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• CHAPTER 3

• HOW ROCK WERE FORMED

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• I. Modern geology- 1795

• A. James Hutton-

• 1. uniformitarianism- "the present is the key to the

past"

• a. the geologic processes now at work were also

active in the past

• b. the present physical features of earth were

formed by these same processes, at work over long

periods of time.

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• B. 3 Groups of Rocks

• 1. rock-defined as a group of minerals bound together in

some way.

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• 2. 3 groups based on how rocks have been observed to

form

• a. igneous rocks- are formed by the cooling and

hardening of hot molten rock (magma) from inside earth.

• b. 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.

• c. metamorphic rocks- are formed when rocks that

already exist are changed by heat and pressure into new

kinds of rocks.

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• C. Recognizing Igneous Rocks

• 1. plutonic or intrusive - rocks that form underground

from cooled magma. These contain distinct mineral grains.

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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.

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• D. Characteristics

• 1. Magma

• a. 2 types are the most common

• 1) both are hot solutions of magma

• 2) differ in chemical composition

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• 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

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• 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

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• 2. Textures of an Igneous Rock

• a. texture-describes the size, shape and arrangement of

the mineral crystals. Description ranges from glassy to

coarse-grained.

• 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

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• 2) Descriptions range from glassy to coarse-grained.

• a) Phaneritic- coarse grained, crystal grain

visible with the naked eye.

• b) Aphanitic- fine grained, difficult to see

individual grains.

• c) Glassy- looks like glass, mineral grains did

not have time to grow, cooled very quickly.

• d) Pegmatite- extremely large crystal grains.

• e) Frothy- volcanic foam that cooled quickly.

• f) Vesicles-larger air bubbles formed in lava

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Phaneritic

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Aphanitic

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Glassy

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Pegmatite

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Frothy

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Vesicles

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• b. Porphyritic Texture

• 1) porphyry-rocks which have 2 distinctly different

textures

• a) ground mass- the majority of the rock material,

usually smaller crystal size.

• b) phenocryst- the larger crystals in the porphyry.

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Phorphyritic

phenocryst

Ground mass

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• E. Families of Igneous Rocks

• There are 3 Families of Igneous Rock

• 1. Granite family-

• 2. Diorite family-

• 3. Gabbro family-

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• E. Families of Igneous Rocks

• 1. granite family-

• a. -felsic

• b. -orthoclase feldspar and quartz

• c. -light in color

• d. -different textures

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• 1) large grain texture

• a) granites

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• 2) fine grain texture

• a) felsites (rhyolite)

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• 3) glassy texture

• a) obsidian

• b) pumice

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• 2. gabbro family

• a. mafic

• b. -plagioclase feldspar and augite

• c. -dark in color

• d. -more dense that granite family

• e. -different textures

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• 1) large grain texture

• a) gabbro

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• 2) fine grain texture

• a) basalt

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• 3) glassy texture with large vesicles

• a) scoria

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• 3. diorite family

• a. -composition and color between granite and gabbro

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• A. Different textures

• 1) large grain texture

• a) diorites

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• 2) fine grain texture

• a) andesites

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ObsidianGlassy

BasaltsAndesitesRhyolitesIndistinct

ScoriaScoriaPumicePorous

PorphyryPorphyryPorphyryDistinct and

indistinct

GabbrosDioritesGranitesDistinct

(large)

Tex

tures

MaficIntermediateFelsicComposition:

GabbroDioriteGraniteFamily: