Review Sheet for Midterm

Hydrologic Cycle

Weathering and Minerals

Soils and Erosion

Rivers

Sedimentary Rocks

Metamorphic Rocks

Igneous Processes

Lecture 1: The Hydrologic Cycle

1. Elements of the hydrologic cycle (Fig. 15.5) - don't get hung up on the numbers

   • evaporation from the ocean (~455 cubic km per year)
   • transport in the atmosphere (~46 cubic km per year to the land)
   • precipitation (~409 cubic km per year back into the oceans) (~108 cubic km per year onto land)
   • evaporation from the land (~62 cubic km per year)
   • transport by rivers (~46 cubic km per year back to oceans)
   • transport by ground water

    

2. Water - the Miracle Substance

   • Where did it come from?
   • What makes it special?
   • Where is it? (Fig. 15.3)

    

3. Evaporation from the ocean

   • The evaporation rate varies with latitude

    

4. Transport in the Atmosphere

   • Winds transport moisture (Fig. 18.19)

    

5. Precipitation

   • Precipitation varies with latitude: see global distribution of deserts for example (Fig. 18.20)
   • Pattern over land not uniform due to topography (Fig. 18.21 showing rainshadow effect)
   • Pattern of US has maxima in northwest and southeast (Map)

    

6. Evaporation from the land

   • Not all water winds up in rivers
   • Relative proportion of runoff and evaporation depends on temperature and can be quantified mathematically:

    

7. Transport by Rivers

   • rivers come in all sizes, and form networks
   • Region drained by a river network called a drainage basin
   • River's velocity, depth, and width vary regularly with its discharge
   • Lag time between runoff and rainfall (Fig. 1, Perspective 15.2)
   • Hydrograph as record of runoff (Fig)
   • Floods have definite statistics (Fig. 2, Perspective 15.2)

    

8. Transport by ground water

   • Flow of ground water depends on permeability and water pressure (head)
   • flow velocity = permeability * (difference in head)
   • Surface flow down slope of water table
   • Deep flow channeled by permeable rocks into aquifers (Fig. 16.12)

Weathering and Minerals - 1

Weathering is the physical breakdown (disintegration) and chemical alteration (decomposition) of rocks and minerals at or near the Earth's surface where they contact the atmosphere, water and organic life. Most rocks are composed of several different minerals. Therefore to understand weathering, we need to know how minerals are defined, what they are composed of, and how they interact with the hydrosphere, atmosphere and biosphere.

We will begin with a review of basic chemistry as it applies to the elements that are important in the crust of the Earth - the raw materials of our physical surroundings.

Si and O are the most common elements in the Earth's crust

Element	Wt.%	Atom.%	Ionic Radius	R/Ro	Charge
O	46.6	62.6	1.40		-2
Si	27.7	21.2	0.42	0.3	+4
Al	8.1	6.5	0.51	0.36	+3
Fe	5.0	1.9	0.64-0.74	0.46-0.53	+2,+3
Ca	3.6	1.9	0.99	0.71	+2
Na	2.8	2.6	0.97	0.69	+1
K	2.6	1.4	1.33	0.95	+1
Mg	2.1	1.8	0.66	0.47	+2
others	1.5	0.1

The ionic radius is given in Angstroms (1Å = 10^-8 cm), the R/Ro column is the radius ratio calculated by dividing the ionic radius of the cation by that of oxygen.

1. Understand and diagram the structure of atoms. (Fig. 2.3)
   • nucleus, protons, neutrons, electrons
   • electrons 'orbit' the nucleus and a predictable number are in each of one to several electron shells

   
   

2. Understand the atomic differences between atoms of different elements.
   • elements are characterized by the number of protons (atomic number) and the number of protons plus neutrons in the nucleus (atomic mass)
   • the number of electrons in a neutral atom is equal to the number of protons

   
   

3. Understand and explain what an isotope is. (Fig. 2.4)
   • varying the number of neutrons in a nucleus (protons remain the same)
   • very important because this leads to the basis of radioactive decay and dating

   
   

4. Understand that most minerals are composed of compounds and recognize some that are not.
   • atoms are joined to one another (bonded) by various kinds of forces ionic, covalent, metallic bonds (Fig. 2.5, 2.6)

   
   

