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actualism

actualism

physical processes that exist now also operated similarly in the past

3 types of geologic contact

1. Fault - surfaces where two rocks have moved into current positions next to each other along a fault

2. Depositional - sedimentary or volcanic rock was deposited on older rocks

3. Intrusive contacts - igneous rocks intrude older rocks

unconformities

gap in geologic record created when rock layers are eroded or when sediment is not deposited for long period of time

disconformity

flat-lying sediments lay on top of flat-lying sediments w/ an unconformity in between

angular unconformity

an unconformity in which younger sediment or sedimentary rocks rest on the eroded surface of tilted or folded older rocks (due to tectonic processes)

Nonconformity

unconformity where metamorphic or igneous rocks come in contact and intrude sedimentary strata, removing time

Five Principles of Relative Age Dating

1. original horizontally

2. superposition

3. lateral continuity

4. cross-cutting relationships

5. faunal succession

Original horizontality

all sedimentary beds are always originally deposited horizontally due to gravity

Superposition

in undeformed stratigraphic sequence, the oldest layer of sedimentary rock will be at the bottom & youngest layer will be at the top

How to tell which way is up?

• Cross-bedding

• Ripples - has wave look that flows in specific direction

• Mudcracks - grooves filling sediment point down

• Graded bedding

• Flame structure - grooves point upwards

Lateral continuity

layers of sediment initially extend laterally in all directions & thinning unless obstructed by topography

Cross cutting relationships

a fault or intrusion is younger than the rock it cuts across

Types of cross-cutting relationships

• dikes cut through sedimentary beds

• sills intruded parallel to bedding planes

• faults displace bedding, dikes, and sills as they shift blocks of rock

Faunal succession

rock layers that have similar fossils occurred during similar ages; can be connected to rocks from long distances

• often supported with index fossils (cover large distance but not long span of time)

What are the 4 intervals of geologic time?

eon = longest division of geologic time, including all below

Eras = subdivision of an eon including multiple periods & epochs

• Paleozoic

• Mesozoic

• Cenozoic

Periods = subdivision of an era w/ various epochs

• EX: Jurassic period

Epochs = subdivision of a period (smallest division)

• EX: Holocene ("completely new") epoch of Neogene period in Cenozoic era

What do geologic time interval boundaries mark?

periods of mass extinction

• EX: meteor that killed all the dinosaurs represented end of Cretaceous period

4 eons of geologic time

1. Hadean (first period of time; 4.6-3.8 bya)

2. Archean (3.8-2.5 bya)

3. Proterozoic (2.5-0.5 bya; most recent precambrian eon)

4. Phanerozoic (540 mya - present); consists of easily identifiable fossils

3 eras of Phaneozoic Eon

1. Paleozoic (540-248 mya; early invertebrates, fish, & reptiles)

2. Mesozoic (245-144 mya; rise of mammals & dinosaurs, including their extinction)

3. Cenozoic (66 mya-present; age of mammals & evolution)

isotopes

Atoms of the same element that have different numbers of neutrons

radioactive decay

spontaneous disintegration of atomic nuclei that emits particles, transforming an atom into one of a different element (protons change)

• original atom = parent

• product of decay = daughter

half-life

length of time required for one half of original number of parent atoms to be transformed into daughter atoms

• parent isotope decays into daughter at constant rate → exponents of 2

• As parent decays, amount of daughter isotope grows (preserves same number of overall atoms)

Why are radioactive isotopes good for clocks?

• Half-lifes do not vary with changes in temperature, pressure, chemical environment, or other geologic processes

• isotopic age of rock corresponds to time when isotopes were locked into the minerals of rock, usually when mineral crystallizes from a magma or recrystallizes during metamorphism

True or False: The number of daughter atoms in minerals always resets to zero in new "clock"

FALSE; actual geologic rocks are never perfect & assuming 0 will over estimate age

• Use stable isotope as a reference to measure initial daughter amount

Isotopic dating methods: Carbon-14

useful for dating organic materials in sediments less than a few tens of thousands of years old

• Nitrogen-14 is daughter isotope

isotopic dating methods: uranium & lead

One of most precise dating methods for old rocks

TWO SETS

• Decay of uranium-238 to lead-206

• Decay of uranium-235 to lead-207

decays behave similarly for having same protons, but vary in half-life → provide consistency check when accounting for weathering, contamination, & metamorphism

Getting actual age using slope of half-life (EXAMPLE)

must solve for T in slope equation

EX: slope = 2^T - 1 = 0.067

solving for T

• 2^T = 1.067

• T*log(2) = log(1.067)

• T = log(1.067)/log(2) = 0.0935 half-life

half-life formula

N(t) = N_o*(1/2)^t/h

• N(t) - quantity of substance remaining

• N_o - initial quantity of substance

• t - time elapsed

• h - half life of substance

Describe the reactions of the Uranium Decay Series (²³⁸U)

