Download Plate Tectonics

Plate Tectonics
Plate Tectonics
► Tectonics
: bending and breaking of the
lithosphere
► Plate
tectonic theory
 explains volcanism, seismic activity, continental
movement, folding and faulting
Lithosphere : 15 plates
See pp 438-439
► Lithospheric
plates rest on soft, plastic
asthenosphere.
► Allows
plates to move away from, towards
and against one another.
 Plate boundaries
► Oceanic
and continental crust at boundaries
Types of Plate Boundaries:
1. Divergent (Spreading)
2.Convergent
3.Transform
1. Divergent (Spreading) Boundaries:
 plates pull apart
a) at oceanic/oceanic crust boundaries (most
common)
* mid-oceanic ridges (“sea-floor spreading”) on ocean
floor
b) at continental/continental crust boundaries
* rift valleys on land
a) oceanic/oceanic crust boundary
P 426
P 429
a. oceanic/oceanic crust boundary
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)
(See undersea topography pp. 430 – 431)
b) continental/continental crust boundary





cause continental rupture
rift valleys form
narrow sea may form
new oceans may form
Examples: East African Rift Valley, Iceland rift
valley, Red Sea
East African Rift Valley
Iceland
2. Convergent Boundaries
a) at oceanic/continental crust boundary
b) at continental/continental crust boundary
c) at oceanic/oceanic crust boundary
a) oceanic/continental
► 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
Andes
Pacific “Ring of Fire”
b) continental/continental
► Plates
collide; crustal rocks fold, break,
become fused in a suture
►mountain
chains
►Example: Himalayas
c) oceanic/oceanic
► Subduction
of one plate beneath another
► submarine
trench and island arc (chain of
volcanic islands)
► Example:
Aleutians
Aleutian Trench
3. Transform Boundaries
 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
Hot Spots (Mantle Plumes)
► Do
NOT occur at plate boundaries
► Individual spots of upwelling molten rock;
randomly distributed
► produce plumes of magma which pierce
lithosphere and create a volcano;
 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