Some note for Geology 105 test 1

geol105-p1.md

Combined, undated notes for Part I - Rocks and Minerals

Some Intro

Philosophies in Geology

1. Scientific method of analysis - ideally, unbiased (but in practice this is impossible). Basic steps:
   1. Observe relative to the question or problem
   2. Hypothesize based on observations
   3. Test hypothesis
   4. Interpret results and modify hypothesis based on the results
2. Doctrine of Uniformitarianism - the processes which act to shape the earth today are the same which have acted throughout the earth's history; i.e. "the present is the key to the past." It is an assumption, but it's a very functional assumption.

Multiple working hypotheses - come up with as many hypotheses as possible to explain a given set of facts. (i.e. keep an open mind)

After verification, hypotheses are elevated to theories. Theories are hypotheses which have withstood the test of time and remain the best available explanation for something.

An example of scientific method application: Plate tectonics going from hypothesis to theory over time.

Age of the earth - 4.6 bn years, just in case you wondered. (Evidence later?)

Realms of the earth:

1. Atmosphere - gases surrounding the earth
2. Hydrosphere - oceans, etc.
3. Geosphere - crust and interior
4. Biosphere - the realm of living organisms

(These are all intricately interconnected - no sharp separation)

More definitions

Rock - a mixture of minerals in a solid state

Mineral - homogenous, inorganic, naturally occuring solid with:

• a definite chemical composition,
• a characteristic atomic structure,
• and a specific set of physical properties (which can be used in identification)

Classifications of rocks:

1. Igneous (once molten)
2. Sedimentary (consolidated sediment)
3. Metamorphic (formerly igenous or sedimentatry, transformed by heat, pressure, and/or 'hydrothermal solutions'

Crystal - Solidified form of a substance whose internal atomic structure is reflected in its exteral shape. Formed by slow change of state (liquid to solid or gas to solid), like slow cooling of melted solids or precipitation of compounds in oversaturated solutions.

The "Rock Cycle" Changing between rock types. @see pretty picture.

"The Organization of Matter" (to make rocks) protons, neutrons, and electrons together form Atoms -> "native mineral" (elemental) ...atoms together form Compunds -> mineral (type of compound) ...compounds together form Mixtures -> rocks (type of mixture)

"The most abundant element in the Earth's crust is Silicon"

Minerals

Fortunately, of the many elements only a handful turn out to be important in geology (phew).

Some definitions from chemistry: Ion - and atom that has gained or lost an electron, giving it and charge. Isotope - atoms of the same element with varied numbers of neutrons (Also, several kinds of chemical bonding, @see ppt)

Mineral Properties

Crystal form

The external shape of the mineral reflects the internal arrangement of its atoms as the crystal grows. Seven crystal systems:

1. Isometric (e.g. Halite)
2. Hexagonal (e.g. Quartz)
3. Tetragonal - 3 axes @ 90deg, two equal and one longer
4. Orthorhombic - 3 axes @90deg, all different lengths
5. Monoclinic -
6. Triclinic -
7. ?

Nice notes you've got there. @see powerpoint

Cleavage and Fracture

Cleavage is the tendency of a mineral to break along certain planes due to internal weaknesses along that plane of atoms.

Fracture is... pretty much the absence of cleavage. It just breaks.

A mineral's cleavage is not necessarily the same as it's crystal shape! Halite has a cubic crystal and breaks into perfect cubes, but in contrast Fluorite grow in cubes but breaks in pyramids, and while Pyrite grows in cubes with striations it breaks w/ irregular fracture.

Cleavage types

• Basal (1 plane) [Biotite, Muscovite]
• Prismatic (2 planes, 90deg angles) [Pyroxene, Feldspar]
• Prismatic (2 planes, !90deg angles) [Amphibole]
• Cubic (3 planes, 90deg angles) [Galena, Halite]
• Rhombohedral (3 planes, !90deg angles) [Calcite, Dolomite]
• Octahedral (4 planes) [Flourite]
• Dodecahedral (6 planes) [Sphalerite]

Striations parallel lines running across a crystal face or cleavage plane. Ex: Pyrite crystal, quars crystals, and plagioclase cleavages

Hardness

The resistance of a mineral to abrasion. So to figure out the hardness, we try to abrade it with another mineral (or other material) of known hardness. See Moh's scale of hardness from lab notes.

