Chapter 3
Plate Tectonics —
a Unifying Theory
Discovering Plate Boundaries
• 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/
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
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 http://earthview.sdsu.edu/trees/tecqest.html
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
3 Main Boundary Types
• Convergent
–
–
–
–
Ocean-continent
Ocean-ocean
Continent-continent
Oceanic crust subducts – cold, dense
• Divergent
– New oceanic crust created
• Transform
– Often difficult to recognize
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
• 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
Our deepest hole:
9 Kilometers ….. we have a
long way to go!
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
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?
Earth - 3 Heat Sources:
• 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
Convection: Driving Force of Plate Tectonics
• Interior of Earth has
sluggish convection in some
regions
• Heat from core rises, creates
convection cells in the
mantle
NOT LIQUID
• Rising hot material at mid-ocean ridges and mid-ocean
volcanic islands
• Descending cooler material at trenches
• Lithospheric plates “carried” with the convection cells
Plate Tectonics as the Unifying
Concept of Earth Science
Accumulation of
Observations Evidence
 Patterns of continents
 Paleontology
 Geology




Patterns of sea floor ages
Patterns of seafloor depth
Patterns of volcanoes
Patterns of earthquakes
• 1912 Continental Drift
Observations
• Fit of Continents
• Geology
• Paleontology
• Climate belts
• Pangaea 200 Ma
• Breakup 180 Ma
• Rigid bodies moving through
Alfred Wegener
yielding seafloor
• No mechanism of movement
Jigsaw-Puzzle Fit of Continents
• Matching mountain
ranges
• Matching glacial
evidence
Matching Fossils
Mechanism for Plate Movement!
• Author Holmes (Early 20’s)
• Interior of Earth has sluggish
convection (transport of heat from
core)
• New ocean crust injected into
ocean floor (where?)
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….”
Seafloor Spreading - Observations
• 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
Magnetism – The Final Piece
• 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
Magnetic Reversals
• 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
Mapping Ocean Basins
• 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
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
Confirmation of Hess’s Hypothesis
• The magnetic anomalies were discovered to
be striped, parallel to the oceanic ridges
and symmetrical with the ridges
How Do Magnetic Reversals
Relate to Seafloor Spreading?
Oceanic Crust Is Young
• 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
Age of Ocean Basins
Building Continental Material:
Observable Trends?
How Fast Do Plates Move?
What Absolute Direction?
How Do We Know?
Intra-Plate Volcanism
• 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
• 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
Pacific
Caribbean
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