5. Definition of the term "mineral".
   • naturally occurring, inorganic, crystalline solid, with a narrowly defined chemical composition and characteristic physical properties

   
   

6. What are the physical properties of "crystalline solids". (Fig. 2.7)
   • atoms are joined to one another (bonded) by various kinds of forces
   • regular 3-D framework; under ideal conditions this may be reflected by the formation of smooth planar surfaces (crystal faces) and sharp corners and edges (Fig. 2.8)
   • contrast with amorphous (glass) solids

   
   

7. What causes minerals to have variable compositions? (Fig. 2.9)
   • nature is impure; some elements have similar ionic sizes and charges and can substitute

   
   

8. Know that silicates are the most common minerals and why this is so.
   • common ions range from 0.3 to 1.8Å in diameter where (Å = 1x10^-8 cm)
   • no real, absolute sizes, atoms are "fluffy" clouds of electrons
   • the sizes control the geometries that atoms/ions can combine in
     • Al similar in size to Si
     • Mg similar in size to Fe
   • Important because similarly sized atoms often substitute for one another
   • Different ways of packing oxygen (big) around cations (small)
     • 6 oxygens with cation in middle
     • 4 oxygens with cation in middle
     • 3 oxygens with cation in middle

   
   

9. Understand and be able to diagram the structure of the silica tetrahedron. (Fig. 2.10)
   • build some basic structures; Can SiO₂ be a simple linear molecule?
   • charge on tetrahedron is -4, so other cations must pack in to balance the charge to zero:
     • Mg or Fe to make (Mg,Fe)₂SiO₄, a mineral called olivine
     • Olivine crystallizes at a very high temperature (>1350°C)
   • In olivine, silicon tetrahedra are disconnected
   • At lower temperatures, silicon tetrahedra can link in chains
     • making minerals called pyroxenes (single chains)
     • making minerals called amphiboles (double chains)
   • Ca, Na, K combine with silicon (and aluminum) tetrahedra to make feldspars
     • Calcium feldspar (anorthite): CaAl₂Si₂O₈
     • Sodium feldspar (albite): NaAlSi₃O₈
     • Potassium feldspar (microcline): KAlSi₃O₈
   • Silicon tetrahedra alone, sharing all four oxygen atoms, make the mineral quartz SiO₂

   
   

Weathering and Minerals - 2

1. List the major groups of minerals and the basis for the recognition of each.
   • silicates, carbonates, sulfides, sulfates, halides, native elements, oxides

   
   

2. Understand and be able to recognize the physical properties of minerals.
   • color: some specific colors are reliable, remember the generalities streak color much more diagnostic
   • luster is the appearance in reflected light: metallic vs nonmetallic vitreous (glassy), greasy, waxy, brilliant, dull (earthy) (Fig. 2.15)
   • crystal form (Fig. 2.16)
   • cleavage vs fracture (Fig. 2.17, 2.18)
   • hardness: resistance to abrasion; scale of 1-10 (Table 2.6)
   • specific gravity: ratio of a minerals weight to an equal volume of water
   • feel, magnetism, taste, plastic vs flexible, double refraction, reaction with acid (Fig. 2.20)

   
   

3. Most rocks are composed of minerals
   • There are thousands of minerals, but only a few compose the bulk of most rocks.
   • The ultimate source of rocky material is magma (molten rock) from within the earth.
   • Eruption of magma on the surface (lava) forms fine-grained extrusive rocks:
     • Rhyolites (rich in Si, Al and K)
     • Basalts (rich in Mg and Fe)
   • When deeply-buried magmas solidify slowly, they make coarse-grained plutonic rocks:
     • Granite, the coarse-grained equivalent to rhyolite
     • Gabbro, the coarse-grained equivalent to basalt

Chemical Weathering:

1. Solution: ions become dissociated from one another in a liquid
   • water is a great solvent because it is a dipole {Fig. 5.14}
   • most minerals aren't very soluble in pure water
     • however a small amount of acid makes water a much better solvent

     
     

2. Oxidation: reaction with oxygen to form oxides or hydroxides
   • especially important in the alteration of Fe-Mg silicates
   • oxidation of pyrite to form sulfuric acid is also very important

    

