MAGMA GENESIS AT DESTRUCTIVE BOUNDARIES
E1:K1:a)
ii)
Melting of rock material at depth to form magma occurs in a
number of different plate environments.
Near to destructive plate margins - partial melting of subducted
oceanic lithosphere and overlying lithospheric wedge generates
andesitic magma.
Preview/Review:
References:
Websites:
Earth: p423,426
Geoscience: p63-65
Inaccessible Earth: Brown and Mussett
http://www.ucl.ac.uk/~ucfbrxs/B164/Plates.htm
CORDILLERAN OCEANIC - CONTINENTAL BOUNDARIES
Cordilleran mountain belts eg Andes - S America, Cascade Range N - America,
New Zealand, Japan, Kamchatka - Russia
Nature/distribution of igneous activity depends on geometry of subducting plate
Volcanoes @200-300km from trench - distance depends on angle of subduction
Descending slab dehydrates - fluids rise into lithosphere, lowering melting
temperature
Water in vesicles and between pillows and dykes also lowers melting
temperature
Heat generated by friction and by conduction from mantle
At depth slab melts, plus wet sediments
Rising magma interact with crust as it rises through a series of magma
chambers
Magma chambers injected by new magma: magma mixing alters composition
The thicker the crust, the greater the assimilation and fractional crystallisation
Thin crust - more basic magmas
Thick crust - more acidic magmas
Andesitic large, high cones: explosive Vulcinian to Plinian style eruptions
Pyroclastics dominant
Andesites: plagioclase, pyroxenes, hornblende, biotite +/- olivine or quartz
Commonly porphyritic
May be basalt, andesite on seaward side, rhyolite on thickened crust
Bulk of upper crust in active belt is vast linear granitic/granodioritic batholith.
Continental crust up to 2x average value of 35km.
Crust thickened by vertical additions: intermediate and acid rocks never
subducted
fig 9.6 p168 B+M fig4.14 p63 Geoscience
ISLAND ARC OCEANIC-OCEANIC BOUNDARIES
a) Smaller volumes of intrusives
b) Extrusive rocks predominate
c) Explosive due to phreatic action and high volatile content
Descending slab dehydrates - rising fluids initiate partial melting
Islands built up by successive eruptions
Thickening of crust slows rise of magma - more time for fractional crystallisation
Subducted material = upper 5-10km basaltic crust overlying depleted peridotite.
Peridotite unlikely to melt because it is already a residuum.
Basaltic oceanic crust metamorphoses to amphibolite.
High geothermal gradients - melts to give andesitic magma + depleted eclogitic
residue.
Lower geothermal gradients - release of hot volatiles leading to partial melting
of overlying mantle + quartz eclogite residue.
Silica enrichment by melting of wet siliceous sediments.
Rising magma 1000'C melts some surrounding crust - becomes more acidic.
Cooling - fractional crystallization - acidic residual liquids
Lower crustal rocks dry - insufficient temperatures for spontaneous melting.
Subducted plate initiates melting as rising body of hot magma leads to partial
melting of lower crust 35-40kkm- 600'C- 10,000 atmospheres.
Batholiths formed by series of teardrop shaped plutons.
Rate of crustal accretion has varied through geological time; slower now than in
past.
Igneous rocks above subduction zones vary considerably in composition.
Increasing age Deeper magma source
Increasing acidity
Increasing ratio of intrusive material
Increase in %potassium K
1
Young arcs - South Sandwich, Marianas, Tongas:
Basalts, Basalt-andesites.
2
Intermediate - West Indies, Central America, New Zealand, Indonesia,
Japan:
Calc-alkaline andesites + dioritic + granodioritic intrusives.
3
Mature arcs - Andes, Rocky Mountains:
Intrusive rocks predominate - gabbro - granodiorite - adamellite.
fig 9.13 p177 B+M.
ESTA GEOTREX The Geology Teachers Resource Exchange Contributor: Ben Church
Establishment: Monmouth Comprehensive School Date:24:05:05