Ch 23 The Ocean Floor Ms. Martel

23.1 – Studying the Ocean Floor Submersible, satellites, and other technology allow scientists to study the structure and composition of the ocean floor.

Echo Sounding In the days of the first oceanographic surveys, scientists measured the distance to the sea floor with a lead weight on a line. They lowered the weight until it touched the bottom, measured the line, and hauled the weight back up. In deep seas, a single depth reading might take an entire day.

Today ships use echo sounding, or sonar, to find the distance to the ocean floor. A special device called a precision depth recorder sends a sound signal through the water to the sea floor. By tracking how long it takes for the signal to reach the bottom and echo back to the ship, scientists can measure the water’s depth.

Multi-beam echo sounding allows scientists to cover an area that is up to twice the depth of the water over which the research vessel is travelling. Accurate maps can then be made more efficiently. Scientists can also measure the intensity of reflected sound beams to determine sea floor composition, ex) rock, gravel, mud.

Sediment Sampling Scientists gather samples of the ocean floor in order to study its composition. By studying the oceans sediment layers, scientists can learn a lot about Earth, including how its atmosphere and climate have changed over millions of years. Scientists obtain some sediment samples through a process called core sampling, where a long cylinder of material is removed from the ocean floor.

Scientists use several kinds of coring devices. One is a gravity corer: a hollow, weighted tube with one open end, attached to a ship by a cable. The gravity corer drops through the water and plunges into the sea floor, driven by it’s own weight.

Satellite Observations Echo sounding is useful for mapping the ocean floor, but it has limitations Ships move slowly and can map only a fraction of the sea floor as they travel. Today, satellites provide greater range and speed in the mapping process. A satellite can gather far more data more quickly than a seagoing vessel.

Map of the Ocean Floor The map of Earth’s ocean topography shown above was generated by combining data taken by radar altimeters and echo soundings. Land is shaded black, red represents the shallowest water, and violet represents the deepest water. The ridge system extending throughout the world’s oceans shows up clearly as lines of green and yellow. Deep trenches are shown as thin lines of purple near Asia and Australia.

23.2 – The Continental Margins The continental margin is the underwater part of the continental crust. It consists of the continental shelf and the continental slope, and, depending on whether it is an active or passive margin, it may lie adjacent to a trench or continental rise.

Parts of the Continental Margin The continental shelf is that part of a continent that extends from the shoreline out to the continental slope. They are very flat, and their widths vary.

The continental slope begins at the shelf edge, where water depth begins to increase rapidly. Sediments tend to build up temporarily on the slope; eventually they become unstable and tumble downward to form the continental rise. The continental rise is generally several kilometers thick, and it lies on the ocean crust. It’s considered part of the ocean basin rather than part of the continent.

Active and Passive Margins Not all continental margins possess the same features of a specific margin depend on where it lies in relation to a subduction zone or transform fault. Continental margins can be active or passive. Active continental margins are where the oceanic plate is sliding beneath, or subducting under, the continental plate. Here the continental shelf is very narrow and the continental slope is steep.

Passive continental margins are located at plate boundaries. Another type of active margin occurs where two plates are sliding past one another. These transform fault boundaries, ex. San Andreas fault, experience many earthquakes. Passive continental margins are located at plate boundaries. Here there are no ocean trenches or rugged coastal mountains.

Submarine Canyons Most small valleys that cut through the continental shelf and the continental slope probably resulted from mudslides. Large underwater valleys, called submarine canyons, have a much different origin. Some of these are larger than the Grand Canyon Some large submarine canyons are too deep to have ever been above sea level. Geologists think these canyons may be the result of powerful turbidity currents. Turbidity currents are great landslides of mud and sand that speed down the continental slopes.

23.3 – The Ocean Basin The ocean basin is not the featureless, underwater expanse people once thought it was. It has a remarkable and interesting terrain that, like all of Earth’s environments, changes over time.

Abyssal Plain and Abyssal Hills One feature of the deep sea floor is the abyssal plain, the flattest of all Earth’s surface areas. Abyssal plains are composed of sediments, most of which came from continents. Turbidity currents carry sediment material down the continental slopes when the sea level rises again and spread it over the continental rise and abyssal plain.

