Earth and Planetary Sciences 101

THE WAY THE EARTH WORKS, Igneous Rocks Fall, 2002

I. Introduction---WHY ARE IGNEOUS ROCKS IMPORTANT?????

A. Volcanic Rocks (Extrusive igneous rocks)

1. processes, often catastrophic (look at Mt. Etna….doing its thing as we speak)

2. landforms

3. soil formation

4. geothermal energy

5. sources of base and precious metals

B. Plutonic rocks (Intrusive igneous rocks)

1. window on how magmas may actually be generated

2. interactions between the crust and magmas

3. sources of base and precious metals

4. geothermal energy

5. crystallization of magmas--fascinating subject

II. Classification of Igneous Rocks

A. Texture

1. aphanitic vs. phaneretic

2. implications for rate of cooling

3. difficulties with fine-grained igneous rocks

B. Chemistry--mineralogical composition

1. modal (volume abundance) mineralogic composition of rocks

2. oxide abundances (silicic vs. mafic)

C. Others????

II. Extrusive Igneous Rocks

A. Products

1. magmas/lavas classification---rhyolites, andesites, basalts

2. pyroclastic deposits

3. related volcaniclastic deposits

4. surface features---volcanoes, calderas, etc..

B. Factors in volcanism

1. Viscosity, defined as the resistance to flow or shear

a. related to polymerization of silicate melts ( a polymer is a substance formed by the union of molecules of the same kind---i.e. strings of SiO₄ tetrahedra)

2. chemistry

a. solids---mafic/intermediate/silicic

b. volatiles

c. textures

3. Solidification temperature range

C. Landforms

1. shield volcanoes/cinder cones/composite or strato volcanoes

2 volcanic domes

3. pyroclastic rocks--associated calderas

4. “flood” or plateau basalts

5. mid-ocean ridge products.

D. Location of Volcanic activity

1. plate margins

2. continental rifts

3. intraplate activity, possibly related to mantle plumes

III. Intrusive Igneous rocks, Introduction

A. General definition--crystallization of magmas within the earth

B. types-related to overall composition--look at the classification scheme

C. Evidence:

1. chemically identical to extrusive rocks

2. require, often, high pressures

3. country rock is forced apart

4. baked contacts

5. xenoliths, foreign rock fragments

6. chill zones of the intrusive rock against contacts

D. Intrusive Bodies

1. shallow-eroded volcanic necks

2. dikes---discordant

3. sills--concordant (laccolith, special type)

4. general plutonic rocks, irregular in character

a. stocks---small

b. batholiths--large

E. Identification

1. coarse-grained igneous rocks/porphyritic texture indicative of at least one phase of crystallization (slowly) at depth.

2. rock classification table, intrusive equivalents of extrusive rocks

3. note that geologists love to invent names for rocks--something like over 4000 separate rock “species” in the literature, just for igneous rocks! Most are of little importance

F. Origin of intrusive igneous rocks

1. abundance and distribution of plutonic rocks is similar to that of volcanic rocks

2. formation of magma (temperature increases, pressure decreases)

a. sources of heat--geothermal gradient--radioactivity--friction--thermal upwelling (plumes)

b. effect of pressure (solid pressure vs. vapor or volatile pressure)

c. effects of added phases

3. theories on the origin of magmas

a. differentiation of a more mafic material --Bowen’s reaction series

--continuous branch--plagioclases

--discontinuous branch--(Fe-Mg silicates)

b. magma processes

--crystal settling

--assimilation

--magma mixing

--partial melting

IV. Sources of Magmas (Magma generation)

A. Upper few hundred km of Earth (base of continental lithosphere, also within lower crust, generally)

B. Relations to plate tectonics

1. asthenosphere----zone of partial melting and generation of mafic magmas

2. role of partial melting

3. effect of pressure

4. effect of fluids (volatiles)

C. Mantle Plumes

1. exception to plate tectonic paradigm for magma generation

2. origin debated

3. core/mantle boundary???? (whole mantle convection)

4. upper mantle (layered mantle convection)

D. Generalizations as to where different magmas come from (but, never completely trust generalizations!!!!)

1. basalts--mid ocean ridges, mantle plumes, ocean/ocean convergence, “back-arc” spreading; represent partial melting of the mantle--hard to directly evolve a magma into a silicic composition from mantle material, without significant contamination.

2. andesites--ocean/continent convergence--direct products of convergence of lithosphere plates and some degree of crustal contamination

3. rhyolites--ocean/continent convergence (with major contamination by crustal materials) mantle plumes (affecting upper crust)