Formation of magmas 3.2.2(a)
Occurs when crust or mantle material rises, undergoes expansion and temperature falls, with no loss or gain of thermal energy.
Define adiabatic cooling.
Occurs when crust or mantle material descends and tempertaure rises as it contracts, with no loss or gain of thermal energy.
Define adiabatic heating.
Thermodynaic process in which no heat enters or leaves the system during expansion or compression.
Define adiabatic process.
Very large igneous intrusion, with undefined depth, in Earth's crust.
Define batholith.
Occurs where 2 plates are colliding and magma is formed above a subduction zone or deep in the crust.
Define convergent plate margins.
Accounts for most volcanism, including MORs and hot spots. Decompression of the ultramafic peridotite causes partial melting and produces mafic magmas by decompression melting.
Define decompression melting.
Occurs where 2 plates are moving apart and magma is rising up between them.
Define divergent plate margin.
Formed by fixed mantle plume bringing magma to the surface.
Define hot spots.
Occurs where some of the minerals in a rock melt to form magma.
Define partial melting.
Basalt may erupt onto surface over very short time scales (<1 million years) to form flood basalts.
How does basalt magma erupt at hot spots?
As lithosphere stretches and thins due to being pulled apart, asthenosphere upwells closer to the surface and pressure is reduced. Decompression of ultramafic peridotite causes partial melting and produces mafic magmas.
How is lava produced at a divergent plate margin?
Neither plate subducted. High pressures and mass of sediments, which have been deformed to form fold mountains, combine to force base of crust down. Partial melting at base of continental crust will produce silicic magma.
How is magma formed at a continental-continental plate margin?
Convection in mantle slowly transports heat from core to surface. Mantle plumes carry heat upwards in narrow, rising columns of hot material, which spreads out when plume head meets base of rigid lithosphere. Lower pressures allow decompression and partial melting of mantle peridotite, forming large volumes of basaltic magma.
How is magma produced at hot spots?
When oceanic plate subducted, water in its rocks is carried down into the hot mantle. As descending plate heats up, water released into mantle rocks above the plate. Water lowers melting point of minerals in mantle rock and partial melting of mantle produces magma. Known as flux melting.
How is magma produced at subduction zones?
Pressure increases with depth and rock then requires a higher temperature to melt.
If temperature decreases with depth, why is molten rock not found at little more than 40km?
Magma must rise through thicker silicic crust, may partially melt due to increasing temperature. Rising mafic magma may be 1000°C and silicic melts at just 800°C. Result is mixing of the mafic and silicic melts to give intermediate to silicic magma.
Type of magma erupted at continental-oceanic plate margin?
Erupting magma is mafic to intermediate in composition, as initially magma has only thin oceanic crust to rise through. If magma rises quickly at shallow depths will be basalt. If crust is thicker may be intermediate.
Type of magma erupted at oceanic-oceanic plate margin?
Magma rising through the thicker continental crust is modified and can result in a wide range of volcanic products.
What can happen to rising magma at divergent plate boundaries in continental settings?
A continental-continental plate margin.
What does this image show?
A continental-oceanic plate margin.
What does this image show?
A divergent plate margin.
What does this image show?
A hot spot.
What does this image show?
As magma rises it intrudes to form granite batholiths.
What forms under fold mountians from rising magma?
Increase in temperature, reduction in pressure or lowering of rock melting point will generate magma.
What in the low velocity zone can generate magma?
Continental crust is silicic and starts to melt at 800°C, this is the temperature below mountians due to the geothermal gradient and the presence of water.
What is temperature below fold mountains and why?
Basalt pillow lavas are produced. Dolerite dykes and sills are intruded below.
What kind of lava is produced at a divergent plate margin? What are intruded below?
Reduction in pressure allows rising rock to expand, but the increase in volume reduces the overall temperature in process known as adiabatic cooling (heat energy within the rising rock remains constant, it is simply spread over greater volume). No melting occurs.
When rock rising there is a reduction in pressure, what process does this result in?
Seismic low velocity zone of the asthenosphere. Whilst still solid, it behaves in a more ductile manner, allowing motion of lithospheric plates above.
Where is the mantle closest to melting?
Silicic magma is too viscous to rise to the surface.
Why are there no volcanoes at continental-continental plate margins?
Geothermal gradient of mantle approaches that of an adiabatic system. As hot mantle rock rises, does not gain or lose heat to the surrounding mantle but remains in equilibrium with it.
Why does convection within the Earth's mantle allow hot, less dense rock to rise and cool, more dense rock to sink without melting?
Different viscosities so mixing difficult. Some magma will reach surface to form intermediate volcanoes. Most will be intruded to form granite batholiths.
Why is mixing of the magmas difficult? What can happen to magma as a result?
Some rock-forming minerals have a lower melting point than others.
Why is partial melting to be expected in the mantle/asthenosphere?
