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157 Cards in this Set
- Front
- Back
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unconformity |
a time gap in the rock record, from non deposition & erosion -3 types - angular, nonconformity, disconformity |
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angular unconformity |
an angular unconformity represents a huge gulf in time. (James Hutton - first to recognize) horizontal marine sediments deformed by orogenesis -mountains eroded completely away -renewed marine invasion -new sediments deposited |
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nonconformity |
igneous/metamorphic rocks capped bysedimentary rocks -crystalline igneous/metamorphic rocks were exposed by erosion. -sediment was deposited on this eroded surface. |
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disconformity |
parallel strata bounding non-deposition, due to an interruption in sedimentation and sometimes -hard to see -pause in deposition -sea level falls, then rises -erosion |
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the principle of fossil succession |
fossils are often preserved in sedimentary rocks, fossils are time markers useful for relative age dating -fossils speak of past depositional environments, specific fossils are only found within a limited time range |
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the principle of fossil succession |
-species evolve, exist for a time, & then disappear -the first appearance, range, & extinction used for dating -fossils succeed one another in a known order -a time period is recognized by its fossil content |
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the principle of fossil succession |
-fossil range- the first & last appearance --each fossil has a unique range, range overlap narrows time -index fossils are diagnostic of a particular geologic time -fossils correlate strata: locally, regionally, globally |
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numerical age (geochronology) |
-many relative ages can now be assigned actual dates -based on radioactive decay of atoms in minerals --radioactive decay proceeds at a known, fixed rate --radioactive elements act as internal clocks |
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absolute dating |
involves a calculation of when something actually occurred. -geologists deal in relative & absolute time -relative dating is simply a sequence of events --knowing when eruptions occurred allows us to determine which volcanoes are the most hazardous (same with floods, earthquakes, landslides) |
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isotopes |
elements that have varying numbers of neutrons -isotopes have similar but different mass numbers --stable- never change (i.e., 13C) --radioactive- isotopes that spontaneously decay (i.e., 14 C) |
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parent isotope |
the isotope that undergoes decay |
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daughter isotope |
the product of the parent isotope decay |
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radioactive decay progresses along a decay chain... |
decay may create new, unstable elements that also decay decay proceeds to a stable element endpoint |
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half life (t 1/2) |
time for half of the unstable nuclei to decay, as the parent disappears, the daughter increases -half life is a characteristic of each isotope --after one, one half of the original parent remains --after three, one eighth of the original parent remains |
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metamorphism may allow the diffusion of material, essentially resetting the "clock"... |
the radiometric age will then give the time of the metamorphism, rather than the age of the original rock |
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a fundamental requirement of radiometric dating is forthe parent & daughter isotopes to remain locked withinthe sample, & are not able to move into or out of thesystem... |
these conditions aretypically best met inigneous rocks, wherethe crystallisation ofthe mineral locks theradioactive isotopesinto the crystal latticeand starts the “clock” |
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assumptions of radioactive dating |
-the rock or mineral must be a "closed system" -we must be able to accurately determine a value for the initial daughter atoms if they were present in mineral or rock sample being dated. -the value of the decay constant (λ) must be known accurately -the measurements of the parent & daughter atoms must be accurate & representative of the rock or mineral to be dated |
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why are mountains high? |
lithosphere thickening |
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isostasy |
balancing of loads on the lithosphere with the flow of the asthenosphere buoyancy analogous to icebergs |
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why are mountains high? especially at convergent plate boundaries? |
the thickest block floats highest & sinks deepest convergent-margin horizontal compression causes horizontal shortening & vertical thickening. these processes create a thick crustal root beneathmountain ranges. |
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why are mountains high? |
surface elevation is a balance between forces of gravitational attraction (whichpulls plates into the mantle) & buoyancy (which floats lithosphere on top of themantle). this balance is termed isostasy. adding or removing weight, or changing lithospheric thickness or density, resets its isostatic equilibrium. |
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causes of mountain building |
