Waves and Tides Earth Science 6th Grade

Waves A wave is a pulse of energy that moves from one particle of water to the next. When you watch a wave, it looks like the water in the waves is rushing toward you. But, it is not. When a wave passes through the water, the water rises and then comes down again in the same place. Only the energy of the waves moves forward.

How Are Waves Made? Most waves are formed by the wind blowing across the surface of the water. The longer and harder the wind blows, the higher the wave. Some of the largest and most destructive waves are not formed by wind. They are formed by earthquakes, underwater volcanic eruptions, or underwater landslides. These waves are called seismic sea waves or tsunamis. Tsunamis can speed through the ocean at nearly 500 miles per hour and can rise as high as a 20 story building.

Parts of a Wave Trough - the lowest point in a wave between crests Crest - the highest point of wave that moves up and down and at right angles to the direction it travels Wave height - the vertical distance between the crest and trough of a wave Wavelength - the horizontal distance between two successive wave crests or troughs

Measuring Waves Wave period - the time it takes for successive wave crests to pass a certain point Wave frequency - the number of waves that pass a given point per second Tsunami - a series of traveling ocean waves of extremely long wavelength generated by disturbances with earthquakes, underwater volcanic eruptions, and landslides occurring below or near the ocean floor (Japanese word)

Beaufort Scale The Beaufort Scale is another way to measure the height of waves. The scales uses the numbers from 0 - 12 to show how strong the wind is and the effect of wind speed on wave height.

Surf The constant wave breaking action is called surf. Over time, the surf wears away at rocks and breaks them down into particles of sand. The surf, storms, large waves, and wind move the sand from spot to another or they carry the sand out to sea. This destroys dunes and beaches and new sand must be brought in to replace the sand that was moved.

Tides A tide is the slow, periodic rise and fall of ocean waters. Tides can be predicted and occur at regular intervals. Tides are caused by 2 factors: gravitational pull of the Moon and Sun inertia of water in oceans Inertia is the tendency of the water in the ocean to stay in motion. Gravity creates a bulge of water on the side of Earth facing the Moon. Inertia creates a bulge of water on the side of Earth facing away from the Moon. The Sun also affects the size and position of the 2 tidal bulges.

High Tide and Low Tide Most coastal areas have 2 high tides and 2 low tides every day. Lunar days are used when recording tides. A lunar day is the time it takes for a specific site on the Earth to rotate from an exact point under the moon to the same point under the moon. A lunar day is 24 hours and 50 minutes. High tides occur 12 hours and 25 minutes apart. It takes 6 hours and 12.5 minutes for the water at the shore to go from high to low, or from low to high.

Spring Tide Twice a month, the Sun, Moon and Earth are in a straight line. This produces very high and very low tides called spring tides. Spring Tides occur during the new moon and full moon phases.

Neap Tide Twice a month, the Sun and Moon are at right angles to each other and the gravity of each pulls the ocean water in different directions. There is very little difference between the depths at high and low tide. Neap Tides occur during the 1st and 3rd quarter phases of the Moon.

Other Factors that Affect Tides Shape of bays and estuaries Narrow inlets Shallow water Strong river flow Local wind and weather patterns