Geology 101: Test #2 – Flashcards
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What is the average rate at which temperature increases with depth in the Earth's crust? a. 10°C/km b. 30°C/km c. 100°C/km d. 300°C/km
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B
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Metamorphism:
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change 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
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Main factors driving metamorphism:
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1. internal heat of the Earth 2. Earth's pressure 3. Earth's fluids
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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 metamorphism)
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Role of Pressure
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- Greater pressure (burial, collision, etc.) - confining pressure) tends to decrease the space available for mineral growth; density - Stress causes preferred orientation of minerals - differential stress (directed pressure)
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Role of Temp
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Heat: greatly affects a rock's mineralogy and texture; increases mobility of elements Geothermal gradient: measure of variation of temperature with depth (average 30°C/km)
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Role of Fluids
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Fluids (H2O): act as a catalyst during metamorphism - aids the exchange of ions between growing crystals
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High metamorphic grade:
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High temperature and pressure
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Low metamorphic grade
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low temp and pressure
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The Amount of Change During Metamorphism Depends on:
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- The grade of metamorphism - The duration of metamorphism - The composition of the rock
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Directed stress will orient minerals in these two ways:
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1. Lineation 2. Foliation (a set of flat or wavy parallel planes produced by deformation)
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Metamorphic rocks result from the recrystallization of existing rocks while they are still in a solid state. a. True b. False
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a. True
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Stability of minerals
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Most minerals are stable over a relatively narrow range of pressure and temperature (e.g., ice unstable above 0°C) The stability range of different minerals sometimes overlap and provide insights into the metamorphic history of rocks
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Metamorphic Facies
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Definition: a set of metamorphic mineral assemblages that were formed under similar pressures and temperatures. - Each facies has certain minerals that are indicative of pressure and temperature. - The minerals in a rock can therefore be clues to the (pressure and temperature) history of the rock
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Exhumation
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Subducted light metamorphic rocks are driven upward by buoyancy
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Which of the following rocks represents the highest metamorphic grade? a. Gneiss b. Phyllite c. Schist d. Slate
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a. Gneiss
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The parent rock of a quartzite is __________. a. shale b. granite c. sandstone d. limestone
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c. Sandstone
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Which mineral is commonly used to determine a metamorphic P-T path? a. Calcite b. Garnet c. Muscovite d. Quartz
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b. Garnet
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Geochronology
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- the study of time in relation to the Earth. (Great "moments" in Earth history lasted millions or tens of millions of years.)
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Historical Geology
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- branch of Geosciences that deals with the Earth's past.
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The two ways to Date Geological events
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Relative Dating: how old a rock is compared to surrounding rocks Absolute dating: actual number of years since the rock formed
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Principle of Superposition
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In a sequence of undisturbed layered rocks, the oldest 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 (parallel to the Earth's surface)
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Fossils
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Consist of the remains of ancient organisms, or other evidence of their existence. (found primarily in sedimentary rocks)
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Paleontology
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The study of life in the past based on the fossil remains of plants and animals
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Fossils that are preserved in sedimentary rocks are used to determine
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1. relative age 2. environment of deposition
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John Drayton
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was one of the first to correlate sedimentary beds regionally using fossils. - founded the USC's department of geology in 1801
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Unconformities
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Surfaces where erosion has removed rock layers. "Missing time"
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Cross-cutting Relationships
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The geometry of rock bodies, and their relationship with adjacent bodies, allows geologists to place rock units in relative chronological order - used for relative dating
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Which of the following is the best statement of the principle of original horizontality? a. Igneous intrusions form horizontal layers b. Metamorphic isograds are horizontal before deformation c. Sediments are deposited as horizontal layers d. Most igneous, metamorphic, and sedimentary rocks in the Earth's crust form horizontal layers
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c. Sediments are deposited as horizontal layers
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Sequence Stratigraphy
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Stratigraphic analysis in which the major geologic units are bound by unconformities (Used widely with seismic data, especially in petroleum exploration)
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Geologic Timescale
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Divisions in the worldwide stratigraphic column based on variations in preserved fossils
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Absolute Geochronology
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Add numbers to the stratigraphic column which was originally based on fossils and correlation - Based on the regular radioactive decay of some chemical elements
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Isotopes
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atoms of the same element with different numbers of neutrons
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Ions
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When an atom loses or gains an electron to or from another atom
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anions
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Negatively charged ions (gain of electron)
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cations
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Positively charged ions (loss of electron)
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isotopes
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Atoms of elements with the same number of protons and varying numbers of neutrons
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Isotopic Dating
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Radioactive elements (parents) decay to stable, non-radioactive elements (daughters) - rate of decay is constant and known
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Half-life
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The half-life of a radioactive isotope is defined as the time required for half of it to decay to the daughter product - rate is constant
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The study of the layers in sedimentary rock is known as _________. a. Stratigraphy b. Paleontology c. Sedimentation d. Geochronology
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a. Stratigraphy
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The principle of superposition states that _______________. a. a fault is younger than the rocks it cuts b. undisturbed sedimentary layers get progressively younger from bottom to top c. the present is the key to the past d. sediments are deposited as essentially horizontal layers
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b. undisturbed sedimentary layers get progressively younger from bottom to top
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Deform -
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change volume or shape of rock by
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stress and the three types
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-force applied to rock Compressional -squeezed Tensional - pull apart rock Shear - sideways slip on parallel planes.
