Chapter 3 - Igneous Rocks.docx

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Western University
Earth Sciences
Earth Sciences 1022A/B
Stephen R Hicock

Igneous Rocks 9/18/2012 9:17:00 AM - Ignis – “fire” - Form from molten material magma (less dense than rocks)  magma is formed through process partial melting o occurs at various levels of crust and upper mantle  magma that reaches earth’s surface = lava, melted portion = melt, solid = silicate minerals  forms at surface o extrusive / volcanic  sometimes loses mobility before reaching surface, igneous rock forms at depth o intrusive / plutonic o occurs in BC  primary magma o from direct melting of rocks, has not evolved yet  secondary magma o changed and evolved Generating Magma from Solid Rock HEAT - Geothermal gradient, hotter deep in the earth  Pressure also increases with depth PRESSURE  Increase in confining pressure causes rise in rock’s melting temperature  Decompression melting is triggered where confining pressure drops  When rock goes up from convective upwelling, moves into zones of lower pressure, thus melts  generates magma along ocean ridges where plates are drifting apart VOLATILES  rock’s water content can affect rock to melt at lower temperatures  wet rock has lower melting temp than dry - Magma due to lighter density, rises through volcano - Volcanoes came from ancient god of fire, Volcan  Figure 1 - Pattern occurs called “texture” How Magmas Evolve (page 45) - Discovered by Norman L. Bowen from Kingston, ON. Carnegie Institute. - Bowen used polarizing microscope, shine through thin sheet of rock to show a ray of colours and you can tell distinction between rocks through Bowen’s Reaction Series, in which crystals settle Crystal settling in a magma chamber. Earlier dark crystals remove heavy elements, leaving melt with light ones (magmatic differentiation) Heavy crystals form first, and take heavy elements with them. The remaining melt is lighter (aluminum
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