DESTRUCTIVE CONVERGENT PLATE MARGINS: SUBDUCTION

INTRODUCTION

G141What is the process where oceanic plates descend into the mantle

called?

Subduction

G142What major topographic feature marks the junction between the

descending oceanic plate and the over –riding plate?

Oceanic trench

G1783What topographic features develop above the subduction zone in:

E512a)Oceanic – Oceanic collisionIsland Arc

b)Oceanic – Continental collisionCordilleran mountain

chain

G1794Describe and explain the gravity anomalies associated with subduction

Zones

-ve at trench+ve at Island arc / Mountain chain

G145Describe and explain the presence of Benioff Zones

Inclined zone of earthquake foci

Shallow focus close to trench

Deep focus below Island Arc / Continent

G1796Why are there no earthquake foci below 700km?

Below 700km r plate totally consumed and merged with mantle

IGNEOUS PROCESSES AT SUBDUCTION ZONES

G631What material is subducted with the oceanic crust?

Water in vesicles and between pillows and dykes

Wet sediments

G632Explain 5 reasons why partial melting and the generation of magmas

occurs in subduction zones.

1Water lowers melting point of rocks

2Friction and seismic activity in Benioff zone generate heat

3Descending slab heated by conduction from hot mantle

4Increasing ‘C and pressure with depth dehydrates oceanic slab, releasing volatiles into the overlying mantle wedge

GS1795Phase change to higher density minerals at depth releases heat

E5123At what depth are magmas generated in the subduction zone?

100- 200km

G634Why 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

G1795What sequence of magma compositions is found after initial

subduction and why?

Diorite – Granodiorite – Granite

Partial melting of early formed diorites

G63-46What is the relative sense of movement between subducting and over-

riding plates?

Subducting plate moves, over-riding plate stationary

G647How 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

G648What rock type is characteristic of volcanic island arcs parallel to

oceanic trenches?

Andesite – Intermediate

G649Why 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

G6410How long does continental crust take to form?

20 million years to produce 30km thickness of continental crust

Africa + Asia 1000million years

G6411Why is volcanism above subduction zones usually explosive?

Silica rich – viscous – traps dissolved gases – explosive eg St Helens

G6412What are volcaniclastic / pyroclastic / tephra rocks?

Fragmental volcanic rocks produced by explosive eruptions

G6413What shape / type of volcano do pyroclastics form?

Conical composite volcanoes

14How does the size of pyroclastic material change with increasing

distance from the vent?

Smaller

G6415What 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

G6516Copy fig 4.16

METAMORPHIC PROCESSES AT SUBDUCTION ZONES

G1341Paired metamorphic belts are typical l associated with subduction

E512Zones. 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

E5122How 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

E5123Describe 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

E5134Describe the paired metamorphic belts found on the Californian coast

and Sierra Nevada and explain their interpretation.

Californian coastMelange Lo temp Hi pressW

Sierra NevadaMagmatism Hi temp Lo pressE

Pacific plate moving E to be subducted under American plate

SEDIMENTARY PROCESSES AT SUBDUCTION ZONES

E2891What type of deposition is typical of deep ocean trenches?

Thick Turbidites

E5122Why are these sediments often intensely deformed?

Low density – not subducted

Crumpled, folded, scraped off subducting oceanic crust

Plastered against edge of over-riding plate

E2813Copy fig 11.33

E5124What term is used to describe the intensely deformed sediments found

at subduction zones?

Melange

CONVERGENT PLATE BOUNDARIES

CONTINENT –CONTINENT COLLISION: OROGENY

G1421What happens to sediments on the continental shelf in continent-

continent collision?

Faulted, folded, uplifted

G1422Give an example of where this has happened

India and Asia collide – Himalayas

G1443What type of deformation is dominant at shllow depths in these regions

and why?

Brittle fracture – Thrust faults – cool

G1444What type of deformation is dominant at greater depths in these

regions and why?

Ductile deformation: folding – hotter

Tight isoclinal folding, nappes

G1445What grade of metamorphism is commonly associated with nappes?

High grade regional

E5156Describe 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