PLATE TECTONICS
OCEANIC/CONTINENTAL CONVERGENCE
THE ANDES
o The Nazca plate moves eastwards towards the south American plate
o Rate of movement: 79mm per year
o Denser oceanic lithosphere is forced down under the more buoyant continental
lithosphere- SUBDUCTION
o Peru- Chile Trench
o Friction of the two plates causes both to fracture and deform
o Frequent shallow focus earthquake that gets deeper as the plate descends and
forms a zone of earthquake foci known as the Benioff zone
o Obduction- sediment that was on the ocean floor accumulates onto the South
American Plate forming an accretionary wedge where layers of the deformed and
metamorphosed sediments and ocean crust are thrust onto the South American
Plate along faults
o Crustal shortening increases the vertical thickness and reduces the width of the
lithosphere in the collision zone- produces the fold mountains of the Andes
o Continued subduction brings sea water into the mantle which lowers the melting
point of the mantle and causes partial melting. The magma rises and erupts
explosively as andesite at the surface
o Andesitic magma can have a temperature of 1000oc and is very viscous trapping
gases as it rises.
o Produces pyroclastic flows
PLATE TECTONICS
CONTINENTAL/CONTINENTAL
HIMALAYAS- FOLD MOUNTAINS
o Tibetan plateau
o Collision of the Indian plate and Eurasian Plate which began 50 million years ago and
continues today
o India used to be separated from Asia by the Tethys Sea.
o Supercontinent Pangea and started a northward drift
o India moved at a rate of 9-15cm per year
o Tethys Sea subducted beneath Asia and the plate margin was convergent (oceanic
continental)
o As the Tethys Sea closed neither continental plate could be subducted due to their low
density
o Continental plates thickened due to folding and faulting by the compressional forces
o Crust is 75km
o Any magma moving upwards solidifies before it could reach the surface
o Still rising by 1 cm per year which causes shallow focus earthquakes
o Weathering and erosion are lowering the Himalayas at the same rate
o Extends 2900km and reaches a maximum elevation of 8848m (Mount Everest)
PLATE TECTONICS
RIFT VALLEY- CONSTRUCTIVE
EAST AFRICAN RIFT VALLEY
o Northern Ethiopia
o A rift valley is a linear-shaped lowland between several highlands or mountain ranges
created by the action of a geologic rift or fault. A rift valley is formed on a divergent plate
boundary, a crustal extension, a spreading apart of the surface, which is subsequently
further deepened by the forces of erosion.
o Y Shaped Rift Valley where the continental lithosphere is being stretched and is splitting
o Arabian plate is rifting away from the African plate along an active divergent ridge system
o Forms the Red Sea and Gulf of Aden
o It is splitting to form two new plates: Nubian and Somalian
o The point in the afar region where the boundaries meet is called a Triple Junction
o Orientation of the rifts, highlands, and associated volcanic activity suggest that the rifting is
the result of a hot spot that may have been formed by a rising mantle plume
o Plume causes a brittle lithosphere to stretch and fracture and causes outpouring of lava
called flood basalts
PLATE TECTONICS
OCEAN TRENCHES- subduction
TONGA
o Runs between the North Island of New Zealand and the island of Tonga
o Subduction zone
o Pacific plate is converging with the Australian Plate
o Closure rates of 24cm per year have been measured with is the fastest plate motion yet
recorded
o Older oceanic crust sinks below the more buoyant younger oceanic crust of the Australian
Plate
o Forms as arcs of volcanoes above the subduction zone
o Rapid speed has caused extension of the overriding Australian Plate and the opening of a
back-arc basin (isolated basin behind a subduction zone) between the two ridges
o 36 inhabited islands
o Polynesian seafaring culture that stretched across the South Pacific
PLATE TECTONICS
CONSTRUCTIVE- OCEAN RIDGE
MID ATLANTIC RIDGE
o South American Plate is moving westward due to sea floor spreading at the Mid Atlantic
Ridge where it meets the Caribbean Plate
o Caribbean subducts beneath the American Plate
o Ocean lithosphere of the South American is cooler and denser than the Caribbean so
subducts
o Floor sediment is scrapped off the surface of the South American Plate and thrust onto the
Caribbean Islands as accretionary wedges by obduction
o Line of subduction: Puerto Rico Trench
o South American Plate drags against the overlying plate and deforms and fractures cause
shallow focus earthquake and has an earthquake foci known as a Benioff Zone
o Partial melting
o Andesitic magma is less dense and is viscous trapping gases as it rises
o Ash and pyroclastic flows
o Examples: Montserrat Volcano
o Caribbean volcanic islands form a curved linear chain or ‘volcanic island arc’ parallel to the
west of the Puerto Rico Trench.
PLATE TECTONICS
CONSERVATIVE
SAN ANDREAS FAULT (1994)
o Location:
California
Runs for 1300km
6th richest economy
o Cause:
Pacific plate moves at a different speed alongside the North American
6.7 magnitude earthquake
Plates move at about 6cm/year
o Efffects:
57 deaths
1500 seriously injured
20,000 homes with no gas
11 major roads collapsed
no transport
$20 billion loss to the economy
o Response:
School closure
United States Geological Society intervened
reconstruction and aims at mitigation