Often for practical identification in the field it is sufficient to divide minerals into hard and soft at 5.5 (glass, knife blade).

Other notable mineral properties:

• Specific Gravity
• Color - non-diagnostic, @see quarts
• Streak - colors of powdered form
• Luster - appearance in reflected light
• Diaphaneity - translucency
• Other "Special" Properties:
  • Magentism,
  • Odor
  • Taste
  • Reaction w/ HCl (acid)
  • Tenacity
  • etc.

You don't need to know every physical property for every mineral, but each mineral has a combination of just a few properties that reveal its identity.

Mineral groups

Major components of the Earth's crust, ordered by percentage of the Earth by weight; together, these 8 make up 98.5% of the crust:

1. Oxygen (O)
2. Silicon (Si)
3. Aluminum (Al)
4. Iron (Fe)
5. Calcium (Ca)
6. Sodium (Na)
7. Potassium (K)
8. Magnesium (Mg)

(You will need to memorize these!)

The Silicates

Oxygen and Silicon make up 46.6% and 27.7%, respectively. Minerals which contain O and Si are called Silicates, and they are (for obvious reasons) the most common. Some Silicates:

• Quartz
• Orthoclase Feldspar
• Sodic Plagioclase
• Calcic Plagioclase
• Mica Group
• Amphibole Group
• Pyroxene Group
• Olivine

All of these except Quartz and Olivine are Aluminosilicates, i.e., they contain aluminum along with oxygen and silicon.

Feldspar refers to internal structure; the mineral itself is just Orthoclase, and the feldspar variety in particular. a.k.a. the "potassium feldspar" (or even "k-spar")

Observations about the list of Silicates:

• From top to bottom, SG increases
• Form top to bottom, Color darkens (generally)
• Orthoclase..Pyroxenes are Alumniosilicates
• Quartz..Micas are called Felsic Minerals
• Amphpibole..Olivine are "mafic" (magnesium and iron)
• Silicates close to Micas and Amphiboles in composition are "Intermediate"

We will see that most of these characteristics can be explained in the context of Bowen's Reaction Series.

Typical Silicate Structures

The Si-O Tetrahedron - the basic building block of silicates. 4 oxygens surrounnding 1 silicon. A variety of ways to combine these Tetrahedra to form more complex structures:

1. Individual Tetrahedra, linked together by some additional atom (we call the glue atom a "cation"). Olivine strcutred this way.
2. Framework Structure - each Oxygen atom in a tetrahedron is shared with another surrounding tetrahedrons, producing a 3d framework (rather like pyramids w/ those awesome magentic toys)
3. Single chain - Chains of SiO tetrahedrons connected at the corners, and the single chains are held together by Ca, Mg, Fe, and/or Al atoms. (Pyroxene group)
4. Double chain - two connected chains of SiO pyramids, chains held connected by Ca, Mg, Fe, Al (Amphibole group)
5. Sheet structure, sheets held together by Ca, Mg, Fe, or Al (Micas)

Pyramid Chaining: single, double, or sheets. Depends on conditions when the mineral forms

Single vs double chains are what give Pyroxenes vs amphiboles a 90deg vs 60deg cleavage angles, and the sheet arrangement of the SiO tetrahedrons in Mica causes them to cleave in sheets. So, you can see that the structure at the molecular level determines the physical properties of the mineral.

Bowen's Reaction Series

Order of crystallization of the 8 most abundant silicate minerals in molten rock as it cools.

This is a nice 2D diagram that would be trivial to sketch but tedious or impossible to reproduce in plaintext. So see the ppt.

Olivine and Plagioclase are High temp, Orthoclase and Quartz form cooler, and tend to be more stable.

Being very visual, it doesn't appear in these notes much, but Bowen's Reaction Series is really important!

Continuous vs. Discontinuous Reaction series: Discontinuous: entire atomic structure changes at each step (left side of the series) Continuous: atomic structure preserved at each step (right side)

Bowen's Hypothesis (origin of igenous rocks) His idea was that all igneous rocks start as Mafic magma. The minerals can:

1. React - transform as the magma cools, thus vanishing, and/or
2. Fractionate - they settle to the bottom, are protected, and do not react. Only exposed by erosion after everything cools.

