G201 Midterm 1 Study Guide
The science of Geology
• Geology is the science that pursues an understanding of planet
Earth
• Physical geology - examines the materials composing Earth and the
processes generating them
• Historical geology - seeks an understanding of the origin of Earth
and its development through time; chronology of events
Geologic time
• Accurate dates to events in Earth history
• Absolute dating
• Relative dating and the geologic time scale
• Relative dating means that dates are placed in their proper sequence or
order without knowing their age in years
• The magnitude of geologic time
• Involves – millions or billions of years
• Geological processes operate
– Gradually over periods as much as millions of years
– Episodic in events that may last only seconds to minutes
Early evolution of Earth
• Origin of planet Earth
• Earth and the other planets formed at the ~same time from
interstellar dust
• Nebular hypothesis
• Layered structure developed by chemical segregation early in the
formation of Earth
A view of Earth
• Earth’s four spheres
•
•
•
•
Hydrosphere
Atmosphere
Biosphere
Solid Earth
Earth as a machine
• Internal forces
–
Powered by heat from the interior
• Leads to convection in the earth
• Moves plates on the earth surface
• Produce volcanoes, earthquakes, and mountains
• External forces
- Powered by the Sun that drives external processes in the
• Atmosphere
• Hydrosphere
• At Earth’s surface
Earth’s surface has two principal divisions
Earth’s internal structure
• “Layercake” Earth
Earth’s internal structure
• Mechanical Subdivision of the upper Earth
• Lithosphere
(rigid)
• Asthenosphere
(ductile, plastic)
• Mesosphere
Earth’s Surface
•
•
•
•
Earth’s crust broken into rigid plates
7 major plates
Where plates meet are called plate boundaries
Three types of plate boundaries
Plate Boundaries
• Divergent (constructive) boundary – plates move apart, resulting in upwelling
of material from the mantle to create crust
• Convergent (destructive) boundary – plates move towards each other;
subduction of oceanic plates or collision of two continental plates
• Transform (conservative) boundary – plates move along each other without
either generating new lithosphere or consuming old lithosphere
Dynamic Earth
• The theory of plate tectonics
• Theory, called plate tectonics, has now emerged that provides
geologists with the first comprehensive model of Earth’s internal
workings
• The theory of plate tectonics
• Involves understanding the workings of our dynamic planet
• Began in the early part of the twentieth century with a proposal
called continental drift – the idea that continents moved about the
face of the planet
The Rock Cycle
• The loop that involves the processes by which one rock changes to another
• Illustrates the various processes and paths as earth materials change both on the
surface and inside the Earth
There are three rock classes
• Igneous (magmatic) rocks
• Sedimentary rocks
• Metamorphic rocks
The science of Geology
• Some historical notes about geology
• The nature of Earth has been a focus of study for centuries
• Catastrophism – earth changes by large events like floods, eruptions,
etc.
• Uniformitarianism – present is key to past; processes same through
time, only rates have changed
The nature of scientific inquiry
• Science assumes the natural world is consistent and predictable
• Goal of science is to discover patterns in nature and use the
knowledge to make predictions
• Scientists collect “facts” through observation and measurements
The nature of scientific inquiry
• How or why things happen are explained using a
• Hypothesis – a tentative (or untested) explanation
• Theory – a well-tested and widely accepted view that the scientific
community agrees best explains certain observable facts
The nature of scientific inquiry
• Scientific methods
• Scientific method involves gathering facts through observations and
