Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Earth Systems 3209 Unit: 3 Earth’s Materials Reference: Chapters 2, 3, 6, 7; Appendix A & B

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Classification of Igneous Rocks (Texture) Unit 3: Topic 3.3 Focus on...  explaining the relationships between geologic environment, rate of cooling and texture.  identifying five common igneous rock textures.  explain how different Igneous rocks can have the same texture.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador  Igneous rocks are further classified according to: 1. Texture  Describes the appearance of an igneous rock, based on the size, shape and arrangement of interlocking crystals. 2. Mineral Composition  Is the mineral makeup of an igneous rock based on the chemical composition of the magma. Text Reference: pages Classification of Igneous Rocks:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Texture a. Size b. Shapeof interlocking crystals c. Arrangement Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador  The texture of an Igneous rock reveals a great deal about the environment in which the rock cooled and solidified.  Crystal size is the most important factor affecting texture and the size of the crystals are determined by cooling rate. Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador  Cooling Rate  Molten rock (magma) can cool beneath Earth’s surface (Plutonic environment).  Large crystals form deep within Earth where magmas may take up to tens of thousands of years to cool and crystallize.  Therefore, the slower the magma cools, the larger the crystals. Called a Coarse Texture.  Gabbro, diorite, and granite are examples of intrusive or plutonic igneous rocks Magma Cools Slowly Coarse Texture Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador  Cooling Rate  Molten rock (lava) can cool on Earth’s surface.  Fine crystals form on or near Earth’s surface where lava cools quickly in the matter of minutes to hours.  Therefore, the quicker the lava cools, the smaller the crystals. Called a Fine Texture.  Basalt, andesite, and rhyolite are examples of extrusive or volcanic igneous rocks. Lava Cools Quickly Fine Texture Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador cooling rate crystal size Slow cooling Larger Crystals Fast CoolingSmaller or No Crystals Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 1.Coarse Grained Texture:  forms deep beneath the surface in a plutonic environment where the molten rock cools slowly.  larger crystals of uniform size.  also called Phaneritic texture.  minerals can be identified with the unaided eye. Examples: Granite Gabbro Diorite Quartz Orthoclase Feldspar Amphibole. Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 2.Fine Grained Texture:  forms at or near the Earth’s surface in volcanic environment where the molten rock cools quickly.  very small crystals (only seen under microscope)  may contain vesicles (gas bubbles)  also called Aphanitic texture.  minerals are not identified with the unaided eye. Examples: Basalt Andesite Rhyolite Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 3.Porphyritic Texture:  this texture results when magma with crystals already formed escape to the surface and cools quickly forming a fine grained igneous rock with large crystals inside.  this texture is a result of two stages of cooling; 1) slow cooling forming the larger crystals. 2) rapid cooling forming the finer crystals. Examples: Andesite Porphyry Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 4.Glassy Texture:  forms when the ions in the molten rock do not have time to organize into an orderly pattern to form crystals because it cools very, very quickly.  the result is volcanic glass.  minerals can not identified with the unaided eye. Examples: Obsidian Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 5.Vesicular Texture:  forms when gas bubbles escape from molten rock and are trapped as it cools and crystallizes.  this texture can form near the top of lava flows.  minerals can not identified with the unaided eye. Examples: Scoria Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Fine-grained Coarse-grained PorphyriticGlassy Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Classification Chart TEXTURETEXTURE Coarse GrainedGraniteDioriteGabbro Fine GrainedRhyoliteAndesiteBasalt Vesicular Pumice (frothy glass) Scoria Glassy Obsidian (compact glass) Igneous Rock Texture:

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Igneous Rock Texture: What are the similarities and differences between the following rocks? SimilarityDifference Gabbro, Diorite, Granite.  texture  all plutonic  intrusive environment  composition  diff. minerals Basalt, Andesite, Rhyolite.  texture  all volcanic  extrusive environment  composition  diff. minerals

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Example 1: Which rock-forming process will result in a glassy texture? (A) alignment of minerals as a result of stress (B) cementing together of large, angular crystals (C) rapid cooling of lava at Earth’s surface (D) recrystallization of a rock during metamorphism

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Example 2: An igneous rock is found to contain both large and small crystals, as shown below. What conditions were necessary for this rock to form? Answer: This texture is a result of two stages of cooling; 1) slow cooling forming the larger crystals. 2) rapid cooling forming the finer crystals.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Your Turn... Take the time and complete the following questions... (Solutions to follow) Question: Describe two things that can be determined by examining the crystal size of minerals in igneous rocks.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Solutions... Question: Describe two things that can be determined by examining the crystal size of minerals in igneous rocks. Crystal size of minerals in igneous rocks can indicate the following: 1)The environment in which the molten rock cooled, intrusive (plutonic) or extrusive (volcanic). 2)The cooling rate of the molten rock. Large crystals suggest a slow cooling rate and small crystals suggest a faster cooling rate.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Summary... Overview of Points covered:  Texture is controlled by cooling rate, which is determined by the environment in which the rock formed. Fine-Grained fast cooling lava. forms at or near surface (volcanic). can’t see individual crystals. Coarse-Grained  slow cooling magma.  forms far below surface (plutonic).  visible larger crystals. Porphyritic magma cooled slowly beneath surface, then erupted and cooled quickly. Two stages of cooling. Glassy very rapid cooling. ions unable to form an orderly crystalline structure.