Download Magma Ascent Topics Magma Generation Magma Rise Energy

Topics
Magma Ascent
• How does magma ascend?
• How do dikes form?
Best & Christiansen
Chapter 9
• How is magma emplaced?
Magma
Generation
• Partial melting
– Upper mantle
– Deep crust
• Magma density
• Less than surroundings
Magma Rise
• Buoyancy
– Driving force is density difference
– Resisting force is the magma viscosity
• Silicic magma
– High viscosity requires large volume
• Mafic magma
– Low viscosity allows small volumes to rise
Energy
Sources
• Thermal energy
– Melting caused by
decompression or volatile
flux
• Gravitational energy
– Driven by density
differential
Stability
• Partial melts may become buoyant and rise
• As magma roses cooling increases viscosity
• This may produce a balance or lava may erupt
onto the surface
• Parsimony dictates that the system takes the path
of least work
Neutral Buoyancy
• Positively buoyant
– Melt that is less dense than surrounding rocks
– Primary basalt magma surrounded by mantle
peridotite
• Negatively buoyant
– Melt that is more dens than surrounding rocks
– Olivine basalt intruded into continental crust
Oceanic Crust
Heat Transfer
• Conduction
• Radiation
• Convection
– Transfer of heat to the
surface by magma is a
major factor in cooling
of the Earth
Buoyancy in the Crust
• Role of P & T important
• Slight density differences important
– A few tenths of a g/cc is significant
• Factors include
– Vesicularity
– % phencryrsts
– Magma composition
Continental Crust
• Density increases with depth
• Crustal rocks are 2.6 to 2.9 g/cc
• Neutral density for olivine basalt at about 1 - 3 km
• Local crust is 2.9 g/cc
• Magma may accumulate at this level
• Magma chambers at spreading ridges
• Less dense evolved magma erupts as lava
– granite and some metamorphic rocks
– Amphibolite or granodiorite
• Andesite and silicic magma + buoyant
• Undersaturated basalt (>2.7 g/cc) can’t rise
– They underplate the crust
Density Filter
• Crustal rocks block the ascent of
denser magmas
• Heat from these magmas melt the
lower crust
• Residual melts may rise
• Exsolved volatiles also facilitate rise
How Can Dense Magma Rise?
• Volumetric expansion on melting?
• Exsolution of bubbles?
• There must be another cause.