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Explore these resources to teach students about marine organisms, their relationship with one another, and with their environment. Along coasts, the water slowly rises up over the shore and then slowly falls back again. The Earth's rotation and the gravitational pull of the sun and moon create tides.
The intertidal zone is an ecosystem found on marine shorelines, where a multitude of organisms living on the shore survive changes between high and low tides. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Great Barrier Reef. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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View Article. Earth's Tides. Intertidal Zone. A wave forms in a series of crests and troughs. The crests are the peak heights of the wave and the troughs are the lowest valleys.
A wave is described by its wavelength or the distance between two sequential crests or two sequential troughs , the wave period or the time it takes a wave to travel the wavelength , and the wave frequency the number of wave crests that pass by a fixed location in a given amount of time.
When a wave travels, it is passing through the water, but the water barely travels, rather it moves in a circular motion. Waves on the ocean surface are usually formed by wind. When wind blows, it transfers the energy through friction.
The faster the wind, the longer it blows, or the farther it can blow uninterrupted, the bigger the waves. Therefore, a wave's size depends on wind speed, wind duration, and the area over which the wind is blowing the fetch.
This variability leads to waves of all shapes and sizes. The smallest categories of waves are ripples, growing less than one foot. The largest waves occur where there are big expanses of open water that wind can affect. These large wave sites attract surfers, although occasionally, waves get just too big to surf. Some of the biggest waves are generated by storms like hurricanes. In , Hurricane Ivan created waves that averaged around 60 feet 18 meters high and the largest were almost feet In , hurricane Dorian also created a wave over feet high in the northern Atlantic.
To sailors, they look like walls of water. No one knows for sure what causes a rogue wave to appear, but some scientists think that they tend to form when different ocean swells reinforce one another. One was recorded by a buoy in and measured A classic tsunami wave occurs when the tectonic plates beneath the ocean slip during an earthquake. The physical shift of the plates force water up and above the average sea level by a few meters. From a single tectonic plate slip, waves radiate outwards in all directions moving away from the earthquake.
When a tsunami reaches shore, it begins to slow dramatically from contact with the bottom of the seafloor. As the leading part of the wave begins to slow, the remaining wave piles up behind it, causing the height of the wave to increase. Though tsunami waves are only a few feet to several meters high as they travel over the deep ocean, it is their speed and long wavelength that cause the change to dramatic heights when they are forced to slow at the shore.
Tsunami waves are capable of destroying seaside communities with wave heights that sometimes surpass around 66ft 20 m. Tsunamis have caused over , deaths since —over , people were killed by the giant earthquake off Indonesia in , and the damage caused to the Fukushima nuclear reactor in Japan by a tsunami in continues to wreak havoc.
The highest tsunami wave reached about 1, ft m , a product of a massive earthquake and rockslide. When the wave hit shore, it was said to destroy everything. There are also other, usually less destructive tsunami waves caused by weather systems called meteotsunamis. These tsunami waves have similar characteristics to the classical earthquake driven tsunamis described above, however they are typically much smaller and focused along smaller regions of the oceans or even Great Lakes.
Meteotsunamis are often caused by fast moving storm systems and have been measured in several cases at over 6 feet 2 meters high. A study found that smaller meteotsunami waves strike the east coast of the U. Tides are actually waves, the biggest waves on the planet, and they cause the sea to rise and fall along the shore around the world. Tides exist thanks to the gravitational pull of the Moon and the Sun, but vary depending on where the Moon and Sun are in relation to the ocean as Earth rotates on its axis.
The Moon, being so much closer to Earth, has more power to pull the tides than the Sun and therefore is the primary force creating the tides. Centrifugal force is the same force that smooshes riders to the outside walls of spinning carnival rides. Meanwhile, Earth continues to spin.
A specific point on the planet will pass through both of the bulges and both of the valleys. When a specific place is in the location of a bulge it experiences a high tide. When a specific place is in the location of a valley it experiences a low tide.
During one planetary rotation or one day a specific location will pass through both bulges and both valleys, and this is why we have two high tides and two low tides in a day.
But, while Earth takes 24 hours to complete one rotation, it must then rotate an additional and 50 minutes to catch up with the orbiting Moon. This is why the time of high tide and the time of low tide change slightly every day. The Sun also has a part to play in causing the tides, and its location in relation to the Moon alters the strength of the pull on the ocean.
This happens when the Moon is either on the same side of Earth as the Sun or directly on the opposite side of Earth. Smaller-than-usual tidal ranges, called neap tides, occur when the gravitational force of the Sun is at a right angle to the pull from the Moon.
The two forces of the Sun and Moon cancel each other out and create a neap tide. But continents obstruct the flow of water, causing this seemingly simple daily cycle to be a bit more complicated. Because of continental obstruction, some locations experience two tides a day that are more or less the same height known as semidiurnal tides , some locations experience one tide at one height and the second at a different height mixed semidiurnal tides , and some locations have so much interference from land that they only experience one high tide and one low tide per day diurnal tides.
The local geography can also affect the way the tides behave in a location. Shores around coastal islands and inlets may experience delayed tides compared to smoother surrounding coasts since the water must funnel in through constrained waterways.
The intertidal zone , the coastal area tides submerge for part of the day, is home to many ocean creatures. It takes a special set of adaptations to live a life half the time scorched by the Sun and the other submerged underwater. Moreover, the incoming tide promises a constant pounding by ocean waves. Shelled mollusks like periwinkles, muscles, and barnacles cling to rocks, sea stars wedge themselves in crevices, and crabs hide in fronds of algae.
A red tide is not a true tide at all but rather a term used to describe the red color of an algal bloom. Algae are integral to ocean systems, but when they are supplied with excessive amounts of nutrients they can explode in number and smother other organisms.
The algae may produce toxins or they can die, decay, and the bacteria decomposing them take up all the oxygen. This massive growth of algae can become harmful to both the environment and humans, which is why scientists often refer to them as harmful algal blooms or HABs.
Tidal movements are tracked using networks of nearshore water level gauges, and many countries provide real-time information with tidal listings and tidal charts. Tides can be tracked at specific locations in order to predict the height of a tide, i. The tides there range from 11 feet 3. This erosion also releases nutrients into the water that help support marine life. The currents associated with the tides are called flood currents incoming tide and ebb currents outgoing tide.
Having reliable knowledge about the tides and tidal currents is important for navigating ships safely, and for engineering projects such as tidal and wave energy , as well as for planning trips to the seashore. Skip to main content. Credit: Pixabay. Are You An Educator? At Smithsonian Ocean, we have lesson plans, activities, and resources to help you engage your students in the wonders of our oceans.
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