As far as we know, this hypothesis is correct. Though plate tectonics (which Bowen didn't know about) also help form intermediates by mixing Mafic and Felsics together.

Fractionation - separation of early forming minerals from the magma before they have chance to react. Two common types:

1. Gravity settling - the dense, early-forming minerals sink to the bottom of the magma chamber
2. Filter pressing - magma is squeezed upward through cracks leaving already formed crystals below

In the case of gravity settling, we get an order like this: (higher) Felsic (granite) Intermediate (diorite) Mafic (gabbro) Ultramafic (peridotite) (lower)

Other Mineral Groups

These are also important but much less common than silicates, so we spend mch less time on them in this class. Know the example minerals given here for each category!

1. Oxides (O + something) ex: Hematite Fe2+O3
2. Sulfides (S + something) ex: Pyrite Fe+S2
3. Carbonates and Sulfates (something + carbonate or sulfate ion) e.g.: Calcite, CaCO3 ; Gypsum , CaSO4-H2O
4. Halides (something + a halogen Cl, I, Br, Fl) Halite, NaCl
5. Native Element Minerals - Gold, Silver, Copper, etc.

Examples of other mineral groups:

1. Oxides
   • Hematite
   • Bauxite
2. Carbonate
   • Calcite
   • Dolomite
3. Halides
   • Halite
4. Sulfides
   • Pyrite
5. Native elements
   • Gold
   • Silver
   • Copper

Rocks

Rocks: Aggregations of Minerals

Main categories are based on formation process:

1. Igneous
2. Sedimentary
3. Metamorphic

Igneous Rocks

Classified based on:

1. Composition (minerals in the rock)
2. Texture (size and relationships of mineral grains)

Igneous Compositions

Igneous rocks composed mostly of Felsic minerals tend to be light colored; those composed of mostly Mafic minerals tend to be dark. (And intermediate compositions tend to have intermediate color)

Felsic minerals nearer the bottom of Bowen's reaction series; Mafic minerals nearer the top of Bowen's reaction series.

Igneous Textures

1. Phaneritic - crystals large enough to be seen by eye. Indicates slow (thousands of years) cooling underground
2. Aphanitic - Crystals not large enough to be seen with the eye. Cooled quickly (days or hours), at or near the surface (like lava) More textures:
3. Porphyritic - mixture of visible crystals surrounded by non- visible crystals (Phaneritic suspended in matrix of Aphanitic) Indicates change in rate of cooling.
4. Glassy - no crystals had time to form, indicates very rapid cooling
5. Cellular (vesicular) - gas escaping from lava leaves bubbles (Rock could be both glassy and cellular), e.g. Pumice
6. Fragmental - solid and semisolid pieces of ash and lava welded together before completely cooling (volcanic erruptions...)

Common igneous rocks

The 6 most common ignenous rocks are these combinations: { Phaneritic, Aphanitic } X { Mafic, Intermediate, Felsic }

Mafic: Fe, Mg, Ca; near the top of Bowen's series Felsic: Al, K; lower down in Bowen's reaction series

The Great Table of Igneous Rock names you Need To Know:

_	Felsic	Intermediate	Mafic
Phaneritic	Granite	Diorite	Gabbro
Aphanitic	Rhyolite	Andesite	Basalt
Vesicular	Pumice	Scoria	Vesicular Basalt

MEMORIZE THE TOP TWO ROWS OF THIS TABLE! (minimum)

There is also an extended, more detailed version in the Powerpoints, if you're curious about examples of the other igneous textures, or specific gravity, and stuff and things.

