geology exam 2 USC – Flashcards
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which mineral is commonly used to determine a metamorphic P-T path?
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Garnet
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metamorphism
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changes affected by pressure, heat, and water that results in a more compact and more highly crystalline condition
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metamorphic rock
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rock that has undergone changes in texture, mineralogical, or/and chemical composition in the solid state (10-30 km in the Earths crust)
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how thick is the thickest crust?
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78 km
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how do rocks metamorphose?
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through partial or complete recrystallization of minerals in the rocks over long periods of time -rocks remain in solid state during metamporhism -matrix can melt due to fluids that lower melting temperature
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what is the role of pressure?
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greater pressure tends to decrease the space available for mineral growth; density -metamorphic minerals created at high pressure tend to be more dense -confining pressure is a general force applied equally in all directions -directed pressure is a force exerted in a particular direction -increases with depth almost steadly
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what is the role of temperature?
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-heat: greatly affects a rocks mineralogy and texture -geothermal gradient: measure of variation of temperature with depth (average 30 degrees Celsius/km) -increases at different rates in different regions of earth
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what are the 3 main factors driving metamorphism?
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-internal heat of the earth -earths pressure -earths fluid
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which of the following tectonic settings will be the coolest at 30-km depth?
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stable continental lithosphere
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prograde
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mineral changes that take place during an increase in temperature
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retrograde
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mineral changes that take place during a decrease in temperature
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what is the role of fluids?
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-fluids (H2O) act as a catalyst during metamorphism -aid the exchange of ions between growing crystals metasomatism: change in a rocks bulk composition by the fluid transport of chemical substances in or out of a rock -ions and atoms dissolved in the fluid can migrate through a rock and react with solids to form a new mineral
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water in rocks
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-all rocks characteristically contain water -in some minerals water is part of their chemical structure, like amphibole or mica
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high grade
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high temperature and pressure
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low grade
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low temperature and pressure
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regional
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Widespread changes in temperature and pressure bring changes in rocks due to tectonic forces. takes place at moderate to deep levels under moderate to ultra-high pressures and high temperatures. Usually related to continental arcs like the Andes.
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Regional high-pressure metamorphism
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along linear belts of volcanic arcs (high pressures and high to medium-temperatures
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contact
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Intrusion of magma against colder rocks. It usually occurs at high temperature, and low pressure, unless it takes place deep in the crust.
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seafloor(metasomatism)
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Changes in rocks at the Mid-ocean ridges associated with chemical reactions promoted by the infiltration of heated seawater. High temperatures, low presures.
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burial (low-grade)
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Changes in a rock due to the gradual changes in temperature and pressure due to successive burial.
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shock
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Changes due to rapid increase in pressure (localized only ). E.g. meteorite impacts can metamorphose rocks immediately around the impact site. Low temperature, high pressure.
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what does the amount of change during metamorphism depend on?
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-the grade of metamorphism -the duration of metamorphism -the composition of the rock
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what is classification based on?
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texture
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foliation
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a set of flat or wavy parallel planes produced by deformation
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order of foliation from lowest to highest
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1. slate 2. phylitte 3. schist 4. gneis 5. migmatite
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stability of minerals
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-most minerals are stable over a relatively narrow range of pressure and temp (ice is unstable above 0 degrees celsius) -the stability range of diff minerals sometimes overlap and provide insights into the metamorphic history of rocks
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where does metamorphism occur?
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-plate interiors -divergent plate margins -convergent plate margins -transform plate margins
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which is the type of metamorphism with the highest pressure and lowest temperature
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shock
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phylite is a rock of which type of metamorphism
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low grade
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outcrop
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exposure of bed rock without being obscured by soil or loose sediment boulders
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folds
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imply that an original planer structure such as sedimentary bed, has been bent -most typically found in mountain belts
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fault
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a fracture in earths crust resulting from the displacement of one with respect to the other -involves relative movement of geologic layers
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how do rocks deform?
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stress- force per unit applied to a rock strain-change in shape and/or volume of a rock body as a result of applied stress strength-ability of an object to resist deformation
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types of stress?
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compression- pushed together tension-pulled apart shearing- moving sideways
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compression
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action of oppositely directed forces acting towards each other
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tension
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acting away from each other
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shear
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action of coinciding and oppositely directed forces acting parallel to each other across a surface
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strain
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any change in original shape or size of an object in response to stress acting on the object
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strength
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ability of an object to resist deformation
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structural geology
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the study of the deformation of rocks and its effects
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how do we measure deformation?
