Overview of Physical Geology•What is Geology?

–Definition Geology is the scientific study of the solid earth or other solid planets, Extends to other aspects, such as atmosphere and oceans, as needed.

–Bases of geology •Physics and Chemistry, Biology, some Meteorology.

•Forms much of the basis for soil science

–Applications of geology •Groundwater

•Oil

•Ore

•Industrial minerals

•Hazards

•Soil

v 0050 of 'Overview of Physical Geology' by Greg Pouch at 2012-08-27 13:08:43 LastSavedBeforeThis 2011-01-19 17:39

Overview of Physical Geology1 What is Physical GeologyKey Concepts

3 Key Concepts > Phase4 Key Concepts > Materials5 Key Concepts > -spheres6 Key Concepts > Geologic Time7 Key Concepts > Plate Tectonics8 Key Concepts>Plate Tectonics>Types of plate boundary9 Key Concepts > "Heat" engines

10 Rheology > Study of deformation of materials in response to stress.11 Chemical formulas

Key Concepts > Phase

–Phase: a homogeneous, physically distinct portion of matter (in a non-homogeneous system)

• Ice and water are two separate phases of water. Both are the same substance.

• Oil and water don’t mix and are two separate phases, both as solid and liquids.

• Water and ethanol mix in any ratio as liquids (miscible liquids) and form a single phase ( a water-ethanol solution), but cannot freely dissolve as solids and form two phases (water-ice, ethanol-ice)

• Iron and nickel mix in any proportion and form only one phase (iron-nickel alloy), both solid and liquid.

• Iron and carbon have limited solubility in each other as solids, and form a variety of phases (never more than two at the same time) in steel.

• Iron and water do not dissolve each other at all as solids (iron, water-ice) and are two phases.

• The difference in physical properties between two phases is usually very distinct, and much of geology is driven by phase changes. Often, near the melting temperature, a substance will have transitional properties. (Ice gets more plastic as the T↑0ºC , and water gets more viscous as T↓0ºC

Key Concepts > Materials–Mineral: naturally occurring inorganic (non-biogenic) crystalline material with a definite composition or range of compositions

–Rock: consolidated (solid, hard, rigid) mixture of minerals• Igneous: Formed of inter-locking grains. Solidified from a hot, siliceous liquid (magma)

• Sedimentary: Formed of loose grains glued together or sometimes of inter-locking grains. Derived from sediments or precipitation from low-temperature fluids.

• Metamorphic: “Altered form” Formed from pre-existing rocks by action of heat, pressure, or fluids with the protolith (rock it was before it was this) still solid.

–Rock “cycle” is a series of pathways of rocks going from one type to another.

Key Concepts > -spheres–Conceptual/Mechanical

•Atmosphere•Hydrosphere•“Petrosphere” = Solid earth•Biosphere

–Compositional•Crust

–Continental about 35 km thick. Upper part is rigid, lower part is plastic–Oceanic about 10 km thick, all rigid

•Mantle Rigid outer, weaker middle part (asthenosphere)•Core Liquid outer, solid inner

•Earth is a little like a Tootsie Pop, –where the Tootsie-Roll center is the core, –the hard-candy shell is the mantle, –and the crust is the wrapper, or maybe a coating of dust.

–Mechanical•Lithosphere Crust plus rigid part of the mantle•Asthenosphere Plastic/weak part of the mantle•Lower mantle is solid•Outer Core is liquid•Inner Core is solid

Key Concepts > Geologic Time•Earth is around 4.55 billion years old. 4.55 Ga [Giga-annum]= 4,550Ma [Mega-annum]

–Assume the room is 45 feet wide and represents the age of the earth (4.5 Ga)–1 foot represent 100 million years 1.2 inches (=1/10 ft) represents 10 million year–0.12 inches ~1/8 represents 1 million years–The last glaciation occurred 10,000 years ago, or 1/100 of 1/8=1/800 inch (smallest dot you can make with a

good laser printer). All of written history is half that distance. Oldest known rocks are seven feet from starting point, and the oldest microfossils at about the same location. Large, hard-shelled fossils (trilobites, crinoids, mollusks…) are in the last 5 to 7 feet.

•The long timespan means that even very slow processes, going on for a very long time, can have enormous consequences.•For historical and convenience reasons, geologist use named time periods rather than absolute dates for “recent” geologic events, like the extinction of the dinosaurs.•Many geologic processes are not the total build up of a long series of small increments, but are the NET result of a long series of large increments adding and subtracting. For example, erosion and deposition oppose each other, and accumulation/denudation of sediment is NET of these.

Key Concepts > Plate Tectonics•"Plate tectonics" refers to a whole set of theories.•The lithosphere is divided into a small number of plates, each of which moves as a rigid body, with deformation concentrated along the boundaries between plates. Typical plate motions are on the order of 5 cm/year, or 50 km/Ma.•Plates come with two types of crust, which behave differently. Some plates are composed entirely of one or the other, but most have both.

–Continental (sediments and granitic igneous rocks, and more-or-less permanent, low density and does not subduct.)

