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100 Cards in this Set
- Front
- Back
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When is wave action on the coast the most regular?
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when there isn't much wind and waves are spread apart
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Why are waves perceived to be complex?
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What we see is the interference pattern of many different waves.
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What are the two general types of waves?
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progressive waves and standing waves
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What are the major features of a wave?
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the crest (elevated line) and trough (depressed line)
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What measurements are taken of a wave?
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wavelength L (between two crests), height H (crest to trough), and frequency f.
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What wave values are derived from measurements?
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amplitude H/2, period T=1/f, wave speed C=L/T, and wave steepness H/L
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What is another name for wave speed?
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celerity (C), because little to no water actually is transported
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How does a wave store energy?
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as kinetic and potential energy
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How much energy does a wave typically store?
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around (gρH^2)/8, where g is the acceleration due to gravity and ρ is the density of the water, but it also increases to a smaller degree with wavelength
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What are the two common restoring forces for waves?
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surface tension and gravity
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Which waves use which restoring forces?
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capillary waves (T<.1 seconds) use surface tension, most other waves use gravity, but the tides use the Coriolis effect
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How are large waves in general created?
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using long periods of time or larger displacing forces
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What can create an ocean wave?
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the winds, impacts into the water, gravity between Earth, moon and sun, and moving vehicles and organisms
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What can create an impact into the ocean?
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earthquakes, volcanic explosions, seafloor slumps, turbidity currents, coastal cliff collapse, and meteorite impacts
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What do ocean impacts cause?
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tsunamis, or seismic sea waves
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What does the gravity of the moon and sun cause?
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the tides
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Describe capillary waves.
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restoring force is surface tension, max wavelength 1.73 cm, round crests, and V-shaped troughs
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Describe the formation of gravity waves.
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Small atmospheric pressure gradients develop, causing winds as well as slight imbalance in ocean surfaces. The winds create a shear stress in the upper layer of water, forming capillary waves. Capillary waves grow because they can accept more energy from the winds, and create their own pressure gradients as wind blows over them, becoming bigger gravity waves.
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What happens to a wave's shape as it absorbs more energy?
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It starts out with a V-shaped trough and rounded crest, becomes close to a perfect sine wave, then trochoidal, with a pointed crest and rounded trough.
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What increases as wind energy is imparted in a wave?
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height, speed, period, and wavelength
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What is sea?
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the mix of waves of different heights, periods, and wavelengths where they are generated
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What is calm?
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when there are no big waves and the sea is flat
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What is swell?
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a region where waves are smooth, from the same direction and of the same wavelength
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What are whitecaps?
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waves breaking in deep water
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When do waves break?
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when steepness exceeds 1/7, or the winds blow the tops off
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Why do waves break?
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They have stored the maximum amount of energy for that wind speed.
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What is a developed sea?
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a sea where the energy lost to turbulence plus the energy needed to maintain the sea equals the energy inputed by the wind
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What determines the maximum wave height formed by a given storm?
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wind speed, wind duration, the distance over which it blows (fetch), and the depth for shallow water
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What is fetch?
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the distance over which a wind blows
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Which waves store the most energy?
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those with long periods
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Why are Pacific waves the largest?
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because the Pacific has a large fetch
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What are the largest wave sizes in each ocean?
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around 15 meters in the Atlantic and Indian; 34 meters in the Pacific
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Who and when observed the largest reliable measured wave?
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1933; the US navy ship Ramapo
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What effect can a current have on a wave?
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If they travel in opposite directions, the current can make the wave steeper, especially on its front face.
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Where do currents interact with waves most noticeably?
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The Agulhas Current opposes waves generated in the westerly wind zone, creating steep waves that can break oil supertankers in two.
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What indicates that wave heights are increasing?
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Near Britain between 1960 and 1985, average maximum wave heights increased from 12 meters to 15 meters (25%).
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What causes waves to dissipate energy before they break?
