Igneous Processes GLG Physical Geology Bob Leighty

These lecture notes are very similar to the ones I use in my traditional classes. You’ll find they are loaded with imagery and streamlined text that highlight the most essential terms and concepts. The notes provide a framework for learning and, by themselves, are not meant to be a comprehensive source of information. To take advantage of the global knowledge base known as the Internet, I have included numerous hyperlinks to external web sites (like the Wikipedia, USGS, NASA, etc.). Follow the links and scan them for relevant info. The information from linked web sites is meant to supplement and reinforce the lecture notes – you won’t be responsible for knowing everything contained in them. As a distance learning student, you need to explore and understand the content more independently than in a traditional class. As always, I will help guide you through this learning adventure. Remember, Dr. Bob if you have any questions about today’s lecture Leave no questions behind! Explore and have fun! These notes and web links are your primary “lecture” content in this class. Additionally, various articles are assigned each week to supplement this “lecture” information. I believe you’ll have enough information to reference without having to purchase a costly textbook.

higher T & P lower T & P  The Earth has a lot of trapped heat (from radioactive decay)radioactive decay deep shallow hotter colder Temperature Depth X Increasing pressure downward typically keeps rocks from melting Temperature increases with depth Heat Within the Earth Igneous Processes

Depth (km) 50 0 Temperature (°C)  C / 50 km = 20  C/km Lower gradients (“cold”) <20  C/km (continental interiors)  Some areas have different geothermal gradientsgeothermal gradients Higher gradients (“hot”) >40  C/km (active volcanic areas) 1000  C / 25 km = 40  C/km Heat Within the Earth Igneous Processes

Depth (km) 50 0 Temperature (°C) solid rocks  Solid rock melts (become magma) when it is heated above a certain temperaturemagma melting temperature (increases with depth) How Does Solid Rock Melt? Igneous Processes geothermal gradient

Depth (km) 50 0 Temp (°C)  Melting temperature is controlled by the composition of the rock solid rocks magma geothermal gradient basalt melting T granite melting T (lower) How Does Solid Rock Melt? Igneous Processes

“dry” basalt melting T (water lowers the melting T – makes it easier to melt) Depth (km) 50 0 Temp (°C) solid rocks magma geothermal gradient “wet” basalt melting T (lower) How Does Solid Rock Melt? Igneous Processes  Melting temperature is also controlled by the presence of water

solid rocks magma Depth (km) 50 0 Temp (°C) geothermal gradient  Rocks can melt by: 1) increasing T 2) decreasing P 3) adding water  Partial melting: only part of the rock melts Partial melting Different Ways Rocks Can Melt Igneous Processes

Divergent Margins (mid-ocean ridges, continental rifts)  Plates pull apart, lets mantle rocks rise & melt (decompression melting) Magmatism & Plate Tectonics Igneous Processes  Forms oceanic crustoceanic crust  Example: Mid-Atlantic Ridge, Red Sea, East Pacific RiseMid-Atlantic RidgeRed Sea East Pacific Rise

 Intense heating melts overlying lithosphere  Rocks in a mantle plume rise & melt (decompression melting)mantle plume Magmatism & Plate Tectonics Igneous Processes Intraplate “Hot spots”  Example: Hawaiian Islands (oceanic crust), Yellowstone (continental crust)Hawaiian Islands Yellowstone

 Most continental crust forms in this waycontinental crust  Hydrous (“wet’) melting above down-going slab Magmatism & Plate Tectonics Igneous Processes Convergent Margins (subduction zones)  Example: Andes, CascadesAndesCascades  Volcanic arcs Volcanic arcs

 Differentiation: magmas may change as they rise Rising Magma Igneous Processes  Magma will start to rise due to a buoyancy contrast with surrounding rockbuoyancy  Magma forms diapirs (magma balloons)

Magma Differentiation Igneous Processes  Magma mixing - magmas may mix & contaminate each other Magma mixing

 Assimilation - contamination of a magma by wall rock Assimilation Magma Differentiation Igneous Processes

 Assimilation - contamination of a magma by wall rock Assimilation Magma Differentiation Igneous Processes

 Assimilation - contamination of a magma by wall rock Assimilation Magma Differentiation Igneous Processes

 Crystallization – Depending on a magma’s composition, certain minerals will begin to crystallize as the magma cools Crystallization Mafic minerals Magma Differentiation Igneous Processes

 Crystal settling – Heavy, early-formed crystals (like olivine, rich in Fe, Mg, & Ca) sink & form layers Magma Differentiation Igneous Processes

WWW Links in this Lecture > Radioactive decay - > Geothermal gradient - > Magma - > Partial melting - > Oceanic crust - > Mid-Atlantic Ridge - > Red Sea - > East Pacific Rise - > Divergent boundary - > Hot spot - > Mantle plume - > Hawaiian Islands - > Yellowstone - > Convergent boundary - > Volcanic arc - > Continental Crust - > Andes - > Cascades - Igneous Processes

WWW Links in this Lecture > Buoyancy - > Magma differentiation - > Magma mixing - > Magma assimilation - > Crystallization Igneous Processes