Igneous Activity

1. Intrusive - magma forced into existing rock but does not reach the surface
2. Extrusive - magma forces its way upward until it escapes at the earth's surface

Intrusive activity

Methods of upward movement:

1. Squeezing into weaknesses in rock layers
2. Stoping (NOT "stopping", say "stohping") - blocks of pre-existing rock break off and fall into magma, and the magma displaced upward to fill the space
3. Melting the overlying rock

Plutons - igneous rocks masses formed when magma solidifies underground (igneous intrusions)

Pluton orientations:

1. Concordant - boundaries parallel to the layering of the country rock
2. Discordant - boundaries of pluton ignore the orientation of the country rock

Pluton shapes:

1. Tabulate - thin-ish layers, 2d
2. Massive - large in all dimensions

Names for igneous intrusions:

_	Tabulate	Massive
Concordant	Sill, Lopolith*	Laccolith
Discordant	Dike	Batholith, Stock*

• A Lopolith is a Sill that has depressed into a spoon-ish shape
• Laccolith might have been a sill, but has bulged up into a dome
• a Stock is a projection off of a Batholith

Extrusive activity

Woohoo volcanoes!

"Ring of Fire" - chain of volcanic and earthquake activity surrounding the Pacific Ocean (a.k.a. the "Andesite Line")

High viscosity is "thicker" than low viscosity

"Flood Basalts" - b/c basaltic lava has such a low viscosity, it can "flood" areas and solidify in layers, like the Colombia Platau or Deccan Plateau

Basalt Lava formations:

• Aa ("Ah-ah") - Blocky, jagged (more gas dissolved in the lava)
• Pahoehoe ("Pah-hoeh-hoeh") - smooth, ropey (not much gas trapped)
• Pillow - rounded and billowy (cooled underwater)

"Columnar Jointing" - as the lava cools it shrinks, and pillars of lava will shrink away from each other, leaving cracks and pillars.

Types of Volcanoes

1. Composite volcano (a.k.a. Strato-volcano)
   • eruption of felsic or intermediate laval
   • alternating layers of lava and volcanic ash
   • very tall, steep volcanic cone - "largest" type
   • Ex: Mt. St. Helens, Mt. Fuji
2. Basaltic Cinder Cone
   • smallest type, just a vent blowing scoria, actually kind of fragile
   • cone of scoria a few hundred ft. tall, <1mi around
   • lavas rarely flow from the top, but may break through vents in the base
   • Ex: Sunset Crater
3. Shield Volcano
   • Broad, flat shape (think slope of 4-5 deg, miles wide)
   • many layers of basaltic lava; very broad but flat shape result of basalt lava's low viscosity (thin, flows well)
   • usually contain a large central depression from collapse after magma is evacuated
   • can often have many smaller craters across surface
   • Ex: Hawaiian Islands (yeah, the whole chain); Olympus Mons (Mars)
4. Caldera
   • technically, any volcano with craters greater than 2km across
   • Usually, occurs when the volcanic vent is plugged and pressure builds up to an enormous explosion.
   • usually, only a little of the original volcano is left
   • can scatter debris for (literally) thousands of miles (these are the ones that tend to kill things and change the climate) ex: Krakatoa, Crater Lake

Some famous eruptions:

• Santorini/Thira
• Mt. Visuvius
• Krakatoa
• Pele
• Mt. St. Helen's

Sedimentary Rocks

Rocks are weathered at the earth's surface because most of the minerals that make up rocks are unstable at surface conditions.

For silicate minerals, Bowen's reaction series can tell us which minerals are the most unstable - the minerals that form first are the least stable and those that form last are the most stable, because they conditions lower in the reaction series are closer to surface conditions.

Categories of Sedimentary rock

1. Clastic (Archaic: "Detrital") - contain intermediate weathering products of clay or unaltered mineral grains
2. Chemical (inorganic) - contain precipitates of minerals that were in solution. Formed without the aid of organisms.
3. Biochemical (organic) - Rocks containing precipitates resulting from the actions of organisms

Clastic sedimentary rocks

Classification of Clastic sedimentary rocks:

1. Texture - size and shape of grains [ Particle name | particle size | rock name ]
   1. Granules (angular) & pebbles (rounded) | > 2mm | Breccia, Conglomerate
   2. Sand | 2 mm - 1/16 mm | Sandstone
   3. Silt | 1/16 mm - 1/256 mm | Siltstone
   4. Clay | < 1/256 mm | Shale
2. Composition (if the grains are large enough to identify) - we don't always bother with this metric.
   1. quartzitic - contains lots of quarts
   2. arkosic - contains much orthoclase and some mica. Few dark minerals. Indicates a granite source
   3. graywacke - complex source, containing dark minerals, mica, feldspar, clay
   4. clean - pure or nearly pure (one mineral; usually used in reference to quartz)

Lithification - the conversion of loose sediment into sedimentary rock. Two factors:

1. Pressure - results in compaction (closer together) and consolidation (squeeze out water)
2. Cementation - grains bound together by minerals forming between them (Cement examples: Calcite, Hematite, Silica)

"Fluvial transportation" - What happens to sediment when moved by a stream?

