24: Day Geologic Cycle
Geologic Time Scale * Major Divisions: 1. eons: based on evidence of life in the geologic record (Precambrian, Phanerozoic) 2. eras: based on major worldwide changes in life forms; major extinctions Paleozoic: "age of fishes" Mesozoic: "age of reptiles" Cenozoic: "age of mammals 3. periods: based on lesser changes in life forms Geologic Cycle * all endogenic and exogenic processes that cause changes in the Earth's lithosphere * consists of the following cycles: 1. hydrologic cycle 2. rock cycle 3. tectonic cycle Rock Cycle * formation and transformation of the materials that comprise the Earth's lithosphere * mineral: naturally-occurring, inorganic solid, possessing a specific chemistry and a specific atomic structure ---silicates: minerals containing silicon and oxygen; most abundant group of minerals at the Earth's surface ---polymorphs: minerals having the same chemistry, but different properties (ex. diamond and graphite) rock: aggregate of minerals The rock cycle describes how different classes of rocks form and change (igneous ---- sedimentary ---- metamorphic)
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Igneous Rocks * rocks formed via cooling and solidification of molten rock * molten rock originates deep within the Earth's interior due to radioactive decay and friction * 2 divisions: 1. intrusive ("plutonic"): igneous rocks formed below the surface = within the Earth's interior, molten rock is insulated by the surrounding rock, resulting in slow cooling = slow cooling allows for large mineral crystals to form in the solidifying rock mass = intrusive rocks have coarse-grained textures (ex. granite) 2. extrusive ("volcanic"): igneous rocks that form at the surface = extrusive rocks form from rapid cooling/solidification of molten rock = rapid cooling does not allow time for large mineral crystals to form = typical textures are glassy, vesicular and fine-grained (ex. Basalt)
Day 25: Rock Cycle (continued)
Igneous Rocks (continued) * magma: molten rock existing beneath the surface; contains abundance of dissolved gases; formation of intrusive igneous rocks lava: molten rock existing on the surface; lower content of dissolved gases; formation of extrusive igneous rocks classifications based on chemistry: 1. felsic: rich in silicon and oxygen; light-colored; low-density 2. mafic: rich in iron and magnesium; dark-colored; high-density Sedimentary Rocks * rocks that form via lithification of sediments and/or organic materials * * lithification: compaction, cementation, and hardening of sediments to form solid rock processes of weathering and erosion wear down the rocks and organics at the surface to form sediments most abundant rocks at the surface sedimentary rocks are the least resistant to weathering and erosion fossils and fossil fuels are generally found in sedimentary rock structures. (exception: anthracite coal = metamorphic rock) Divisions: 1. detrital (clastic): composed of solid rock fragments that have been lithified (ex. conglomerate and sandstone) 2. chemical and biochemical: minerals that were derived from solutions, or lithified organic remains (ex. limestone and coal)
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Metamorphic Rocks * * rocks that have been altered due to excessive heat and pressure parent rock: the original rock that existed before metamorphism (ex. gneiss........parent rock is often granite) Divisions: 1. foliated: metamorphic rocks exhibiting a crude layered or banded structure (ex. gneiss) 2. nonfoliated: mass of interlocking crystals (ex. marble)
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Day 26: Tectonic Cycle Earth's Layers
Metamorphic Rocks (continued) * Metamorphic Environments 1. Regional = large segments of Earth's crust subjected to intense heat and pressure in association with mountain building = formation of the greatest volumes of metamorphic rocks 2. Contact (Thermal) = zones of rock are altered due to intense heat around an intruding body of magma 3. Hydrothermal = rocks altered as hot, ion-rich fluids circulate through fissures and cracks that develop in rock = often closely associated with igneous activity that leads to contact metamorphism 4. Burial = occurs as sedimentary rock layers become buried by thick accumulations of sediment = low-grade metamorphism caused by increased confining pressure 5. Fault Zones (Dynamic) = shallow zones: rocks are crushed and acted upon by groundwater action = deep zones: rocks acted upon by high temperatures and minerals are elongated as large slabs of rock move in opposite directions 6. Impact (Shock) = energy from rapidly-moving meteorite or asteroid is transformed into heat energy and shock waves that pass through surrounding rocks = rocks are pulverized, shattered and sometimes melted
