DESTRUCTIVE CONVERGENT PLATE MARGINS: SUBDUCTION
INTRODUCTION
G14 1
What is the process where oceanic plates descend into the mantle
called?
Subduction
G14
2
What major topographic feature marks the junction between the
descending oceanic plate and the over –riding plate?
Oceanic trench
G178 3
E512
What topographic features develop above the subduction zone in:
a)
Oceanic – Oceanic collision
Island Arc
b)
Oceanic – Continental collision
Cordilleran mountain
chain
G179 4
Describe and explain the gravity anomalies associated with subduction
Zones
-ve at trench +ve at Island arc / Mountain chain
G14
Describe and explain the presence of Benioff Zones
Inclined zone of earthquake foci
Shallow focus close to trench
Deep focus below Island Arc / Continent
5
G179 6
Why are there no earthquake foci below 700km?
Below 700km r plate totally consumed and merged with mantle
IGNEOUS PROCESSES AT SUBDUCTION ZONES
G63
1
What material is subducted with the oceanic crust?
Water in vesicles and between pillows and dykes
Wet sediments
G63
2
Explain 5 reasons why partial melting and the generation of magmas
occurs in subduction zones.
1
Water lowers melting point of rocks
2
Friction and seismic activity in Benioff zone generate heat
3
Descending slab heated by conduction from hot mantle
4
Increasing ‘C and pressure with depth dehydrates oceanic slab,
releasing volatiles into the overlying mantle wedge
5
Phase change to higher density minerals at depth releases heat
GS179
E512 3
At what depth are magmas generated in the subduction zone?
100- 200km
G63
Why do magmas produced at subduction zones not have the same
compositions the original basaltic rocks of the oceanic crust?
Partial melting: lowest ‘C of melting minerals
4
G179 5
What sequence of magma compositions is found after initial
subduction and why?
Diorite – Granodiorite – Granite
Partial melting of early formed diorites
G63-4 6
What is the relative sense of movement between subducting and overriding plates?
Subducting plate moves, over-riding plate stationary
G64
7
How and why does the composition of magmas generated at
subduction zones change with time?
Increasing silica
Magma rises through same zone of over-riding plate
Progressive heating
Addition to magma by partial melting / removal by crystallisation
G64
8
What rock type is characteristic of volcanic island arcs parallel to
oceanic trenches?
Andesite – Intermediate
G64
9
Why are andesites described as an ‘irreversible addition to continental
crust’?
Low density – cannot be subducted
Island arcs collide to form mini-continents
Accretion forms continents
G64
10
How long does continental crust take to form?
20 million years to produce 30km thickness of continental crust
Africa + Asia 1000million years
G64
11
Why is volcanism above subduction zones usually explosive?
Silica rich – viscous – traps dissolved gases – explosive eg St Helens
G64
12
What are volcaniclastic / pyroclastic / tephra rocks?
Fragmental volcanic rocks produced by explosive eruptions
G64
13
What shape / type of volcano do pyroclastics form?
Conical composite volcanoes
14
How does the size of pyroclastic material change with increasing
distance from the vent?
Smaller
G64
15
What are nuess ardentes / pyroclastic flows and what deposits do the
produce?
Cloud of hot gas and ash flowing fast down side of volcano following
collapse of ash plume – deposit ash and welded tuff – ignimbrite
pumice
G65
16
Copy fig 4.16
METAMORPHIC PROCESSES AT SUBDUCTION ZONES
G134 1
E512
Paired metamorphic belts are typical l associated with subduction
Zones. Describe the temperature and pressure conditions and
metamorphic facies that are produced in the metamorphic belt close to
the oceanic trench.
Low geothermal gradient / high tectonic forces
Low temperature / High pressure metamorphism
Blueschist facies rocks
Blue due to Na rich amphibole Glaucophane
E512 2
How does blueschist facies metamorphism lead to further magma
genberation above the subduction zone?
Metamorphism releases water into overlying asthenosphere
Increases partial melting above descending slab
E512 3
Describe the temperature and pressure conditions and 3 metamorphic
facies associated with rising magma causing metamorphism of igneous
rocks above subduction zones
High temperature Low pressure
Increasing temperature: Greenschist – Amphibolite – Granulite
E513 4
Describe the paired metamorphic belts found on the Californian coast
and Sierra Nevada and explain their interpretation.
Californian coast
Melange
Lo temp Hi press
W
Sierra Nevada
Magmatism Hi temp Lo press
E
Pacific plate moving E to be subducted under American plate
SEDIMENTARY PROCESSES AT SUBDUCTION ZONES
E289 1
What type of deposition is typical of deep ocean trenches?
Thick Turbidites
E512 2
Why are these sediments often intensely deformed?
Low density – not subducted
Crumpled, folded, scraped off subducting oceanic crust
Plastered against edge of over-riding plate
E281 3
Copy fig 11.33
E512 4
What term is used to describe the intensely deformed sediments found
at subduction zones?
Melange
CONVERGENT PLATE BOUNDARIES
CONTINENT –CONTINENT COLLISION: OROGENY
G142 1
What happens to sediments on the continental shelf in continentcontinent collision?
Faulted, folded, uplifted
G142 2
Give an example of where this has happened
India and Asia collide – Himalayas
G144 3
What type of deformation is dominant at shllow depths in these regions
and why?
Brittle fracture – Thrust faults – cool
G144 4
What type of deformation is dominant at greater depths in these
regions and why?
Ductile deformation: folding – hotter
Tight isoclinal folding, nappes
G144 5
What grade of metamorphism is commonly associated with nappes?
High grade regional
E515 6
Describe the Wilson cycle of plate tectonics
Opening of ocean basins by sea floor spreading – closure of ocean –
continent continent collision – formation of intracontinental orogenic
mountain belt