1. Grains become smaller and more rounded
2. Unstable minerals less abundant, quarts more abundant
3. Grains become more uniform in size (well sorted)

Transportation of sediment: Sourceland -> Downstream -> Beach -> Offshore

Typical Depositional Environments for Clastic rocks Breccia and Congolmerate - Alluvial fans, river headwaters Graywacke - Steep ocean slopes close to volcanic islands Arkose - Alluvial fans, river channels Dirty Sandstones - river channels, deltas Clean sandstones - beaches, offshor barse Silstone and Shale - quiet areas (swamps, lagoons, floodplains) Limestone - warm, shallow water free of sediment

"...what the heck is an alluvial fan?" (Easy to visualize, hard to describe) Think delta right next to sourcelands?

Sediment Maturity - a measure of the degree of weathering and transportation a sediment has undergone. (Only applicable to breccias, conglomerates, and sandstones.) @see table in ppt.

Chemical sedimentary rocks

Formed by the precipitation of compounds from ions in solution when oversaturated, and stuff.

Types:

1. Evaporites - formed when water rich in certain minerals evaporates
   • Rock salt (halite + impurities)
   • Gypsum
2. Silica Rich - form when there is an excess of Si ions in the sediment
   • Chalcedony (flint and chert)
3. Carbonates - contais the carbonate ion
   • Dolomite (Dolostone) - (Ca, Mg) carbonate + impurities
   • Oolitic Limestone - Ca carbonates + impurities
   • Travertine - Ca carbonate deposits in hot springs and caverns

Biochemical sedimentary rocks

Formed as a result of plant and/or animal life

1. Carbonates - contain CO_3 + impurities (will fizz)
   • Chalk - microscopic shells cemented together by calcite
   • Coquina - macroscopic shells cemented by calcite
   • Limestone - formed from limey mid (calcite plus clay); actually a broad category more than one specific type.
   • Fossiliferous limestone - limstone containing fossils or fossil fragments)
2. Pure organics - contain hydrocarbons (Hydrogen, Carbon, Oxygen)
   • Peat
   • Lignite
   • Coal
   • Petroleum

Sedimentary Structures

Patterns formed by sedimentary particles as they are deposited (Generally water, but this happens with wind as well - think sand dunes!)

1. Asymetric ripple marks - indicate water flowing in one direction
2. Symmetric ripple marks - indicates oscillating water currents
3. Crossbedding - indicates shifting currents flowing in one general direction
4. Bioturbation - disruption of sediment by burrowing or crawling animals
5. Laminar bedding - very thin layers of fine sediment; indicates quiet water
6. Graded bedding - sorting of sediment with largest grains on the bottom, grading up into finer grains. Indicates rapid deposition or stream deposition
7. Mudcracks - shrinkage in fine grained sediments; indicates ares frequently exposed to air
8. Current marks - groooves scoured in mud by turbulent bottom currents
9. Oolites - small spheres of calcite formed by agitation in warm water

Weathering

"the chemical and physical breakdown of rocks. The principle products of weathering are soil and rounded rock forms"

Chemical weathering

degeneration of rock due to chemical changes in the composition of the rock

1. Hydrolisis: addition of water to the molecular structure, usually through a reaction with weak carbonic acid (produced from decaying organic matter)

   Produces minerals which are soft and sticky when wet, called "clay minerals" Examples; happens to feldspars:

• Orthoclase -> kaolinite

• Plagioclase -> montmorillonite

  Because clay minerals are small, they can be transported long distances by water. But because they are sticky, they are not picked up by wind. Clay minerals will suspend in water; called "colloids".