–Oceanic (basaltic rocks, and very thin sediments, and extruded and subducted)

•Plate boundaries come in 3 or 4 flavors, with different sorts of geologic features

–Divergent (plates move apart)–Convergent (plates move together)

• Subduction where one plate subducts because it's oceanic• Collision where neither plate subducts because they're both

continental–Transform (plates slide past each other)–A boundary can be mixed, but this seems rare and may be where

exotic terranes come from. (Look at NA+Eurasia northern)• If motion at a convergent boundary is not perpendicular to the

boundary, both convergent and transform co-occur.• If a chunk of continental crust gets carried into a subduction

zone, you have subduction of oceanic crust and {collision of two continental masses, or a reversal of subduction}

Key Concepts>Plate Tectonics>Types of plate boundary•Divergent

– Plates move apart, lava fills the gap and new ocean floor is created at the mid-ocean ridge. As the crust cools, it subsides. Basaltic volcanoes, usually underwater, and small, shallow earthquakes. North America and Europe in the Atlantic.

•Convergent– Plates come together. Usually has some degree of transform motion.– Continent-ocean

• Continental plate rides over the oceanic one (which is subducted, and returns to the mantle to be melted/resorbed). Leads to partial melting of subducted oceanic crust and sediments -> andesitic volcanoes. Big earthquakes very deep. Trench and island arc. South America and the Nazca Plate

– Ocean-ocean• One of the plates is subducted by the other. Trench and island arc, andesitic volcanoes. Pacific

and Philippine– Continent-continent

• Neither subducts. High, folded mountains and thick section of continental crust. Granites. India and Eurasia

•Transform– Two plates slide past each other. BIG earthquakes near surface. Pacific and North America in

California, African and Eurasian in Turkey

Key Concepts > "Heat" enginesAn “engine” converts energy from some other form to mechanical energy, as an electric engine, steam engine, or internal combustion engine. Most earth processes seem to be driven by one of two major engines: internal motions driven by density differences (often due to temperature differences, but sometimes due to chemical differences) and external motions driving the atmosphere and ocean, also due to density differences, and mainly due to heat differences (some driving by humidity, salinity, and other compositional factors.)

–Book override: it is more useful to think of the earth as operated by two density or gravity engines: you can almost always find the driving density contrast, but it is sometimes very hard if not impossible to find a driving thermal gradient.

Rheology > Study of deformation of materials in response to stress.

• Stress is the applied force divided by the area. (Force/area)– Stresses can Compressive (pushing together), Tensile (pulling apart), or Shear (sliding past)– Pressure is stress uniform in all directions (homogenous, pressure) – Stresses can be decomposed into pressure (the average) and stresses deviating from uniform.– Effective stress = (stress on grains)-(fluid pressure) [the effect of increasing fluid pressure is equivalent to decreasing confining pressure

• Strain is dimensional change and is like displacement. Change in length divided by the initial length. – Causes of strain (Deformation)– Strain can result from an applied stress or force. (For example, stepping on mud applies a force or stress, and the soil strains (elastically, plastically, or viscously).– Strain can occur and cause a stress (When something cools, it contracts, and causes a tensile force/stress. When

something is heated, it expands, causing a compressive stress.)

–Styles of strain• Elastic strain is reversible. Important special case: strain is linearly proportional to stress Wood under low stress• Brittle general term for irreversible strains that includes jointing and faulting. In brittle deformation, there are

discontinuities. Breaking glass is brittle. Rock under sudden, high stress, like hammer blow.• Ductile (continuous, dispersed deformation)

– Viscous the material responds to any shear stress by flowing to eliminate the stress and deformation rate is proportional to applied stress.. Fluid. Water

– Plastic the material has a level of stress required to initiate movement (plastic limit), after which it strains at a uniform rate. Mashed potatoes

• What controls the style of strain?– Material– Magnitude of stresses (At low stress, most materials deform elastically (they bounce back). At higher stresses, failure

—brittle or ductile—occurs.)– Confining pressure (Brittle failure common at low pressure, plastic common at high pressure)– Temperature (Cold Brittle->Plastic->Viscous Hot)– Fluid pressure (pressure increases have the same effect as decreasing confining pressure)– Fluid composition (reactive fluids, like water and saltwater, can influence the mode of deformation. Fluid viscosity can

influence the style of deformation.)– History of the material (pre-existing cracks and crystal defects)– Stress rate

Chemical formulas• The letters represent elements. Most elements have a two-letter symbol, like Silica Si, some have a one-letter

symbol, like Oxygen O and potassium (Kalium in German) K. • Subscripts indicate the number of atoms of the element or group in the molecule. No subscript is equivalent to

1. (Usually, lack of a number is an implicit 1.)• Superscripts indicate charge on an ion, and are usually omitted.• Groups are either substitutions, where one element can substitute freely for another and are separated by

commas like (Mg+2, Fe+2), or complex ions, where an complex of atoms acts like a single atom like Fe(OH)3. Often, parentheses are not shown for a complex ion.

• These are all ways of writing the formula for the mineral olivine

(Mg, Fe)2SiO4 Usual way to write it. Indicates that Mg and Fe can freely substitute for each other, does not explicitly show charges or that ( SiO4 )-4 is a complex ion.

(Mg+2, Fe+2)2(SiO4)-4 Shows charges on everything and groups silica ion.

Mg2SiO4 and Fe2SiO4 Pure magnesian and ferruginous end-members

Mg0.5Fe1.5SiO4 or (.25)Mg2SiO4(.75)Fe2SiO4 or (Mg0.25Fe0.75)2SiO4 show a particular composition where one fourth of the A sites are filled with Mg and three fourths with Fe

• Compound Ions–Silica ion SiO4

-4 (may bind with another SiO4-4 at the oxygens)

–Carbonate CO3-2 Common in solids

–Bicarbonate HCO3-1 Common in solution

–Phosphate PO4-3

–Sulfate SO4-2