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-many rocks occur naturally in layers: sedimentary, lava flows, metamorphic -many features within rock bodies are either planar or linear: minerals, foliation
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dip
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the angle of steepest descent of the bed from the horizontal
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strike
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line at the right angles to the dip direction
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elastic
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temporary change in shape or size of a body that is recovered when the stress is removed
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ductile(plastic)
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permanent change in shape or size of a body that is not recovered when the stress is removed (smooth, continuous plastic deformation)
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brittle(rupture)
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the body undergoes little change under the stress, until it breaks suddenly (earthquakes)
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rock deformation
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-deep crustal rocks are more likely to defrom ductilely than shallow crustal rocks -hotter rocks are more likely to deform ductitlely -most sedimentary rocks are more deformable than igneous rocks
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normal fault
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-the rocks above the fault plane move down in relation to the rocks below the fault plane, causing an extension -results from tension
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reverse fault
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-the rocks above the fault plane move upward in relation causing a shortening -results from compression
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strike slip fault
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the blocks move horizontally, sliding past each other -results from shearing -san adreas fault
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thrust faults
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low angle reverse faults
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what type of material undergoes smooth, continuous deformation
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ductile
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geologic maps
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represents the rock formations exposed at the earths surface
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common scale
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-1:24000 -1 cm on map is 240 meters -eliminate 2 zeros
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geologic cross sections
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diagrams showing the features that would be visible if vertical slices were made through part of the crust
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upfolds
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arches of layered rocks called anticlines -a sequence of folded rocks with the oldest rocks inside of the fold
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downfolds
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throughs called synclines -a sequence of folded rocks with the youngest rocks on the inside of the fold
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types of folds
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-symmetrical folds(anticlines and synclines) -asymmetrical folds -overturned folds -plunging folds
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an anticline is a sequence of folded rocks with the youngest rocks on the inside of the fold
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false
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factors that affect deformation
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-temp -pressure -rock type
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competent rocks
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rocks that deform only under great stresses
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incompetent rocks
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rocks that deform under moderate to low stresses
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joints
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-fractures in rocks along which there has been no appreciable displacement -rocks on either side of a fault have moved, whereas rocks on either side of a joint have remained stationary
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reverse faults form in response to
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compressive
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the study of layers in sedimentary rocks is known as
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stratigraphy
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geochronology
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the study of time in relation to the earth
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relative dating
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-fossils, structure, cross-cutting relationships -how old rock is compared to surrounding rocks
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absolute dating
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-isotopic, tree rings -actual number of years since the rock was formed
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principle of superposition
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-in a sequence of undisturbed layered rocks, the older rocks are on the bottom
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principle of original horizontality
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-layered strata are deposited horizontally or nearly horizontally or nearly parallel to the earths surface
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fossil
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comes from the latin, fossilus meaning "something dug up" -consists of remains of ancient organisms or other evidence of their existence -found primarily in sedimentary rocks
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paleontology
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-study of life in the past based on the fossil remains of plants and animals -fossils that are preserved in sedimentary rocks are used to determine 1. relative age 2. the environment of depostion
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correlation
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-no single area has a complete record of earth
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unconformities
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-surfaces where erosion has removed rock layers -"missing time"
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types of unconformities
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-nonconformity -angular unconformity -disconformity
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nonconformity
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-sedimentary layers deposited on older igneous and metamorphic rocks
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angular unconformity
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younger sedimentary layers deposited on older, tilted sedimentary rocks
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disconformity
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younger sedimentary layers deposited on older, horizontal sedimentary layers
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cross cutting relationships
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geometry of rock bodies that allows geologist to place rock units in relative chronological order -used for relative dating
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what is the relative timing between faulting and intrusion of granite
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-the relative timing between faulting and intrusion of the granite cannot be determined from the info provided
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sequence stratigraphy
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-stratigraphic analysis in which the major geologic units are unconformities
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the principle of superposition states that
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-undisturbed sedimentary layers get progressively younger from bottom to top
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absolute age
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-geologic event in the rock record is the number of years that has elapsed between the event and the present day
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isotopic dating
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-radioactive elements (parents) decay to stable, non radioactive elements (daughters) -the rate at which decay occurs is constant and known
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isotopic dating methods
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-uranium-lead -potassium-argon -rubidium-strontium -carbon-nitrogen
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the half-life of a radioactive isotope is
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the time it takes for half of the radioactive atoms to decay
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what is the order of geologic time from largest to shortest?
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eons, eras, periods, epochs
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which of the following is the largest division of geologic time?
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eon
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under intense pressure and high temp, hydrogen atoms combine to form helium. what is this process?
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nuclear fusion
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nuclear fission
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splitting of an atom into two or more smaller ones
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nuclear fusion
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the fusing of two or more smaller atoms into a larger one
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big bang theory
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started 14 billion years ago
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nebular hypothesis
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-formation of the solar system -due to gravity the mass started to drift toward the center creating the proto-sun -compressed under its own weight the material in the proto-sun became hot and dense
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inner planets
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rocks and metals
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outer planets
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ice and gases
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definition of a planet
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-in orbit around the sun -has sufficient mass so that it assumes a hydrostatic equilibrium -has "cleared the neighborhood" around its orbit
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continents
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solidified magma that floated up from the mantle
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oceans and atmospheres
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fluid outer layer derived from volatile transfer of gases from the interior
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what caused dust and condensing material to accurate into planetesimals
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gravitational attraction and collisions
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volatile transfer
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photosynthesis by microorganisms removed CO2 and added oxygen to the primitive atmosphere
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the four inner planets are
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-small, rocky, and relatively lacking in volatile elements ----- venus, mars, earth, mercury
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what mountain chain runs along the eastern margin of north america?