2. Oxidation: formation of Oxide minerals, usually involving ions freed by hydrolysis Oxide minerals tend to accuulate as hard masses in and below the soil and are not transported far

Differential Erosion Weathering occurs in the opposite order of Bowen's Reaction series, and is called the Goldich Stability Series. So rocks made of different minerals will tend to weather and erode at different rates

(The closer the conditions in which a mineral formed are to the conditions at the Earth's surface, the more stable the mineral will be at the earth's surface and the less it will chemically weather.)

Physical weathering

a.k.a. "Mechanical" weathering; caused by:

1. Temperature effects
   1. expansion and contraction
   2. ice wedging
2. Exfoliation - curved slabs/flakes of rock stripped off the parent rock, producing rounded shapes; probably caused by internal pressures in the rock (think Half Dome in Yosemite)
3. Plants and animals (including humans)
4. Impact (very generic/catch-all - anytime anything hits a rock)

Soil formation

Soil = weathered rock, usually mixed with organic material When rock weathers faster than it erodes, you can form soil. Type factors:

1. Climate (major)
2. Length of time weathered
3. type and amount of vegetation
4. topography ('relief')
5. 'parent' rock

Weathering vs. erosion: Weathering breaks a rock down into pieces. Erosion moves those pieces away. Hence above, if weathering is faster than erosion, then a layer of sediment will accumulate

A few common types of soil...

1. Pedalfer - Al, Fe, collect beneath the surface (humid environments)
2. Pedocal - Calcite collects beneath the surface (arid environments)
3. Laterite - Extreme soil leaching, bauxite collects beneath surface (tropical)

Metamorphic rocks

Start with an igneous or sedimentary rock, and add either/and/or:

1. Prolonged high temperature (250-950 *C; NO MELTING, otherwise we'd be back to igneous)
2. High pressure
3. Metasomatism - chemical reaction resulting from contact with hydrothermal solutions

Carbonates are more sensitive to metamorphism

Metamorphic Grade: "how metamorphosed are we talking?"

Some minerals like Kyanite, Staurolite, and Garnet only form under metamorphic conditions, and can be used as flags for metamorphic activity and metamorphic grade

Metamorphic "Fabric" (a.k.a. Texture)

1. Foliated - banded or layered appearance due to 'platy' minerals
2. Lineated - rod-like minerals all approximately parallel
3. Granular - no banding, layering, or lineation ('massive' appearance)

Metamorphic "scopes"

Contact Metamorphism

Country rock touches magma as the magma rises (very localized)

• grade decreses with distance from magma intrusion
• only granular metamorphic rocks are formed (pressure is not aligning minerals to produce foliation; there's only heat)
• Hornfels is diagnostic of contact metamorphism

Zones of contact metamorphism:

• Metasomatic Aureole - closest to intrusion, thar' be hydrothermal solutions
• Thermal Aureole - outside a bit, typically contains
  • metaconglomerate
  • hornfels
  • marble
  • quartzite

Regional Metamorphism

Things like continents smashing into each other (high temps and pressures over a larger area). Metamorphism of deeply buried rock by large scale folding and faulting (mountain building); large batholiths

• Foliated rocks are diagnostic of regional metamorphism
• Granular metamorphic rocks can also occur (but not Hornfels!)
• Soapstone is diagnostic of regional metamorphism
• grade decreases w/ distance from area of collision
• Coal is very sensitive to metamorphism

Protoliths

Protolith = the rock that was before it was metamorphosed; the 'source' or 'parent' of a metamorphic rock. The more complex the composition of the source rock, the more complex the composition of the possible metamorphic rock.

Foliated Metamorphic rocks and their protoliths:

Rock	Protolith(s)
Slate	Shale
Phyllite	Shale
Schist	Shale
Gneiss	Shale, Arkose, Greywacke, or Granite
Graphite	Anthracite coal
Amphibolite*	Pretty much any Mafic Igneous rock

Granular Metamorphic rocks and their protoliths:

Rock	Protolith(s)
Hornfels	Shale
Quartzite	Quarts Sandstone
Marble	Limestone or Dolostone
Soapstone	...some Mafic Igneous rock
Anthracite coal	Bituminous coal

*Amphibolite may be granular or faintly foliated

Shale is very chemically complicated, so depending on the grade or conditions you can produce a variety of metamorphic rocks. Outcomes depend on the exact composition of the protolith and exact metamorphic conditions

Looks like tables aren't working for some reason?