Does not decay automatically to Pb-206; starts w/ U-238 and goes through series of daughter products before reaching stable Pb (lead)

Processes that can reset/disturb system and cause open system behavior

• Heating

  • Minerals w/ parent/daughter atoms diffuse out of mineral lattice

• Metamorphism (recrystallization)

  • Causes daughter/parent atoms to leak through lattice

• Weathering

  • Minerals open up or break down

• Interaction with fluids

  • Introduce parent or take away daughter through chemical interactions

Essential Requirement for Chronometric System

NEEDS to be a closed system

• No Parents/daughters lost/gained from systems other than original decay which would offset time

  • Introducing parents from elsewhere = rock is artificially younger

  • Introducing daughters from elsewhere = rock is artificially older

Closure

occurs when temp decreases to point where diffusion of atoms in/out of mineral is minimal; closed system

Chronometric Examples - Igneous Rock (Zircon U-Pb)

• Magma placed in crust as large igneous body

• Magma cools & crystallizes

• Mineral zircon grows, temp of melt at time of growth is below closure of U-Pb (closed)

• U-Pb age indicates time when zircon grew

Chronometric Examples - Metamorphic rock (biotite Ar-Ar)

• metamorphism causes recrystallization —> new minerals (biotite) form above closure temp as atoms diffuse

• decay happens in mineral, but daughters lost since it is above closure temp; age reads as zero 

• As metamorphic rocks rise, they cool; clock restarts when minerals cool below closure temp

• Age dates time of cooling

Chronometric Examples - Resetting ages (apatite [u-th]/he)

• Sedimentary deposit has minerals that record age of cooling; youngest mineral = max deposition age

• Burial causes lithification → produces sedimentary rock —> increases temp until mineral system opens; daughter atoms are lost → age is reset to zero from loss

• rising rocks result in cooling below closure temp, clock restarts; age records time of cooling of sedimentary rock

To know type of date, you have to know…

what mineral and what system (meeting requirements) is used

• What system?

  • Crystallization = time at which mineral grew

  • Cooling = time at which mineral cooled below closure

  • Exposure = time since rock was at/near surface

planes in space: dips vs. strikes

dips = angle at which a plane of interest is inclined to the horizontal plane

strikes = direction of the line formed by the intersection of a fault, bed, or joint (plane of interest) and a horizontal plane (such as water)

geographic up vs. stratigraphic up

Geographic up = perpendicular to surface of earth (pointing to sky)

Stratigraphic up = perpendicular to bedding (pointing old to young)

What is overturning?

beds rotated past 90 degrees and are now upside down

What is a fault?

Break in the rock that separates it into 2 separate blocks, along which there is motion (slip, displacement, offset)

• primarily caused by tectonic forces & movement → development of fractures in lithosphere

Strike-slip faults

fault blocks are moving horizontally (along line of strike) caused by shear stress

Right-lateral vs left-lateral strike slip fault (with example)

Right-lateral strike slip fault = standing on one side of fault and looking across it, the opposite side appears to move right

Left-lateral strike slip fault = standing on one side of fault and looking across it, the opposite side appears to move left

Example for both: San Andreas Fault

Hanging wall vs. footwall

Hanging wall = block of rock that lies above fault plane

footwall = block of rock that lies below fault plane; stationary block against which hanging wall moves

What are the 2 types of dip-slip faults (vertical movement along fault plane)?

Normal = hanging wall moves down relative to footwall; lengthening of Earth’s crust in surrounding area

Reverse = hanging wall moves up relative to footwall; shortening of Earth’s crust in surrounding area

Folds: Anticlines vs. Synclines

Anticline = where beds arch upwards (like an “A” shape)

• oldest strata in center, youngest on the outside edges

Syncline = where bends arch downwards (like a “U” shape)

• youngest strata in center, oldest on the outside edges

What do most/all folds accomodate?

shortening of the crust

Symmetric vs. Asymmetric Folds

Symmetric = limbs slope at same angle and axial plane is vertical (2 equal halves)

asymmetric = one side of limbs is steeper than other; axial plane is inclined (unequal halves)

overturned folds

axial plane inclined to such an extent that the strata on one limb are overturned

What is rheology?

the study of how materials deform & flow under influence of stress

• Temp, pressure, composition dependent

Brittle deformation

tendency of materials to fracture/break when subjected to stress; exhibit little or no plastic deformation before breaking

• EX: quartz, olivine, and feldspars

• often results in formation of fractures, faults, & shear zones

Ductile deformation

ability of materials to deform plastically without fracturing; can undergo lots of plastic deformation before failure

