Ch 1: Introduction
How is science of geology different than other sciences?
What is “Physical Geology”
What do Geologists do?
Basic principles:
 Catastrophism vs Uniformitarianisn
 Law of Superposition
 Law of Faunal Succession
 Absolute vs relative dating
 Magnitude of geologic time
Geologic time scale. Know the eons, eras and periods. Know the ages of the eons and eras.
Scientific inquiry
 Hypothesis
 Theory
 Paradigm
A definition of science:
“Science is the acceptance of what works and the rejection of what does not”
Earth as a system
 Hydrosphere
 Atmosphere
 Biosphere
 Solid Earth
 Outer space?
Big Bang
 According to NASA WMAP mission – occurred at 13.7 Ga
 First stars appears 200 m.y. later
Early evolution of the earth
 Nebular hypothesis
• Assumes a flat, disk shape with the protosun (pre-Sun) at the center
• Inner planets begin to form from metallic and rocky substances
• Larger outer planets began forming from fragments of ices (H2O, CO2, and others)
Formation of Earth’s layered structure: the “Great Catastrophe”
 Metals sank to the center
 Molten rock rose to produce a primitive crust
 Chemical segregation established the three basic divisions of Earth’s interior
 Primitive atmosphere evolved from gases in Earth’s interior
Earth’s internal structure
 Layers defined by composition
• Crust
• Mantle
• Core
 Layers defined by physical properties
• Lithosphere
• Asthenosphere
• Mesosphere
• Inner and Outer Core
The face of the Earth
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Earth’s surface
• Continents: average elevation 0.5 miles
• Oceans: average depth 2.4 miles
 Continents
• Mountain belts
– Most prominent feature of continents
• The stable interior
– Also called a craton – composed of shields and stable platforms
 Ocean basins
• Continental margins
– Includes the continental shelf, continental slope, and the continental rise
• Deep-ocean basins
– Abyssal plains
– Oceanic trenches
– Seamounts
• Oceanic ridge system
– Most prominent topographic feature on Earth
– Composed of igneous rock that has been fractured and uplifted
Rocks and the rock cycle
 Basic rock types
• Igneous rocks
– Cooling and solidification of magma (molten rock)
– Examples include granite and basalt
• Sedimentary rocks
– Accumulate in layers at Earth’s surface
– Sediments are derived from weathering of preexisting rocks
– Examples include sandstone and limestone
• Metamorphic rocks
– Formed by “changing” preexisting igneous, sedimentary or other metamorphic rocks
– Driving forces are increased heat and pressure
– Examples include gneiss and marble
 The Rock Cycle: One of Earth’s subsystems
• One rock changes to another and back with time
• Materials change both on the surface and inside the Earth
Ch 2: Plate Tectonics
Continental drift, Alfred Wegener, 1915
 Continents assembled into Pangaea by fitting the shorelines
 Fossil evidence: Mesosaurs
 Reconstructed mountain belts
 Reconstructed glacial deposits
Breakup of Pangaea, last 200 m.y.
Wegener’s continental drift idea was ridiculed because there was no mechanism. So, how did the idea
come to be accepted? The key is rock magnetism or “remanent magnetism,” which led to the discovery
of polar wandering in the 50’s and seafloor spreading in 1962.
The earth is divided into seven major lithospheric plates and many smaller ones. The lithospheric plates
move by sliding over the weaker asthenosphere.
Plate margins are
 spreading centers (divergent boundaries). Includes oceanic spreading ridges and continental
rifts
 subduction zones (convergent boundaries)
 transform faults (plates slide past each other)
Convergent Boundaries:
 Subduction of oceanic lithosphere beneath a continent leads to the development of continental
volcanic-plutonic arcs and orogenic belts.
 Subduction of oceanic lithosphere beneath oceanic lithosphere leads to volcanic island arcs.
 Collision of continentals leads to broad high-elevation orogenic belts.
 Does continental lithosphere ever subduct beneath oceanic lithosphere?