subduction (convergent) boundaries create mountains Subduction-related volcanic arcs grow on overriding plate. Accretionary prisms (off-scraped sediment) grow upward. Compression shortens and uplifts overriding plate. A fold-thrust belt develops landward of the orogen. |
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causes of mountain building |
continental collision creates a welt of crustal thickening. --thickening due to thrust faulting & flow folding --center of belt consists of high-grade metamorphic rocks -fold-thrust belts extend outward on either side, the resulting high mountains may eventually collapse |
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causes of mountain building |
continental rifting creates mountains -normal faulting creates fault-block mountains & basins -decompressional melting adds volcanic mountains -increased heat flow expands & uplifts rocks |
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geologic time |
provides a frame of reference for understanding rocks, fossils, geologic structures, landscapes, tectonic events, & change |
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geologic time (age of the earth, evolution of humans, recorded history) |
age of the earth - 4.55 billion evolution of humans - 2 million recorded history - 5 thousand |
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relative dating |
determining the order of geologic events based on logical analysis - event a happened before event b |
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absolute dating |
determining an approximate numerical age of a geologic event - event a happened - 103.4 million years plus/minus 0.3 million years ago |
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relative vs. absolute time |
Relative dating of geologic materials is a qualitative method that wasdeveloped hundreds of years ago. Numerical age dating is aquantitative method that was developed over the last 60 years. |
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rock types tell us about the geologic history |
igneous/sedimentary/metamorphic all tell us different things sandstone - beach granite - convergent margin |
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deformation/structures also tell us about geo history |
-faults tell us about directions of stress set up about tectonics -metamorphism tells us about mountain building, etc |
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physical principles allow us to sort out relative ages with ease |
this is possible even & especially in complex situations |
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uniformitarianism |
the present is the key to the past but some catastrophic processes have occurred in the past -the past has not always been the same as today- erosion rates, sea levels, composition of the atmosphere |
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original horizontality |
sediments are laid down close to horizontal any strata that are not horizontal have been disturbed by later movements |
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superposition |
undisturbed strata have the oldest units at the base |
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lateral continuity |
sediments continue in all directions until it thins & pinches out at the edge of the basin -removal of material at a later time |
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cross cutting relationships |
any strata that truncate others must be younger, intrusions (plutons, dikes etc.) & faults are cross cutting & must therefore be younger than the rocks into which they intrude |
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inclusions |
fragments of pre existing rock so must be older than the rocks in which they are incorporated -xenoliths in igneous rocks -cobbles in conglomerates |
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mountain |
a landform that rises above the surrounding landscape |
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mountain range |
a cluster of mountains within a confined distinct geographic area |
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mountain belt |
a broader geographic area containing a series of mountains & ranges with origins of the same geologic forcing |
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orogenesis |
mountain building! mountains reflect geologic processes of uplift, deformation, & metamorphism at work; they are vivid evidence of tectonic activity. constructive processes build mountains up; destructive processes tear them down mountains occur in elongate, curvilinear belts |
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force |
defined as that physical quantity, which changes or tends to change the state of rest or state of uniform motion of a body in a straight line |
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stress |
defined as the distribution of force per unit area (stress defines the "intensity of force") |
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deformation |
defined as a change in shape due to an applied force |
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strain |
the measure of deformation relative to the original size of the sample |
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stress vs. strain |
a force acting on a rock is calledstress & is applied across a unit area. strain is the result of deformationcaused by stress. (a large force per area results in much deformation or strain) (a small force per area results in little strain) |
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deformation |
change in shape due to stress |
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unstrained vs strained rocks |
