How do mountains form?
• What is a mountain?
• Geologically speaking = where the crust is thicker….
• Crust made thicker and shorter
How do mountains form?
• Geosynclines - mountain belts
• Thick sedimentary packages and crustal warping
• Cooling and shrinking earth
• Assumes that the continents are stationary
How do mountains form?
• Why are most mountain belts situated near the edges of the continents?
• Drifting Continents
Continental Drift
• Wegener’s Hypothesis (1912)
• Continents have drifted
• Pangaea
Continental Drift
• Wegener’s (and others) evidence:
• Jigsaw fit of the continent’s edges
Continental Drift
• Paleoclimate indicators at wrong latitudes
- glacial deposits (till & striations)Continental Drift
• Paleoclimate indicators at wrong latitudes (cont.)
- Coal
- Reefs
- Eolian Deposits
- Evaporites
Continental Drift
• Fossil Evidence => fossil species match between continents separated
by oceans
Problems with Wegener’s theory
• an inadequate mechanism
• floating continents - plowing through thin brittle crust
• movement by centrifugal force (earth’s rotation)
• How to explain the oceans
Problems with Wegener’s theory
• Wegner’s Pangaea did exist…….
- continents didn’t drift apart - moved apart by sea floor spreading
Historical development of Plate Tectonics
• For 30 years - stationary continent model (geosynclinal theory)
• Continued study of the earth eventually led to discovery of plate tectonics
• Sea Floor Topography
• Earth’s Magnetic Field
The earth’s magnetic field
• Movement of iron alloy in the outer core = magnetic field
• The earth’s dipole - an arrow that points from the north pole
towards the south
– How does a compass work?
• Magnetic declination
• Magnetic inclination
Earth’s past magnetic field preserved in rocks
• Rocks that contain iron-rich minerals can preserve the magnetic field
of earth that exists at the time that they form
– dipole
• Basaltic lava flows
• Ocean crust
• Direction & inclination
• Paleogeographic tool
Apparent polar-wonder
• Paleomagnetism measured in samples from around the world - began to
see interesting pattern
• The dipole (direction & inclination) preserved in lava rocks from
different ages and places appeared to show that the position of the north pole
had changed over time
• Continents changed position through time - not the poles
The solution = sea floor spreading
• 1960 - Harry Hess proposes sea-floor spreading
Magnetic Anomalies and Reversals - the final straw
• Magnetic anomalies discovered on the ocean floor
– Imaginary stripes of + and - parallel to the mid-ocean ridges
• Paleomagnetism demonstrates that the polarity of the earth’s magnetic
field changes over time = magnetic reversals
Magnetic anomalies and reversals = related
• Oceanic crust recording magnetic reversals
• As new oceanic crust added at the mid-ocean ridges -material acquires
polarity that exists at the time
• Illustrates sea-floor spreading
Sea Floor Spreading=> oceans = natural strip recorders
• Magnetism, age, etc.
Parts of the earth involved
- Crust and outer Mantle
- Lithosphere = rigid plates
- Asthenophere = plastic
- Plates of lithosphere float in gravitational balance = isostacy
Divergent Boundaries = Formation of the Oceans........
• Sea Floor Spreading
• Divergent Boundaries => tensional forces
• ocean-ocean divergence => mid-ocean ridges form....
• ex.: Mid-Atlantic Ridge
How do Divergent Boundaries Form?
• continental rifting
• continent-continent divergence
• most oceans began this way - proto MOR’s
• ex.: Arabian Shield & East African Rift
Transform Boundaries
• shear forces
• plates slide past one another
• transform faults
• form perpendicular to MOR’s
• ex.: San Andreas Fault
Convergent Boundaries
• compressional forces
• ocean - continent convergent boundaries (subduction zones)
• Andean Arcs
• subduction of denser crust at trench
• causes melting and volcanism• ocean - ocean convergent boundaries
(subduction zones)
- Island Arcs
Other Convergent Boundaries
• continent -continent => continental crust clogs a subduction zone
• Begin as ocean-continent convergence….ocean crust is completely
subducted in front of a trailing continent
• continental collision
• Suture Zone
What Drives Plate Motion
• Mantle Convection
• Ridge-push
• Slab-pull
Rates of Plate Motion
• Plate velocity = slow
• Typical rates = 1-15 cm/yr
Plate Tectonics and Sedimentary Basins
• Basin Types
• Trench
• Forearc
• Foreland
• Intracratonic
• Rift Basin - passive margin
• Requires subsidence
Plate Tectonics and Sedimentary Basins
• Types of subsidence
• Subduction subsidence
• Crustal-thinning subsidence
• Crustal loading - sediment, thrust
• Bassin reflects tectonic setting