Download Plate Boundaries

Plate Boundaries
Lecture #5
What Is a Plate?
• Tectonic plate: section of lithosphere
! Outer shell of the Earth
! Cold
! Brittle
! Includes crust and part of mantle
(to ~100 km)
What Is a Plate? (cont.)
• Asthenosphere
! Layer under lithosphere
! Hotter
! Softer
! Able to flow
! ~100 km to ~400 km
(all in mantle)
Lithosphere & Asthenosphere
What Is a Plate? (cont.)
• May contain:
! Only oceanic lithosphere
! Oceanic + continental lithosphere
• Changes size with time
Plates of the World
What Is a Plate? (cont.)
• Tectonic activity:
! Occurs mainly at plate boundaries
o Volcanoes
o Earthquakes
o Mountain building
• Plates outlined by EQs
Earthquakes Outline Plates
Plate Boundaries
• 3 types
• Stand on 1 plate, by boundary. Other plate?
! Moves away from you
! Moves toward you
! Slips past you
1. Divergent Boundaries
• Plates move apart
• Mid-ocean ridge
! Underwater volcanic mtns
! Central rift valley
• Molten rock rises from asthenosphere
! Cools > new lithosphere
• Oceanic rock moves
! Away from MOR
! On both sides
1. Divergent Boundaries (cont.)
• At MOR
! Non-explosive lava flows
! Minor earthquakes
• Ocean basins grow wider
• Continents move apart
1. Divergent Boundaries (cont.)
• Shape of MOR
! Many segments
! From rifting
of Pangaea
! Faults between
Examples
Chile Rise
Red Sea Rift
Central Indian Ridge
Plate Boundaries
• 3 types
• Stand on 1 plate, by boundary. Other plate?
Moves away from you
! Moves toward you
! Slips past you
2. Convergent Boundaries
• Plates move toward each other
• Marked by trench
• Also called subduction zone
! Old oceanic lithosphere bends…
! Sinks into mantle…
! Is destroyed
New o.l. at MOR ! old o.l. at trench
Exploration Qs: How Far
Down Do Plates Go?
• Debate
! Asthenosphere
" Young hot plate
! Lower mantle
" Old cold plate
! Resorbed into mantle
2. Convergent Boundaries (cont.)
• Subduction zones
! Explosive eruptions
! Major EQs
• Oceans get smaller
• Continents move closer together
A Convergent Boundary
• New continental crust forms here
2. Convergent Boundaries (cont.)
• Three types:
a) Ocean-ocean
! 2 oceanic plates
! One subducts - the older (heavier) one
! Volcanic island arc forms
Ocean-Ocean Convergence
2. Convergent Boundaries (cont.)
• Three types:
b) Ocean-continent
! 1 oceanic plate + 1 continental plate
! Oceanic plate subducts
! Continental volcanic arc forms
Ocean-Continent Convergence
2. Convergent boundaries (cont.)
• Three types:
c) Continent-continent
! 2 continental plates - Which subducts?
! Neither subducts - they collide!
! Mountain range forms
Two Stages of ContinentContinent Convergence
Examples
Continent-continent (Alps)
Oceanic-continental (Andes)
Oceanic-oceanic (Japan)
Plate Boundaries
• 3 types
• Stand on 1 plate, by boundary. Other plate?
Moves away from you
Moves toward you
! Slips past you
3. Transform Boundaries
• 2 plates slide past one another
• Marked by transform fault
! No volcanoes
! No new crust forms
! No old crust destroyed
! Major EQs
A Transform Fault
Another Look
3. Transform Boundaries (cont.)
• Most under oceans
• Connect segments of MOR
• Or connect MOR to subduction zones
Offset of MOR
Examples
Dead Sea Fault
Queen Charlotte Fault
Alpine Fault