Introduction
Plate Tectonics
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?
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
convection in some regions
• Heat from core rises, creates
convection cells in the
mantle
• 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
Alfred Wegener
Puzzle Fit of Continents
• Matching mountain • Matching glacial
ranges evidence
Matching Fossils
Mechanism for Plate Movement!
• 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
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
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
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
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
Theory of Plate Tectonics
Fit of continents Volcanism at ridge axes / hot spots
Patterns of heat flow Magnetic stripes
Ocean floor topography/Sediment “Polar” wander
patterns Seismic zones
Age patterns of seafloor 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 – Example: Isthmus
– intraplate volcanoes of Panama creates
a barrier to marine
– island arcs organisms
– mid-ocean ridges
– mountain ranges
– subduction zones
Pacific Caribbean
Summary
• Plate tectonic theory
– widely accepted by the 1970s
• It helps explain
– volcanism
– earthquake activity
– mountain building
– global climate changes
– distribution of biota and resources
