Igneous Rocks

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Queen's University
Geological Sciences and Geological Engineering
GEOL 104
Michael T.Mc Bride

Igneous Rocks Magma: The Parent Material of Igneous Rocks:  Igneous rocks form as molten rock cools and solidifies  rises to the surface as magma is buoyant  When molten rock breaks through, producing a volcanic eruption, magma reaches the surface and becomes Lava  Igneous rocks that form when magma solidifies at the surface are classified as extrusive or volcanic  common in the Western Americas and Hawaii  Igneous rocks that form below the surface are called intrusive or plutonic  body of plutonic rock is called a pluton  common in Canada (ie. BC, Canadian shield)  Magma is believed to be formed from melting in the mantle, more actively at divergent plate boundaries (ie. seafloor spreading)  based on scientific evidence, Earth's crust and mantle are composed primarily of solid, not molten rock  Geothermal Gradient is the change in temperature (increase) as depth increases  Volatiles (fluids/gases) or a drop in confining pressure around rocks almost at their melting point cause rocks to melt  pressure increases as depth increases causing rock to melt at higher temperatures because of greater confining pressure  reducing confining pressure lowers a rocks melt temperature (causes decompression melting when rock ascends due to upwelling/moving into low pressure zones)  water and volatiles cause rocks to melt at lower temperatures  When mafic (high in magnesium and iron) magma forms, it rises towards the surface  on land, mafic magma may pond below crustal rock and cause some of it to melt, forming a secondary felsic (feldspar and silica rich) magma  Magma endures crystallization (forms crystalline structures) when cooling and form various silicate minerals  silicon-oxygen tetrahedra tend to form first in cooling magma  as magma loses more heat, tetrahedra join with each other and other ions  eventually forms an igneous rock (solid mass of interlocking silicate minerals) How Magmas Evolve:  Magma crystallizes through a wide range of temperatures in a systematic fashion based on their melting points  Bowen's Reaction Series shows the sequence of crystallization in 2 branches:  Discontinuous branch:  First mineral from mafic magma to crystallize is Olivine  Further cooling generates calcium-rich plagioclase feldspar as well as pyroxene Bowen's Reaction Series:  Crystal settling: occurs when earlier-formed minerals are denser than the liquid magma and sink towards the bottom of the magma chamber  forms rocks with different chemical composition than the parent magma  formation of secondary magmas from a single parent magma is called magmatic differentiation  If solid components of a magma remain in contact with the remaining melt, they will chemically react and evolve into the next mineral in the sequence  Quartz, potassium feldspar, and muscovite are typically found together as major constituents of the plutonic igneous rock granite  Assimilation: as magma migrates upward, it may incorporate some of the surrounding host rock  occurs near surface where rocks are brittle  Magma Mixing: occurs when one magma body intrudes another  Bowen's Reaction Series demonstrates through crystal settling and magmatic differentiation, a single parent magma can generate several mineralogically different igneous rocks Partial Melting and Magma Formation:  Mafic magmas probably originate from partial melting of igneous rock that is thought to make up most of the upper mantle  mafic magmas that originate from direct melting of mantle rocks are called primary magmas since they have not yet evolved  melting occurs due to reducing confining pressure which allows mafic magmas to appear at the surface  in some situations, mafic magmas that are highly dense collect under crustal rocks and crystallize  Evidence suggests that interactions between mantle-derived mafic magmas and more silica-rich components of the Earth's crust generate Intermediate and Felsic magmas  Intermediate and Felsic rocks are not erupted from volcanoes in ocean basins, but are found only within/near continents  As mafic magma migrates upward, it may melt and assimilate some crustal rocks forming a more silica-rich magma of intermediate composition or andesitic composition (between mafic and felsic)  Felsic melts are higher in silica and are more viscous (thicker) and are formed at lower temperatures  usually lose mobility before reaching the surface, producing large plutonic bodies  when silica-rich magmas reach the surface, explosive eruptions occur (ie. Mt. St. Helens)  Mafic magmas frequently produces vast outpourings of lava Igneous Compositions:  Ferromagnesian silicates are rich in iron or magnesium or both and comparatively low in silica  Includes Olivine, Pyroxene, Amphibole, Biotite Mica  Non-ferromagnesian silicates contain greater amounts of potassium, sodium and calcium and contain more silica  Includes Quartz, Muscovite Mica, and Feldspars = most abundant mineral in igneous rocks Igneous Textures:  Three factors contribute to the texture of igneous rock:  the rate at which magma cools (fast cooling= glassy texture)  the amount of silica present  the amount of volatiles in the magma  Slow coolin
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