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Here we go again, guys!!!! More geology notes!!! sweatdrop sweatdrop sweatdrop sweatdrop sweatdrop PLEASE, wish me luck on my test tomorrow!!!!!!! smile whee
Crustal Deformation - (two driving forces - weather and plate tectonics) - due entirely to plate tectonics - the change in rock shape due to a stress - application of stress that changes the shape of rocks - Stress = Force/Unit Area - Stress = F/A - uniform stress and deformation - dividing by the unit area always gives the same type of deformation - force is nothing more that a push, pull, or other sort of change in energy - result of stress is strain - Strain = % Deformation - Types of Differential (Dynamic) Stress - Compression ( --> O <-- ) - ( --> 0 <--) - Tension (pulling apart instead of pressing together, but the result looks the same as Compression) - Sheer Stress (push in or pull out on one side at the bottom and at one side at the top) - Torque (basically, a twisting of the rock) - Different Types of Strain (Deformation) - Elastic (pulling and letting go - stress is released and the material returns to its natural shape) - Plastic (molecules start to slide past each other and the material remains deformed) - Brittle (rock breaks under stress) - Relationship between Types of Stress and Types of Strain - continue to add stress - addition of strain - elastic à plastic à brittle - different types of rocks strain different ways - Controls Types of Deformation - Temperature (cooler objects tend to be more brittle - hotter objects tend to be more plastic) - Pressure (Static Stress - increase - becomes more plastic) - Composition (certain materials deform brittley and other materials deform plastically - depends on the material) - Rate of Stress (speed of stress applied though with the same amount - slow results in elastic - fast results in brittle) - Results of Stress - Elastic (no record that it ever occurred) - Plastic (the rock stays deformed - folding) - Brittle (faulting), etc.
Deformation - deformation is a general term that refers to all changes in the original form and/or size of a rock body - most crustal deformation occurs along plate margins - Deformation involves - Force - that which tends to put stationary objects in motion or changes the motions of moving objects - Stress (the force applied to a given area) - Compressional Stress, Tensional Stress, and another one - Strain is the percent change in shape (Elastic Deformation, Plastic Deformation, and Brittle Deformation) - Mapping geologic structures - once a rock has been deformed, description of deformation is in order - Strike and Dip - Strike is the first measurement taken - the compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane (generally expressed in an angle relative to north) - Dip is the second measurement taken - the angle of inclination of the surface of a rock unit or fault measured from a horizontal plane (direction of the steepest descent of an inclined plane) - includes both an angle of inclination and a direction toward which the rock is inclined - The Dip is always perpendicular to the Strike - A long line (horizontally) represents Strike, while a shorter line (vertical) and perpendicular to the long line represents Dip measurement - Folds - characteristics of Folds - Parts of a Fold- Limbs (refers to the two sides of a fold) - Axis (a line drawn down the points of maximum curvature of each layer) - Axial Plane (an imaginary surface that divides a fold symmetrically) - The Limbs are separated by the Axial Plane - the Axis is basically the center and the part where the Axial Plane divides the two Limbs - Common Types of Folds - Plastic - Anticline (upfolded or arched rock layers - dips away from the axis - oldest rocks are in the middle) - Syncline (downfolds or troughs of rock layers - dips towards the axis- youngest rocks are in the middle) - Depending on their orientation, anticlines and synclines can be describe as symmetrical, asymmetrical, recumbent (an overturned fold - one of the limbs has been turned over), or plunging (forms a sort of parabola) - Monoclines (large, step-like folds in otherwise horizontal sedimentary strata) - Other Types of Folds - Dome (up warped displacement of rocks - circular or slightly elongated structure - oldest rocks in the center and youngest or younger rocks on the flanks) - Basin (down warped displacement of rocks - circular or slightly elongated in structure - youngest rocks in the center and oldest or older rocks on the flanks) - Brittle Deformation - Faults are fractures in rocks along which appreciable displacement has taken place - sudden movements along faults are the cause of most earthquakes - classified by their relative movement which can be horizontal, vertical, or oblique - the Strike and Dip symbol is used but an arrow is put on the end of the Dip symbol to indicate that it is a Fault - Rocks moving along the Dip are called Dip-Slip Faults - Rocks moving along the Strike are called Strike-Slip Faults - Footwall (the part of a fault which one can stand on in an ore vein) - Hanging Wall (one can hang something from this sort of wall in an ore vein and fault) - If the Hanging Wall goes down, it is called a Normal Fault (Dip-Slip Fault) -If the Footwall goes down and the Hanging Wall goes up, it is called a Reverse Fault - A Thrust Fault is a shallowly dipping reverse fault, etc.