formulation of hypotheses and theories
• There is no fixed path that scientists follow that leads to
scientific knowledge
Minerals: Building blocks of rocks
• Definition of a mineral:
•
•
•
•
Naturally occurring inorganic solid
Result of crystallization
Ordered internal molecular structure
Definite chemical composition
• Definition of a rock:
• Aggregate of crystals composed of one or multiple mineral types
• Composed also of mineraloid and glass (other building blocks)
Composition of minerals
• Chemically:
–
–
–
Elements
Ions (charged elements)
Molecules (group of elements)
• Physically:
–
Atoms
• Composed themselves of particles
Structure of Atoms
(Atomic Structure)
• Central region called the nucleus
• Consists of protons (positive charges) and neutrons (neutral charges)
• Outer area called electron shells
• Electrons are negatively charged particles
• Electrons are located in discrete energy levels called shells
Idealized structure of an atom
Bonding among mineral components
• Chemical bonding
• Ionic bonding
• Attraction due to oppositely charged components (single ions or
charge molecule)
• Covalent bonding
• Atoms share electrons to achieve electrical neutrality
• Metallic bonding
• Outermost electrons are free to migrate among atoms
Ionic bonding - attraction of opposite charges
Structure of minerals
• Minerals consist of an orderly array of atoms chemically bonded
to form a particular crystalline structure
• For ionic compounds, the internal atomic arrangement is
primarily determined by the size of ions involved
Classification of Minerals
• Nearly 4000 minerals have been identified on Earth
• Rock-forming minerals
• Common minerals that make up most of the rocks of Earth’s crust
• Only a few dozen members
• Composed mainly of the 8 elements that make up over 98% of the
continental crust
• Ore-forming minerals
• Gemstones
Classification of Minerals
• Several major groups exist including
–
Non-silicates
– Native Elements
– Sulfides
– Oxides
– Halides
– Carbonates
– Sulfates
– Phosphates
–
Silicates
Physical properties of minerals
•
•
•
•
•
Hardness - Resistance of a mineral to abrasion or
scratching
Luster - Shine in reflected light
Cleavage - Tendency to break along planes of weak bonding
Fractures - Absence of cleavage when a mineral is
broken
Color
– Streak: Color of a mineral in its powdered form
Mineral group: Native Elements
Mineral Group: Sulfides
Galena is a lead sulfide that
displays metallic
luster
Mineral Group: Oxides
Mineral Group: Carbonates
Mineral Group: Sulfates
Mineral Group: Phosphates
Mineral Group: Halides
Mineral Group: Silicates
• Most important mineral group
– Comprise most of the rock-forming minerals
– Very abundant due to large amounts of silicon and oxygen in Earth’s crust
• Basic building block is the silicon-oxygen tetrahedron molecule
– Four oxygen ions
smaller silicon ion
• All silicate minerals types
they are composed of Si, O
surrounding a much
have in common that
Silicate Structures
• Building block: silicon-oxygen tetrahedron
• 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
Silicate Minerals
• Isolated tetrahedra
–
Olivine (
• Ferromagnesium mineral
• Elements: Fe, Mg
• Typical mineral in igneous(magmatic) rocks
• Gem: Peridot
–
Garnet family
• Ferromagnesium mineral
• Elements: Fe, Mg, Al
• Typically occurs in metamorphic rocks
Silicate Minerals
Tetrahedra linked to form chain structures
• Single chain
–
Pyroxene family
• Ferromagnesium mineral
• Elements: Fe, Mg, Ca
• Typical mineral in igneous rocks (Augite)
• Gem: Jade
• Cleavage at 900
• Double chain
–
Amphibole family
• Ferromagnesium mineral
• Elements: Fe, Mg, Ca, Na, H2O
• Typically mineral in
igneous rocks (Hornblende)
metamorphic &
Hornblende crystals
Silicate Minerals
• Sheets or layers of linked tetrahedra
–
Mica family
• Elements: Fe, Mg, K, Al, H2O
• Biotite - black color, Ferromagn. Min.