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viscosity of water and air resistance
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What ultimately happens to wave energy?
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The wave reaches a shore, and its energy is dissipated as heat in the wave breaking and used to erode the shore and establish the longshore currents.
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Why does wave breakage not increase the coastal water's temperature by much?
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Water has a large heat capacity and this energy is spread over a large volume of water.
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What is deep water?
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depth > L/2
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What pattern do individual water molecules at the surface of a deep water wave follow?
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a roughly circular orbit, with a slight net forward movement
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What happens to water molecules below a wave?
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They rotate in similar circular orbits, with smaller diameters as you go down: H/2 at depth L/9, ~0 at L/2
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What is the speed of a deep water wave in terms of wavelength?
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C=sqrt(gL/(2π))=1.25sqrt(L) if in m/s and m
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What is the wavelength of a deep water wave in terms of period?
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L=1.56T^2 if in m and s
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What are wave trains?
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groups of waves with the same characteristics
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How do wave trains travel?
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Each wave transfers some of its energy forward and backward, so energy is lost in front of the wave train and gained at the back. The front wave diminishes while a new wave is built at the back. The overall group speed is half of that of one of its waves.
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How can waves interfere?
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They can add and subtract (constructive and destructive interference), sometimes creating extra-large waves that wash people out to sea
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What is wave dispersion?
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the sorting of waves by wavelength (period) at a distance away from their source
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How do meteorologists use wave records?
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to pinpoint storm locations from a long way out to sea using short- and long-period waves
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What causes waves to change from sea to swell?
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As waves disperse and spread their energy over a larger area, their steepness decreases, making them more sinusoidal.
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At what depth do waves begin to interact with the seafloor?
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L/2
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What does interaction with the seafloor do to a wave?
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The circular orbits are compressed to ellipses, slowing the wave down.
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What are intermediate waves?
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waves in water of depth between L/2 and L/20
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What are shallow water waves?
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waves in water of depth less than L/20
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What is the speed of a shallow water wave in terms of depth?
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C=sqrt(gD)=3.1sqrt(D) if in m/s and m
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Which waves become shallow water waves farthest from shore?
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waves with longer periods (because L α T^2)
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What happens to a wave's period as it becomes a shallow water wave?
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It stays the same, because the wave stays in phase.
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What happens to a wave's speed as it becomes a shallow water wave?
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It decreases due to friction with the sea floor.
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What happens to a wave's wavelength as it becomes a shallow water wave?
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It decreases because speed decreases and period stays the same.
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What happens to a wave's steepness as it becomes a shallow water wave?
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It increases because above L/10 depth the wavelength decreases faster than height and below L/10 depth the height increases.
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What is wave refraction?
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the effect on waves coming in at an angle that makes wave rays bend toward the shallow water, becoming closer to perpendicular to the coast
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What are wave rays or orthogonals?
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lines drawn perpendicular to the wave crests and troughs that show the direction a wave travels
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What makes bays sandy and headlands rocky?
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Wave refraction concentrates more energy at the headlands, eroding them, than in the bay, depositing eroded particles there.
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What happens to a wave's height as it becomes a shallow water wave?
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Starting at L/2, it is reduced gradually to about 90% at L/10, then rapidly increases when kinetic energy is turned into potential energy.
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What is the surf zone?
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the area offshore where some waves break
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What are the four types of breakers?
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spilling breakers, plunging breakers, collapsing breakers, and surging breakers
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When do spilling breakers form?
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when the seafloor is practically flat, and water depth reaches ~1.2H
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Describe a spilling breaker.
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The crest tumbles down the front face, filling with air bubbles seen as white foam, maintaining the steepness at 1/7 without destroying the wave.
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When do plunging breakers form?
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when the coast is moderately steep
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Describe a plunging breaker.
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The bottom of the wave is slowed more quickly than the top, making the crest curl out in front of the wave.
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When do collapsing breakers form?