Two types of surface waves are Love and Ralleigh in earthquakes - the primary wave is fastest and always arrives first, the secondary wave is slower and arrives second, and the surface waves arrive last - secondary waves sometimes do not arrive - the secondary waves cause the most damage - Focus (the place within the Earth where an earthquake originates) - Epicenter (the place above the focus on the surface of the Earth) - Locating the epicenter of an earthquake - three station recordings are needed to locate an epicenter - each station determines the time interval between the arrival of the first P wave and the first S wave at their location - a time-travel graph is used to determine each station’s distance to the epicenter - Triangulation of an epicenter occurs - Earthquake belts - about 95% of earthquakes originate along plate boundaries For divergent plate boundaries, one gets shallow earthquakes - small earthquakes due to weak rocks - rocks under tension, being pulled apart, are weak - shallow, weak earthquakes at mid-ocean ridges - at transform plate boundaries (like in San Andreas fault) almost always shallow but devastating - at convergent plate boundaries (like the Japanese Island Arc) the earthquakes occur shallow to medium to deep and can be weak to medium to very strong - strongest earthquakes are found in subduction zones - Benioff Zone (the place between convergent plate boundaries where one plate slides underneath another) - Continental crust-Continental crust - shallow and intermediate earthquakes - Locating the source of earthquakes - five to seven-hundred kilometers in depth - Measuring the size of earthquakes - two measurements that describe the size of an earthquake - Intensity - Mercalli Scale - (a measure of the degree of earthquake shaking at a given locale based on the amount of damage) - uses Roman numerals - Magnitude - Richter Scale - (estimates the amount of energy released at the source of an earthquake) - uses Arabic numerals - Modified Mercalli Intensity Scale was developed using California buildings as its standard - the drawback of intensity scales is that destruction may not be a true measure of the earthquake’s actual severity (assign a number - Roman numeral - from one to twelve to indicate intensity) - Richter magnitude (concept introduced by Charles Richter in 1935 - based on the amplitude of the largest seismic wave recorded and accounts for the decrease in wave amplitude with increased distance (know how much energy is produced) - each number is 30 times more energy than the previous number - Largest magnitude recorded on a Wood-Anderson seismograph was 8.9 - Magnitudes < 2.0 are not felt by humans - each unit of Richter magnitude increase corresponds to a tenfold increase in wave amplitude and a 32-fold energy increase - a Richter 1 is about 20 million Ergs of energy - a Richter 6 has about the amount of an atomic bomb test - a Richter 9 is about like the 1964 Alaskan Earthquake - the amount of damage can vary according to the sediment - Other magnitude scales - several “Richter-like” magnitude scale have been developed - Earthquake Destruction - destruction from seismic vibrations - Seiches - the rhythmic sloshing of water in lakes, reservoirs, and enclosed basins - waves can weaken reservoirs and cause destruction - Tsunamis (seismic sea waves) - destructive waves that result from vertical displacement along a fault located on the ocean floor or a large undersea landslide triggered by an earthquake (often inappropriately called “tidal waves”) - Landslides and ground subsidence - Fire - Can earthquakes be predicted? - not really - a good fit exists between the plate tectonics model and the global distribution of earthquakes, etc.
