Igneous Processes, Igneous Intrusions I. Processes that change the composition of the magma A. Fractional crystallization (crystal settling) B. Assimilation C. Magma mixing Online rocks and mineral table. Viscosity & melts Partial melt Bowen’s reaction series

1. What three factors control the melting of rocks? Question of the Day 1. What three factors control the melting of rocks? Use 8 1/2” x 11” paper. Put your name and class on the top of your paper.

Plate tectonics and Igneous Rocks Basics Basalt and Gabbro— Low viscosity, so often reach the surface (most common rock) 1. Oceanic Basalts Mid ocean ridge basalts (MORBs) 65% of Earth’s surface Divergent boundaries b. Ocean Island Basalts (OIBs) Over hot spots

Plate tectonics and Igneous Rocks Basalts and Gabbros Oceanic Basalts 2. Continental Basalts (rare) a. continental rifting b. continental hot spots c. over subduction zones rare because often undergo fractional crystallization  more felsic rocks

Plate tectonics and Igneous Rocks B. Andesite and Diorite Forms over subduction zones Basalt forms from partial melting of ultramafic mantle Fractional crystallization and assimilation of crust  intermediate magmas Also, melting of sediment on top of slab  more intermediate magmas

Plate tectonics and Igneous Rocks C. Rhyolites and Granites Continental hot spots Over subduction zones So the melts are very viscous, so they rarely reach the surface, usually plutonic/intrusive. Rhyolite and granite form by: Melting of crust Heat source from intermediate and mafic magmas. Fractional crystallization and assimilation are important

Plate tectonics and Igneous Rocks III. Intrusive Igneous Rocks (plutonic rocks) Plutons—formations created by emplacement of magma at depth Tabular 1. Dike—discordant, cuts cross pre-existing layers Look lie sheets and generally dip steeply or vertical Dikes are magma injected into fractures - pressure of magma may open up small fractures Range in size from < centimeter to hundreds of kilometers Often more resistant to erosion than surrounding rock, so remains like a wall Spanish Peaks, southern Colorado, Shiprock Peak, NM, all over SW US

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Plate tectonics and Igneous Rocks III. Intrusive Igneous Rocks (plutonic rocks). Plutons—formations created by emplacement of magma at depth Tabular 2. Sill: a concordant tabular pluton Lies parallel to layers of preexisting rock. Intrudes between layers and pushed overlying rock up, so shallow where overlying pressure is low. Generally basalt (low viscosity), form uniform thickness layers/large extent

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Plate tectonics and Igneous Rocks

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Plate tectonics and Igneous Rocks III. Intrusive Igneous Rocks (plutonic rocks). Plutons—formations created by emplacement of magma at depth Tabular 3. Laccolith: concordant, similar to sill, but mushroom shaped/domed Formed by more viscous magma: intermediate - felsic Shallow depth = low pressure Henry Mountains of SE Utah (draw erosion of overlying rock to form mts)

Plate tectonics and Igneous Rocks III. Intrusive Igneous Rocks (plutonic rocks). Plutons—formations created by emplacement of magma at depth A. Massive plutons—several km thick and 10’s-100’s km across Batholiths—massive discordant plutons When exposed, surface areas of >hundred km2 Intermediate-felsic (too viscous to reach surface) Form ~25 km below the surface Examples: Sierra Nevada Mts., Yosemite, White Mts in NH, Pikes Peak CO, Idaho Batholith

Plate tectonics and Igneous Rocks III. Intrusive Igneous Rocks (plutonic rocks). Plutons—formations created by emplacement of magma at depth A. Massive plutons—several km thick and 10’s-100’s km across Batholiths: massive discordant plutons When exposed, surface areas of >hundred km2 Intermediate-felsic (too viscous to reach surface) Form ~25 km below the surface Examples: Sierra Nevada Mts., Yosemite, White Mts in NH, Pikes Peak CO, Idaho Batholith

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Plate tectonic setting and igneous rock types 06_19.jpg

Summary Intrusive rocks Cool slowly Phaneritic texture Extrusive rocks (volanic rx) Cool rapidly Aphanitic texture Types of igneous rocks and tectonic settings Divergent boundary Convergent boundary Concepts Partial melting Fractional crystallization & Crystal settling Assimilation Magma Mixing Igneous rocks on and in the Earth Dikes, sills, batholiths