Oceans
CITE
We have made every effort to follow citation style rules, but there may be some minor differences. If in doubt, please refer to the appropriate citation style manual.
Tides. (2024). In Q-files Encyclopedia, Earth, Oceans. Retrieved from
https://www.q-files.com/earth/oceans/tides
"Tides." Earth, Oceans, Q-files Encyclopedia, 12 Jan. 2024.
https://www.q-files.com/earth/oceans/tides.
Accessed 19 Mar. 2024.
Tides 2024. Earth, Oceans. Retrieved 19 March 2024, from
https://www.q-files.com/earth/oceans/tides
Earth, Oceans, s.v. "Tides," accessed March 19, 2024.
https://www.q-files.com/earth/oceans/tides
Tides
Sea levels rise and fall twice a day. This pattern is known as the tides. The tidal range is the difference between high and low tide. Seas that are almost surrounded by land, such as the Mediterranean Sea, have a very small tidal range. Tides are caused by the the combined gravitational pull of the Moon and the Sun and the rotation of the Earth. Most coastlines have two low tides and two high tides each day, while others have only one of each or as many as four.
How are tides caused?
Tides are caused by the pull of gravity by the Sun and Moon on the Earth. As the Earth spins, the ocean waters on the side of the Earth closest to the Moon (and the opposite side) bulge outwards, causing a high tide. At the same time, the rest of the Earth has a low tide.
When the Sun, Moon and Earth are in line, the Sun’s gravity combines with that of the Moon to increase the pull on the water. This makes high tides higher and low tides lower. These are called spring tides (1). When the Sun and Moon are at right angles, the difference between high and low tides is at its lowest. These are neap tides (2).
Tidal bore
A tidal bore is a type of wave. It forms when the leading (front) edge of the incoming tide meets the current in a river, estuary or narrow bay flowing in the opposite direction. The resulting wave travels up the river or bay against the direction of the current, accompanied by a powerful roar.
Bores occur in only about 60 locations around the world, usually where there is a large tidal range and where incoming tides are funnelled into a shallow, narrowing river via a broad bay. The world's largest bore is on the River Qiantang in China. It can reach nine metres (30 feet high). Perhaps the most famous is the pororaca, which travels up the Amazon in South America.
The bore on Turnagain Arm, an inlet or fjord branching east from Cook Inlet near Anchorage in Alaska, may be more than two metres (about 7 feet) high and travel at 24 km/h (15 mph) on high spring tides. Turnagain Arm has the largest tidal range in United States, and the fourth highest in the world.
Consultant: Ian Fairchild