• Muscovite - clear
–
Clay family
•
•
•
•
Elements: K, Al, H2O
Can’t see individual crystals
Typical minerals for sedimentary rocks
Weathering minerals
Silicate Minerals
• Complex 3-dimensional structures (frameworks)
–
Quartz
• Elements: only Si, O
• Transparent
• Varieties
– Rose quartz - pink
– Amethyst -purple
– Smoky quartz - gray - black
–
Feldspar family
•
•
•
•
Elements: K, Na, Ca
50% of all minerals
Orthoclase (potassium)
Plagioclase (calcium & sodium)
Physical properties of minerals
• Luster
• Appearance of a mineral in reflected light
• Two basic categories
– Metallic
– Nonmetallic
• Other terms are used to further describe luster such as vitreous,
silky, or earthy
Physical properties of minerals
• Color
• Generally an unreliable diagnostic property to use for mineral
identification
• Often highly variable for a given mineral due to slight changes in
mineral chemistry
• Exotic colorations of some minerals produce gemstones
Physical properties of minerals
• Streak
• Color of a mineral in its powdered form
• Helpful in distinguishing different forms of the same mineral
• Hardness
• Resistance of a mineral to abrasion or scratching
• All minerals are compared to a standard scale called the Mohs scale
of hardness
Physical properties of minerals
• Cleavage
• Tendency to break along planes of weak bonding
• Produces flat, shiny surfaces
• Described by resulting geometric shapes
– Number of planes
– Angles between adjacent planes
Three examples of perfect cleavage – fluorite, halite, and calcite
Physical properties of minerals
• Fracture
• Absence of cleavage when a mineral is broken
• Specific Gravity (Density)
• Ratio of the weight of a mineral to the weight of an equal volume of
water
• Average value is approximately 2.7
Conchoidal fracture
Physical properties of minerals
• Crystal Form
• External expression of the orderly internal arrangement of atoms
• Crystal growth is often interrupted because of competition for space
and rapid loss of heat
The mineral garnet often exhibits good crystal
form
Structure of minerals
• Polymorphs
• Two or more minerals with the same chemical composition but
different crystalline structures
• Diamond and graphite are good examples of polymorphs
– The transformation of one polymorph to another is called a phase change
Diamond and graphite –
polymorphs of carbon
Physical properties of minerals
• Other properties
•
•
•
•
•
•
•
Magnetism
Reaction to hydrochloric acid
Malleability
Double refraction
Taste
Smell
Elasticity
Igneous Rocks
• Igneous rocks form as molten rock (magma) cools and solidifies (through
crystallization, mostly)
• Characteristics of magma
•
•
•
•
Magma that reaches the surface is called lava
Have different compositions, high Mg - low Mg
Liquid, Temperature = ~ 700 to 1300 oC
Forms from partial melting of rocks inside the Earth
The Nature of Magma
• Consists of three components: melt (liquid), crystals (solids), volatiles
(gases)
• Melt: Same 10 elements that make up main igneous minerals are elements
that make up magmas: O, Si, Al, Fe, Mg, Ca, Na, K, Ti, P
• Crystals: variable, typical: feldspars, pyroxenes, olivine, biotite, quartz,
magnetite
• Volatiles: water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide
(SO2)
• Magma is: liquid, liquid + crystals, liquid + crystals + gases
Types of Igneous Rocks
• Rocks formed from lava at the surface are classified as extrusive, or
volcanic rocks
• Rocks formed from magma that crystallizes at depth are termed intrusive,
or plutonic rocks
Crystallization of Magma
• Cooling of magma leads to crystallization of mineral
• If cooling is very quickly, no time for crystallization, magma “frozen” as
glass
• Magma composition and environment decide which minerals crystallize
• Mostly silicate minerals (olivine, pyroxenes, feldspars, biotite,
hornblende, quartz)
• Short time to cool ==> small crystals, glass
• Long time to cool ==> larger crystals
Bowen’s reaction series
• As a magma cools, minerals crystallize in a systematic fashion based on their melting
points
Bowen’s reaction series
• During crystallization, the composition of the liquid continually changes
(differentiation) due to removal of elements by earlier-forming minerals
• Systematic changes in the melt
• Minerals in the melt can chemically react and change
Igneous Compositions - mineralogical
• Mineral components - mainly silicates
• Dark (or ferromagnesian) silicates
– Olivine
– Pyroxene
– Amphibole
– Biotite mica
• Light silicates
– Quartz
– Feldspars
– Plagioclase
– Orthoclase
