Igneous.pdf - Igneous Rocks Magma is molten rock below the...

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Page 127Igneous RocksMagmais molten rock below the surface.Lavais molten rock that is now exposed at the surface.Igneous rocksare rocksthat form from the solidification of magmas. (All igneous rocks were once molten.)Generating magmasMost magmas form at great depth when mantle rock partially melts. Mantle rock is ultramafic in composition and solidunder its usual pressure and temperature conditions. Its minerals are stable in solid form (like food in an icebox). To meltmantle rock, there must be a change in the normal conditions. If temperature increases for example, some of the mineralsin the mantle rock will start to melt (not all, just somethink of increasing the temperature in your freezernot everythingstarts to melt at once). If pressure decreases or water is added (or both), the melting point of a mineral actually decreasesand it will start to melt. As small amounts of melt migrate and accumulate, they form a magma.Imagine taking a jar and filling it with marshmallows, water, and syrup. Now freeze it. The composition of the jar is thecombined composition of all three ingredients. But if you increase the temperature of the jar slightly, so that somecomponents are still frozen and others not, (say the water melts only), then you get a melt forming that has a differentoverall composition from thejar’s materials. The original composition was marshmallows, water, and syrup. The melt thatforms from a slight increase in temperature is pure water. The residual material left behind is marshmallows and syrup (nowater).The same thing happens in the mantle. When ultramafic mantle rock begins to melt, only minerals with the lowest meltingtemperatures go into the magma phase. Others remain behind. We call thispartial melting. We can study the compositionof mantle rocks and tell if they’ve produced melts before. (If our jar contains only marshmallows and syrup, then we know itpartially melted and lost its water.)To melt mantle rock, there must be a drop in pressure (which is what happens under oceanic ridges where seafloorspreading occurs), a rise in temperature (which is what happens when plumes migrate from the deep in the Earth, throughthe mantle and the crust), or an influx of water (which happens where subduction zones reinject water-rich oceanic crustback into the mantle).Partially melted mantle tends to produce mafic compositions. Fully melted mantle produces ultramafic magmas. Afterthese magmas are produced at depth, they rise because of buoyancy.Changing magma compositionAs magmas rise, they mix with other magmas (magma mixing) or melt rocks that they are moving through (assimilation),both of which processes will change magma composition. In addition, as magmas rise to the surface, they gradually cooldown. When magmas cool, crystals start to form. Bowen’s Reaction Series is a graphic representation of the order in whichminerals crystallize during the cooling of a typical ultramafic magma (see image ahead). If after a mineral forms it is

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