Undeformed(unstrained) sed rocksdisplay horizontal beds, spherical sand grains,no folds or faults. Deformed(strained) rocks show tilted beds,metamorphic alteration, folding, & faulting. |
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what are the three forms of stress? |
compression tension shear |
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compression |
stress that occurs when an object is squeezed. deformation shortens & thickens the material (plate tectonic collision) |
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tension |
stress that occurs when the ends of an object are pulled apart, which stretches & thins the material (horizontal tension drives crustal rifting) |
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shear |
stress that develops when surfaces slide past one another (neither thickens or thins the crust) |
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stress leads to strain |
two major deformation styles (shallow - brittle/deeper - ductile) small stress-increase stress-too much stress=failure/strain |
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brittle deformation |
occurs in the shallower crust, rocks break by fracturing shattering of a porcelain plate is brittle brittle failure- faulting, jointing/fracturing |
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ductile deformation |
occurs at the higher P & T conditions, which causes rock to deform by flowing & folding flattening a ball of dough is an apt analogy. |
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what dictates which type of fault will develop? |
stress orientation normal fault - tension reverse fault - compression strike-slip fault - shear know : dip slip, strike slip, thrust |
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______ forces add _______ (compression, tension, or shear) to rock. |
tectonic, stress -the rock bends slightly without breaking (elastic strain) -continued stress causes cracks to develop & grow -eventually, cracking progresses to the point of failure -stored elastic energy is released at once, creating a fault |
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rocks ______ past one another along a fault. |
slide -fault motion cannot occur forever |
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fault motion is arrested by _________. |
friction -friction is the force that resists sliding on a surface. -friction is due to asperities (bumps) along the fault. |
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faults move in _________. |
jumps -once movement starts, it quickly stops due to friction. -over time, strain builds up again leading to repeat failure. -this behavior is termed stick-slip behavior -- stick - friction prevents motion -- slip - friction is briefly overwhelmed by motion |
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_____ are waves of energy that travel through the earth as a result of earthquakes |
seismic waves They are what shake the earth They can travel far distances, detectable byscientific equipment There are different types of seismic waves |
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body waves |
pass through earth's interior -P-waves (primary or compressional waves). ---Waves travel by compressing & expanding material. --Material moves back & forth parallel to wave direction. --P-waves are the fastest. --They travel through solids, liquids, & gases. |
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body waves |
pass through earth's interior -S-waves (secondary or shear waves). --Waves travel by moving material back & forth. --Material moves perpendicular to wave travel direction. --S-waves are slower than P-waves. --They travel only through solids, never liquids or gases. |
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seismographs |
instruments that record ground motion -A weighted pen on a spring traces movement of the frame. --Vertical motion—records up-and-down movement. -- Horizontal motion—records back-and-forth motion. |
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what are damaging seismic waves? |
surface waves |
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surface waves |
travel along earth's exterior -Surface waves are the slowest & most destructive. --L-waves (Love waves) ---S-waves that intersect the land surface. ---Move the ground back and forth like a writhing snake. |
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surface waves |
travel along earth's exterior -R-waves (Rayleigh waves) --P-waves that intersect the land surface. --Cause the ground to ripple up & down like water. -Surface waves die out with depth. |
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what kind of eruptions are largest? |
FELSIC ex. Yellowstone, Santorini (Greek Islands), Krakatau (Indonesia) |
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how do we monitor volcanoes? |
increased seismic activity, increased gas activity, changes in topography, changes in temperature |
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earth shaking caused by a rapid release of _______. |
energy Energy moves outward as an expanding sphere of waves. This waveform energy can be measured around the globe. |
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earthquakes are ______ on this planet. |
common They occur everyday. More than a million detectable earthquakes per year. |
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most earthquakes are a result of ________ _________ __________. |
tectonic plate motion |
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most earthquakes are _______. |
small |
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large earthquakes... |
destroy buildings & kill people :D - 3.5 million deaths in the last 2,000 years several hundred large earthquakes per year |
