Magma Flashcards
Adiabatic process
A thermodynamic process in which no heat enters or leaves the system during expansion or compression
Adiabatic cooling
It occurs when crust or mantle rises, undergoes expansion and the temperature falls, with no loss or gain of thermal energy
Adiabatic heating
It occurs when crust or mantle material descends and temperature rises as it contracts, with no loss or gain in thermal energy
Divergent plate margin
They occur where two plates are moving apart and magma is rising up between them
Partial melting
It occurs where some of the minerals in a rock melt to form a magma
Hot spots
an area of high heat flow above a mantle plume
Convergent plate margins
Are where two plates are colliding and magma is formed above a subduction zone or deep in the crust
Decompression melting
Decompression of the ultramafic peridotite causes partial melting and produces mafic magmas
How is magma produced at a convergent plate boundary?
Two plates collide, one is forced down, the geothermal gradient increases and partial melting results
Describe the process of flux melting
An oceanic plate is subducted, water in the rocks is carried down into the hot mantle. As the descending plate heats up, the water is released into the mantle rock above the plate. The water lowers the melting point of the minerals in the mantle rock and partial melting produces magma.
How is basaltic magma produced at an oceanic-oceanic crust?
The magma is mafic to intermediate in composition, initially the magma has only thin oceanic crust to rise through. If the magma rises quickly at shallow depths then it will be basalt.
How is intermediate to silicic magma produced at a continental- oceanic plate?
Magma rises through thicker silicic crust and this may partially melt due to increasing temps. Rising mafic magma has temp of 1000+ and silicic material melts at just 800. The resulting mixing gives intermediate to silicic magma.
How is silicic magma produced at a continental-continental plate margin?
Neither plate is subducted. High pressures and the mass of sediment (formed fold mountains) combine to force the base crust down. Partial melting at base produces silicic magma.
Magma
Molten rock which is beneath the earth’s surface
What determines the density of magma?
composition, pressure and temperature
What is the effect of Composition on the density of magma?
Mafic magma is rich in iron compared to silicic magma and this makes it more dense
What is the effect of Pressure on density of magma?
An increase in pressure causes the magma to compress, which increase its density making it less buoyant
What is the effect of Temperature on the density of magma?
An increase in temperature cause the magma to expand, which decreases its density making it more buoyant
What is the viscosity of magma determined by?
Composition- mafic magma low viscosity as silica content is low vies versa
Temperature- increase in temp reduces viscosity
How can magma recharge be detected?
Microgravity- sensors detect local changes in subsurface density from magma movement
Seismometer- tracks migration of small earthquakes from depth towards surface
Ground water heated- increase in pressure can cause crustal deformation and release of gases can also be measured
What factors can prevent magma mixing?
density contrast
viscosity contrast
thermal barrier
How are geysers produced?
Groundwater is heated at depths. Convection causes the gas-rich, super heated water to rise. As it rises, the pressure decreases and ‘flash’ boiling into steam result. The column of cooler water above it is thrust up explosively. The water then drains back into the ground
How can gases exsolve from magma?
As magma rises to the surface, the reduction in pressure allows dissolved gases to exsolve. This happens because there is less magma above it pressing down. Bubbles can also form in a magma due to recharge and mixing.
What can provide the earliest indication of magma moving?
Earthquakes