Tectonic Cycle
* endogenic forces reshaping the Earth's lithosphere from within * build-up of the landscape * movement of Earth's layers * knowledge of Earth's layered structure comes from analysis of seismic waves Earth's Internal Layers 1. Core: innermost Earth; one-third of Earth's total mass * inner core: solid iron and nickel * outer core: liquid iron and nickel; responsible for 90% of Earth's magnetic field 2. Mantle: most extensive portion of Earth's structure (approx. 80% of total volume) * Gutenberg Discontinuity: zone of transition between the outer core and the mantle * Asthenosphere: "plastic" layer of the upper mantle; not truly liquid, but still capable of slow movement or flow * Lithosphere: rigid, brittle layer comprised of the upper mantle and crust; fractured into large sections that move in response to movement of the asthenosphere below 3. Crust: outermost layer; thinnest layer consisting of rigid, brittle rock * Mohorovicic Discontinuity: zone of transition between the mantle and the crust; based on rock density * continental crust: low-density rock, rich in silicate mineral * oceanic crust: high-density rock, rich in iron and magnesium * isostasy: concept of Earth's crust "floating" in a gravitational balance atop the mantle
Day 27: Continental Drift Theory and Plate Tectonics
* * * * CONCEPT PROPOSED BY ALFRED WEGENER IN THE EARLY 1900S The earth's land masses were once joined in a single "supercontinent" that Wegener called Pangaea Approximately 200 million years ago, Pangaea began to break apart; the continents moved into the locations that are recognized today Continental Drift states that Earth's continents are "drifting" within the ocean basins
Evidence for Continental Drift * * Wegener provided an abundance of geological evidence supporting his concept of Pangaea and continental drift some of his evidence: 1. fit of the continents 2. fossil associations between the continents 3. structural associations between the continents 4. glacial evidence * * * Wegener provided no explanation for a driving force behind continental drift Wegener had no way of analyzing the sea floor; he only looked at the Earth's continents as being in motion The scientific community was locked in debate until the 1950s
Plate Tectonics * * * * advances in oceanography in the 1950s led to detailed analysis of the sea floor revival of Wegener's ideas of the Earth's moving continents new showed evidence the entire Earth's lithosphere is divided into a number of large sections, or "plates", that are in motion by the 1960s, the original concept of continental drift was refined into Plate Tectonics
Day 28: Plate Tectonics
Sea Floor Spreading and Plate Tectonics * * * evidence of sea floor spreading led to the idea of plate tectonics spreading center: boundary along which the sea floor is being forced apart (divergence on the sea floor) evidence of spreading centers: a) increasing rock ages away from spreading center b) geomagnetic reversals indicated in rocks on the sea floor * * * * spreading centers are created due to magma upwelling from Earth's interior supporting evidence of upwelling in the form of pillow lavas found along spreading centers upwelling magma indicates convection within Earth's interior; the driving force behind plate tectonics Earth's internal convection is poorly understood; several key models
Plate Boundaries Divergent: plates are moving apart from each other; spreading center * rift valley * mid-ocean ridge
Convergent: plates are colliding; moving toward each other * continental-oceanic: = subduction of oceanic plate = deep ocean trench offshore; volcanic mountains onshore * oceanic-oceanic: = subduction of oceanic plate = deep ocean trench with adjacent chain of volcanic islands continental-continental: = no subduction; crust is warped and buckled into a large area of high mountains
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Transform: plates move laterally past one another; horizontal displacement of structures along boundary
Day 29: Crustal Deformation
* * Changes in the form of the crust due to excessive stress types of stress: a) compressional b) tensional c) shear deformational structures: 1. folds: rock structures that have been warped, or bent due to compressional stress = synclines: troughs, or downwarping, in the rock structure = anticlines: ridges, or upwarping, in the rock structure 2. faults: fractures in the crust along which movement periodically occurs a) normal: tensional stress b) reverse: compressional stress c) strike-slip: shear stress crustal deformation is driven by the same tectonic forces behind plate tectonics results in the build-up of landforms