– Muscovite mica
Igneous compositions - chemical
• Silica (SiO2) content as an indicator of composition
• High silica (felsic rocks) (>70% SiO2 )
• Rhyolite, Granite
• Low silica (basic, mafic rocks) (<51% SiO2 )
• Basalt, Gabbro
• Intermediate silica
• Andesite, Diorite, Dacite, Granodiorite
• Very-low silica (<45% SiO2 )
• ultramafic rocks, Peridotite
Felsic igneous rocks
• Granite
–
–
–
–
Phaneritic texture - mineral grains observable with naked eye
Over 25 percent quartz, about 65 percent or more feldspar
May exhibit a porphyritic texture
The term granite covers a wide range of mineral compositions
• Rhyolite
–
–
–
Aphanitic texture - mineral not observable
Extrusive equivalent of granite
May contain glass fragments and vesicles
Glassy felsic igneous rocks
• Obsidian
–
–
–
Volcanic
Dark colored
Glassy texture
Intermediate igneous rocks
• Diorite
–
–
–
–
Phaneritic texture - mineral grains observable with naked eye
Intrusive origin; plutonic equivalent of andesite
Contain at least 25 percent dark silicate minerals
Composed mainly of plagioclase and amphibole
• Andesite
–
–
Typically porphyritic texture w/ plagioclase phenocrysts
Volcanic origin
Mafic igneous rocks
• Basalt
–
–
–
–
–
Aphanitic - porphyritic texture
Volcanic origin
Composed mainly of pyroxene and calcium-rich plagioclase feldspar
Most common extrusive igneous rock
Scoria with vesicles (former
gas bubbles)
• Gabbro
–
–
Similar to diorite
Intrusive equivalent to basalt
Origin of Magma
• Melting process: Generation of magma from solid rocks
–
–
–
Produced from partial melting of rocks in the crust and upper mantle
Rocks in the lower crust and upper mantle are near their melting points
Melting controlled by:
• Heat
• Pressure
• Composition
Geothermal Gradient
Melting regimes
• Mantle beneath divergent plate boundaries
• Mantle beneath convergent plate boundaries
• Continental crust
Melting at divergent plate boundaries
Melting at convergent plate boundaries
Melting in the continental crust
Evolution of magmas
• Processes responsible for changing the composition of a magma
–
–
–
Magmatic differentiation
• Separation of a melt from earlier formed crystals to form a different
composition of magma
Assimilation
• Changing a magma’s composition by the incorporation of foreign matter
(surrounding rock bodies) into a magma
Magma mixing
• Involves two bodies of magma intruding one another
• Two chemically distinct magmas may produce a composition quite different
from either original magma
Processes that change magma compositions
Igneous textures
• Texture is used to describe the overall appearance of a rock based on the
size, shape, and arrangement of interlocking minerals
• Texture in igneous rocks is determined by the size and arrangement of
mineral grains
• Factors affecting crystal size
• Rate of cooling
– Slow rate promotes the growth of fewer but larger crystals
Types of Igneous Textures
• Aphanitic (fine-grained) texture
– Rapid rate of cooling of lava or magma
– Microscopic crystals
– May contain vesicles (holes from gas bubbles)
• Phaneritic (coarse-grained) texture
– Slow cooling
– Crystals can be identified without a microscope
Aphanitic texture
Phaneritic texture
Types of Igneous Textures
• Porphyritic texture
– Minerals form at different temperatures as well as differing rates
– Large crystals, called phenocrysts, are embedded in a matrix of
smaller crystals, called the groundmass
• Glassy texture
– Very rapid cooling of molten rock
– Resulting rock is called obsidian
Porphyritic texture
Glassy texture
Igneous textures
• Factors affecting crystal size
• Rate of cooling
– Fast rate forms many small crystals
– Very fast rate forms glass
• Amount of silica (SiO2) present
• Amount of dissolved gases
Volcanic (Extrusive) Rocks
• Magma that erupts at the surface (volcano) forms volcanic rocks
• Volcano
• Place where eruption took place
• Accumulation of volcanic rocks
• Volcanic activity
Volcanic Activity
• Effusive: Magma erupts as lava flows
• Explosive: Magma erupts as volcanic fragments (pyroclasts)
Viscosity
• Viscosity is a measure of a material’s resistance to flow (e.g., Higher viscosity
materials flow with great difficulty)
• Factors affecting viscosity
– Temperature - Hotter magmas are less viscous
– Composition - Silica (SiO2) content
– Higher silica content = higher viscosity
(e.g., felsic lava such as rhyolite)
– Lower silica content = lower viscosity or more fluid-like behavior (e.g.,
mafic lava such as basalt)
– Dissolved Gases
