Plate Tectonics 2 Flashcards
Give two types of divergence at constructive plates
Seafloor spreading
Rift Valley
What type of lava is found at constructive late margins
Basaltic Lava (runny) - low silica content
Give two examples of constructive margins
Mid Atlantic Ridge
African Rift Valley
Is old or young rock found at constructive margins
Young - New crust is being formed as the gap created by the spreading plates is filled by the magma rising from the asthenosphere.
As the magma cools it solidifies to form new rock
Describe the characteristics of Ocean ridges
Can rise up to 4000m above the ocean floor - middle of the ridges are marked by deep rift valleys
Over centuries the rift valleys are widend by magma rising from the asthenosphere, which cools and solifies to form new crust
Volcanic eruptions along the ridges can build submarine volcanes - which may eventually grow to rise above sea level creating volcanic islands
Can constructive plate bpundires (ocean ridges) cause earthquakes
Yes
Regular breaks (transform faults) cut across ridges at right angles to the plate boundary - seperating sections of the ridge
They may widen at different rates which leads to frictiona stesses huilding up - shallow focus earthquakes
Describe the location of the Mid-Atlantic Ridge
Submerged mountain range
15,000km through Atlantic Ocean from iceland to a point 7,200km east of Argentina
Giev some of the volcanic islands formed by the Mid-Atlantic Ridge
Iceland
Tristan du Cunha
How fast is the Mid Atlantic Ridge widening
3cm per year
Explain sea floor spreading
Hot buoyant mantle rises up a mid-ocean ridge, causing the ridge to rise upward.
The hot magma at the ridge erupts as lava that forms new seafloor.
When the lava cools, the magnetite crystals take on the current magnetic polarity and as more lava erupts, it pushes the seafloor horizontally away from ridge axis.
The magnetic stripes continue across the seafloor. As oceanic crust forms and spreads, moving away from the ridge crest, it pushes the oceanic lithosphere away from the ridge axis.
What is a rift valley
Continental divergence
Lithosphere streches, causing it to fracture into sets of parallel faults.
The land between these faults collpases into deep, wide valleys that are seperated by upriht blocks of land called horsts
Minor earthquakes + risingmagma forming shield volcanoes (Mt Killimangaro)
How do rift valleys form a new sea
The deep valleys fall bellow sea level so a linear sea starts to form. As the plates continue to split this will become a new ocean
At destructive plate bounders, if both are oceanic plates ten which one will subduct
Same density so the one with the largest mass (pacific plate)
What landfroms are created at destructiove margins
Composite cone volcanoes (oceainic continental + oceanic - oceanic)
Fold mountains (continental - continental)
Is old or young rock created at destructieb boudnies
At subductive (oceainic continental + oceanic - oceanic) young rock will be created. Subduction esnures mantel material rises above the plate surface and cools to form new rock
At fold mountians, old rock will be formed as their is nto vlcanic activity (magma not present)
What is the type/charatristic of lava found at destructive
Viscious, high levels of ash/silica, explosive (andesitic)
Why is destruictive lava explosive
Sea water dragged into mantle
creates steam and makes lava more acidic
Give an example of an oceanic - continental destructive margin
Nazca - South American plate
Give an example of an oceanic - oceanic destructive margin
Carabein - South American
Give an example of an continental - continental destructive margin
Indian - Eurasian
Explain the surface proccesses occuring at oceanic - cointinetal destructive plate margin
Oceanic lithospher subducts beneath the continetal plate beacuse it is denser
The exact point of collsiion is marked by bending of the oceanic plate to form a deep ocean trench such as the Peru-Chilie trench
As the two plates converge, the continetal land mass is uplifted, compressed, buckled and folded into chains of fold mountains (Andes)
Explain the melting proccesses occuring at oceanic - cointinetal destructive plate margin
The descending oceanic plate starts to melt at depths iof 100Km +. The zone of melthing is caused the Benioff zone. Melting is caused by increased heat and friction
This friction may also lead to tension building up, which may lead to deep focus earthquakes
The melted oceanic plate creates magma which is less dense tahn the sorrounding asthenosphere ans, thereofre rises in great plumes
Passes through faults in buckled continetal plate, magma edventially reaches the sufcae to form explosive eruptions
Explain the proccess occuirng at Oceanic- oceanic destructive plate margins
The oceanic plate with a higher mass or speed subducts beneath the other leading to the formation of a deep oceanic trench and melting
Resulting rising magma from the benioff zone forms crescents of submarine volcanoes along the plate margin which may grow to form island arcs
Continued subduction of the oceanic plate brings sea water, locked in the oceans crust deep into the mantle. Explosive andesite lava formed
What are island arcs
Long chains of volcanic islands - typically formed where oceanic crust meets oceanic rust at destructive margins
Explain the formation of island arcs
From subduction - hot magma leaks out of fractures in the crust- cuaisnga. series of volcnoes to form an island arc
Magma plumesm, an upswelling of abnormally hot rock within the mantle can also form island arcs through hotspot actvity. The plate passes over the magma plume ans weaknesses in the crust allow it to excape onto the surface as an active volcano
Decribe the charaticristcs of anesitic lava
Less dense than the sorrounding mterial and can have a tempature of 1000ºC
Viscious, trpaping gas inside at it rises. This water and gas accounts ofr its exploive activity
Explain continetal continetal destructive plate margins
Continental plates have a much lower density than the asthesnopsher beneath them so subduction does not occur
They become uplifted, and buckle to form high fold mountains (Himmilayas)
Volcanci activity does not occur an there is no subduction, but shallow focused earhquakes can be triggered
Himmilayas still rising by more than…cm per year as India continues to move north into Eurasia
Why then are the Himmilayas not rising
1
FOrces of weatehringa nd erosion are lowerig the Himmilayas at about the smae rate
What proccess occurs at conservatiev plate
Smae directions t different speeds
or oppositie directions
Give an example of a conservative plate
Pacific - North America (San Andres Fault)
Conservative plate margins are also known as………………
trasnform faults
Describe conservative margins
Mainly found ont eh ocean floor where they offset mid ocean ridges and enable the ocean to spread at different rates
Relative movemnt is horizontal
Lithosphereis neither crated or subducted
No volcanic activity or formation of new crust
Site of extensive shallow earthquakes
Why is the relative movemnt of the North AMerican Plate to the SE
Although both plates move West, the Pacific plate is moving faster that the N.A plate (6cm per year comapred with 3cm) so the relatiev movemnt = South East
Exlain Conservative margins
Friction between the plates stop them moving. Fores build up to overcome the friction. Pressure suddenley relaeased casuing plates to jolt past each other
Cause shallow focus earthquakes - exacerbating the impact felt
How do magma plumes form
Radioactive decay in the Earth’s core generates hot tempatures.
If the decay is concentrated, hots;ots will form aroudn the core
Heat the lower mantle creating localised thermal currents where magam plumes rise vertically
Burn throught eh lithospehere to create volcanic activity on the surface
How don hotspots create islands arcs
Hotspot remains staionary, the movement of the overlying plate reults in formation of a chain of active volcanoes. Hawii
As oceanic lithosphere oves away, volcanic activity ceases, cools and becomes an island
Dexcirbe Hawwi’s volcanoes
Often very wide, shied volcanoes
Basaltic lava flows
Mauna Loa
