Physical Geology - Exam 2

What is the distinction between weathering and erosion?

Weathering is the breakdown of rocks while erosion is the removal from its location

what are the two types of weathering?

Physical weathering and chemical weathering

How do mechanical weathering and chemical weathering enhance eachother?

mechanical weathering breaks down rocks into smaller pieces with greater surface area.

clues left by weathering

• differential weathering

• rounding

• weathered surface on a rock

mechanical weathering

• pressure release

• frost wedging

• plant growth

• animal burrowing

• thermal cycles

• salt growth

why do granite outcrops form a dome shape when exposed to the surface?

exfoliation

atoms

smallest assemblies of matter

electrons

positively charged particles

protons

negatively charged particles

ion

an atom that has an excess or deficiency of electrons in the outer shell relative to number of protons in the neucleus

cations

positively charged ions

anions

negatively charged ions

chemical weathering agents

• oxygen

• acids

strong acids

give off large quantities of hydrogen ions (H+)

weak acids

give off low quantities of hydrogen ions (H+)

Olivine

With water→limonite

without water→hematite

feldspar

kaolinite (clay mineral)

calcite

ions in solution

in what part does chemical weathering take place?

Fission a nuclear reaction in which an atomic nucleus

sediment

an accumulation of loose grains

cementation

the phase of lithification in which cement, consisting of minerals that precipitate from ground water, partially or completely fills the spaces between clasts and attaches each grain to its neighbor

lithification

the transformation of loose sediment into solid rock through compaction and cementation

detrital rocks

consisting of cemented-together detritus derived from the weathering of pre-existing rock: produced from eroded mineral grains

bioclastic rocks

produced directly by accumulation of organic remains of plants or animals

chemical rocks

produced by precipitation of dissolved ions in water

sediment sizes of detrital sediments

1. gravel-sized grains

2. sand-sized grains

3. silt-sized grains

4. clay-sized grains

compaction

a shift to a tighter packing of the grains due to increasing overburden

crystallization

precipitation of minerals from solution

what happens to sediment when it is transported?

• rounding: grains display more rounding further from the source

• sorting: sediment grains are separated according to size and shape. sediment sizes decrease with increased transport distance

why are sedimentary structures important to recognize?

what are rock contacts?

where two different types of rock touch each other

metamorphic textures

• slaty foliation → metamorphism

• schistose foliation → more heat and/or pressure

• gneissic foliation → temperature change

regional metamorphism

• high pressure

• differential stress

• results in foliated texture

• occur in wide temperature range

hydrothermal metamorphism

• precipitated from or altered by hot water

• metasomatism: water adds new atoms to the rock

partial melting metamorphism

produces migmatites: exhibit both intrusive igneous and foliated metamorphic textures

shock metamorphism

produced by rapid application of extreme pressure

contact metamorphism

due to proximity to or contact with an igneous intrusion

prograde metamorphism

a metamorphic rock re-crystallizes into a higher-grade metamorphic rock

shale→slate→phyllite→schist→gneiss→migmatite→granite

what is a parent rock?

original rock before metamorphism

radioactivity

radiometric dating (isotopic dating)

determining the absolute age of an igneous rock through its radioactive elements

isotopes

an element with the same number of protons but different number of neutrons

same atomic number, different atomic mass

radioactive decay

the spontaneous nuclear loss or gain of protons or neutrons at a constant and measurable rate

parent isotope

the original isotope before radioactive decay

daughter isotope

the product of radioactive decay

half-life

the time required for half of the parent isotopes to decay into daughter isotopes

stable isotopes

unstable isotopes

alpha decay

daughter nucleus has atomic number 2 less and mass number 4 less than parent nucleus

beta decay

daughter nucleus has atomic number 1 higher than parent nucleus. no change in mass number

electron capture

daughter nucleus has atomic number 1 lower than parent nucleus. no change in mass number

why is it a reliable time piece for age dating igneous rocks?

it is consistent

closure temperature

the temperature of an igneous melt at which it becomes a closed system

closed system

melt is sealed off so neither parent or daughter isotopes can leave

different isotopic pair

has different closure temperatures

what isotope is used for radiocarbon dating?

Carbon-14

how long is Carbon-14’s half-life?

5,730 years

about how long is carbon dating accurate for?

40,000 years

what is carbon dating more useful for than geology?

paleontology

disconformity

an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition

nonconformity

the contact surface between younger sedimentary beds and eroded older intrusive igneous or metamorphic rocks

Angular unconformity

contact between rock layers where the lower, underlying rocks are tilted and the younger, overlying rocks are flat

Law of original horizontality

beds that are deposited in deep, calm water formed horizontal or near horizontal layers

Law of superposition

in a sequence of undisturbed beds, the oldest is at the bottom and the the beds get progressively younger and upward

Law of lateral continuity

sedimentary or volcanic layers extend laterally until it tapers out or hits an obstruction

Law of inclusion

any clast that has become included into another rock must be older than the rock into which it has been incorporated

Law of cross-cutting

any geologic feature that cuts across another geologic feature must be younger than that which it cuts through

conformable sequence

sequence of sedimentary rock with complete record of depositional history

unconformities

erosional contact between rock layers that represent a gap in geologic time (lost time)

Index fossil

a fossil used to date a geologic structure

index fossil requirements

• short geologic timespan

• broad geographic range

sedimentary structures

features found within sedimentary rocks that form during or after deposition but before lithification

deformational structures

features that result from the deformation of the earth’s crust after lithification

compressional stress

forces push the rock together from opposite directions

tensional stress

forces pulling away from each other in opposite directions

shear stress

stresses act parallel to a plane

elastic phase of strain

deformed material that recovers its original shape after stress is reduced or removed

ductile phase of strain

material will bend while under stress but will NOT return to its original shape after stress is removed

brittle phase of strain

material fractures at stresses higher than the strength of the material

Tilted sedimentary bed (strike measurement)

a line formed by the intersection of an inclined bedding plane with a horizontal plane perpendicular to the dip direction

• angle: how many degrees off due north

• direction: what direction off of due north

Tilted sedimentary bed (dip measurement)

angle that a bedding plane dips below the earth’s surface

• angle: number of degrees

• direction: compass direction the bed dips

anticline fold

• folds shaped like an arch

• oldest rocks in center of the fold getting progressively younger toward the edge

• bed dips away from hinge line

syncline fold

• fold shaped like a trough

• youngest rock are in the core of the fold and get progressively older toward the edge

• rock beds dip toward the hinge line

structural domes

• folds in which the bed dips away from a center point

• oldest rocks are exposed at the core and get progressively younger toward the edge

structural basin

• folds in which the beds dip toward a center point

• youngest rocks are exposed at the core

Dip-slip faults

movement is parallel to dip direction

normal dip-slip fault

• hanging wall down relative to foot wall

• tensional stress

reverse dip-slip fault

• hanging wall moves up relative to the foot wall

• compressional stress

strike-slip fault

• movement is parallel to the strike of the fault

• horizontal or lateral faults

• shear stress

right-lateral strike-slip fault

If the block opposite an observer looking across the fault moves to the right

left-lateral strike-slip fault

If the block opposite an observer looking across the fault moves to the left

oblique-slip fault

dip-slip and strike-slip components of movement

clues of fault movement

• fault-zone

• slickensides

• drag folds

fault-zone

zone of broken rock that was ground up by fault movement

slickenside

polished and striated rock surface

drag folds

minor folds produced by movement