Abyssal hills are another part of the deep ocean basin. These small rolling hills often occur in groups next to oceanic ridge systems. Individual hills are typically several hundred meters to tens of km across the rise no more than a few hundred meters above the abyssal plain.

Deep-Sea Trenches Deep-sea trenches are long, narrow, steep-sided troughs that run parallel to continental margins or to volcanic island chains called island arcs. These trenches exist at subduction zones – convergent plate boundaries where subduction occurs. Deep-sea trenches are common sites of earthquakes and volcanic activity.

The bottoms of deep-ocean trenches are narrow, flat, and filled with sediment. On the descending plate, there is a bulge in the sea floor, which scientists think results from the bending of the plate as it subducts beneath the overriding plate. If one plate is oceanic and the other continental, a marginal trench forms. If the plate involved in the subduction are both oceanic plates, an arc of volcanic islands form on the overriding oceanic plate.

Deep-Ocean Vents A deep-ocean vent is a geyser that erupts under water. Cold ocean water seeps through the sea floor and is heated by molten material. The hot water then gushes back into the ocean through sea floor cracks. Most deep-ocean vents are closely associated with mid-ocean ridges.

Mid - Ocean Ridges The most obvious features of the ocean basin are mid- ocean ridges, great undersea mountain ranges. Mid-ocean ridges form at divergent plate boundaries where two lithospheric plates are moving apart. Magma rises between the plates and cools, forming a ridge of new sea floor.

Ocean ridges are not seamless Ocean ridges are not seamless. Hundreds of strike-slip faults, called transform faults, break them into separate pieces; some of the pieces are 100’s of km long. Together with the rugged terrain around them, the transform fault make up fracture zones. The pieces of ridge are offset relative to each other. Between them, the crustal plates are moving in opposite directions. The grinding and straining that results from this movement causes earthquakes along these sections of the faults.

Seamounts and Guyots Seamounts are cone-shaped mountain peaks that rise high above the deep ocean floor. These peaks are typically found in clusters or rows. Seamounts occur in all ocean but are most abundant in the Pacific. Volcanic in origin, seamounts seem to be related to plate boundary activity.

Some seamounts look as though their tops have been sliced off. These flat-topped seamounts are called guyots. Wave action removed their tops when they projected above sea level. Later, the sinking of the oceanic crust lowered the tops of the guyots, some as deep as 2 km below sea level.

Corals and Coral Atolls Another product of crustal sinking is the coral atoll, a ring- shaped coral island. An atoll begins to form when a coral reef develops around a volcanic island. As the ocean crust beneath the volcano sinks, the corals sink with it, but new corals continue to form on top of the old. Eventually the mountain is completely below sea level, leaving behind a coral atoll.

24.3 – Tides Tides are the periodic rise and fall of the ocean surface due to the gravitational pulls of the moon and sun. The positions of the moon and sun relative to Earth affect both the time and height of the tides. The heights of tides along the coast also depend on the shape of the shoreline. Because tides can have a significant effect on shorelines, knowledge of tides is important to people who live on the coast.

The Moon and Tides Around the world, tides go in and out throughout the day. When the tide reaches its highest point on the shore, it is called high tide. When the tide reaches its lowest point, it is called low tide. This movement is influenced by the gravitational pulls of the moon and the sun. When the moon is new or full, tides rise higher than normal due to the gravitational pull

The Sun’s Effect on Tides The sun has the same kind of effect on Earth’s waters as the moon does. However, because it is so much farther away, the sun’s tidemaking effect is only about half that of the moon. Tides are always high in line with the moon and low midway between the high-tide points. During these times, high tides are especially high, and low tides are especially low. These tides occur twice a month and are called spring tides.

At quarter phases, the moon and the sun are not in line with Earth. As a result, the sun’s entire tidemaking effect is subtracted from the moon’s. The outcome is high tides that are not very high and low tides that are not very low. These tides also occur twice a month and are called neap tides.

When the moon is at perigee, the closest point to Earth in its orbit, the tidal effect is greater, especially if perigee occurs during the new or full moon phases. If the moon is at apogee, its farthest point from Earth, the tidal effect is less.

Tidal Range If you were sailing on the ocean, you would not notice the tides much, it at all. But if you were standing on shore you would notice it much more. The difference in the water level between high tide and low tide is called the tidal range. Tidal ranges vary from one body of water to the next and tend to be more noticeable on oceans than on lakes.