MECHANISMS OF IGNEOUS ACTIVITY
E1: K1: b)
Mechanisms of emplacement and extrusion of magmas depend
on
Magma density, Viscosity and Fluid pressure;
Rise due to buoyancy forces, rate of rise controlled by viscosity;
Forcible intrusion into surrounding rock if fluid pressure exceeds
confining pressure;
Importance of gas content.
Preview/Review: Geoscience p56
References:
Websites: www.geo.ua.edu/volcanology/lecture_notes_files/viscosity
MAGMA DENSITY
Rhyolite 2.2gm/cm to Basalt 2.8gm/cm
Contrast due to composition: higher Fe, Mg, Ca in Basalts
Density decreases with
a)
Increasing temperature
b)
Increasing gas content
Densities increase by a few % between liquid /crystalline phases
VISCOSITY
Viscosity is a fluid's internal resistance to flow
Measured in poises
Viscosity depends on several variables which effect the melt structure:
a)
Composition
High silica: many framework tetrahedral + strong bonds
Rhyolite up to 6 times as viscous as basalt!
b)
Temperature
As temperature increases, viscosity decreases due to
increased distance between cations and anions,
decreasing Si-O bond strength
Lavas typically 700-1200'C
Low 'C Rhyolites - viscous
High 'C Basalts - fluid
c)
Fluid pressure
Volume of magma with dissolved gas is less than that
of melt and separate volatile phase
Volatiles solubility increases as pressure increases
Viscosity decreases with increased pressure at temperatures above
liquidus
Dissolved water decreases viscosity
PLUTONS
Granitic magma density 2.4-2.6 formed at base of continental crust density
2.9
Magma less dense than surrounding rocks - rises due to buoyancy forces
Rate of rise controlled by high magma viscosity
Magma stops rising when:
a) It reaches rocks of the same density
b) It cools and crystallizes @700'C 3-5km
DYKES
Discordant
- cut through boundaries of country rock
- often follow joints
Wall-like body of rock usually 20cm-2m thick
Often occur in large numbers as a DYKE SWARM
May radiate or be concentric around a large pluton as RING DYKES or
CONE SHEETS (may be associated with piston faulting)
Cool fast
- thin dykes : fine grained basalt (rarely
rhyolite)
- thick dykes: medium grained dolerite (rarely microgranite)
Fastest cooling at dyke margins - fine grained/glassy CHILLED
MARGIN
Relatively small intrusions, so narrow contact metamorphic aureole: or
BAKED MARGIN
Cooling leads to contraction, forming polygonal COLUMNAR JOINTS
at 90' to the walls of the dyke
SILLS
Concordant
- follow boundaries of country rock
- often follow bedding planes
- often appear almost horizontal
Few metres up to hundreds of metres thick, can have large lateral
extent
Because generally thicker than dykes - cool slower - commonly
medium grained dolerite
Thick sills may show gravity settling of early formed crystals to form
CUMULATES in a LAYERED INTRUSION eg Palisades Sill New York
Slow cooling allows better development of polygonal columnar jointing
at90' to the cooling surface
Sills often more resistant to weathering and erosion than surrounding
country rocks and commonly lead to waterfall development eg High
Force on the Whin Sill, which underlies a large area of N England
EXTRUSIVE IGNEOUS ACTIVITY
Nature of volcanic eruptions depends on lava VISCOSITY
Viscosity depends on:
a) Temperature
High temperature - low viscosity
b) Gas content
High gas content - low viscosity
c) Crystal content
Low crystal content - low viscosity
d) Composition
Low silica content -low viscosity
ie Basic lava: Basalt generally low viscosity
Low viscosity basaltic lava (1000-1200'C) tends to be erupt EFFUSIVELY relatively quietly due to easy escape of gas
Low viscosity lavas flow long distances at a few kmph before cooling, forming
a) low angle
'SHIELD VOLCANOES'
eg Hawaii
b) widespread
FLOOD BASALTS
eg Deccan Plateau -India
Mostly lava with few pyroclastics
Surface textures may be
a) ropy PAHOEHOE, due to lava flowing under a 'skin'
b) blocky Aa due to gas escaping and breaking 'skin'
very rough surface – painful to walk on!
High viscosity andesitic or rhyolitic lava (700-1000'C) tends to erupt
EXPLOSIVELY due to trapping of gas and build up of pressure
Very high viscosity lava piles up around vent to form lava DOME eg Katmai
(Alaska)
High viscosity lava flows short distances forming steep sided volcanic cones
eg Fuji, Japan
Mix of alternate pyroclastics followed by lava
Pyroclastics include Bombs, Blocks, Lapilli, Scoria, Pumice, Ash and dust
Nuee Ardente - pyroclastic flow of hot dust and gas, forming WELDED TUFF
(Ignimbrite)
Very explosive eruptions can empty the magma chamber, leading to roof
collapse forming a CALDERA (huge crater) eg Krakatoa, Indonesia 1883
ESTA GEOTREX The Geology Teachers Resource Exchange Contributor: Ben Church
Establishment: Monmouth Comprehensive School Date:24:05:05