HOW CHAPTER 5 ROCK WERE FORMED 1
I. Modern geology- 1795 A. James Hutton- 1. uniformitarianism- "the present is the key to the past" a. b. the geologic processes now at work were also active in the past the present physical features of earth were formed by these same processes, at work over long periods of time. 2
B. 3 Groups of Rocks 1. rock-defined as a group of minerals bound together in some way. 3
2. 3 groups based on how rocks have been observed to form a. b. c. igneous rocks- are formed by the cooling and hardening of hot molten rock (magma) from inside earth. sedimentary rocks- are formed by the hardening and cementing of layers of sediments. The sediments may consist of rock fragments, plant and animal remains or chemicals that form on lake and ocean bottoms. metamorphic rocks- are formed when rocks that already exist are changed by heat and pressure into new kinds of rocks. 4
C. Recognizing Igneous Rocks 1. plutonic or intrusive - rocks that form underground from cooled magma. These contain distinct mineral grains. 5
2. volcanic or extrusive- rocks that form when lava cools, (lava is the term that describes magma that reaches the surface of the earth) they also may form out of volcanic dust and ash. These lack distinct mineral grains. 6
D. Characteristics 1. Magma a. 2 types are the most common 1) 2) both are hot solutions differ in chemical composition 7
a) felsic magma- -has a high percentage of silica -does not have much calcium, iron, or magnesium -thick and slow flowing. -light colored -granite is a common example -most plutonic rocks are felsic 8
b) mafic magma- - has a lower percentage of silica -higher percentage of calcium, iron and magnesium -hotter, thinner and more fluid -dark-colored -basalt is a common example -most volcanic rocks are mafic 9
Texture and composition are two characteristics used to classify igneous rocks. Texture describes the appearance of an igneous rock based on its size, shape, and the arrangements of its interlocking crystals The composition classes of igneous rocks are based on the proportions of light and dark minerals in the rock. 10
Texture 11
2. Textures of an Igneous Rock a. texture-describes the size, shape and arrangement of the mineral crystals. Description are Coarse-Grained, Fine- Grained, and Glassy 1) crystal size- most important factor a) size depends on; how fast the crystal develop (time in cooling) how much gas is dissolved in the magma 12
2) Additional descriptions: a) b) c) d) e) f) Phaneritic- coarse grained, crystal grain visible with the naked eye. Aphanitic- fine grained, difficult to see individual grains. Glassy- looks like glass, mineral grains did not have time to grow, cooled very quickly. Pegmatite- extremely large crystal grains. Frothy- volcanic foam that cooled quickly. Vesicles-larger air bubbles formed in lava 13
Phaneritic 14
Aphanitic 15
Glassy 16
Pegmatite 17
Frothy 18
Vesicles 19
b. Porphyritic Texture 1) porphyry-rocks which have 2 distinctly different textures a) b) ground mass- the majority of the rock material, usually smaller crystal size. phenocryst- the larger crystals in the porphyry. 20
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Phorphyritic Ground mass phenocryst 23
Composition 24
E. Families of Igneous Rocks There 1. 2. 3. are 3 Families of Igneous Rock Granitic family- Basaltic family- Andesitic family- 25
E. Families of Igneous Rocks 1. granitic family- a. b. c. d. -felsic -orthoclase feldspar and quartz -light in color -different textures 26
1) large grain texture a) granites 27
2) fine grain texture a) felsites (rhyolite) 28
3) glassy texture a) b) obsidian pumice 29
2. Basaltic family a. b. c. d. e. mafic -plagioclase feldspar and augite -dark in color -more dense that granite family -different textures 30
1) large grain texture a) gabbro 31
2) fine grain texture a) basalt 32
3) glassy texture with large vesicles a) scoria 33
3. Andesitic family a. -composition and color between granite and basalt 34
A. Different textures 1) large grain texture a) diorites 35
2) fine grain texture a) andesites 36
Read about ultra-mafic rocks on page 73 of your textbook. Review table 1 pg. 74. 37