180 Ma Rigid bodies moving through yielding seafloor No mechanism of movement Alfred Wegener

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

Matching Fossils

Author Holmes (Early 20’s) Interior of Earth has sluggish convection (transport of heat from core) New ocean crust injected into ocean floor (where?) Mechanism for Plate Movement!

Harry Hess and Seafloor Spreading Crust moves with convection currents New ocean crust at MOR’s Ocean crust dragged down at trenches; mountains form here Continental crust too light; remains at surface 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 Similar to a giant dipole magnet –magnetic poles essentially coincide with the geographic poles –may result from different rotation of outer core and mantle Magnetism – The Final Piece

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

Measuring paleomagnetism and dating continental lava flows lead to –the realization that magnetic reversals existed –the establishment of a magnetic reversal time scale Magnetic Reversals

Ocean mapping revealed –a ridge system 65,000 km long –the most extensive mountain range in the world The Mid-Atlantic Ridge –is the best known –divides Atlantic Ocean basin in two nearly equal parts Mapping Ocean Basins

Atlantic Ocean Basin Mid-Atlantic Ridge

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

How would you test this?

Confirmation of Hess’s Hypothesis

The magnetic anomalies were discovered to be parallel to the oceanic ridges striped, and symmetrical with the ridges Confirmation of Hess’s Hypothesis

How Do Magnetic Reversals Relate to Seafloor Spreading?

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

Age of Ocean Basins

Building Continental Material: Observable Trends?

How Fast Do Plates Move? What Absolute Direction? How Do We Know?

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

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??)

Theory of Plate Tectonics Fit of continents Patterns of heat flow Ocean floor topography/Sediment patterns Age patterns of seafloor Volcanism at ridge axes / hot spots Magnetic stripes “Polar” wander Seismic zones Patterns of mountains

How Does Plate Tectonics Affect the Distribution of Life? Present distribution of plants and animals –climate –geographic barriers Barriers create biotic provinces –distinctive assemblage of plants and animals Plate movements largely control barriers –when continents break up, new provinces form –when continents come together, fewer provinces result –as continents move north or south they move across temperature barriers

How Does Plate Tectonics Affect the Distribution of Life? Physical barriers caused by plate movements include –intraplate volcanoes –island arcs –mid-ocean ridges –mountain ranges –subduction zones –Example: Isthmus of Panama creates a barrier to marine organisms CaribbeanPacific

Summary Plate tectonic theory –widely accepted by the 1970s –overwhelming evidence supporting it It helps explain –volcanism –earthquake activity –mountain building –global climate changes –distribution of biota and resources