Solidus is the highest temperature at which a rock is solid, where melting begins. Liquidus is the temperature at which a rock is completely melted.
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Describe how mantle melting occurs by decompression melting and identify triggers
Rock is moved towards the surface, either at a mantle plume or in the upwelling part of a mantle convection cell. If a rock is hot enough to be close to its melting and it is moved towards the surface, the pressure is reduced.
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Describe how mantle melting occurs by fluid flux melting and identify triggers
When a rock is close to its melting point and water is added, the melting temperature is reduced and partial melting starts. E.g. wet, subducting oceanic crust at subduction zones
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Give examples of geological settings where mantle melting occurs
Juan de Fuca ridge, Mt St Helens, Merapi, Hawaii
4 of 17
Explain the effect of Si, Na, K, Ca on viscosity of magma
Si is a network former and makes magma more polymerized, therefore more viscous (e.g. Rhyolite). Na, K and Ca are network modifiers and disrupt the polymers, making the magma less viscous.
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Describe the magmatic composition of lava domes
Highly viscous, gas-poor andesitic and rhyolitic lava. Viscosity is so high that lava piles up over the vent rather than flowing away.
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Explain how rhyolitic obsidian flows are formed
High silica content causes high viscosity, which, upon rapid cooling results in a natural glass forming from the lava.
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Explain how basaltic lava flows are formed
Low viscosity due to low silica content, resulting in rapid lava flows before cooling and solidification
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Explain how shield volcanoes are formed
Low viscosity basaltic magma flows easily downslope from summit vent on a gentle slope. Form by non-explosive eruptions
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Explain how composite volcanoes are formed
Thick, viscous lava flows that do not flow far downslop from vent. Usually andesitic to rhyolitic magmas. Form by highly-explosive eruptions
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Explain how cinder cones are formed
Small volume cones formed from tephra from strombolian eruptions. Usually consist of basaltic to andesitic material.
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Explain why SiO2-rich magmas erupt violently
Thick, silica-rich magma does not release gas easily so it builds up inside the magma chamber, causing pressure to rise. When the pressure gets to a certain point, the magma chamber cannot contain it and erupts violently
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What are the 4 stages of 'First Boiling' during depressurisation?
1. Nucleation of bubbles 2.Growth 3. Saturated froth 4. Magma propelled upwards
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What happens during second boiling when the gases in magma are depressurised?
The crystallisation of minerals increases the gas concentration in the magma.
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What is Fractional Crystallisation?
The process of magmatic differentiation that accompanies the failure of early-forming crystals to react to the melt that remains
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What happens during fractional crystallisation?
Minerals with the highest melting points or lowest solubilities crystallise out first, leaving minerals with lowest melting points behind in the melt to freeze out last
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Explain why ores and gems are associated magmatic intrusions?
The last dregs left after crystallisation of a magma is generally a magmatic fluid fraction rich in gases, water and rare element, giving rise to gemstones and ore deposits
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Other cards in this set
Card 2
Front
Describe how mantle melting occurs by decompression melting and identify triggers
Back
Rock is moved towards the surface, either at a mantle plume or in the upwelling part of a mantle convection cell. If a rock is hot enough to be close to its melting and it is moved towards the surface, the pressure is reduced.
Card 3
Front
Describe how mantle melting occurs by fluid flux melting and identify triggers
Back
Card 4
Front
Give examples of geological settings where mantle melting occurs
Back
Card 5
Front
Explain the effect of Si, Na, K, Ca on viscosity of magma
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