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Earthquakes * sudden release of stress along a fault * * * focus: point of release of built-up stress; occurs deep within the lithosphere epicenter: point on the surface directly above the focus elastic rebound: the concept that rock units store huge amounts of energy = rocks change in volume and shape = rocks "snap back" to their original form when stress is released * seismic waves: shock waves released during an earthquake 1. body waves: waves that travel through Earth's interior a) primary ("P-waves"): compressional waves; fastest of all seismic waves that travel through solid and liquid rock b) secondary ("S-waves"): rock units are displaced away from the path of the wave; absorbed in liquid materials 2. surface waves: damaging waves moving across the surface * Measurement and Classification * seismometer: device used to detect and record seismic waves; printed time record of seismic activity that is produced is called a seismograph * Modified Mercalli Scale = used to classify earthquake intensity = ranges I-XII, base on damage done * Richter Scale = used to classify earthquake magnitude = ranges 1-10, based on the amplitude of seismic waves from a seismograph
Day 30: Earthquakes (continued)
* Hazards 1. initial quake 2. aftershocks: weaker quakes following the initial quake; caused by further adjustments of rock units along the fault = weakened structures may collapse, resulting in further injuries, loss of life and hampering rescue efforts 3. fire: gas lines rupture, downed power lines ignite fires 4. tsunami: seismic sea waves caused by an undersea earthquake = open ocean: tsunami has extremely long wavelength but very low amplitude (height); travels extremely fast (400-600 mph) = near shore: friction with sea floor causes wave to slow down and grow in height (possibly 100+ feet) = travel great distances 5. landslide and land subsidence = landslide: sudden slope failure; rock and soil break loose and move downslope due to seismic activity = land subsidence: sinking of the land surface; depends on the magnitude and the surface and subsurface materials 6. disruption of infrastructure = utilities, transportation arteries, communications, etc. are damaged or destroyed during the quake = causes problems with extinguishing fires, rescue efforts, etc. * Earthquake Distribution = most earthquakes occur along plate boundaries; greatest amount of stress builds near plate boundaries due to internal convection = many active faults are located far from major plate boundaries; potential for earthquakes * Prediction and Preparedness = seismic risk analysis: determination of the potential of an earthquake occurring in a given area a) seismic gap theory: sections of a given fault that have been inactive have the greatest potential of a quake b) monitoring precursory phenomena: changes that occur in the rock structures, water table, etc. prior to a quake = land use policies, building codes/designs, etc. are affected by seismic risk analysis
Day 31: Volcanoes
* * * Landform resulting from the accumulation of rock material ejected from Earth's interior Volcanoes vary in shape, size and nature of eruption General Types of Eruptions 1. explosive: violent, sudden blast from within the volcano 2. effusive: lava freely flowing onto the surface; possibly lava fountains * Types of Ejected Materials A) lava: molten rock = viscosity: a liquid's resistance to flow; a big factor in the eruptive characteristics of a volcano B) pyroclastics: solid rock fragments; often associated with explosive eruptions * pyroclastic flow: cloud of pyroclastics and superheated gases C) toxic gases: various harmful gases are released from molten rock as a volcano erupts * Types of Volcanoes 1. composite cone ("stratovolcano") = tall, steep-sided volcano; composed of layers of high-viscosity lava and pyroclastics = often explosive in nature = examples include Mount St. Helens, Mount Pinatubo (Philippines), Krakatau (Indonesia) and Mt. Fuji (Japan)
Day 32: Volcanoes (continued)
* Types of Volcanoes (continued) 2. shield cone = broad, gently-sloping mountain = composed of low-viscosity lava = generally effusive in nature = examples include the volcanoes of the Hawaiian Islands (Mauna Loa, Kilauea, Mauna Kea) = largest volcano in the Solar System, Olympus Mons on Mars 3. cinder cone = smallest type of volcano = very steep-sided = composed of pyroclastics ("cinder") = generally, effusive eruptions; fountains of cinder * Fissures = long fractures in the Earth's crust from which low-viscosity lava flows = effusive eruptions; lava fountains = flood basalts: layers of low-viscosity, basaltic lava that cover large areas of land (ex. Columbia Plateau; northwestern U.S.) * Distribution of Volcanic Activity a) plate boundaries = usually, convergent or divergent b) hot spots = upwelling of magma beneath a plate = chains of volcanoes form as plate moves across hot spot