3. Hydrolysis: chemical reaction between hydrogen (H⁺) and hydroxyl (OH^-) ions of water and a minerals ions.
   • Cations in minerals are replaced by H⁺ and OH^- radicals
   • actual replacement that liberates soluble salts to the surrounding water
   • Cations and Si go into solution, leaving insoluble aluminum silicate - "clay"
   • Clay structure is "sheet-like" - makes small platy grains

Mechanical Weathering:

1. Frost action: repeated freezing and thawing of water in cracks.
   • Water expands about 9% upon freezing
   • may form talus slopes - Fig. 5.5
   • particularly effective in jointed rocks - Fig. 5.4
   • Related process of frost heaving: source of rocks in New England fields
2. Pressure release: differential confinement and release of deep seated pressure
   • exfoliation, sheet jointing - Fig. 5.6, 5.7, 5.8
3. Thermal expansion and contraction: differential change in volume
   • has not been conclusively proven in lab but natural evidence suggests that it may be important
4. Activities of organisms: burrowing, root pressure - Fig. 5.9

Factors Controlling the Rate of Weathering

• Particle size, climate, parent material - Fig. 5.13, 5.14
• World distribution of rainfall important for weathering, because weathering is driven by water delivery to the land
• Streams are main agent for moving materials by either solution or particles
• Stream discharge roughly proportional to surface area for world's continents
  • except Australia and Antarctica: low because of especially low rainfall
• Chemical weathering per unit area is 20-40 metric tons/km²/year and is pretty constant for continents
• Mechanical weathering has more variation (20-300 metric tons/km²/year), because of large variation in slopes. Asia is largest, because of large mountain ranges (e.g. Himalayas).
• Continents with high mean elevations have high mechanical weathering.
• Very high erosion where there are mountains and high rainfall (e.g. Indus from Himalayas and Amazon from Andes: both are in areas of high rainfall and have high sediment load).
• Mean rate of denudation 3-4 cm per thousand years. But this is a misleading figure, since it mostly comes from mountains, which have a small surface area. Mountains are being worn down very quickly. Implies recent geologic process must have made mountains.

Soils and Erosion

Factors Controlling the Rate of Chemical Weathering:

• particle size (edges and corners), climate (water and temperature), parent material
  • the smaller the particles, the higher the ratio of surface area to volume
  • hot humid climates increase speed and depth of weathering
  • inverse of Bowen's reaction series (see Igneous Rock section below)
  • igneous minerals are out of equilibrium at surface
  • spheroidal weathering

Soil:

• regolith is the general term for broken up rock material
• regolith + water + air + organic matter = soil
• humus: partly decayed organic material
• residual vs transported soils

Soil layering or profile: O, A, B, C

• leaching vs. accumulation

Factors controlling soil formation:

• climate
  • pedalfers: Al and Fe-rich, water flow downward, >60 cm/yr precip.
  • pedocals: Ca or calcite-rich, water flow upward, <60 cm/yr precip.
    • extreme example: caliche soil in desert setting
  • laterites: extreme leaching of everything except Fe or Al (bauxite), tropical
• parent material: e.g. quartzite vs granite
• organic activity: animals, organic acids
• relief and slope: steep vs shallow, high vs low elevations, facing direction
• time: average of 2 cm/century - non-renewable resource

Soil Degredation:

• erosion: wind or water; sheet erosion, rill erosion
• chemical deterioration
  • nutrient depletion
  • insufficient fertilizer
  • clearing land: low quality soils in rainforests
  • pollution
• salinization, especially in desert settings
• physical deterioration: compaction

Desertification: positive feedback process

• erosion in the Sahara actually feeds the Amazon rainforest
• soil erosion can poison coral reefs (the ocean equivalent to the rainforest)

Weathering and Mineral Resources:

• bauxite, iron, manganese, clays, tin, gold, diamonds
• direct shipping ores (iron, Ladysmith Cu)
• supergene enrichment­ gossans

