Plate Tectonics

Nota:

• _Two plates move apart, resulting in upwelling of material from the mantle to create new plates.  _ Zone of tension and crustal formation.  _Mostly occurs at mid ocean ridges (Atlantic Ridge) _ A few are found within the continents. (Great Rift Valley of East Africa)

1. Continental-Continental Plates

   Nota:

   • _Magma swells up below continental crusts, forcing continental plates to drift apart. 
   1. Rift Valleys

      Nota:

      • _Elongated depression in which the entire thickness of the lithosphere has been deformed. Often a pre-cursor to the development of a new ocean basin. 
      2. Global Distribution

         Nota:

         • Divergent plate boundaries within continents.
         1. Eg. Great Rift Valley of East Africa: Arabian plate moves away from African plate
         2. East African Rift Valley, the Rhine Valley (Northwest Europe), The Basin and Range province in Western United States, the Red Sea Rift
      3. Characteristics

         Nota:

         • _Long narrow depression bounded by straight, steep slopes, and normal faults where the entire lithosphere is deformed.  _Sides are characterised by steps and terraces. _Main physical features: Fault scarps, Horst and Graben. _If tensional forces are maintained, rift valley will lengthen and deepen -> Extending outwards the the margin of the plate -> breakup of the continent.  _ Early in the rift formation, streams+rivers flow into the low valleys -> long, arrow lakes created.
         1. Volcanicity

            Nota:

            • _Volcanoes are more composite, more explosive eruptions. Due to rhyolites in the magma by the melting of the continental crust (Rich in silica).
            1. Eg. Mount Kilimanjaro at the East African Rift Valley
         2. Seismicity

            Nota:

            • Shallow-focus earthquakes are dominant. 
   2. Block Mountains

      Nota:

      • _Block mountains are uplifted blocks with steep fault scarps. 
      1. Formation
         1. Mantle Convection Current

            Nota:

            • _Rift valleys are produced by tensional tectonic forces that occur at continental divergent plate boundaries, which tends to elongate and thin the lithosphere.  _Mantle reaches the base of the lithosphere -> Overlying crust to dome and weaken -> Mantle spread laterally as it reaches the base of the cool lithosphere -> upper crust broken down along faults and sretched. Basaltic magma is injected into the rift system. _two divergent convection flows -> plates move from the area of uprising -> broken slabs are displaced downwards -> Down-faulted valley = rift valley. _Remaining standing blocks = block mountains. _Spreading continues -> rift valley lengthen and deepen -> ocean invade the rift zone + injection of basaltic magma = new oceanic plates. 
         2. Mantle plumes at hot spots

            Nota:

            • _Rising mantle -> up-warping, arching and doming, crustal stretching, cracks formation, and eventually, rifting. _upwarping -> surface fractures into 3 parts, which radiate out to form a triple junction. _ 3 blocks move apart, central part collapse downwards -> rift valley. 3 blocks = Block mountain. _Rift valley will widen and deepen over time. Eg. The Red Sea. (Resulted from the drifting apart of the African, Arabian, and Somali plate) wide enough to be invaded by the sea. 
      2. Global Distribution

         Nota:

         • Rift Valley regions. 
      3. Two types of block mountains
         1. Horst

            Nota:

            • Horst: Block mountains with horizontal surfaces, with two pronounced fault scarps.
         2. Tilt Block

            Nota:

            • Tilt Block: Block mountains with tilted surfaces, with one pronounced fault scarps.
2. Oceanic-Oceanic Plates

   Nota:

   • Seafloor spreading
   1. Ocean Ridges
      1. Eg. Mid Atlantic Ridge: American Plates moved away from Eurasian and African plates
      2. Eg. East Pacific Rise: Nazca plate is moving away from Pacific plate
      3. Formation

         Nota:

         • Seafloor Spreading _Basaltic magma from the mantle rises to create new ocean floor at mid-oceanic ridges. On each side of the ridge, sea floor moves from the ridges towards the deep-sea trenches, where it is subducted and recycled back into the mantle.
      4. Chracteristics

         Nota:

         • _Huge features generally >1500k wide, peaks rising as much as 3Km above the surrounding ocean floor.  _Composed of prominent oceanic rift valleys as well, which can exceed 30Km in width and have walls that tower 2000km above the valley floor.  NOTE: Topography differs cuz of spreading rates. 
         1. Spreading rates

            Nota:

            • The main factor affected by spreading rate is the amt of magma generated at the rift zone. 
            1. Slow 1-5cm/year

               Nota:

