Name:______

Features on the Ocean Floor (ch. 18)

(Textbook p. 322-337)

  1. The H.M.S. Challenger measured the ocean depths using a______on a line, while modern ships continuously measure the oceans depths using ______recording the time taken to reach the bottom and return.
  1. An undersea feature that is actually the first part of the continental margin is the ______that extends to a depth of approximately ______.
  1. The continental slopes are cut by many huge gullies called ______that may have formed during the ______when sea level was perhaps 100 meters lower than today.
  1. Some geologists think that the gullies and valleys on the continental slope may have formed when landslides of mud and sand called ______rushed at high speed down the slopes.
  1. Some seamounts that appear to have their tops sheared off are called ______. They seem to be seamounts that may have once risen above sea level and have been worn down by ______.
  1. Fractures which cut across sections of the mid-ocean ridges where it bends around regions, like the bend in Africa, are called ______.
  1. Continental rise: (p. 329)
  1. What is it: ______
  2. They are found on (active/passive) margins only because on (active/passive) margins the sediments are trapped in the ______.
  1. Circle the correct word that makes each statement true.
  2. Abyssal plains in the ocean show the remains of many (volcanoes/ earthquakes)
  3. The world’s largest mountain range is the (continental shelf/ the mid-ocean ridge)
  4. The mid-ocean ridge was formed where tectonic plates (push together/ move apart)
  5. Undersea earthquakes occur at (seamounts/ the mid-ocean ridge)
  6. The continental slope rises sharply from the (deep plains/ continental shelf)
  1. What are seamounts? In which ocean are most seamounts formed?

______

  1. Match each term in Column A with its description in Column B. You will use some terms more than once.

Column A / Column B
Trench / ______1. Tectonic plates moving apart form a canyon
Rift / ______2. Dense sediment filled water forms a canyon
Submarine canyon / ______3.The sinking of part of the ocean floor forms a
canyon at a subduction boundary
______4. A canyon found on the continental shelf
______5. Deepest of all canyons
  1. How does sediment from land get to deep plains thousands of kilometers from the shore?

______

SECTION B: IDENTIFICATION OF OCEAN FEATURES

For each of the following terms, locate where it would be on the profile of the seafloor, and next to each word write the letter

SECTION C: MAPPING THE OCEAN FLOOR

Oceanographers study the ocean floor using sonar. Sonar, or sound waves, are sent from a ship to the ocean bottom. The waves bounce off the bottom and back up to a receiver located on the ship at the surface. The deeper the ocean, the longer it takes to receive the returning sonar vibrations. Oceanographers know how fast sound waves travel in water. Thus, they know how far the waves travel during their round trip to the surface. Half this distance is the depth of the ocean at that spot. The formula used to find ocean floor depth is: D = ½ t X v

“D” stands for depth, “t” is the time between the sending and receiving, and “v” is the velocity or speed of the sound waves in water. The speed of sound waves in water is 1524 meters per second.

Example: A research ship sends a sonar signal to the bottom. The ship receives the signal back in 4 seconds. How deep is the ocean at this point?

D = ½ t * v

D = unknown

t= 4 sec.

v = velocity (1524 meters per second)

The table below gives you some sonar sounding data. The data was gathered on a cruise from South America to Africa. Calculate the ocean floor depth for each sounding. Then plot these depths on the graph which follows. Smoothly connect the depth points and you will have a cross-sectional map of the South Atlantic Ocean floor.

Station / Time for signal to return (sec) / Depth of Ocean (m) / Station / Time for signal to return (sec) / Depth of Ocean (m)
A / 6.0 / F / 2.4
B / 5.2 / G / 4.0
C / 4.0 / H / 5.2
D / 2.4 / I / 6.0
E / 3.2

Cross-sectional Map of South Atlantic Ocean

A B C D E F G H I

West3000kmEast

  1. From what point do oceanographers measure ocean depth? (ocean floor/sea level)
  2. At which station (s) is the ocean the deepest? ______
  3. At which station(s) is the ocean the most shallow? ______
  4. What large ocean floor feature is evident in the cross-sectional map? ______
  5. What is the name of the ocean floor feature located directly below station E? ______