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the Appalachians
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nature of continents
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-continents are made and deformed by plate motions -continental rocks are generally older than oceanic rocks -lithosphere floats on viscous layer below the continents, the asthenosphere
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continental lithosphere
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-continental crust: sedimentary cover and crystalline basement -uppermost mantle
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stable cratons
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-shields -platforms -oldest part of the continent
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orogeny
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-general term for mountain building processes -folding and thrusting of rock layers
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continental shields
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-oldest rocks come from Australian shield -canadien shield: oldest rock in n america -low elevation and relatively flat -not covered by sediments: only "crystalline basement" at surface
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stable platforms
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-shields covered with a sequence of horizontal sedimentary rocks -sandstones, limestones, and shales deposited in ancient shallow seas -many transgressions and regressions caused by changes in spreading rate
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continental keel
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-older they are, the thicker they are -strong, cool, thick
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folded mountain belts
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-relatively narrow zones of folder rocks with associated margins -formed at convergent plate boundaires -two major active belts: cordillera(Rockies-andes) and alps-himalaya
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what is meant by the tectonic age of a region
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the youngest major deformation event in the region
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how do continents grow?
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-magmatic differentiation: magma transferred to continents at subduction zones -continental accretion: buoyant fragments of crust attached to continents as a result of plate motions
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how continents are modified
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-orogeny -epeirogeny: vertical motions of largely flat-lying rocks without faulting or significant folding
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the wilson cycle
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1. continental rifting 2. continent-continent collision 3. subduction 4. continental rifting
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what is metasomatism?
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a change in the bulk composition of a rock during metamorphism
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what type of metamorphism is caused by igneous intrusions
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contact metamorphism
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which if the following rocks represents the highest metamorphic grade?
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gneiss
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which of the following processes will cause metamorphism?
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increase in pressure and temp -interaction with hydrothermal fluids
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at what type of plate tectonic boundary would you expect shearing forces to occur?
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transform
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what determines whether a rock deforms in a brittle fashion versus a ductile fashion?
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temperature, rate of deformation and rock type
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which of the following is used by geologists to determine the relative ages in a rock sequence?
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cross cutting relationships, fossils, and stratigraphy
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which is the most likely parent rock of marble?
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limestone
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purely tensional stresses associated with a _____ plate boundary will generate a _____ fault
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divergent; normal
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which is older, the intrusion or the fault?
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intrustion
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the horizontal line labeled X-X' is an
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angular unconformity
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which principle did you use to identify the oldest rock unit?
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superpostion
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when considering the effects of faulting on the lithosphere; it is clear that normal faults serve to effectively:
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thin and extend crust
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when did the mesozoic era begin?
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250 million years ago
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which planet is closest in size to the earth?
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venus
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current evidence indicates that the moon was produced by
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the collision of a mars-sized body into earth
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which list places the four inner planets in order of increasing distance from the sun?
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mercury, venus, earth, mars
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cratons are made up of?
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platforms and shields
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the study of geobiology is defined as
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the study of how organisms influence and are influenced by earths environment
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the radiation of life at the beginning of the phanerozoic is known as that
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cambrian explosion
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ecosystems
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composed of organisms and geologic components that function in a balanced, interactive way
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organisms that gain energy from feeding directly or indirectly on "producers" are referred to as
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heterotrophs
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what life is made of:
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-carbon(the building block of life) -nutrients -water -energy
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processes and outputs
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how organisms live and grow
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metabolism
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the process that all organisms use to convert inputs to outputs -photosynthesis -respiration
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biogeochemical cycle
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-a pathway through which a chemical element or molecule moves between biologic and environmental components of an ecosystem
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the sulfur cycle
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-Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids. -tectonic processes uplift rocks, and weathering breaks down sulfur-bearing minerals -humans burn fossil fuels giving over sulfur compounds
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microbes
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-single celled organisms including bacteria, some fungi and algae, and protozoa -most genetically diverse group -can grow in hostile enviroments -most important for biology
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universal tree of life
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-the hierarchy of ancestors and descendants of all life on earth -bacteria -archaea -eukaryote
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stromatolites
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sedimentary features that form from the interaction of microbes with their environment
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in which geologic environment would you expect to find thermophiles?
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-mid ocean ridge (divergent)
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ciento bacteria
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started putting oxygen in atmoshphere
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photosynthesis arose in a group of bacteria known as
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cyanobacteria
