Igneous Rocks and Processes (A2)
Generation of Magma at Divergent Boundaries
Factors that Control Melting:
• Temperature – hotter temperatures lead to more melting taking place
• Pressure – increasing pressure increases the melting point
• Water/Volatiles – water lowers the melting of a rock because helps to break Si-O bonds in silicate
minerals
• Mineral Mix - mixtures lower the melting temperature
- Temperature will depend upon the % of the minerals involved
- Related to Bowen’s Reaction Series – some minerals have a higher melting temperature than
others, so if you have more of these minerals then the mix will melt at a higher temperature
Decompression Due to Convection
• Convection can be induced if the temperature gradient is high enough that material at depth expands
so that its density is lower than the material above it
• This is an unstable situation and the hotter, lower density material will rise to be replaced by
descending cooler material in a convection cell
• The rate of convection depends on both the temperature gradient and the viscosity of the material
- Solids convect but the rate is lower than in liquids as solids have higher viscosity
Decompressional Melting
1) Anywhere there is a rising convection current, hotter material at depth will rise, carrying its heat with
it
2) As it rises to lower pressure it will cool somewhat but will still have a higher temperature than the
surroundings
3) Decompression will result in raising the local geothermal gradient
4) If this new geothermal gradient reached temperatures greater than the peridotite solidus, partial
melting and the generation of magma can occur
Generation of Basalt at Divergent Plate Boundaries
1) Mantle rock rises to surface as it is less dense
2) Pressure reduces so mantle partially melts (in the asthenosphere)
3) Lower density melt rises and collects in the lithosphere, forming a magma chamber
4) Some melt continues to rise and escapes magma chamber in dykes
5) Some melt erupts at the surface forming pillow lavas
Structure of Ocean Crust
Serpentinite – chemically weathered gabbro or peridotite through the hydrothermal alteration of olivine
Current Model New Model
Gabbro is considered to make up the majority of The mantle (as it contains more olivine) has been
oceanic crust. P wave velocity when fresh is more susceptible to serpentinisation, slowing down
approximately = 6- 8kms-1 the waves. This could mean the 6- 8kms-1 layer was
originally peridotite and not mantle.
, Fast vs Slow Spreading Ridges
Fast Spreading Ridges Slow Spreading Ridges
East Pacific Rise (EPR) Mid Atlantic Ridge (MAR)
Example
1km below sea level 3km below sea level
Topography Positive Relief Rift Terrain
Speed 7.5cm/year 2.5cm/year
Size of
Large Small
Intrusion
Shape of
Onion Shaped Leek Shaped
Intrusion
Permanence
Permanent More Intermittent
of Intrusion
Depth of
100s m 1-2km
Faulting
Symmetrical vs Asymmetrical Spreading Ridges
Symmetrical Ridge Asymmetrical Ridge
• As the plates separate, ductile • Less melt is produced in these regions, so
asthenosphere wells up to fill the gap and faults are much larger
partially melts (up to 20%) • The faults may accumulate normal
• The melt rises and solidifies to form an displacement of 10s of km and can
ocean crust made of layers of gabbro penetrate through the lithosphere
plutons, doleritic dykes and basaltic lavas • The hanging wall plate receives less melt
• Sufficient melt is produced to ma a simple, than normal, so its crust is thinner
regular, layered igneous ocean crust • The footwall plate is formed by pulling
• Once the crust has formed, the stresses mantle material up, and forms an irregular,
pulling the plates apart produce normal discontinuous crust (or none at all)
faults with displacements of ~100m which • If all of the plate separation is taken up by
add 5-10% of strain slip on the detachment fault, then no new
• New material is added equally to both sides material will be added to the hanging wall
plate, resulting in an Ocean Core Complex
Ocean Core Complex – uplifted footwalls of very large offset, low angle normal faults that exhume lower crust
and mantle rocks onto the sea floor at slow-spreading ridges
Generation of Magma at Divergent Boundaries
Factors that Control Melting:
• Temperature – hotter temperatures lead to more melting taking place
• Pressure – increasing pressure increases the melting point
• Water/Volatiles – water lowers the melting of a rock because helps to break Si-O bonds in silicate
minerals
• Mineral Mix - mixtures lower the melting temperature
- Temperature will depend upon the % of the minerals involved
- Related to Bowen’s Reaction Series – some minerals have a higher melting temperature than
others, so if you have more of these minerals then the mix will melt at a higher temperature
Decompression Due to Convection
• Convection can be induced if the temperature gradient is high enough that material at depth expands
so that its density is lower than the material above it
• This is an unstable situation and the hotter, lower density material will rise to be replaced by
descending cooler material in a convection cell
• The rate of convection depends on both the temperature gradient and the viscosity of the material
- Solids convect but the rate is lower than in liquids as solids have higher viscosity
Decompressional Melting
1) Anywhere there is a rising convection current, hotter material at depth will rise, carrying its heat with
it
2) As it rises to lower pressure it will cool somewhat but will still have a higher temperature than the
surroundings
3) Decompression will result in raising the local geothermal gradient
4) If this new geothermal gradient reached temperatures greater than the peridotite solidus, partial
melting and the generation of magma can occur
Generation of Basalt at Divergent Plate Boundaries
1) Mantle rock rises to surface as it is less dense
2) Pressure reduces so mantle partially melts (in the asthenosphere)
3) Lower density melt rises and collects in the lithosphere, forming a magma chamber
4) Some melt continues to rise and escapes magma chamber in dykes
5) Some melt erupts at the surface forming pillow lavas
Structure of Ocean Crust
Serpentinite – chemically weathered gabbro or peridotite through the hydrothermal alteration of olivine
Current Model New Model
Gabbro is considered to make up the majority of The mantle (as it contains more olivine) has been
oceanic crust. P wave velocity when fresh is more susceptible to serpentinisation, slowing down
approximately = 6- 8kms-1 the waves. This could mean the 6- 8kms-1 layer was
originally peridotite and not mantle.
, Fast vs Slow Spreading Ridges
Fast Spreading Ridges Slow Spreading Ridges
East Pacific Rise (EPR) Mid Atlantic Ridge (MAR)
Example
1km below sea level 3km below sea level
Topography Positive Relief Rift Terrain
Speed 7.5cm/year 2.5cm/year
Size of
Large Small
Intrusion
Shape of
Onion Shaped Leek Shaped
Intrusion
Permanence
Permanent More Intermittent
of Intrusion
Depth of
100s m 1-2km
Faulting
Symmetrical vs Asymmetrical Spreading Ridges
Symmetrical Ridge Asymmetrical Ridge
• As the plates separate, ductile • Less melt is produced in these regions, so
asthenosphere wells up to fill the gap and faults are much larger
partially melts (up to 20%) • The faults may accumulate normal
• The melt rises and solidifies to form an displacement of 10s of km and can
ocean crust made of layers of gabbro penetrate through the lithosphere
plutons, doleritic dykes and basaltic lavas • The hanging wall plate receives less melt
• Sufficient melt is produced to ma a simple, than normal, so its crust is thinner
regular, layered igneous ocean crust • The footwall plate is formed by pulling
• Once the crust has formed, the stresses mantle material up, and forms an irregular,
pulling the plates apart produce normal discontinuous crust (or none at all)
faults with displacements of ~100m which • If all of the plate separation is taken up by
add 5-10% of strain slip on the detachment fault, then no new
• New material is added equally to both sides material will be added to the hanging wall
plate, resulting in an Ocean Core Complex
Ocean Core Complex – uplifted footwalls of very large offset, low angle normal faults that exhume lower crust
and mantle rocks onto the sea floor at slow-spreading ridges