Sediment and Running Water

1. Agents for transport of mechanically and chemically weathered materials include rivers, glaciers, wind and groundwater.
2.  
3. Erosion by rivers
   • Mechanically weathered sediments, consisting of rock particles of various sizes, called clastic sediments.
   • Names given to different size ranges (from big to small):
     • boulders, cobbles, gravel, sand, silt, clay
   • Transport processes by air and water often sort sediments according to size. (Glaciers do not generally sort sediments because of the high viscosity of the ice).
   • Hjulstrom curve (Fig. 15.14): Different combinations of particle size and river velocity divided into three fields
     • Transportation: material being moved by river
     • Depositon: material falling out of river and collecting on bottom
     • Erosion: material being scoured from bottom and added to river's sediment load
   • Sand-sized particles most easily eroded. (Clay is cohesive because of electrostatic attraction between platy grains, and boulders are heavy).
   • Types of transportation
     • Dissolved load: material in solution
     • Suspended load: particles held in water by turbulence (compare to laminar)
     • Bed load: particles rolled along bottom or moving by saltation
   • Ways of describing a river's ability to carry sediment:
     • Competance: ability of stream to move large particles - depends on velocity
     • Capacity: how much load a stream can carry - depends on discharge of stream
   • Lower Mississippi: large capacity but small competance
   • Effect of transport on sediments
     • Rounding of grains
     • Sorting by size: suspended clays take longer time to settle out than sands which are transported along the streambed.
   • Floods may often carry the bulk of sediment - other times of year may carry very little sediment

   
   

4. Streams as Self-Adjusting Systems (Fig. 15.29, 15.30)
   
5. Deposition
   • Depositon occurs when water velocity decreases:
     • Bends in rivers may grow, due to faster erosion on outer bank, forming a meander. After flood, velocity of water in flood plain drops, river deposits sediments.
     • Rivers emptying into still water (lake or ocean) form deltas
       
6. Formation of sedimentary rocks
   • Cementation of grains by CaCO₃ or SiO₂ or ?
   • Names of clastic rocks depend on size of grains (from big to small)
     • Conglomerates - tend to be poorly sorted
     • Sandstones - often well-sorted
     • Shales - usually made of clay minerals
   • Non-clastic rocks: from dissolved load
     • Limestones - CaCO₃, usually removed from water by a biological process (e.g. corals and sea-shells)
     • Evaporites - NaCl and CaSO₄ from evaporation of seawater in enclosed basins (e.g. Utah's Great Salt Lake and the Mid-East's Dead Sea).

Sedimentary Rocks and Continental Margins

1. Weathering, Transport, Deposition, Lithification
   • Sedimentary Facies, Environmental Analysis, Resources

   
   

2. Review of mechanical and chemical weathering

   • Definitions:
     • Mechanical weathering - reduction into physically smaller particles
     • Chemical weathering - chemical reactions in presence of water, oxygen and carbon dioxide
   • Examples:
     • Granite (mostly quartz + feldspar + amphibole or biotite mica):
       • quartz weathers mechanically to produce sand
       • feldspar weathers chemically to produce clay and dissolved silica, calcium and sodium
       • Fe-Mg minerals weather chemically to produce iron oxide and dissolved silica, calcium, magnesium, and sodium.
     • Basalt (mostly pyroxene + feldspar + olivine):
       • pyroxene weathers chemically to produce iron oxide and dissolved silica, calcium, magnesium, and sodium.
       • feldspar weathers chemically to produce clay and dissolved silica, calcium and sodium
       • olivine weathers chemically to produce iron oxide and dissolved silica and magnesium

   
   

3. Formation of sedimentary rocks - sediments must be deposited in a deep hole (or a hole that is deepening), otherwise they will be eroded again.

   • One such deep hole is the deep ocean basin (or abyss), at the edge of continents (Fig 6.5)
   • Delivery of sediments to the sea by rivers (Table in Lecture 5)
     • Bed load and suspended load (Fig. 15.13)
     • Dissolved load
   • Redistribution of sediments by waves
     • Concept of wave base: (see Waves in Chap. 19) - deepest water depth where sediments are moved by wave action
     • Wave base is determined by the largest storms

   
   

4. Delivery of sediments to the abyss

   • Turbidites: rapid flow of sediments down submarine canyons, like an avalanche (Figs. 6.19, 11.10)
     • Coarse sediments moved quite far off shore
     • Telephone cable breakage after the Grand Banks earthquake (1929) indicated that the flow velocity was up to 80 km/hr
   • Suspended sediment: fine particles
   • Wind blown sediment: fine clay brought to the middle of the ocean
   • Dissolved sediments: especially limestone, deposited by corals and other organisms that make limey shells.
     • Form mainly where there is little clastic deposition (clear water)
     • Florida Keys and Bahama Banks are two areas of the US where limestone is being laid down
     • Corals may be ground up and reworked into lime sand before deposition