Earth’s Interior - Two pieces of information to get knowledge about the interior of the Earth - Travel times of Compressional and sheer (P and S) waves through the Earth vary depending on the properties of the materials (Seismic Waves) - Variations in the travel times correspond to changes in the materials encountered (Meteorites?) - Seismic Waves - the speed of the wave depends on the density of the rock - the density of the Earth increases as you go further into the Earth and an increase of seismic wave activity - “Rho” is the standard symbol for density - if a seismic wave goes through the low density layer, it continues on straight - if it hits the higher density layer, some of the wave will continue straight, penetrating the next layer, and some of it will bounce off of the layer and back up into the lower density layer - Indecent Ray to start with - Reflected Ray goes up - Refracted Ray goes down - In a continuous increase of density in the layers of the Earth, the seismic wave will bend on its way downward - Compressional Waves (P Waves) are able to propagate through liquids, solids, and gases - S Waves can only go through solids - When seismic waves pass from one material to another, the path of the wave is refracted (or bent) - Abrupt changes indicate changes in the Earth - Shadow Zone - a zone in the Earth where no earthquakes are recorded - The P-Waves Shadow Zones occur because of refraction of seismic waves off of the core - The wave velocity gives us the density of the layer - Reflection surfaces tell us a change in layer - Whether or not the S Wave passes through the layer tells us whether the layer is liquid or solid - Meteorites - Two types - Stony and Iron/Nickel - Stony Meteorites have the composition of the Earth’s Mantle - We assume that Iron/Nickel Meteorites have the same composition as the Earth’s Core - The asteroid probably had the same composition as the Earth at some point in time - Layers are defined by composition - three principal compositional layers - Crust - the comparatively thin outer skin that ranges from three kilometers (two miles) at the oceanic ridges to seventy kilometers (forty miles) - Mantles - the solid rocky (silica-rich) shell that extends to a depth of about 2900 kilometers (1800 miles) - Core - the iron-rich sphere having a radius of approximately 3486 kilometers (2161 miles) - Layers defined by physical properties or state - depending on the temperature and depth, a particular Earth material may behave like a brittle solid, deform in a plastic-like manner, or melt and become liquid - Main layers of Earth’s interior are based on physical properties and hence mechanical strength - Lithosphere - brittle solid, Earth’s outermost layer, consists of the crust and uppermost mantle, relatively cool, rigid shell - Asthenosphere - beneath the Lithosphere, in the upper mantle to a depth of 600 kilometers, solid, and plastic - Mesosphere (lower mantle) - Rigid Layer between the depths of 660 kilometers to 2900 kilometers - rocks are very hot and capable of very gradual flow - semi-plastic - Outer Core - composed mainly of iron-nickel alloy (liquid iron) 2270 kilometers thick and generates the Earth’s magnetic field - Inner Core - acts like a solid and is stronger than the outer core - between the Asthenosphere and the Mesosphere, olivine collapses and becomes much more dense - this layer of the Mantle is very erratic and what-not - the Core of the Earth is larger than the planet Mars - Earth’s dense central sphere - Fastest route for seismic waves is shifting eastward indicating the inner core is rotating faster than the rest of the planet - the outer core convects because it is molten liquid - Radioactive decay of Isotopes causes convection cells - rotation of the Earth creates convection cells in the outer core, generating the magnetic field of the Earth, etc.
There is a boundary between the crust and the mantle - The Moho (Mohorovicic discontinuity) - discovered in 1909 by Andriaja Mohorovicic - separates crustal materials from underlying mantle - identified by a change in the velocity of primary waves - Low Velocity Zone is right below the Moho - both P and S waves change in velocity - the low velocity zone is at the top of the mantle - partial melting of the mantle - about 5% liquid - enough to be the source of virtually every magma in the world - after that, increase in seismic wave velocity - Asthenosphere - continues from the low velocity zone to 1000 kilometers, roughly - starts collapsing at 410 kilometers deep - the Transition Zone - between the Asthenosphere and the Mesosphere - at 3000 kilometers depth, there is a huge decrease in seismic wave velocity - the outer core is liquid (this is the reason for the decrease) - the S waves cannot travel through it - the inner core has more increase in seismic wave activity and the S waves reappear - Discovering Earth’s major boundaries - the core-mantle boundary - discovered in 1914 by Beno Gutenberg - based on the observation that P waves die out at 105 degrees from the earthquake epicenter and reappear at about 140 degree - 35 degree wide belt is named the P-wave-shadow-zone - characterized by bending (refracting) of the P waves - discovery of the inner core - Lehman in 1936 - the inner core actually spins faster than the rest of the Earth - the rotation of the Earth is slowing down - something to do with the tides and the rotation of the moon - density and composition - mostly iron but 10% nickel - Origin - most accepted explanation is that the core formed early in Earth’s history - as Earth began to cool, iron in the core began to crystallize and the inner core began to form - the outer core convects - Earth’s magnetic field - magnetic precession (convection cells move around on the outer core) - mostly stabilized - the source of the convection cells is due to the rotation of the Earth and its inner core - Earth’s internal heat engine - Earth’s temperature gradually increases with an increase in depth at a rate known as the geothermal gradient - varies considerably from place to place - averages between 20 degrees Centigrade and 30 degrees Centigrade - Major processes that have contributed to Earth’s internal heat - heat emitted by radioactive decay of isotopes of potassium, thorium, and uranium - heat flow in the crust - process called conduction - rates of heat flow in the crust varies - Mantle convection - heat moves - there is not a large change in temperature with depth in the mantle - mantle must have an effective method of transmitting heat from the core outwards - Earth’s gravity - changes at the surface are due to Earth’s rotation - rotation causes a centrifugal force that is proportional to the distance from the axis of rotation - shape is flattened slightly at the poles, resulting in weaker gravity at the equator, etc.