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Dip:
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angle of steepest descent of the bed from the horizontal
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Strike:
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at right angles to the dip direction
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Types of Deformation
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Elastic Ductile (plastic) Brittle (rupture)
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Elastic deformation
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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) deformation
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permanent change in shape or size of a body that is not recovered when the stress is removed
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Brittle (rupture) deformation
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the body undergoes little change under the stress, until it breaks suddenly (earthquakes)
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Stregnth
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Ability of an object to resist deformation (Compressive or tensile)
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Ductile Deformation
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Ductile materials undergo smooth, continuous plastic deformation and do not spring back to their original shape when the deforming force is released
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Symmetrical folds -
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vertical axial planes
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Asymmetrical folds
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inclined axial planes
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Folds
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caused by compression - permanent, plastic deformation.
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Anticline -
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up-arched fold (oldest strata in middle).
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Syncline -
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down-arched fold (youngest strata in middle).
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Monocline
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a bend in flat or uniformly dipping layers.
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Overturned fold
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both limbs dip the same direction.
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Where does most of the worlds petroleum come from?
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Most of world's petroleum and natural gas production comes from anticlinal traps.
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domes
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circular anticlines
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basins
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circular synclines
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Joints
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- fractures with no displacement.
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Faults -
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fractures with movement along fracture (fault plane).
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Reverse faults -
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compression, hanging wall up
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Normal faults
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- extension, hanging wall down.
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Strike-slip
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shear, slide sideways. - Left-lateral - opposite block moves left. - Right-lateral - opposite block moves right (San Andreas fault).
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The dip of a layer in a geologic formation can vary between ___________. A) 0 - 45 degrees B) 0 - 180 degrees C) 0 - 90 degrees D) 0 - 360 degrees
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C
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Lithosphere floats on a ______________________.
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viscous layer below the continents, the asthenosphere, within the Earth's mantle
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Archean cratons
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granite-greenstone terrains high-grade metamorphic terrains
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Continental Shields
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• Central, oldest rocks in continents (up to 3.96 byo) • Low elevation and relatively flat • "Crystalline basement" metamorphic and igneous rocks • once tectonically active
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Canadian Shield
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oldest rocks of N. America
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Stable Platforms
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- Shields covered with horizontal sedimentary rocks - Sandstones, limestones, and shales deposited in ancient shallow seas
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How Continents Grow
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Magmatic differentiation Continental accretion
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Magmatic differentiation
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magma transferred from mantle to continents at subduction zones
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Continental accretion:
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buoyant fragments of crust attached to continents as the result of plate motions - amalgamation or accretion of terranes - exotic terranes (seem out-of-place)
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Orogeny
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- A general term for mountain-building processes - Folding and thrusting of rock layers - Often accompanied by magmatic activity and metamorphism
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Divergent boundaries
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- Oceanic spreading centers - world's longest mountain ranges. - Continental rifts - E. African Rift, Basin & Range - US
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Convergent boundaries
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- Ocean - ocean boundaries - Island arcs - Japan, Philippines, Carolina slate belt. - Ocean - continent boundaries. Volcanic arc - Andes.
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• What mountain chain runs along the eastern margin of North America? A) the Andes B) the Appalachians C) the Caledonides D) the Cordillera
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B) the Appalachians
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Continents are made up of: A. Ancient granitic cratons B. Island arcs accreted to the edges of the cratons C. Exotic terranes (microplates) D. All of the above
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D
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The Carolina Piedmont is made of: A. Passive margin beach sands B. Island arc rocks accreted to North America C. The ancient drowned land of Atlantis
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B
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Elastic Rebound: Explains Why Earthquakes Occur
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Stress builds up in rocks until it exceeds the strength of the rocks and the rocks break Elastic deformation is expressed in rocks between earthquakes Elastic rebound: return of rocks to original shape after an earthquake
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Earthquake
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vibrations of the Earth caused by the sudden release of energy as rocks move along fractures or faults.
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ELASTIC REBOUND THEORY
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Energy is stored in rocks as they bend. When rock strength is exceeded, they break and snap back or "rebound" = earthquake.
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Slip
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the amount of ground displacement in an earthquake
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Body waves
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P-waves (Primary waves) - fastest, compressional, like sound waves. S-waves (Secondary waves) - shear waves, can not travel through liquids or gases.
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Seismograph
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instrument that records and measures earthquake vibrations. -A seismogram shows the varying amplitude of ground oscillations beneath the seismograph Determine time, location, and magnitude
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Locating Earthquakes
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- Seismic waves from an earthquake move out concentrically from the focus and arrive at distant seismographic stations at different times. - At least 3 recording stations are necessary to determine the location of an epicenter
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Intensity
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is a measure of the damage done by an earthquake.
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Magnitude
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the amount of energy released by an earthquake.
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Logarithmic scale:
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Amplitude of magnitude 6 wave is 10 X larger than a magnitude 5
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Distribution of earthquakes
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Most earthquakes occur at plate boundaries. The largest occur at convergent boundaries.
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Destructive effects of earthquakes
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- Ground shaking - Tsunami(seismic waves) - fire - ground failure (landslides and liquification)
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Earthquake Prediction
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Seismic risk maps ( based on distribution and intensity of past earthquakes.) Long-range prediction (regulate zoning and building codes saves lives and money.)
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Short-term prediction of earthquakes
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- Precursor earthquakes - Land deformation - Water level in wells - Magnetic field - Electrical resistance of ground - Animal behavior - Radon gas - Low frequency radio waves
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The size of a magnitude 8 earthquake is how much larger than that of a magnitude 6 earthquake? 1 x 10 x 100 x 1000 x
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100x
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Earthquakes that originate at depths greater than 100 km are associated with ______ plate boundaries. a. Convergent b. Divergent c. Transform d. Convergent, divergent, and transform
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a. Convergent
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Structures built on what suffer the most damage in an earthquake? a. Wet mud or sand b. Granite c. Bedrock d. Basalt
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a. Wet mud or sand
