Geography - Physical Geography Elective

CHAPTER 1 - GATEWAY 1

• What is a natural hazard?
• naturally occurring events that threaten human lives and causes damage to property
• Tectonic natural hazard
• caused by plate movements when continental crusts and ocean floors move.
• earthquake
• volcanic eruptions
• Climate-related natural hazard
• caused by severe and extreme weather and climate conditions
• storms
• floods

INTERNAL STRUCTURE OF EARTH

• Core
• mostly iron and nickel
• separated into liquid outer core and solid inner core
• about 3500km thick
• temperature between 3000 and 5000-degree celsius
• Mantle
• mostly solid rock that flow under high temperature and pressure
• 2900km thick
• temperature between 800 and 3000-degree celsius
• Crust
• basalt and granite rocks
• a few km to more than 70km thick
• !! lithosphere - crust & uppermost mantle
• rocks in lithosphere melt
• magma formed
• TYPE OF CRUST
• 1. Oceanic crust
• beneath deep oceans
• between 5 and 8km
• consists mainly basalt, heavy and dense rock formed by cooled magma
• rocks less than 200 million years old
• 2. Continental crust
• beneath earth's continental land masses and under shallow seas close to continents
• between 35 and 70 km
• consists of lighter rocks including granite
• rocks from recently to 4 billion years old

WHY DO TECTONIC PLATES MOVE?

• 1. Convection currents
• movements of heat within the mantle.
• materials in the mantle heated by core
• mantle material expand, rise and expand beneath the plates
• -this causes plates to be dragged along
• -and move away from each other
• >hot mantle material cools slightly
• >sink and pull plates along
• -sinking mantle material heats up again near core
• -whole process repeats
• 2. Slab-pull force
• occurs when the dense, sinking oceanic plate at subduction zone pulls the rest of the plate behind it
• -subducting or sinking plate drives downward-moving portion of convection current
• >mantle material away from where plates subduct drives rising portion of convection current
• !! USE "PLATES" INSTEAD OF "CRUST"
• tectonic plates move a few cm a year

PLATE BOUNDARIES

• 1. DIVERGENT
• Plates move away from each other
• Magma move to surface
• Cools to form new oceanic crust
• 2. CONVERGENT
• Plates move towards each other
• become faulted, folded and sometimes subducted
• 3. TRANSFORM
• 

DIVERGENT

• 1. OCEANIC-OCEANIC PLATE
• when 2 oceanic plates diverge, fractures are formed
• magma rises at zone of divergence to create ridge of new oceanic floor; sea-floor spreading
• when 2 oceanic plates diverge, mid-oceanic ridge is formed
• at various point along MOR, magma builds up and solidify forming undersea volcanoes; shield volcanoes
• when these volcanoes grow above sea level, they are known as volcanic island
• EXAMPLE mid-atlantic ridge: north american plate & eurasian plate
• azores: a chain of volcanic island in north atlantic ridge
• !! youngest rocks are closest to middle of ridge
• why? new crust form continuously. old crust pushed further away from plate boundary
• 

DIVERGENT

• 2. CONTINENTAL-CONTINENTAL PLATE
• when plates diverge, they are stretched, causing fractures to form; faulting
• when faulting occurs, tensional forces can cause a central block of land to subside between parallel faults, forming rift valley
• block mountains can be formed too
• EXAMPLE east african rift valley: somalian & nubian plate boundary of african plate
• hutt valley, new zealand: australian plate & pacific plate
• black forest, germany: eurasian plate & north american plate
• 

CONVERGENT

• 1.OCEANIC-OCEANIC PLATE
• when 2 oceanic plates converge & collide, the denser oceanic plate will subduct beneath the less dense oceanic plate; subduction zone
• a deep oceanic trench is formed at the point of subduction
• the subducted ocean plate causes mantle material above it to melt, forming magma
• magma rises through the cracks in the crust and form volcanoes; eventually a chain/arc of island
• earthquake may occur due to friction created
• EXAMPLE mariana island & mariana trench: pacific plate & phillipine plate
• 

CONVERGENT

• 2. CONTINENTAL-CONTINENTAL PLATE
• 2 continental plates may collide and push against one another
• they resist subduction because both plates are too thick and buoyant for subduction to occur
• this causes plates to break and slide along fractures in the crust
• layers of rock on the upper part of the crust are then compressed together, they fold sideways/upwards creating fold mountains; folding
• EXAMPLE himalayas: indian plate & eurasian plate
• 

