Igneous rocks and melt chemistry physical geology

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Igneous Rocks and Melt Chemistryphysical geologychapter 4d. altinRelease Pressure.Remember there is also an increase in pressure within the earth—SOLID rocks under pressure.Confiningpressure of atoms at great depths pushes them closer together (than if the rocks were at the earth’ssurface).To change from a solid to a liquid requires the atoms to spread apart.Well we can spread apartatoms by heating them OR by releasing the pressure.
Igneous Rocks and Melt Chemistryphysical geologychapter 4d. altinRocks deeper in the earth are already at a higher temperature.Example:Let’s say that to melt basalt (mafic) at the earth’s surface would require us to heat it to 100degrees.Under confining pressure deep within the earth atoms are packed closer than at the earth.At depths, I may have to heat the basalt to 110 degrees to overcome the pressure and be able to spreadthe atoms further apart to liquify them.Therefore, the melting point of rocks increases with increasing pressure. (under pressure rocks will remainsolid even if the temperature is higher).Cracking the lithosphere at MOR releases pressure and allows for melting without changing thetemperature.This is why we have constant volcanism along mid-ocean ridges—continuously spewing out basalt creatingoceanic crust.MAGMA CRYSTALLIZATIONCrystallization occurs when molten rock cools.As the melt begins to cool, movement of atoms slow down toarrange themselves into certain minerals until the crystals are closely packed with no spaces between them.
Igneous Rocks and Melt Chemistryphysical geologychapter 4d. altinThe first chemical bonds to form are Si and O forming the tetrahedra.Then, whatever remaining ions areavailable, will determine the silicate structure (isolated, single chains, double chains, sheets or framework).High amounts of iron and magnesium in the melt should produce darker, mafic minerals such as augite,hornblende and olivine.Melts with little iron and magnesium (but perhaps K, Na, Ca, Al) will produce felsicminerals (less dense continental crust).The chemistry of the melt/rock (i.e. how much Si, O, Fe, Mg) will determine:The temperature it will melt or crystallize.The order that minerals will melt or crystallize.Which minerals will crystallize.Recall our silicate minerals and go get that mineralogy table I gave you.Pay close attention here.BOWEN’S REACTION SERIES: THE DISCONTINUOUS BRANCHAs a melt cools, minerals will crystallize in a specific ORDER based on their melting points which is relatedto their SILICATE STRUCTURE.REGARDLESS OF THE COMPOSITION OF THE MELT, minerals willcrystallize in this order.Olivine (isolated), Augite (single chain), Hornblende (double chain), Biotite (sheet) and finally Quartz(framework).The first elements to BOND as a melt cools is the silica tetrahedron.After each silica
Igneous Rocks and Melt Chemistryphysical geologychapter 4d. altintetrahedra form, Fe and Mg are sucked out of the melt to fill up the tetrahedra.The goal is to have Feand Mg equally distributed amongst as many silica tetrahedra (but we know different minerals containdifferent combinations of each).

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