• EX: clay minerals, micas, and calcite

• Leads to development of folds, cleavage, & ductile shear zones

pure elastic materials

materials that return to their original shape and size once strain and stress are removed if they do not fracture; strain is recoverable

pure plastic materials

materials that undergo permanent deformation when force (strain and stress) are applied; strain is nonrecoverable

what are elastoplastic materials?

materials that behave pure elastic at one strain-stress point and transition to pure plastic further down

Describe the large-scale deformation of fold-thrust belts

driven by compressional forces (shortening), resulting in folding and thrust faulting

• Folding → rocks fold under stress & create anticlines

• Thrust Faulting → low-angle reverse fault where movement is more horizontal

Describe the large-scale deformation of basin & range provinces

province shaped by extensional forces (lengthening), leading to normal faulting and block faulting that create alternating mountain ranges & valleys

• Block Faulting → series of tilted fault blocks

deformation

general term that encompasses folding, faulting, shearing, compression, & extension of rock by plate tectonic forces

What type of deformation forces occur at each plate boundary?

divergent boundaries: tensional forces (stretch & pull rocks apart)

convergent boundaries: compressive forces (squeeze & shorten rocks)

transform-fault boundaries: shearing forces (shear two parts of a rocks in opposite directions, leading to rocks changing shape)

4 key considerations for rock deformation

1. same rock can be brittle at shallow depths (low PT) and ductile deep in crust (higher PT)

2. Rock type affects deformation

3. rock formation that behaves as a ductile material if deformed slowly may behave as a brittle material if deformed more rapidly

4. rocks break easier when subjected to tensional (pulling and stretching) forces than when subjected to compressive forces

basins

syncline structure; bowl-shaped depression of rock layers in which beds dip toward central point (often where sediments deposit)

domes

anticline structure; broad circular/oval upward bulge of rock layers

joint

crack in rock formation along which there is no substantial movement

• Caused by tectonic forces & non-tectonic expansion/contraction of rocks, such as cooling and erosion

cataclastic textures

grains are broken & angular from shearing where rocks are more brittle (usually in upper crust)

mylonites (textures)

grains are smoother & more round from shearing where rocks are more ductile (deeper in crust where PT is higher)

horizontal vs plunging folds

plunging fold = fold with tilted fold axis

horizontal fold = fold with horizontal fold axis

folds

features that occur when sedimentary beds (or other flat surfaces) are bent into a curved structure

fold axis

line made by lengthwise intersection of axial plane with the rock layers

Tensional tectonics (broad scale)

• create normal faults in brittle crust that split plates apart, forming rift valleys (which include mid-ocean ridges)

• Rocks in shallow continental crust create steep normal faults, while deeper rocks create curved (listric) faults

Compressive tectonics (broad scale)

Where thrust faulting occurs in continental compression, forming overthrust structures

• includes subduction zones → oceanic lithosphere sliding beneath an overriding plate on megathrust fault

• Colliding continents can create fold and thrust belts, leading to mountain building

Shearing tectonics (broad scale)

Transform faults (strike-slip faults forming plate boundaries)

• can have bends and jogs, changing forces from shearing to compressive or tensional → complex deformation patterns, secondary faulting, & folding

• EX: San Andreas Fault experiences the "Big Bend"

Tectonic provinces

large-scale regions formed by distinctive tectonic processes that reflect long-term tectonic history

tectonic ages (on map)

Ages of most recent deformation

Cores of continents are oldest, edges are younger

shield

tectonic province within continent that is stable & consists of ancient crystalline basement rocks at surface

platforms

region where Precambrian basement rocks of earlier deformation are overlain by layers of flat sediments

Where are the youngest orogens found?

in active margins, such as North American Cordillera

cratons

stable interior of ancient continental crust, often made up of continental shields & platforms

active vs passive margins

Active margins = continental margin where tectonic forces caused by plate movements are actively deforming the continental crust

Passive margins = continental margin far from a plate boundary; attached to oceanic crust as part of same plate & zones of extended crust

Continental basin

region of prolonged caving (deep holes) in where thick sediments have accumulated during the Phanerozoic eon & dip into center

EX: Michigan Basin

Phanerozoic orogen

region where mountain building has occurred during the Phanerozoic era (younger mountains)

• EX: Appalachian Fold Belt

Extended crust

region where most recent deformation has involved large-scale crustal extension (crust stretches)

EX: Basin and Range Province

Terranes

fragment of crustal material formed on, or broken off from, one tectonic plate and "sutured" to crust of another plate, preserving own geologic history

• suture zone between terrane & crust often causes a fault

accretion

process by which fragments of tectonic plates are added to continent at a plate tectonic boundary (continental growth)

How do continents grow?