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rarely, when the seafloor is steep
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Describe a collapsing breaker.
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The bottom of the wave is slowed rapidly, making the front of the wave collapse in front of the crest, making lots of foam.
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When do surging breakers form?
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on very steep shores
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Describe a surging breaker.
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The wave doesn't actually break; it is partially absorbed but mainly reflected by the steep slope.
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How can seawalls and cliffs create hazardous waves?
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The waves are reflected nearly perfectly off these solid objects, and reflected waves interfere with incoming waves.
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What is wave diffraction?
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when a wave bends around a solid object, radiating out in all directions
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How can waves break far off shore?
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A fringing reef or longshore bar can create a large zone of shallow water, so the waves break on the bar or reef first, delivering less energy to the coast.
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How does surfing work?
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The surfer must be on the front of a wave, balancing the force on the board from the orbital motion of the water with gravity.
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How do waves kill people and what can be done to prevent that?
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Incoming waves knock people over, then the trough sucks them out to sea where they meet the next wave, etc. They can get back to shore by riding a wave and digging in when it tries to pull them back out to sea, making sure to breathe in between waves.
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What are rip currents?
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Waves transport water toward the shore, and it must return somewhere. Rip currents are fast-moving narrow corridors of water that go out to sea.
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How can rip currents be dangerous, and what can be done to avoid them?
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Rip currents can take people rapidly out to sea, overcoming even the best swimmers, exhausting them, and drowning them. To get out of a rip current, simply swim parallel to the shore.
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What can be used to identify locations of rip currents?
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depressions in the beach, plumes of increased turbidity, lowered wave heights, or lines of floating debris and foam
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When do tsunamis form?
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when a volcano or earthquake displaces a section of seafloor or coast and the water above it
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What physical characteristics do tsunamis have?
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long wavelengths (100-200 km) and periods (10-30 minutes) and fast speeds (200 m/s at 4 km ocean floor depth)
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What kind of waves are tsunamis?
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shallow water waves
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What heights do tsunamis have?
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Out at sea, they are 1-2 meters high, so they don't affect ships, but increase rapidly as they approach shore.
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How can a tsunami cause destruction?
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It floods low-lying areas with water for several minutes before receding, stranding boats far inland and destroying buildings and other structures.
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What are examples of destructive and deadly historical tsunamis (as of 1998)?
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Krakatau 1883 blew its island apart and created a tsunami with an hour-long period, with waves of 30 meters on Java, 60 km away, killing 35k and recorded in Panama. Aleutian Trench 1946 earthquake made a tsunami that destroyed a lighthouse 10 meters above sea level and killed 150 people in Hawaii, resulting in a tsunami warning system that saved lives 11 years later.
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Why are most tsunamis in the Pacific?
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The Pacific has most of the subduction zones.
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What are storm surges?
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the elevation of the sea surface caused by lowered atmospheric pressures near the center of a storm
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What is required for a wave to develop?
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a sharp density difference, such as the atmosphere and ocean, or the pycnocline
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What are thought to be the primary causes of internal waves?
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tidal motions and the thermohaline circulation
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What heights do internal waves have?
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up to 100 meters because the density difference is smaller than across the surface
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What properties do internal waves have?
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high heights, speeds about 1/8 surface waves of the same period, 5-8 minute periods, and .6-1 km wavelengths
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Where do internal waves break?
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on the outer part of the continental shelf, possibly mixing deep water and shallow water off NY and NJ
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What do internal waves matter to?
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sealife, submarines and offshore oil platforms
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What are seiches?
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standing waves, when waves oscillate back and forth
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What features do standing waves exhibit?
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nodes, where no vertical motion takes place, and antinodes, which experience the maximum vertical displacement
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Where do standing waves occur?
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lakes and enclosed or nearly enclosed ocean basins
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What is tuned oscillation?
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when progressive waves are in phase with standing waves in a basin, important when dealing with tides
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