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seismicity |
earthquake activity |
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seismicity occurs due to |
***fault slip is the most common cause but... Sudden motion along a newly formed crustal fault. Sudden slip along an existing fault.A sudden change in mineral structure. Movement of magma in a volcano. Volcanic eruption.Giant landslides.Meteorite impacts.Nuclear detonations. |
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hypocenter |
(focus) - the place where the fault slip occurs -usually occurs on a fault surface -earthquake waves expand outward from the hypocenter |
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epicenter |
land surface right above the hypocenter -maps often portray the location of epicenters. |
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Faults in the Crust |
Faults are like planar breaks in blocks of crust. Most faults have a slope (rarely, they can be vertical). On a sloping fault, crustal blocks are classified as: --Footwall (block below the fault). --Hanging wall (block above the fault). |
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normal fault |
-the hanging wall moves down relative to the footwall. -results from extension (pull-apart or stretching) *the fault type is based on relative block motion* |
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reverse fault |
-the hanging wall moves up relative to the footwall. -results from compression (squeezing or shortening) -the slope (dip) of fault is steep. *the fault type is based on relative block motion* |
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thrust fault |
-a special kind of reverse fault -the slope (dip) of fault surface is much less steep -common fault type in compressional mountain belts *the fault type is based on relative block motion* |
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strike-slip fault |
-one block slides laterally past the other block. -there is no vertical motion across the fault. -the fault surface, however, is nearly vertical. *the fault type is based on relative block motion* |
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earthquakes occur as a result of _______ ________. |
fault motion -energy creating earthquakes originates when: --rocks break to form a new fault, or --a preexisting fault is reactivated. -once created, a fault remains a zone of weakness. |
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displacement |
the amount of movement across a fault. -during earthquakes, fault blocks move. -displacement, also called offset, is shown by markers. displacement is cumulative over time. |
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what tends to form with basaltic magma? :P |
scoria cones shield volcanoes large fissures (flood basalts) |
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hazards associated with basaltic eruptions are... |
lava fountain volcanic ash lava-caused fire lava flow -eruptions near ice sheets or snowy volcanoes melts ice & causes huge floods (jokulhlaup) |
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andesitic lava flows... |
higher SiO2 content makes it viscous -unlike basalt, do not flow rapidly -mound around the vent & flow slowly the crust fractures into rubble (blocky lava) andesitic lava flows remain close to the vent. |
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rhyolitic lava flows |
highest SiO2 content, most viscous lava rarely flows plugs the vent as a lava dome sometimes, lava domes are blown to smithereens |
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_______ & ________ flow tend to form with fells magma |
andesitic & rhyolitic (more viscous, more gas more silica) -volcanic domes -composite/strato volcanoes -calderas |
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composite volcanoes |
large traditional volcano looking volcanos like mount fuji & kilimanjaro |
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pyroclastic flow is common in.... |
strato/composite volcanoes |
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how are volcanic domes formed? |
grow from the inside as magma is injected into interior of dome grow as magma breaks through surface |
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how are volcanic domes destroyed? |
when steep flanks collapse explosions from buildup of gases |
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hazards associated with felsic eruptions include |
ash fall pyroclastic flow lahars landslides |
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composite/strato volcano rock types |
andesite mudflow deposits various types of ash, lava, mudflows tephra from eruption column tuff from pyroclastic flows |
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rocks associated with volcanic domes |
rhyolite flows volcanic breccia from breakup of flowing lava & collapse tuff & volcanic breccia associated with dome collapse |
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what is a volcano? |
an erupting vent through which molten rock surfaces a mountain built from magmatic eruptions |
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volcanoes are a clear result of ___________ activity. |
tectonic |
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volcano eruptions are _________ & dangerous. |
unpredictable -build large mountains -blow mountains to bits |
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volcanic eruptions can: |
provide highly productive soils to feed a civilization. can extinguish a civilization in a matter of minutes. also affects climate |
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mafic volcanoes |
scoria cone shield volcano |
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felsic volcanoes |
