Plate Tectonics
A. Plate Tectonics:
defintions
B. Plate Boundaries
1. Divergent (Spreading)
a. continental / oceanic
b. oceanic / oceanic
2. Convergent
a. oceanic/continental
b. continental/continental
c. oceanic/oceanic
3. Transform
C. Hot Spots (Mantle Plumes)
A. Plate Tectonics Definitions
Tectonics : bending and breaking of the lithosphere
Plate tectonic theory
explains volcanism, seismic activity, continental movement, folding and faulting
Lithosphere : approx. 15 large plates (many small plates)
Lithospheric plates rest on soft, plastic asthenosphere.
Allows plates to move away from, towards and against one another.
at plate boundaries.
B. Types of Plate Boundaries:
1. Divergent (Spreading) Boundaries: plates pull apart
a) at oceanic/oceanic crust boundaries (most common)
* mid-oceanic ridges (“sea-floor spreading”) on ocean floor
sea-floor spreading: as plates beneath oceans spread, magma wells up from mantle and
solidifies as new ocean floor
resulting ridge of igneous rock: mid-oceanic ridge (axial rift)
b) at continental/continental crust boundaries
* rift valleys on land
continental rupture
rift valleys form; narrow sea may form; new oceans may form
Examples: East African Rift Valley, Iceland rift valley, Red Sea
2. Convergent Plate Boundaries – plates move toward one another
a) at oceanic/continental crust boundary
Oceanic crust is thinner and denser; it plunges into the soft asthenosphere beneath continent in
a process called subduction.
Ocean floor trench forms at subduction zone
On land, a chain of volcanic mountains parallels the subduction zone
Earthquakes
Examples: Andes, Cascade Range, Pacific “Ring of Fire”
b) at continental/continental crust boundary
Plates collide; crustal rocks fold, break, become fused in a suture : forms mountains
Example: Himalayas
c) at oceanic/oceanic crust boundary
Subduction of one plate beneath another
submarine trench and island arc (chain of volcanic islands)
Example: Aleutian Islands, Aleutian Trench
3. Transform Boundaries: plates move laterally past one another
2 plates move past one another in opposite directions laterally; plates “stick” as they move;
tremendous strain builds up and is released in earthquakes
most transform boundaries occur along mid-oceanic ridges, parallel to direction of plate
movement
Example: San Andreas Fault
C. Hot Spots (Mantle Plumes)
Individual points of upwelling molten rock; randomly distributed
Do NOT necessarily occur at plate boundaries.
plumes of magma pierce lithosphere, creating volcanoes
as plate moves, chain of volcanic islands develops
sea mounts (guyots) :volcanoes which do not surface above sea level
Examples of hot spots: Hawaiian Islands, Yellowstone, Iceland