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Figurę 1.21

Basic and acidlava

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Silica

content

Basic Acid

45%    52%    66%    75%

1 Basalticor basie łava	Andesiticoracid lava
Has Iow viscosity, is hot (1200°C) and runny, likewarmtreade	Viscous, less hot (80O°C), flows morę slowly and shorter distances
Has a lower silica content	Hasa highersilica content
Tales a longer time to cool and solidify, so flows considerable distances as rivers of molten rock	Soon cools and solidifies, flowing very short distances
Produces extensive but gently-sloping landforms Produces steep-sided, morę localised features
Eruptions are frequent but relatively gentle	Eruptions are less freguent but are yiolent due to thebuild-upofgases
Lavaand steamejected	Ash, rocks, gases, steam and lava ejected
Found at constructive piąte margins where magma rises from the mantle e.g.fissures along the Mid-Atlantic Ridge (Heimaey) e.g. over hot spots (Mauna Loa, Hawaii)	Found at destructive margins where oceanie crust is destroyed (subducted), melts and rises e.g. subduction zones (Mount St Helens) e.g.as island ares (Mt Pelee.Martinigue)

Platę tectonics, earthquakes and volcanoes