Magma Formation and Behavior Questions What causes mantle rock to melt, resulting in magma formation? Why is magma formation restricted to specific plate tectonic settings? Why are mafic (basaltic) magmas erupted along mid-ocean ridges if they form by the melting of ultramafic rock (peridotite) Why do granitic (felsic) magmas usually crystallize before reaching Earth s surface? (Most intrusive rocks are granite) Why are mafic (basaltic) magmas usually erupted as lavas at Earth s surface? (Most volcanic rocks are basalt) Subduction Zones According to plate tectonic theory, most magmas form in the following settings: Subduction zones, along convergent plate boundaries Rift zones, along divergent plate boundaries Mantle plumes (hot spots) Rift Zone Mantle Plume 1
How can magma form by melting of mantle rock? Increase Temperature: Least important cause of melting within the Earth There is no source of heat to increase temperature of mantle/lower crustal rocks to their melting temperatures. How can magma form by melting of mantle rock? Decrease Pressure High pressures atoms in minerals are so tightly compacted that chemical bonds cannot be broken to transform the solid minerals to a liquid (a magma) Low pressure atoms in solid minerals are spaced apart so that atoms have room to vibrate and chemical bonds between them can be broken to form a liquid (a magma) At high pressures deep within the mantle, atoms in minerals are so tightly compacted that chemical bonds cannot be broken to transform the solid minerals to a liquid (a magma). At low pressures, atoms in solid minerals are spaced apart so that atoms have room to vibrate and chemical bonds between them can be broken to form a liquid (a magma) Melting by Decrease in Pressure Occurs Along Mid-Ocean Ridges Melting by Decrease in Pressure Occurs Within Mantle Plumes 2
Peridotite ultramafic rock which makes up Earth s mantle Phase Diagram A graph which shows the various physical conditions under which a phase (such as a solid, liquid, gas) can exist Phase Diagram for Melting of Mantle Rock (peridotite) by Decrease in Pressure solidus Why do magmas form along subduction zones? Rocks within the mantle wedge are squeezed, resulting in an increase in pressure. Therefore, decompression melting cannot explain the formation of magmas within a subduction zone What unique feature of a subduction zone might cause melting of mantle rock in the mantle wedge? 3
How can magma form by melting of mantle rock? Melting by Addition of Water Occurs Within Subduction Zones Addition of Water Small amount of water acts as a catalyst to lower the melting temperature of most minerals. Unequal distribution of electrons around the water molecule (electrical polarization) results in decreased anion-cation attraction within minerals, so that chemical bonds in the mineral structure are more easily broken Phase Diagram for Melting of Mantle Rock by Addition of Water Why does cold beer sometimes partially freeze when the can is opened? solidus Why are mafic (basalt) lavas erupted along mid-ocean ridges if the rock being melted to form the magma is ultramafic (peridotite)? Partial Melting Minerals with highest Si content melt at the lower temperatures than minerals with low Si content (reverse of Bowen s Reaction Series) Silicate rocks melt over a range of temperatures. Magmas produced by partial melting of a rock will have a higher Si content than the parent rock which underwent melting. Example: partial melting of an ultramafic rock forms a mafic magma Explains that mafic lavas (basalts) erupted along MOR s form by partial melting of ultramafic mantle rock (peridotite) 4
Low temperature minerals at the bottom of BRS melt first, producing a partial melt with a higher Si content than that of the rock being melted. Partial Melting Silicate rocks melt over a range of temperatures. Magmas produced by partial melting of a rock will have a higher Si content than the parent rock which underwent melting. Partial melting of an ultramafic rock forms a mafic magma Explains that mafic lavas (basalts) erupted along MOR s form by partial melting of ultramafic mantle rock (peridotite) Partial melting of a mafic rock produces a magma of what composition? Why are most mafic (basaltic) magmas erupted at volcanoes, while most felsic (granitic) magmas crystallize beneath the surface? Felsic Magma vs. Mafic Magma Felsic Magmas most crystallize as intrusive rocks (granite) beneath Earth s surface High Si content results in greater viscosity (resistance to flow); flows to surface very slowly High dissolved water content water escapes as steam as magma ascends, increasing temperatures at which minerals will start to crystallize Higher crystallization temperatures increase the likelihood that complete solidification will occur before reaching the surface 5
Why does cold beer sometimes partially freeze when the can is opened? CO 2 Rich Beer (liquid) CO 2 Dry Beer (solid) Felsic Magma vs. Mafic Magma Mafic Magmas most are erupted as basaltic lavas at Earth s surface Low Si Content results in lower viscosity (flows to surface quickly) Low dissolved water content loss of water as magma ascends has little effect on mineral crystallization temperatures Summary To Crystallize a Magma: Decrease temperature Increase pressure Remove water To Melt a Rock: Increase temperature Decrease pressure Add Water 6
Fractional Crystallization Equilibrium Crystallization Involves removal of crystals from the surrounding melt Chemical reactions between crystals and surrounding melt cannot occur Results in solid rock of entirely different chemical composition than that of the melt from which it formed Occurs when crystal remain in contact with the surrounding melt Chemical reactions between solid crystals and surrounding melt can occur Results in solid rock of the same chemical composition as that of the melt from which it crystallized Fractional Crystallization Equilibrium Crystallization 7