CONVERGENT

• 3. OCEANIC-CONTINENTAL PLATE
• when an oceanic plate converges with a continental plate, the denser oceanic plate subducts beneath the less dense continental plate
• an oceanic trench is formed at subduction zone
• the edges of continental plate buckles and folds, forming fold mountains
• at subduction zone, sinking plate causes mantle material to melt, forming magma
• magma rises through the cracks, giving rise to volcanic eruptions
• EXAMPLE sunda trench: australian plate & eurasian plate
• 

TRANSFORM

• 1. TRANSFORM
• plates slide past each other along transform plate boundaries
• this results in the formation of a transform fault
• as they do so, tremendous stress builds up
• this stress is eventually released as violent earthquake and tsunami
• EXAMPLE san andreas fault, usa: pacific plate & north american plate
• anatolian fault, turkey: eurasian plate & anatolian plate
• 

LANDFORMS AT PLATE BOUNDARIES

• 1. DIVERGENT PLATE BOUNDARY
• rift valley
• -a valley formed with steep slope
• -formed along faults
• block mountain
• -a block of land with a steep slope
• -when sections of crust along fault lines sink due to tensional force
• 

LANDFORMS AT PLATE BOUNDARIES

• 2. CONVERGENT PLATE BOUNDARY
• fold mountains
• -when there is increasing compressional force on 1 limb of a fold, the rocks may buckle until a fracture forms.
• -the limb may then move forward to ride over the other limb
• !! sedimentary rocks: formed from multiple layers of sediments
• igneous rocks: formed when molten rocks cool and solidify
• metamorphic rocks: formed when rocks are charged by high temperatures/ pressure
• 
• Caption: syncline- v anticline-A

VOLCANOES

• DEFINITION
• landform formed by magma ejected from the mantle onto the earth's surface
• TYPES
• shield volcano
• -low silica, low viscosity
• -gently sloping sides, wide summit
• -non-explosive (gas escapes easily)
• EXAMPLE mount washington
• !! flows over long distance
• ​​​​​​​​​​​​​stratovolcano volcano
• -high silica, high viscosity
• -steeper at top, gentler at base
• -explosive (traps gas) ; pyroclastic flow, lahars
• EXAMPLE mount pinatubo
• !! flows over short distance + ash
• ?? viscosity: stickiness of the lava/ its resistance to flow​​​​​​​

VOLCANOES

• FORMATION
• more magma seeps into magma chamber, amount of pressure in chamber builds up giving rise to volcanic eruption
• when stratovolcano erupts, pyroclasts are released; force of volcanic eruption depends on amount of pressure being built up
• new eruption of lava covers pyroclasts and builds up volcano
• lava builds up around the vent, solidifying to form a small volcanic cone; crater
• during formation, vent may become blocked; secondary cone develops as magma is forced to find new exit route
• summit of volcano may be blown off during an explosive eruption; sides of crater collapse inwards due to loss of structural support; caldera
• 

SHIELD VOLCANOES

• formed where low silica lava ejected
• low silica lava less viscous and flows easily, spreading out over larger area
• results in gently sloping sides and broad summit
• less viscuos, does not trap much gas, not explosive
• divergent plate boudaries
• EXAMPLE mount washington, usa
• 

STRATOVOLCANOES

• formed where high silica lava ejected
• high silica lava more viscous and traps heat more easily
• results in pressure building up
• as magma rises to surface, gases able to expand, causing outward explosion
• eruption ejects lava and pyroclasts
• successive erueption builds high volcano
• with steeper side and gentler base
• when pyroclasts mix with super heated gases; pyroclastic flow, 200m/sec
• pyroclasts mix with water; lahars, fast flowing muflow, 40m/sec
• EXAMPLE mount pinatubo, philippines, 1991, 10m tonnes of lava and 20m tonnes of ash
• mount merapi, indonesia
• 

DISTRIBUTION OF VOLCANOES

• most earthquake and volcanic eruption; pacific ring of fire
• PROF found along convergent plate boundary
• volcanoes not only PROF but also other places like atlantic ocean and east africa
• hence, there is a close correlation between locations of plate boundaries and distribution of major active volcanoes