theory of plate tectonics Flashcards
what is lithosphere
lithosphere is made up of solid crust and solid upper most layer of the mantle. this layer is broken up into large units called lithospheric plates
what is asthenosphere
is the layer beneath the lithosphere and is much hotter than the lithosphere. it is ductile and capable of slow flowage at a rate of few centimeters per year making plate movements possible
difference between oceanic and continental crust
continental is between 25-70km thick, very young to nearly 4 billion years old, made of Granite, density is 2.7g/cm3, pale coloured.
oceanic is between 5 to 8km thick, less than 200 million years old, made of basalt, density is 3.0g/cm3, dark coloured
mantle convection current
thought to be the most significant forces driving plate motion. the most basic source of energy for tectonic movement is earth’s internal heat released by the core mantle boundary. the heat released causes rocks near the boundary to expand due to increase in temperature and slight density decrease. the subsequent increase in buoyancy causes hot mantle rock to rise through the asthenosphere to the base of lithosphere. at the base of lithosphere, the hot mantle rock move laterally, releases heat and cools. the lateral movement of the mantle rock in the asthenosphere drags the lithospheric plate above along. plates move apart from each other when convection currents diverge and towards each other when convection currents converge. heat loss from the rock causes it to contract to a smaller volume, hence density increases and the hot mantle rock sinks beneath the asthenosphere to remix with the deeper mantle. once sinking begins, the rock rises slowly from deep inside the earth the balance downward flow again, maintaining convection motion,
slab pull force
other than mantle convection current, slab pull force is said to be the key driving force for tectonic movement. it occurs when a denser oceanic plate is formed beneath a less dense oceanic/ continental plate. when the lithospheric oceanic plate subducts, it directly drags the rest of the plate along the subduction zone as it is denser than the underlying asthenosphere. plate movement is thus partly driven by weight of cold, dense plate sinking into the mantle at the ocean trench.
what are the three types of plate boundaries
divergent, convergent and transform plate boundaries
what is divergent plate boundary
also termed constructive plate boundaries as new crust is formed at this location. they are zones of tension where plates split and are pulled apart
rift valleys and block mountains
where continental-continental divergence occurs, large tract of land may be broken up by faults. Rift Valley and block mountains are produced when vertical displacement occurs along these faults.
Rift valleys are lowlands formed when land between two continental crust subsides during rifting
block mountains are the slabs of crust left upstanding by the subsidence of rift valley floors
examples of rift valley
most extensive rift valley system is the East African Rift System. At many points along the rift valley, there are bold and high cliff like fault scarps standing at about 600m or more above the valley floor. more commonly, parallel faults are also found on each side of the valley, these can take the form of step faulting where the spaces between each parallel fault is bigger or grid faulting where the spaces between faults are smaller.
volcanoes are frequently found on the floor of rift valleys. The earth crust thins and stretches due to divergence which causes crustal weaknesses causing lava to move up to the surface to form volcanoes such as mount killimanjaro and mount kenya
a series of lake too are founded along the length of the rift valley. They can either be extremely alkaline and saline like Soda lakes where sodium carbonate concentration is the highest like mount natron or freshwater lakes such as lake Malawi
mid ocean ridges
mid ocean ridges are the most prominent structures seen on the sea floor produced by oceanic oceanic divergence. these ridges are tens to thousands of kilometers long, hundreds of kilometers wide and stands at 0.3km to 6km above the seafloor. Basaltic lava flow occurs at the central rift valley. rapid cooling underwater of these lava forms pillow lava.
when two oceanic plates diverge, magma rises to fill the gap. new seafloor is formed when magma cools and solidifies. this process is known as sea floor spreading. this crust accumulates overtime to form mid ocean ridges. newly formed rocks are closer to the plate boundary and are younger than rocks that are further away
examples of mid ocean ridges
the mid Atlantic ridge is the most prominent mid ocean ridge in the world and part of the longest mountain range. it is formed due to the divergence of the North America and Eurasian plate. and that of the South America plate and the Africa plate are roughly equidistant between continent to either side of it. Another dominant feature is the great transform faults which give the ridges their characteristics of staggered paths like stiches on a baseball. volcanic islands are also located on or close to the ridges like the azores and ascenzions island of the mid Atlantic ridge.
Iceland is the only mid ocean ridge which appears above sea level as it sits above a mantle plume. this accounts for alot of the basaltic magma that can lead to basaltic crust that is twice as thick as normal oceanic crust. this abnormally hot mantle plume causes the land to be elevated so much that it rises above sea level
what is convergent plate boundary landform
convergent plate boundaries also known as destructive plates as old crust is being destroyed at these sites. plates move towards each other. main stresses and dominant features of these sites are due to compression
state and explain the different type of convergence
OO convergence: both plates at a convergent boundary contains oceanic crust. the denser of the two subducts under the other and descends to the asthenosphere to form ocean trenches and volcanic arcs
OC convergence: when continental and oceanic plate collide with each other. thinner and denser basaltic oceanic plate will subduct under the thicker and less dense granitic continental plate and give way the formation of ocean trenches and volcanic arcs. the continental plate folds and forms Andean type fold mountains
CC convergence. when both converging plates contain continental crust, neither can subduct into the mantle hence continents are compressed leading to formation of Himalayan type fold mountains
what are ocean trenches
ocean trenches are long narrow throughs in the ocean bed that are usually 7500m to 10000m deep. it is formed when denser oceanic plate subducts into the mantle after converging with another oceanic or continental plate
CO convergence: peru chile trench which borders the west coast of South America. the trenches, coastlines and andes mountains all follow a roughly parallel course
OO convergence: Mariana trench is the deepest ocean trench in the world known for charateritstic arch form and flaked by volcanic islands
volcanic island arcs
when two oceanic plates collide, the heavier of the two subducts producing a chain of volcanic islands parallel to the ocean trench.
increased temp and pressure experienced by the rock as heavier oceanic plate descends causes it to rise to the above mantle. the presence of water in the oceanic plate locked up in the basaltic rock reduces the melting point of the rock causing some of it to melt. part of the magma generated rises above the mantle and the overlying lithosphere to erupt. a chain of volcanic island is formed constituting island arcs that are about 100km away from the plate boundary.
violent volcanic eruptions on these islands due to the production of andesitic lava. as magma ascends, it becomes less dense but more vicious. as pressure is relieved, dissolved water in magma expands quickly causing violent volcanic eruptions. Japanese islands and Aleutian islands are two examples of island arcs
