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 http://earthview.sdsu.edu/trees/tecqest.html

http://earthview.sdsu.edu/trees/tecqest.html

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

http://earthview.sdsu.edu/trees/tecqest.html

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

Convergent - Plates Moving Together

http://earthview.sdsu.edu/trees/tecqest.html

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/cm3

– Crust 2.5 to 3 g/cm3; mantle 3.3 g/cm3 to 5.5 g/cm3

– Density of core at least 10 to 11 g/cm3 (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• 3.3 - 5.5 g/cm3

• 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

CrustMantleCore

(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

• 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 continentsPatterns of heat flow Ocean floor topography/Sediment patterns Age patterns of seafloor

Volcanism at ridge axes / hot spotsMagnetic stripes “Polar” wanderSeismic zonesPatterns 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