Divergent Boundaries - Origin and Evolution of the Ocean Floor - Continental Margin to Abyssal Plain (Ocean Basin Floor) to Mid-Oceanic Ridge (convection cells occur here) back to Abyssal Plain and then Continental Margin - Continental Margins - A submerged portion of the continental crust (consists of the continental shelf, the continental slope that is about three degrees between the shelf and the slope, and the continental rise) - Passive Continental Margins - found along most coastal areas that surround the Atlantic ocean - not associated with plate boundaries - experience little volcanism and few earthquakes - Features comprising a passive continental margin - Continental Shelf - flooded extension of the continent - varies greatly in width - gently sloping - The continental slope is where the true continental crust ends - thirty to forty degree slope - the continental rise is basically debris and sediments that exists between the continental slope and the oceanic crust - Deep-Sea Fans cause turbidites - Active Continental Margins - continental slope descends abruptly into a deep-ocean trench - oceanic crust subducts under continental crust - Fore-Arc Basin is the place between the continental margin and the Trench - Compressional stress system (forms a crumple zone, often mountains) - on the ocean side of the Fore-Arc Basin there is a steep slope - this forms an Accretionary wedge - like the Andes Mountains - Transform Plate boundary on the edge of the continent at the San Andreas Fault - short, small continental shelf, over-steepened slope, and not much of a continental rise, straight from continental crust to the abyssal plain - Features of the Deep-Ocean Basin - Deep-Ocean Trench - the sites of subduction zones - long, relatively narrow features - these represent plate boundaries - very active - volcanism and large earthquakes - Abyssal plains - the flattest places on the Earth - sites of thick accumulations of sediment - found in all oceans - Sea-Mounts - isolated volcanic peaks - many form near oceanic ridges - extinct hot-spots - may emerge as an island - may sink and form flat-topped sea-mounts called guyots - vast outpourings of basaltic lavas on the ocean floor create extensive volcanic structures called oceanic plateaus - Fringing reefs occur on extinct volcanoes - as the volcanoes erode, the reef grows - barrier reefs and lagoons - submerged volcanoes with coral reefs and lagoons in the center have reefs that grow all around that are called atolls - Anatomy of the Oceanic Ridge - Mid-Atlantic Ocean Ridge is slow-moving - Pacific Ocean Ridge is fast-moving - Up-welling plastic solids - pressure decreases - the state of the rock melts - mantle up-welling, depressurization melting, magma chambers - partial melting - Rift Valleys are when it pulls apart - the mantle material physically pulls the crust up - transform faults in the Mid-Atlantic Ridge - Slow-Spreading Oceanic-Ridge - characterized by well-developed rift valleys - Grabens occur - a central block of rock drops down - transform faults - Fast-Spreading-Oceanic-Ridges - no rift valley - rift bulge (the exact opposite) - more partial melting - well-developed magma chamber - pushed up by the mantle pushing up the rock - Peridotite (upper-most mantle - ultra-mafic) - Gabbro (formed from partial melting of the mantle - mafic) - Sheeted Dike Complex (ocean crust cracked between gabbro and basaltic lava pillows) - Basaltic Pillow Lavas - Deep-Sea Sediments - Ophialite Sequence (a cross-section of the ocean floor) - East African Rift Valley is a relatively young oceanic ridge - Triple Junction ( a place on the Earth’s surface where three plates come together) - two of the plates remain active, and one dies out (the failed one) - also called arms - Mechanisms for Continental Rifting - slab pull and slab suction - subduction of old oceanic lithosphere may pull a continent attached to a subducting slab and create a rift - Destruction of Oceanic Lithosphere - why oceanic lithosphere subducts - Oceanic lithosphere subducts because its overall density is greater than the underlying mantle - subduction of older, colder lithosphere results in descending angles of nearly ninety degrees - Oceanic crust is thin (about five kilometers thick) - it is also dense - gets pushed underneath the continental crust - Why the oceanic lithosphere subducts - younger, warmer oceanic lithosphere is more buoyant and angles of descent are small - the faster it’s going the more deep it’s slant is going to be - Subducting plates - the demise of an ocean basin - plate movements have been reconstructed for the past 200 million years using magnetic stripes on the ocean floor - Horst and Graben features - like big double-u’s - the Horst is the risen part - the Graben is the dropped-down part - Basin and Range System (the entire Western coast of the United States) - If the process continued, an ocean basin would occur on the other side of the mountain ranges - Demise of the Farallon Plate - California formed by subduction of the Farallon Plate - formed the basin and lake features of the Western United States, etc.