   
   

5. Environments of deposition

   • Biogenic Carbonate deposits (Limestone)
     • Warm, shallow water, absence of clastics.
     • Examples:
       • Florida shelf
       • Yucatan shelf
       • Bahama banks
     • map symbol: brick pattern
   • Fine-grained sediments (shale) (e.g. Fig. 6.15)
     • Deposited in quiet water - below wave base
     • Often some distance from shore
     • Examples:
       • Prodelta sediments
       • Marsh environments
       • Deep water deposits
   • Coarse clastics
     • Higher energy environments - near shore- above wave base, wind deposits
     • Examples:
       • Delta mouth bar
       • Point bar
       • Beach deposits
       • Desert deposits: fine material blown away by dunes
   • History in the sequence of sediments - more detail in upcoming lecture on Geologic Time
     • Lithology - facies - paleoenvironments
     • Superposition of sediments - a sequence in time (e.g. Fig. 6.16)
       • Idea first developed by Nikalaus Stensen (Steno 17th century)
     • Fossils - their evolution provides a unidirectional vector in time
       • William Smith (early 18th century) showed that the ages of rocks could be characterized by the fossils that they contained
       • Called the law of Faunal succession
     • The Geologic Column (see Fig. 8.2)
       • Division of the geologic record
       • Eons, Eras, Periods, Epochs
     • The major geologic Eras (Paleozoic, Mesozoic, Cenozoic) cover only the most recent 12% of the earth's age, but this time is important because it spans the time when multicellular life has flourished.
     • You should learn the Eons and Eras and the ages of the boundaries of each.

    

Metamorphism & Metamorphic Rocks

Metamorphism refers to a set of processes that result in changes in mineralogy and texture accompanying changes in temperature and pressure.

In other words, transformation of existing rock, usually beneath the Earth's surface, as a consequence of one (or a combination) of three agents: heat, pressure, and fluids.

The boundary between diagenesis (sedimentary process) and the onset of metamorphism is one of semantics.

Metamorphic Types

Contact Metamorphism - nearby heat source - an intrusive igneous body is injected into a colder, older rock - heat flows from the intrusive body into the country rock

May also involve fluid flow or exchange

Regional Metamorphism - no obvious, local heat source - increasing depth of burial plus deformation results in an increase in temperature and pressure

Remember the geothermal gradient

Dynamic Metamorphism - variable pressure at relatively low temperatures - often associated with fault zones

Commonly reduces grain size

High Pressure - Low Temperature Metamorphism - associated with subduction zones

Characterized by unusual minerals - blueschists

Effects of Metamorphism

Increasing Grain Size:

Increasing Temperature and Pressure may aid in the recrystallization of minerals in the rock

Small grains become larger - oriented with respect to direction of applied pressure(s) - stress

Clay minerals are often enlarged with increasing metamorphism

Growth of New Minerals:

New minerals may grow during metamorphism

CaCO₃ + SiO₂ = CaSiO₃ + CO₂

The presence of wollastonite can be used as an indicator of the degree of metamorphism

ISOGRAD - a line on a map connecting points of equal degrees of metamorphism

Classification

Is the rock banded? - each band is often a single mineral - GNEISS

Does the rock exhibit foliation - parallelism of the cleavage of micas?

Schist - coarse flakes

Phyllite - fine - barely visible flakes, shiny surface

Slate - very fine

If the rock is neither foliated nor banded it is called a granofels if it is coarse grained or a hornfels if it is fine grained

Marble - a metamorphosed limestone

Quartzite - a metamorphosed quartz sandstone

Degree of Metamorphism

A function of the pressure, temperature and composition of the parent rock - the Protolith

Marble - had a parent rich in carbonate

Quartzite - quartz sandstone parent

Slate, Schist, Gneiss - clay mineral rich parent

Metamorphic Facies - attempts to deduce degree of metamorphism by looking at index minerals, rock fabric

Plate Tectonics and Metamorphism

Divergent Boundaries: contact metamorphism, basaltic lavas in contact with sediments; dikes in contact with host rock

Convergent Boundaries: Subduction Zones: high pressure/low temperature metamorphism

Convergent Boundaries: Continent/Continent collision - regional metamorphism

Transform boundaries: two plates sliding past one another can generate higher pressures without much heat - dynamic metamorphism

IGNEOUS ROCKS and IGNEOUS ACTIVITY

Introduction:

Both temperature and pressure increase with increasing depth and it is the rate of increase that is important.