composite volcano volcanic dome |
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big volcanoes (mafic & felsic) |
fissure & caldera mafic & felsic |
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characteristic features of volcanoes |
magma chamber, fissures & vents, craters, calderas, distinctive profiles - shield volcanoes, cinder cones, stratovolcano |
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_____ & _____ of volcano are governed by magma type. |
shape & size -volcanoes come in many shapes & sizes |
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categories of volcanoes (largest-smallest) |
shield volcanoes - largest stratovolcanoes - intermediate in size cinder cones - smallest |
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effusive eruption |
produces lava flows |
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explosive eruptions |
blow up |
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conditions influencing what kind of volcano forms |
gas content of the magma viscosity of the magma |
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how do gases affect magma? (what happens when you open a soda?) |
propels eruption & forms ash under less pressure, gas forms bubbles dissolved gas held in magma by pressure |
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viscosity |
gooeyness or resistance to flow viscosity increases with increasing silica content due to silica chains influenced by composition influenced by temperature |
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how does viscosity affect volcanoes? |
ability for lava to flow low viscosity - low sloping mountain high viscosity - steep mountain |
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how does viscosity affect eruptions |
more viscous: difficult to flow & trap gases less viscous: flows easier & gas can escape |
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eruption types |
lava flow, lava fountain, dome, eruption column |
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mafic lava |
very hot, low silica, & low viscosity BASALTIC LAVA FLOWS |
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basaltic lava flows |
mafic lava - very hot, low silica, & low viscosity basalt flows are often thin & fluid -they can flow rapidly (up to 30 km per hour) -they can flow for long distances (up to several hundred km) --most flows measure less than 10km --long-distance flow facilitated by lava tubes |
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basalt flow characteristics |
pahoehoe - ropy, fluid texture of pahoehoe lava a'a flow - crumbly, blocky texture of a'a flow pillow basalt - basalt flows into water |
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columnar basalt |
solidified flows may contract, creating vertical fractures that are hexagonal in cross-section. this feature, columnar jointing, indicates former lava. |
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changes in mineral assemblage & texture that result from subjecting a rock to high pressure, temperatures is called: |
metamorphism |
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rocks can be subjected to higher temperatures & pressuresas rocks become buried deeper in the Earth such as near ___________ ___________ or _________. |
continental collisions or subduction |
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a general term for describing the relative temperature & pressure conditions under which metamorphic rocks form |
metamorphic grade |
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low grade metamorphism is characterized by an abundance of _______ minerals, minerals that contain water, H2O, in their crystal structure. |
hydrous -clay minerals, serpentine, chlorite -slate roof is hydrous clay minerals |
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as grade of metamorphism increases, ________ minerals become less _______, by losing H2O & non-______ minerals become more common. |
hydrous, hydrous, hydrous ex. garnet schist |
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muscovite |
hydrous mineral that eventually disappears at the highest grade of metamorphism |
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biotite |
a hydrous mineral that is stable to very high grades of metamorphism |
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serpentine |
a hydrous mineral |
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garnet |
a non hydrous mineral |
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When pressure & temperature change, chemical reactionsoccur to cause the minerals in the rock to change to anassemblage that is stable at the new pressure & temperature conditions. |
true ex. chlorite <---- -----> jade |
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temperature increases with depth in the earth along the |
geothermal gradient -thus higher temperature can occur by burial of rock temperatures can also increase due to igneous intrusion |
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the increasing temperature with depth in the earth is called the: |
geothermal radiant |
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if the stress is not equal from all directions, then the stress is called a |
differential stress |
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minerals that crystallize or grow in the differential stress field can have a preferred orientation. such a structure is called a |
foliation |
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Slate Slates form at low metamorphic grade by the growth offine grained chlorite & clay minerals. The preferredorientation of these sheet silicates causes the rock to easilybreak along the planes parallel to the sheet silicates, causing a |