Mantle plumes and hotspots
Driving forces for plate motion:
1. Slab pull
2. Ridge push
Consequences of subduction
1. Slab suction
2. magmatism
Ch. 3: Matter and Minerals
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By definition a mineral is
• Naturally occurring
• Inorganic solid
• Ordered internal molecular structure
• Definite chemical composition
Rock
• A solid aggregate of minerals
Physical Properties of minerals
• Magnetism
• Reaction to hydrochloric acid
• Malleability
• Double refraction
• Taste
• Smell
• Elasticity
• Sectility
• Crystal form
• Color
• Streak
• Hardness
• Cleavage
• Fracture
• Specific gravity: average = 2.7
•
Luster
• Metallic, adamantine, vitreous, silky, earthy, others
Mineral Groups
 Of the nearly 4000 minerals have been named, only a few dozen are common. Those are
composed mainly of the 8 elements that make up over 98% of the continental crust
 Silicates
• Most important mineral group
• Silicon-oxygen tetrahedron
• Joining silicate structures
– Single tetrahedra are linked together to form various structures including
– Isolated tetrahedra
– Ring structures
– Single and double chain structures
– Sheet or layered structures
– Complex 3-dimensional structures
 Common silicate minerals
• Light-colored silicates:
• Feldspar group
– Most common mineral group
– Exhibit two directions of perfect cleavage at 90 degrees
– Orthoclase (potassium feldspar) and Plagioclase (sodium and calcium feldspar) are
the two most common members
• Quartz: SiO2
– Only common silicate composed entirely of oxygen and silicon
– Conchoidal fracture
– Often forms hexagonal crystals
• Muscovite
– Common member of the mica family
– Excellent cleavage in one direction
• Clay minerals
– Clay is a general term used to describe a variety of complex minerals
– Clay minerals all have a sheet or layered structure
– Most originate as products of chemical weathering
• Dark silicates:
• Olivine group
– High temperature Fe-Mg silicates
– Individual tetrahedra linked together by iron and magnesium ions
– Forms small, rounded crystals with no cleavage
• Pyroxene group
– Single chain structures involving iron and magnesium
– Two distinctive cleavages at nearly 90 degrees
– Augite is the most common mineral in the pyroxene group
• Amphibole group
– Double chain structures involving a variety of ions
– Two perfect cleavages exhibiting angles of 124 and 56 degrees
– Hornblende is the most common mineral in the amphibole group
• Dark silicates: Biotite
– Mica – one excellent cleavage
• Important nonsilicate minerals
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Typically divided into classes based on anions
Comprise only 8% of Earth’s crust
Carbonates
– Primary constituents in limestone and dolostone
– Calcite (CaCO3) and dolomite CaMg(CO3)2 are the two most important carbonate
minerals
Oxides. Examples: Hematite: Fe2O3, magnetite: Fe3O4
Halides. Example: Halite: NaCl
Sulfides
Sulfates
Composition of minerals
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Elements
• Basic building blocks of minerals
• Over 100 are known (92 naturally occurring)
 Atoms
• Smallest particles of matter
• Retains all the characteristics of an elemen
 Atomic structure
• Central region called the nucleus
• Consists of protons (+ charges) and neutrons (no charge)
• Electrons
• Negatively charged particles that surround the nucleus
• Located in discrete energy levels called shells
 Chemical bonding
• Formation of a compound by combining two or more elements
 Ionic bonding
• Atoms gain or lose outermost (valence) electrons to form ions
• Ionic compounds consist of an orderly arrangement of oppositely charged ions
 Covalent bonding
 Atoms share electrons to achieve electrical neutrality
 Generally stronger than ionic bonds
 Both ionic and covalent bonds typically occur in the same compound
 Other types of bonding
• Metallic bonding
• Valence electrons are free to migrate among atoms
• Weaker and less common than other bonds
• Van der Waals bonding
• Held together by stray electrostatic forces – very weak
• Isotopes and radioactive decay
• Mass number = sum of neutrons + protons in an atom
• Isotope = atom that exhibits variation in its mass number
• Unstable isotopes emit particles and energy in a process known as radioactive decay
Structure of Minerals
 Minerals are lattices of anions and cations
 Anions are close-packed or almost close-packed
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Atomic arrangement in ionic compounds is determined by ionic size
Polymorphs
• Minerals with the same composition but different crystalline structures
• Examples include diamond and graphite
• Phase change = one polymorph changing into another
CH 4. Igneous rocks
General characteristics of magma
 Igneous rocks were once melted
 Magma at surface is called lava