The geothermal gradient measures the rate at which temperature increases within the Earth.

• Roughly 25-30°C per kilometer.
• This gradient must "flatten out" or lessen with increasing depth.

Pressure increases at a rate of about 333 bars per kilometer in the crust. A bar is about one atmosphere. Therefore the pressure gradient is about one-third of a kilobar (1000 bars) per kilometer.

• Diamonds require about 100 kilobars to form, what depth of burial is required?

Partial Melting:

Most rocks are mixtures of minerals and each mineral has its own set of physical characteristics.

Quartz melts at about 1725°C at one atmosphere total pressure; here, melting is defined as the temperature at which solid and liquid of the same composition are in equilibrium.

In general we must specify the pressure in order to state a unique melting point.

If Quartz is mixed with Alkali Feldspar in some proportion (e.g. 70% feldspar and 30% quartz) melting occurs but not in the same way that the melting of a pure compound occurs. In general, there is no single temperature at which any mixture of minerals goes from solid to liquid. Rather, there is a range of temperatures at which liquid and solid are present. This is the interval of partial melting or partial crystallization.

1000°C all liquid

900°C less solid + more liquid

800°C solid + liquid

700°C more solid + less liquid

600°C all solid

The amount of liquid decreases as the temperature drops until all of the liquid is used up in producing solids.

Here partial melting is initiated at about 700 degrees Centigrade and completed by 1000°C.

Cooling is the reverse. This mixture would be 100% liquid until a temperature of about 1000°C. Crystallization begins and the amount of solids increase and the amount of liquid decreases as the temperature cools. At about 700° all of the liquid is gone.

The concept of partial melting plays a crucial role in igneous processes.

Essentially all magmas are formed by partial melting which did not reach the temperature at which all of the parent material was molten.

In general, liquids tend to be less dense than the solids that crystallize from them. In a mixture of crystals and liquids the liquid (less dense) will attempt to migrate upwards whereas the crystals may sink.

Initiation of Melting

Melting temperatures rise with increasing pressure:

• Therefore a relatively sudden reduction in pressure on an already hot rock can initiate melting.
• Similarly, the addition of water (a bond breaker) to a hot rock can cause melting to begin.

When magma reaches the surface it is called lava. Magmas that cool at the surface of the Earth are extrusive whereas those that cool within the Earth are intrusive.

Plate Tectonics

Review the relationships between plate boundaries and igneous activity.

Classification of Igneous Rocks

Why do we classify things?

• Identify important attributes
• Efficiently communicate information.

Two properties of igneous rocks that we will focus on are texture and mineralogy.

Texture refers to the size, shape and arrangement of the grains in the rock.

• phaneritic - coarse grained - you can see the individual crystals
• aphanitic - fine grained - you can't see the individual crystals or grains
• porphyritic - big grains and small grains
  • phenocryst - big
  • groundmass - small

Minerals in igneous rocks have an interlocking texture.

Minerals crystallize and compete for space.

Rapid cooling leads to fine-grained aphanitic rocks — extrusive.

Slower cooling in an intrusive mass (lower temperature contrast with surroundings) should lead to a phaneritic texture.

Mineralogy - Remember that the most abundant mineral groups in the crust are the plagioclase and alkali feldspars. Norman Bowen (about 1915) proposed the following sequences of crystallization of silicates from a magma.

Bowen's Reaction Series Here

• With the exception of quartz, the other phases represent solid solution series.
• The viscosity (resistance to flow) of a melt (magma/lava) increases with decreasing temperature.
• The complexity (amount of sharing of the oxygens of the silicon-oxygen tetrahedrons) increases with decreasing temperature.
• high temperature - olivine, pyroxene and Ca-rich plagioclase;
• intermediate temperature - amphibole, biotite and Na-rich plagioclase; and
• low temperature - muscovite, alkali feldspar and quartz.
• Bowen's Reaction Series points out that there are commonly occurring mineral assemblages (based on similar temperatures of formation/crystallization). For example, quartz and olivine (at least the magnesium-rich variety) are not expected to occur together as an equilibrium assemblage.

