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CHAPTER 9 Tides

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Chapter OverviewTides are the rhythmic rise and fall of sea

level.Tides are very long and regular shallow-

water wavesTides are caused by gravitational attraction

of the Sun, Moon, and Earth

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What are Tides?Tides – periodic raising and lowering of

ocean sea levelOccurs dailyCaused by combination of gravity and motion

between Earth, Moon, and SunIsaac Newton’s gravitational laws explain

relationship

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Tide-Generating ForcesBarycenter between

Moon and EarthCommon center of

mass or balance point

Beneath Earth’s surface because of Earth’s greater mass

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Gravitational ForcesEvery particle

attracts every other particle

Gravitational force proportional to product of massesIncrease mass,

increase force

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Centripetal ForceCenter-seeking

forceTethers Earth and

Moon to each other

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Resultant Forces Mathematical

difference between gravitational and centripetal forces

Relatively small

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Tide-Generating ForcesResultant force has

significant horizontal component

Pushes water into two simultaneous bulgesOne toward MoonOne away from

Moon

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Tidal PhenomenaTidal period – time between high tidesLunar day

Time between two successive overhead moons24 hours, 50 minutes

Moon orbits EarthHigh tides are 12 hours and 25 minutes

apart

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Tidal Bulges – Sun’s EffectSimilar to lunar

bulges but much smaller

Moon closer to Earth

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Earth’s Rotation and TidesFlood tide – water moves toward shoreEbb tide – water moves away from shore

Earth’s rotation moves different geographic locations into bulges

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Monthly Tidal CycleSpring tides

New or full moonsTidal range greatest

Neap tidesQuarter moonsTidal range least

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Earth, Moon, and Sun Positions Relative to Spring and Neap Tides

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Idealized Tide PredictionTwo high tides/two low tides per lunar daySix lunar hours between high and low tides

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Real TidesContinents and friction with seafloor modify

tidal bulgesTides are shallow-water waves with speed

determined by depth of water

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Tidal PatternsDiurnal

One high tide/one low tide per daySemidiurnal

Two high tides/two low tides per dayTidal range about same

Mixed Two high tides (1 higher than the

other)/two low tides (1 lower than the other) per day

Tidal range differentMost common

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Tidal Patterns

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Monthly Tidal Curves

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Tides in Coastal WatersStanding Waves

Tide waves reflected by coastAmplification of tidal rangeExample: Bay of Fundy maximum tidal range

17 meters (56 feet)

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Bay of Fundy – World’s Largest Tidal Range

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Tides in Coastal WatersTidal Bores

Wall of waterMoves up

certain riversTide-generated

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Coastal Tidal CurrentsWhirlpool

Rapidly spinning seawater

Restricted channel connecting two basins with different tidal cycles

Moon and Tides