Term 2: Intrusions

Question 1

Why are intrusions important?

Answer 1

• Magmatic rocks are an abundant rock-type in continental crust
• reflect the process of thermally - driven differentiation of the Earth’s crust
• derived from mantle and lower crust and can give information on the nature of that crust
• associated with important mineral deposits
• Reflect a melting, transport and crystallisation process that is associated with tectonic belts
• Changing rheology of melt is important factor in mid to lower crust tectonic processes

Question 2

General concept: Magma segregation

Answer 2

How do we move the magma out of source region?
• Gravity driven compaction
– too slow for viscous melts
- May account for some segregation, granitic melts are highly viscous and calculations suggest that compaction alone cannot be used to explain the separation of large melt volumes

• Fracturing – more likely to be the principal mechanism
– Hydraulic fractures in source region can become conduits
– Melt accumulates in fractures
- These fractures can remove melt from the source very quickly, the lifespan of any one fracture is short as they are dependent on high melt pressure - once melt is extracted the fractures will close

Question 3

General concept: Magma intrusion driving forces

Answer 3

• During melting the source region becomes porous and dilates
• Fluid absent melting can cause up to 15% volume increase that produces a hydraulic overpressure in the source – leads to fracturing
• This is relaxed in the form of doming of the source and ultimately uplift of the Earth’s surface
• The doming sets up a radial stress field with vertical tensile cracks/dykes/conduits forming
• Buoyancy forces drive magma up the conduit

Question 4

General concept: Ascent and emplacement

Answer 4

• Initial dyke feeds a sill thus releasing EMV (excess magma volume) in source
• However, once the initial sill forms the EMV is relaxed and the system would quickly stop however….
• The sill now supports its roof but is the floor is decoupled and then will subside back into its source
• This will expel magma from its source, increase sill size, dilate conduit, provide more material for melting – a feedback loop is set up.

Question 5

Salt Tectonics

Answer 5

Linear and vertical structures

Two things make salt special
• Salt flows easily – low viscosity
• Salt has low density

Salt intrusion – diapirs result from upward mobilisation of salt layers