through magmatic addition and terrane accretion

• magmatic addition = low-density, silica-rich rock differentiates in mantle and rises as felsic material to crust (cools and adds new crust)

• terranes accretion = small crustal fragments transported by plate boundaries, colliding and merging with continents (sutured)

exotic terrane

block of land that collided with a continent along a convergent margin & attached to it; not originally part of continent

accretion processes - buoyant fragment to continent

Buoyant crust pieces that can't sink get transferred from one plate to another (can be small land bits or thickened oceanic crust sections)

accretion processes - Island arc to continent

sea between an island chain and a continent disappears as island's crust combines with the advancing edge of continent

accretion processes - Along a transform fault

2 plates slide past each other → strike-slip faulting which can move chunks of land from one plate to the other

accretion processes - Continental collision & rifting

2 continents collide and are sutured together, then break apart later at different location

how are continents modified?

through orogeny (mountain building), the Wilson cycle, & epeirogeny

Wilson Cycle

The cyclical opening and closing of ocean basins caused by movement of Earth's tectonic plates from diverging to converging

Cycle of:

Continental break-up

Rifting, MOR spreading

Subduction, collision

Closure of ocean basins

epeirogeny

Gradual downward & upward movements of broad regions of crust without significant folding or faulting

• downward movement - leads to flat sediments

• upward movement - causes erosion & unconformities

Steps of Wilson cycle:

1) First Form of Ocean Basin (Stable Craton on supercontinent with a Hot Spot):

• hot spot underneath stable craton causes supercontinent to swell & crust thins from heat

• supercontinent eventually breaks into two & forms small ocean (East African Rift Valley)

2) Young Ocean Basin (Early Rifting & Continent Separation):

• Plates start spreading, creating a small ocean

• cooling edges of continents sink below sea, forming divergent boundary (Red Sea)

3) Mature Ocean Basin (Full Ocean Basin):

• large ocean forms between continents from ongoing spreading w/ mid-ocean ridge (Atlantic Ocean)

4) Declining Ocean Basin (Subduction Zones):

• Subduction zone formation as ocean begins to close, due to convergent boundary

• ocean eventually disappears, leaving a remnant ocean basin. (Pacific Ocean)

5) Dead Ocean Basin (Closing Remnant Ocean Basin):

• Continents are nearly colliding, causing magma to form, orogeny, & suture to form

6) continent erodes & crust thins overtime → process eventually restarts

Glacial rebound

continental lithosphere pressed down by weight of a large glacier rebounds upward for tens of millennia after that same glacier melts

Phanerozoic History of North America: Appalachians

Large eastern mountain range dating back hundreds of mya that eroded over time; resulted from collision between Laurentia and Baltica

Phanerozoic History of North America: Cordillera

• Shaped by Sevier-Laramide orogeny, which resulted from a collision between the North American Plate & Pacific Plate

• caused crust to be compressed and uplifted, leading to many prominent mountain ranges in Cordillera, including Rocky Mountains & Sierra Nevada

Phanerozoic History of North America: San Andreas

• During paleozoic era, was part of western North America's tectonic evolution as Pangea assembled but not as prominent

• became more defined & active during Mesozoic and Cenozoic eras, driven transform boundary between Pacific Plate & North American Plate along fault

Phanerozoic History of North America: Basin & Range

• began to take shape during the early Phanerozoic Eon (~540 mya) as part of flat continental platform

• Landscape transformed during Mesozoic & Cenozoic eras (~250-20 mya) through extension forces that created faults and fractures

• lead to formation of long, narrow valleys (basins) & high mountain ranges (ranges)

Complete Paleogeography timeline

470-440 Ma: Arc accretion (Taconic orogeny)

• addition of landmasses & geological history of North America

400 Ma: Laurussia supercontinent forms (Caledonian & Acadian orogenies)

340-300 Ma: Pangaea forms (Variscan, Appalachian, Urals mountain ranges)

180 Ma: Atlantic starts opening as continents move apart

160-40 Ma: Cordillera (Sevier-Laramide orogenies)

25 Ma: San Andreas becomes significant transform boundary

15 Ma: Basin & Range

• in western North America

• elongated valleys & mountain ranges

Caledonian Orogeny

• closed ocean between Laurentia and Baltica, forming the larger continent, Laurasia

• formed Caledonian Mountains in northern Europe ~490-390 mya

Variscan Orogeny

mountain-building event caused by late Palaeozoic collision between Euramerica and Gondwana to form Pangea; created Variscan Mountains in Europe ~380 - 280 mya

Appalachian Orogeny

built Appalachian Mountains in eastern North America ~ 480 and 280 mya

Himalayan Orogeny

Cenozoic episode of mountain building (still occuring) that began ~50 - 40 mya (Eocene) when the Indian plate collided with Asia