On the discontinuous, left side of Bowen’s reaction series are minerals rich in iron and magnesium:

high temperature

olivine

pyroxene

amphibole

biotite

low temperature

On the continuous side of Bowen’s reaction series:

high temperature

calcium-rich plagioclase

calcium-sodium plagioclase

sodium-rich plagioclase

low temperature

On the bottom portion of Bowen’s reaction series the following minerals crystallize:

high temperature

muscovite

alkali feldspar

quartz

low temperature

Generalize from the feldspars:

high temperature

calcium-rich plagioclase

sodium-rich plagioclase

alkali feldspar (a solid solution between K and Na feldspars)

low temperature

Classification scheme for igneous rocks using texture and mineralogy.

Temperature estimated by the feldspar(s) present

Cooling rate (and thus extrusive vs intrusive) estimated from the texture

	Alkali Feldspar	Sodium Plagioclase	Calcium Plagioclase
Phaneritic	Granite	Diorite	Gabbro
Aphanitic	Rhyolite	Andesite	Basalt
Silica Content	>65 %	53-65%	45-52%

Granite is a coarse grained igneous rock which contains abundant alkali feldspar. Granites also contain quartz. This is a relatively low-temperature assemblage.

Rhyolite is the mineralogical equivalent of granite but it formed as a result of rapid cooling giving the rock the fine-grained texture.

Think about the analogous relationships between the pairs Diorite and Andesite and Gabbro and Basalt.

Remember the steps on the west side of the Memorial Library, the rim of the Library Mall fountain and the base of the Lincoln statue - what name would you give these rocks?

If the rock is a granite but with a porphyritic texture it would be a granite porphyry. It if is a rhyolite but with a porphyritic texture it would be a rhyolite porphyry.

Viscosity is a measure of "resistance to flow".

A liquid with high viscosity flows with difficulty. In general, as the temperature of the liquid increases the viscosity of the liquid decreases and the liquid flows more easily. Magmas with lower silica contents also flow more readily - these effects are both in the same direction for igneous melts and make basalts much more fluid than rhyolites.

Shapes of Igneous Bodies

• Tabular Bodies - relatively low viscosity to allow magma to follow relatively narrow openings.
• Dikes - tabular bodies that cut across the "structure" of the enclosing rock.
• Sills - tabular bodies that are oriented parallel to the "structure of the enclosing rock".
• Laccoliths - bodies that "dome up" the overlying rocks
• Pipes and Necks - the fossil remnant of the plumbing system under a volcano
• Irregular bodies include stocks and batholiths that typically were formed from highly viscous (silica-rich) melts.

Mechanics of Batholith Emplacement

• Granitization: NO in almost every case - local phenomena if at all
• Forceful injection: driven by density variations and possibly tectonic forces
• Diapirs: analogy to salt dome formation
• Stoping: commonly good evidence for this

Source of Heat to Partially Melt Solid Rock

Radioactive elements (U235, U238, Th 232 and K40) decay giving off heat. Each decay gives off a very small amount of heat but over long time periods, this heat can result in temperature increases sufficient to initiate partial melting.

Extrusive Igneous Activity and Features

Volcanoes

• Differences between craters and calderas
• Shield volcanoes: 2-10° slopes, low viscosity magmas
• Composite cones: layers of lava and ash - Pacific NW
• Cinder cones: up to 33° slopes (angle of repose)
• Lava domes: Mt. Pelee, nuee ardente
• Fissure eruptions: basalt plateaus - Columbia flood basalts
• Pyroclastic sheet eruptions: welded tuffs

Features of extrusive igneous rocks:

• Columnar joints
• Pahoehoe and aa flows
• Pillow lavas
• Pyroclastics: ash, cinders, bombs
• May contain xenoliths — samples from lower crust or mantle

Magma-forming Environments

• Subduction zones
• Frictional heating
• Circulation of the asthenosphere
• Addition of water
• Rifting
• Pressure release melting

Hot spots