Convergent Boundaries (Origin of Mountains) - Mountain Building - Mountain building has occurred during the recent geologic past - American Cordillera - the western margin of the Americas from Cape Horn to Alaska which includes the Andes and the Rocky Mountains - Alpine-Himalayan Chain - Mountainous terrains of the western Pacific - the Ural Mountains of Russia and the Appalachian Mountains are very similar, both are small mountain chains, and they share many similar features, including height - both were the result of continental collision - sutures (Appalachians in Canada that are like erosions) - the elevation of mountain ranges can be a determinate of the age - higher is younger - Orogenesis - the processes that collectively produce a mountain belt - includes folding, thrust faulting, metamorphism, and igneous activity - Orogenic Belts are mountain ranges - the first physiological part of crust - Second is the stable continental interior - cratons is what they are called - two parts of Cratons - Pre-Cambrian Shields (600 million or more years old - exposed to surface) - Platforms (Pre-Cambrian Shields covered by sedimentary rocks) - Convergence and subducting plates - forearc is between the trench and the volcanic island arc - the backarc is behind the volcanic arc - the backarc is more stable - Andean-Type Plate Margins - Ocean crust goes under a continental crust - volcanic mountain range occurs - The Western United States - the Sienna Nevada Cascades - The Great Valley - The California Coast Range - California was an ancient island arc, but sort of plastered onto the rest of the margin of the United States - that is what an exotic plain is - the boundary between plates are called sutures (when two plates come together) - Compressional Mountain Belt - Continental Crust collides with another Continental Crust - like the Himalayas - thrust faults occur (very common in collision) - the thickness of the mountain range is much larger - no active subduction zone associated with this type of collision - the Aleutian Islands are Ocean-Crust-Ocean-Crust convergent plate boundaries - The Evolution of the Appalachian Mountain Range - Beginning of Cambrian Time - two subduction zones - one formed a small volcanic island arc - the other formed a micro continent (continental volcanic arc) - oceanic crust gets consumed - the micro continent collides with North America, forming the Blue Ridge Mountains (Eastern Tennessee, etc.) - the boundary on the western side of the collided micro continent is east of Knoxville and west of the North Carolina border - the volcanic island arc collides with the Blue Ridge Mountains and forms the Carolina Slate Belt (Eastern Piedmont) - the subduction zone sort of flipped from eastward-dipping to westward-dipping - Africa collided with the Carolina Slate Belt - three Orogenic events (at least) - nasant Atlantic Ocean begins to open between Africa and the collision Valley and Ridge Province - Coastal Plain is formed - North America moves west - Africa moves east - Exotic terrain - pieces of continental crust that get welded on from another continent or an inactive volcanic island arc collides with a continent - Greenstone Belts - forms a chlorite schist - really from the ocean floor - terraces are ancient shorelines in an active plate boundary which is lifted up in sequential bumps - active plate margins like the western United States - mountain ranges are a gravity low - composed of granite (continental crust) - Himalayas are a gravity low, but around it is a gravity high - mountain ranges float on the mantle - erosion of the top of the mountain causes the roots of the mountain to rise and float - isostatic adjustment - convergent plate boundaries are compressed but with a lot of elastic stress - if the stress is released, it goes back out like molasses and is called ductile spreading - gravitational collapse, etc.
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