               • _Rifting appears to be more episodic, where segments of ridge may remain dormant for extended intervals. _Mid-Atlantic and Mid-Indian ridges. _Oceanic ridge is higher, more rugged _Prominent rift valley develops along the axis
            2. Intermediate 5-9cm/year

               Nota:

               • _Occurs at southern part of Indian Ocean Ridge. _First valleys are shallower, often less than 200m deep. _Topography less rugged. 
            3. Faster >9cm/year

               Nota:

               • _Spreading appears to be a relatively continuous process where rifting and upwelling is occurring along the entire length of the ridge axis.  _Lower rate of cooling. _East pacific rise _Rift valleys are generally absent _Gradual profiles, gently slopping, less rugged portions of the ridges are called RISES. 
         2. Vulcanicity
            1. Basaltic shield volcanoes

               Nota:

               • Usually aligned along the rifts where the youngest rocks are found. 
            2. Black Smokers (Hot Springs)

               Nota:

               • Surface water seeps thru cracks downwards -> Heated by hot rocks lying above the magma chambers -> hot thermal waters ascend back thru the overlying crust -> leach out silica and numerous metals from the basaltic lava -> hot spring created at the surface. 
         3. Seismicity

            Nota:

            • _One of the longest active seismic belts where most of the earthquake epicentres are located continuously withing a narrow zone.  _Narrow-belt of shallow-focus earthquakes coincides with the crest of the oceanic ridge = boundaries between the divergent plates.  _Earthquakes are typically small in magnitude. 
         4. Materials

            Nota:

            • _Basaltic volcanic lava _Lava ejected into water -> Cools rapidly -> pillow lavas.

Nota:

• _Two plates move towards each other.  _One plates sinks under another.  _Subduction zone

1. Continental-Oceanic Plates (Subduction Zone)

   Nota:

   • PROCESS _Two plates converge due to the existence of the descending limbs of convection currents.  _Oceanic plates are denser than the continental crust therefore it subducts underneath and descends into the asthenosphere.  _Descending ocean plate is heated by heat from the interior and from friction created at the subduction zone and melts due to the heat and increasing pressure. 
   1. Volcanoes/Island arc

      Nota:

      • _Newly formed magma(Melted subducting plate) is less dense than the surrounding material and therefore rises through weaknesses towards the surface- Intruding the crust/Break through the surface. 

      Adjunto:

      • Volcanology
   2. Fold mountains

      Nota:

      • _Forces of collision of the two plates, cause the continental crust to be uplifted and folding and faulting results in the creation of fold mountains.  _Formed mainly by the effects of folding on layer within the upper part of the Earth's crust.
      1. Global Distribution

         Nota:

         • 2 Major Young fold mountain belts: _One belt is found surrounding the pacific Ocean. Fold mountains like Rockies and Andes are found along the eastern edge of the Pacific Ocean. _The other belt is the Eurasian-Indonesian belt stretching from Northern part of Africa (Atlas mountain), through Southern Europe (Alps) and Northern India (Himalayas) to south-east Asia.
      2. Formation
         1. Eg. Andes Fold Mountains, Nazca plate compressed the sediments of the continental shelf of south American plate
      3. Characteristics
         1. Features
            1. Fold

               Nota:

               • _Fold is a bend in rock strata formed by compression.
            2. Anticline

               Nota:

               • Upward bending arc of the fold
            3. Syncline

               Nota:

               • Trough
            4. Limbs

               Nota:

               • 2 sides of the fold
         2. Young

            Nota:

            • Characterised by high pointed peaks, steep slopes, active glaciers, deep valleys and torrential rivers. Eg. Himalayas, Rockies, Andes. 
         3. Old

            Nota:

            • _Lower in elevation because these are exposed to weathering and erosion over a longer period of time. Eg. Appalachian Mountains in USA. 
   3. Earthquakes

      Nota:

      • _Forces of collision and subduction also trigger major earthquakes within the subduction zone, along the benioff zone (Boundary between the overlying crust and the plate undergoing subduction). 
   4. Oceanic Trench

      Nota:

      • _Deep linear depressions in the ocean floor.  _Natural boundaries between two lithospheric plates.  _Lies in the subducting plate margins, these are usually flanked by volcanic arcs. Eg. Kuril trench flanked by Kuril Island Arc.
      1. Global Distribution

         Nota:

         • _50,000Km of convergent plate margins, mostly around the pacific ocean but they are also in the eastern Indian Ocean., with relatively short convergent margin segments in the Atlantic Ocean and in the Mediterranean Sea. 
         1. Major Oceanic Trenches

            Nota:

            • _The Mariana Trench - Pacific Ocean (11,033m) _Philippine Trench - Pacific Ocean (10,540m) _Japan Trench - Pacific Ocean (9000m) _Puerto Rico Trench - Atlantic Ocean (8,800m) _Peru-Chile Trench - Pacific Ocean (8065m)
         2. Oceanic-Oceanic
            1. Eg. The Marianas Trench: Fast moving Pacific Plate converges against the slower moving Philippine Plate
         3. Continental -Oceanic
            1. Eg. Peru-Chile Trench, Oceanic Nazca plate subducts below continental South American Plate
            2. Eg. Cascadian Trench, Oceanic Juan De Fuca plate subducted below North American Plate
            3. Eg. Aleutian Trench, oceanic Pacific plate subducted below North American Plate
      2. Formation

         Nota:

         • _Denser oceanic plate sink or slip beneath lighter oceanic or continental plate at the subduction zone.  _Trech is formed at the interface with the adjoining plate. _Trench is created by the friction of the cooler subducting plate pulling the leading edge of the overriding plate downward.  )As the subducting plate approaches the trench, it is first bent down to form the outer trench swell, then descends to form the gentle outer trench slope. 
      3. Characteristics

         Nota:

         • _Oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. Deepest parts of the ocean floor.  _Trench marks the position at which the flexed, subducting slab begins to descend beneth another lithospheric slab.  _Generally parallel to a volcanic island arc and about 200km from a volcanic arc. Typically extend 3-4 km below the level of the surrounding oceanic floor. 
         1. Volcanicity

            Nota:

            • Flanked by volcanic arccs
         2. Seismicity

            Nota:

            • Trenches are areas of shallow earthquake. Subduction zones are areas of shallow, intermediate and deep focus earthquakes.
2. Continental-Continental Plates (Collision Zone)

   Nota:

   • _Two continental plates meet head-on, neither is subducted as the continental rocks are relatively light. (2 plates are rigid, buoyant and of low density = no subduction) _Instead the crust buckle and are pushed either upwards or sideways. Forces of collision causes the crust to deform and any ocean sediments between the two plate are forced up -> OROGENESIS (Mountain building) _No volcanoes due to absence of a magma source due to lack of subduction.  _Deep focus earthquakes do occur.
   1. Fold Mountains
      1. Eg. Himalayas and the Tibetan Plateau: Collision of Australian-Indian Plate and Eurasian Plate
      2. Formation

         Nota:

         • _Continental crust is not recycled into the mantle but fused together and pushed upwards to form the Himalan-type of fold mountains.  _Continental crust thickens as the two continents are compressed together. _Earthquake will continue as long as the two plates move towards each other.  _Mountain building = Orogenesis. 
3. Oceanic-Oceanic Plates (Subduction Zone)

   Nota:

   • _One oceanic plate subduct under the other forming a trench.  _Older, denser plate is suducted. (Same process as Oceanic-Continental) 
   1. Oceanic Trench
   2. Island Arcs/Volcanic Arcs

      Nota:

      • _Volcanoes in a chain _Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. _Offshore volcanoes form islands, resulting in a volcanic island arc. 
      1. Continental Arc

         Nota:

         • Oceanic plate subducts below a continental plate on an adjacent plate, creating an arc-shaped mountain belt. (Not all mountain belts are formed this way)
         1. Eg. Andes volcanic arc (South America), Cascadian volcanic arc (U.S. Pacific northwest), Central America Volcanic arc
      2. Oceanic arcs

         Nota:

         • Oceanic crust subducts beneath other oceanic crust on an adjacent plate, creating a volcanic island arc. (Not all island arcs are volcanic island arcs)
         1. Eg. The Marianas and The Aleutian Islands, Japan island arc
      3. Formation

         Nota:

         • _Subducting oceanic plate is saturated with water, as in descends to greater depths -> greater temperatures and pressures -> Slab realease water into the mantle wedge overlying the descending plate-> melting of mantle (Water can lower the melting point of the mantle) _Magma rises upward to produce a linear belt of volcanoes parallel to the oceanic trench = island arc. _Oceanic lithosphere subducts beneath adjacent of continental lithosphere -> Similar belt of volcanoes will be generated on continental crust = volcanic arc

Nota:

• _Refers to a fault which runs along the boundary of a tectonic plate. _The relative motion of such plates is horizontal = left and/or right lateral motion of one plate against another along transform faults.  _Two plates grind past each other without the production or destruction of the lithosphere. (Crustal rocks are neither created nor destroyed) _Most transform faults join two segments of an oceanic ridge where they are part of prominent linear breaks in the oceanic crust known as fracture zones. 

1. Transform Fault