Tides bore forms. A can have a breaking crest or it can be a smooth wave. Tidal bores usually are found in places with large Throughout the day, the level of the rises and falls. This rise tidal ranges. When a tidal bore enters a river, its force causes water and fall in is called a . A tide is caused by a giant in the river to reverse its flow. Waves in a tidal bore might reach 5 m wave produced by the gravitational pull of the and the . in height and speeds of 65 km/h. Although this wave is only 1 m or 2 m high, its wavelength is thousands of kilometers long. As the crest of this wave How does the Moon affect ? approaches the shore, sea level seems to rise. This rise in sea level is called high tide. When the trough of this huge wave nears The Moon and the Sun exert a gravitational pull on . The Sun the shore, sea level appears to drop. This drop in sea level is is much bigger than Earth, but the Moon is much closer. The Moon referred to as low tide. has a stronger pull on Earth than the Sun. Earth and the water in Earth’s respond to this pull. The water bulges outward as the What is the ? Moon’s gravity pulls it. This results in a high tide. The process is shown in the figure below. As Earth rotates, Earth’s surface passes through the crests and troughs of this giant wave. Many coastal areas, such as the The Moon’s gravity pulls at Earth. This creates two bulges of Atlantic and Pacific of the United States, have two high water. One bulge is on the side of Earth closest to the Moon. The tides and two low tides each day. But because basins vary other bulge is on the opposite side. The high tide on the side of in size and shape, some coastal locations, such as many along the Earth near the Moon happens because the water is being pulled Gulf of , have only one high and one low tide each day. A away from Earth towards the Moon. The high tide on the side of tidal range is the difference between the level of the ocean at Earth opposite the Moon happens because the gravitational pull on high tide and the level of the ocean at low tide. that part of Earth is greater than the pull on the water on that side of Earth. The areas of Earth’s oceans that are not toward or away Why do tidal ranges vary in different locations? from the Moon are the low tides. As Earth rotates, the bulges follow the Moon. This results in high and low tides happening around the Most shorelines have tidal ranges between 1 m and 2 m. world at different times. However, tidal ranges can be as small as 30 cm or as large as 13.5 m. What effect does the Sun have on tides?

The shape of the seacoast and the shape of the ocean floor The Sun’s pull can add to or subtract from the gravitational pull of both affect the ranges of tides. A wide seacoast allows water to the Moon. Occasionally during Earth’s revolution, Earth, the Sun, spread out farther. At high tide, the water level might only rise a and the Moon are lined up together. Then, the Moon and the Sun few centimeters. In a narrow gulf or bay, the water cannot spread both pull at Earth. The combined gravitational pull results in spring out. The water will rise many meters at high tide. A narrow gulf tides on Earth. During spring tides, high tides are higher and low or bay will have a greater tidal range than a smooth, wide area of tides are lower than usual. When the Sun, Earth, and the Moon shoreline. form a right angle, high tides on Earth are lower than usual. Low tides are higher than usual. These are called neap tides. What are tidal bores?

Sometimes, a rising tide enters a river from the sea. If the river is narrow and shallow and the sea is wide, a wave called a tidal

23. Define all words in bold. 24. How many tides do we have each day? Where only has one high and one low tide each day? Why? 25. How do tides differ depending upon where you are? 26. How does the Moon affect tides? 27. How does the sun affect tides? 28. Draw a picture the shows the relationship between sun, moon, and earth and when neap tides and spring tides occur.

What is the ? ocean floor changes. When plates separate, hot magma from inside Earth erupts through small cracks, forming new crust. This process Ocean basins are low areas of Earth that are filled with water. is called . The hot lava is cooled by ocean water They have many different features. Beginning at the ocean and hardens to solid rock. shoreline is the continental shelf. The continental shelf is the gradually sloping end of a continent that extends under the What are zones? ocean. While seafloor is being formed along mid-ocean ridges, it is being On some coasts, the continental shelf extends a long distance. destroyed in other parts of the ocean. Areas where old ocean floor This is true for North America’s Atlantic and Gulf coasts. The slides beneath another plate and moves into Earth’s mantle are continental shelves on these coasts stretch 100 km to 350 km called subduction zones. into the sea. On the ocean floor, subduction zones are found within deep On the Pacific , the coastal range mountains are close to ocean trenches, as shown in the figure on the previous page. A the shore. The shelf there is only 10 km to 30 km wide. The ocean trench is a long, narrow, steep-sided depression where one plate covering the continental shelf can be as deep as 350 m. sinks beneath another. Most trenches are found in the Pacific Basin. Ocean trenches are usually longer and deeper than any valley on What other features are found in the ocean? land. The Mariana Trench reaches about 11 km below the ocean The continental slope extends from the outer edge of the surface and is the deepest place in the Pacific Ocean. The Mariana continental shelf down to the ocean floor. Beyond the Trench is so deep that Mount Everest, the tallest mountain on Earth, continental slope lie the trenches, valleys, plains, mountains, and could easily fit into it. The figure below shows what it would look ridges of the ocean basin. like if Mt. Everest were placed in the Mariana Trench. How do abyssal plains form? In the deep ocean, sediment that comes mostly from land, settles constantly on the ocean floor. This sediment fills in valleys in the ocean floor and creates flat seafloor areas called abyssal (uh BIH sul) plains. Abyssal plains are from 4,000 m to 6,000 m below the surface of the ocean. In the Atlantic Ocean, some abyssal plains are large and extremely flat. The Canary has an area of approximately 900,000 km2. Sometimes abyssal plains have small hills and . Seamounts are underwater, inactive volcano peaks. Seamounts are most commonly found in the 29. Define all words in bold (7 in total). Pacific Ocean. 30. Draw the figure above and label it correctly. Ridges and Trenches 31. How deep is the Mariana Trench and what could fit in it? A mid-ocean ridge is a part of an ocean basin area where new ocean floor is formed. Mid-ocean ridges are found at the bottom of all oceans and form a continuous underwater ridge about 70,000 km long. Crustal plates, large section of Earth’s crust and upper mantle, are constantly moving. As these plates move, the