 Rocks formed from lava = extrusive, or volcanic rocks
 Rocks formed from magma at depth = intrusive, or plutonic rocks
 The nature of magma
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Consists of three components:
 Liquid portion = melt
 Solids: silicate minerals
 Volatiles = dissolved gases in the melt, including water vapor (H2O), carbon dioxide
(CO2), and sulfur dioxide (SO2)
 Crystallization of magma
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Minerals crystallize in a predictable order, as described by Bowen’s reaction series
(discussed later on)
Igneous Textures
 Rate of cooling
 Slow rate = fewer but larger crystals
 Fast rate = many small crystals
 Very fast rate forms glass
 % of silica (SiO2) present
 Dissolved gases
 Types of igneous textures
 Aphanitic (fine-grained) texture
 Rapid rate of cooling
 Microscopic crystals
 May contain vesicles (holes from gas bubbles)
 Phaneritic (coarse-grained) texture
 Slow cooling
 Large, visible crystals
 Porphyritic texture
 Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)
 Glassy texture
 Very rapid cooling of lava
 Resulting rock is called obsidian
 Pyroclastic texture
 Fragmental appearance produced by violent volcanic eruptions
 Pegmatitic texture
 Exceptionally coarse grained
 Form in late stages of crystallization of granitic magmas
Igneous compositions
 Minerals in igneous rocks:
 Dark (or ferromagnesian) silicates
 Olivine, pyroxene, amphibole, and biotite mica
 Light (or nonferromagnesian) silicates
 Quartz, muscovite mica, and feldspars
 Granitic versus basaltic compositions
 Granitic composition
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Light-colored silicates
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Termed felsic (feldspar and silica) in composition
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High amounts of silica (SiO2)
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Major constituent of continental crust
 Basaltic composition
 Dark silicates and calcium-rich feldspar
 Termed mafic (magnesium and ferrum, for iron) in composition
 More dense than granitic rocks
 Comprise the ocean floor and many volcanic islands
 Other compositional groups
 Intermediate (or andesitic) composition
 Contains 25% or more dark silicate minerals
 Ultramafic composition
 Composed entirely of ferromagnesian silicates
 Silica content ranges in igneous rocks:
 45% to 70%
 Silica content influences magma behavior
 Granitic magmas = high silica content and viscous
 Basaltic magmas = much lower silica content and more fluid-like behavior
Naming igneous rocks of granitic composition
 Granite
 Phaneritic
 Over 25% quartz, about 65% or more feldspar
 Very abundant - often associated with mountain building
 Rhyolite
 Extrusive equivalent of granite
 May contain glass fragments and vesicles
 Aphanitic texture
 Less common than granite
 Obsidian
 Dark colored
 Glassy texture
 Pumice
 Volcanic
 Glassy texture
 Frothy appearance with numerous voids
Naming igneous rocks of intermediate intermediate composition
 Andesite
 Volcanic origin
 Aphanitic texture
 Diorite
 Plutonic equivalent of andesite
 Coarse grained
Naming igneous rocks of basaltic composition
 Basalt
 Volcanic origin
 Aphanitic texture
 Composed mainly of pyroxene and calcium-rich plagioclase feldspar
 Most common extrusive igneous rock
 Gabbro
 Intrusive equivalent of basalt
 Phaneritic texture consisting of pyroxene and calcium-rich plagioclase
 Significant % of the oceanic crust
Naming pyroclastic igneous rocks
 Composed of fragments ejected during a volcanic eruption
 Varieties
 Tuff = ash-sized fragments
 Volcanic breccia = particles larger than ash
Origin of Magma
 Role of heat
 Temperature increases in the upper crust (geothermal gradient) average between 20˚C to
30˚C per kilometer
 Rocks in the lower crust and upper mantle are near their melting points
 Role of pressure
 When confining pressures drop, decompression melting occurs
 Role of volatiles
 Volatiles (primarily water) cause rocks to melt at lower temperatures
Evolution of Magmas
 A single volcano may extrude lavas exhibiting very different compositions
 Bowen’s reaction series
 Describes what happens when igneous rocks crystallize.
 Magmas change composition via
 Magmatic differentiation
 Melts change as solids crystallize from it
 Assimilation
 Magma can melt country rock
 Magma mixing
 Two chemically distinct magmas may produce a composition quite different from either
original magma
 Formation of basaltic magmas
 Decompression partial melting of ultramafic rock in the mantle
 Formation of andesitic magmas
 Produced by basaltic magmas assimilating silica-rich rocks in the crust
 May also evolve by magmatic differentiation
 Formation of granitic magmas
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End product of crystallization of andesitic magma