slatey cleavage |
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slatey cleavage generally forms on planes that follow planes perpendicular to: |
maximum stress direction |
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The size of the mineral grains tends to enlarge withincreasing grade of metamorphism. Eventually the rockdevelops a near planar foliation caused by the preferredorientation of mica. |
schist |
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As metamorphic grade increases, the sheet silicatesbecome unstable and dark colored minerals like hornblende andpyroxene start to grow. These dark colored minerals tend tobecome segregated in distinct bands through the rock, giving therock a |
gneissic banding |
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in schist & gneiss, the lighter colored bands consist of what minerals? |
quartz & feldspars |
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At the highest grades of metamorphism all of thehydrous minerals and sheet silicates become unstable and thusthere are few minerals present that would show a preferredorientation. The resulting rock will have a _______ ________ that is similar to a phaneritic texture in igneous rocks. |
granulitic texture |
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marble |
Since limestonesare made up of essentiallyone mineral, Calcite, & calcite is stable over a widerange of temperature andpressure, metamorphism oflimestone only causes theoriginal calcite crystals togrow larger. Since no sheet silicates arepresent the resulting rock, amarble, does not showfoliation. |
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marble is a metamorphic rock consisting of large crystals of |
calcite |
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Metamorphism ofsandstone originally containingonly quartz, results inrecrystallization & growth of thequartz, producing a non foliatedrock called a |
quartzite |
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types of metamorphism |
dynamic (cataclastic) contact regional |
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This type of metamorphism isdue to mechanical deformation, like when two bodies ofrock slide past one another along a fault zone. |
cataclastic metamorphism |
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Occurs adjacent to igneous intrusions & results from high temperatures associated with the igneousintrusion, forming a metamorphic aureole. |
contact metamorphism |
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high temperature-low pressure favors ______ metamorphism |
contact |
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This type of metamorphismoccurs over large areas thatwere subjected to high degreesof deformation underdifferential stress. Thus, it usually results informing metamorphic rocksthat are strongly foliated, suchas slates, schists, & gniesses. The differential stress usuallyresults from tectonic forcesthat produce a compression ofthe rocks, such as when twocontinental masses collidewith one another. |
regional metamorphism |
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Mineral Assemblages that reflectphysical conditions in which they formed. |
metamorphic facies |
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Blueschist facies high pressure-low temperature metamorphism occurs where? |
in the crust during mountain building |
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what is a radiometric date? |
radiometric dates give the time a mineral began to preserveall atoms of parent-and-daughter isotopes. (requires cooling below a "closure temperature") (If rock is reheated, the radiometric clock can be reset) Igneous/metamorphic rocks are best for geochronologic work.Sedimentary rocks cannot be directly dated. |
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half life (t1/2) |
time for half of the unstable nuclei to decay (the half-life is a characteristic of each isotope) (after one t1/2 one-half of the original parent remains) (after three t1/2 one-eighth of the original parent remains) as the parent disappears, the daughter increases. |
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assumptions of radiometric dating... |
-the rock or mineral must be a "closed system" we must be able to accurately determine a value for the initial daughter atoms if they were present in mineral or rock sample being dated. -the value of the decay constant (λ) must be known accurately. -the measurements of the parent & daughter atoms must be accurate & representative of the rock or mineral to be dated. |
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what is a fundamental requirement of radiometric dating? |
the parent & daughter isotopes remain locked within the sample, & are not able to move into or out of the system. (these conditions are typically best met in igneous rocks, where the crystallization of the mineral locks the radioactive isotopes into the crystal lattice & starts the "clock" |
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the radiometric age will give the time or the _____________, rather than the age of the original rock. |
metamorphism; metamorphism may allow the diffusion of material, essentially resetting the "clock" |
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how old are the oldest rocks on earth's surface? |
- 4.03 Ga (btw Ga means one billion years)
zircons in ancient sandstones date up to 4.4 Ga - age of earth is 4.54 Ga based on correlation with: - meteorites - moon rocks |
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numerical ages are possible without _________. |
isotopes; - growth rings - annual layers from trees or shells - rhythmic layering - annual layers in sediments or ice |
