111-13 - GY 111 Lecture Notes D. Haywick (2007-08) 1 GY 111...

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GY 111 Lecture Notes D. Haywick (2007-08) 1 GY 111 Lecture Note Series Intrusive Igneous Rocks Lecture Goals A) Intrusive igneous rocks and the bodies they form B) Intrusive igneous Rocks that Suck C) Summary diagram of all igneous rock types Reference: Press et al., 2004, Chapters 5 and 6; Grotzinger et al., 2007, Chapter 4 A) Intrusive igneous rocks and the bodies they form As we have mentioned on several occasions, plutonic rocks form deep below the earth’s surface (see cartoon 1 below). The principle emplacement agents are plutons; those inverted, tear drop- shaped bodies that may measure kms in width and depth. Magma makes it from these plutons to the surface of the Earth through dikes (sub-vertical cracks that cut across horizontally-orientated sediment- ary rocks) and volcanic pipes. Sometimes the magma will be injected between horizontal layers. Thin plutonic bodies are called sills and thicker horizontal igneous bodies are called laccoliths. . Dikes, sills and pipes are usually filled by aphanetic or porphyritic igneous rocks because they are relatively thin and cool relatively fast. Laccoliths are usually filled with phaneritic igneous rocks. Plutons often merge with one another giving very wide composite igneous bodies called batholiths (see cartoons to left) . When a pluton cuts across another pluton, it often rips off pieces of the earlier pluton. These fragments are called xenoliths and they are important for dating purposes 1 . Because they form deep in the interior of the Earth, many millions of years (and thousands of feet) of erosion are needed to expose them at the surface. Batholiths normally form 1 In GY 112, you will learn several geological principles that are used for dating rocks. One of these, the Principle of Inclusions uses xenoliths to date igneous bodies.
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GY 111 Lecture Notes D. Haywick (2007-08) 2 topographic highs (i.e., mountains) because they are composed of very hard crystalline rocks that are usually much more resistant to the forces of erosion than the softer sedimentary rocks which
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111-13 - GY 111 Lecture Notes D. Haywick (2007-08) 1 GY 111...

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