• Viscosity controls explosivity and lava type
Volcanic Eruptions and viscosity
Basalt
(high T,
low SiO2)
Lava Flows
•
•
•
•
Viscosity will control shape (thickness/length ratio) of lava flow
Long thin flows from basalt magma
Thick stubby flows from rhyolite magma
Lava types
•
•
•
•
•
Pahoehoe
AA
Block lava
Pillow lava (submarine lava type)
Lava dome
Pahoehoe lava flow
AA lava flow
Pyroclastic Material (Rocks)
• Pyroclasts: volcanic fragments
• Volcanic ash: < 2 mm
• Lapilli: 2 mm to 7 cm
• Bombs, Blocks: > 7cm
• Tuff: rock resulting from pyroclastic deposit
• Specific pyroclast types
• Pumice - rhyolite with lot’s of vesicles
• Scoria - pumice of basaltic composition
• Cinder - lapilli-size, basaltic pumice
Pyroclastic Transport
• Pyroclastic fallout
• Ejection into the air vertical
• Pyroclasts “rain” out (like snow flakes)
• Pyroclastic flow (ash-flow)
• Ejection typically more horizontal
• Pryoclasts ride on hot gases laterally near the surface
• Lahar
• Volcanic mudflow
• Chaotic mixture of water and debris
• Follow drainages
At Mt. St. Helens
A volcanic bomb
Volcanoes
• Circular opening at the summit of a volcano
• Crater - steep-walled depression at the summit, generally less than
1 km diameter
• Caldera - a summit depression typically greater than 1 km
diameter, produced by collapse following a massive eruption
• Vent – opening at the surface that connects crater with the magma
chamber via a pipe
• Long narrow opening: Fissure
Types of volcanoes
•
•
•
•
•
•
•
Shield volcano
Composite volcano (Stratovolcano)
Cinder (Scoria) cone
Maar Volcano
Dome
Caldera Volcano
Flood basalt
Shield volcano
• Broad, slightly domed-shaped, low angle slope
• Composed primarily of basaltic lava
• Generally cover large areas
• Produced by frequent eruptions of fluid lava
• Mauna Loa on Hawaii is good example and largest volcano on earth
• Example in Oregon: Newberry volcano
Composite volcano
•
•
•
•
•
Large, classic-shaped volcano (1000’s of m high & several miles wide at base)
Composed of interbedded (alternating) lava flows and layers of pyroclastic debris
Steep sided (30 degrees)
Mainly andesite (Andesite: from Andes)
Example: Mt. Hood
Cinder Cone
•
•
•
•
•
•
Built from ejected lava (mainly cinder & bomb-sized) fragments
Steep slope angle (up to 40 degrees)
Small size
Frequently occur in groups
Composed of basaltic material
Example:
Lava Butte, S of Bend,
Pilot Butte, E. Bend
Cinder cone and volcanic activity
Maar Volcano
•
•
•
•
Formed through explosion “explosion crater”
Explosivity due to interaction of lava with water (e.g.lake; groundwater)
Mainly basaltic, but also rhyolitic
Examples: Hole in the Ground, E of La Pine, OR
Volcanic Dome
•
•
•
•
Mainly rhyolitic lava
Smaller volcano
No Crater, is
concealed by
lava
Often after large, violent eruptions, e.g.
Mt. St. Helens
Caldera Volcano
• Collapse depression due to roof collase of a emptied magma chamber
• If “rhyolitic”, then violent eruption first to form ash-flow tuff, e.g. Crater Lake,
Yellowstone
• “Basaltic” examples found atop of shield volcanoes, e.g. Hawaii
Flood Basalt
• Form large lava plateaus
• Vast outpourings of basalt lava
• Eruptions from fissures
• Example: Columbia River Basalt (Gorge)
Size comparison among volcano types
Other volcanic landforms
• Volcanic pipes and necks
• Pipes are short conduits that connect a magma chamber to the
surface
• Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents
left standing after erosion has removed the volcanic cone
Formation of a volcanic neck
Shiprock, NM – a volcanic neck
Plutonic igneous activity
• Most magma is emplaced at depth in the Earth
• An underground igneous body, once cooled and solidified, is called a pluton
• Classification of plutons
• Shape
– Tabular (sheetlike)
– Massive
• Orientation with respect to the host (surrounding) rock
– Discordant – cuts across sedimentary rock units
– Concordant – parallel to sedimentary rock units
Types of intrusive bodies
• Dike – a tabular, discordant pluton
• Sill – a tabular, concordant pluton (e.g., Palisades Sill in New York)
• Laccolith
– Similar to a sill
– Lens or mushroom-shaped mass
– Arches overlying strata upward
• Batholiths
– Multiple plutons, largest intrusive body
Plate tectonics and igneous activity
• Global distribution of igneous activity is not random
–
–
–
Mid-oceanic ridges (divergent plate bound.)
Subduction zones (convergent plate boundaries, “Ring of Fire”
volcanisms along the margin of the Pacific ocean
Intra-plate within a tectonic plate
• Hot spot (e.g. Hawaii, Yellowstone)