LAB H: Oceanography                                                          




If all the water was drained from the ocean basins, what kind of surface would be revealed?  It would not be the smooth uniform topography as was once thought, but a surface characterized by a great diversity of features:  towering mountain chains, deep canyons, and flat plains.  In fact, the scenery would be just as varied as that on the continents.


Figuring out the depths of the ocean floor and the landforms that are present used to be a very difficult task.  Until the late 1920’s the only method was to use a weighted line.  In deep water the time required for a line to reach bottom can exceed 1 ½ hours, while the task of pulling it up is even more time consuming.  Therefore, prior to the late 1920’s relatively few depth recordings had been made and little was known about the topography of the ocean floor.


Since the late 1920s scientists have developed and continually perfected an instrument known as a FATHOMETER.  It records the time for a sound signal to travel from a ship to the ocean floor and back again.  Knowing that sound waves move through sea water at an average speed of 1500 meters per second, it is easy to compute the depth to the ocean floor using the formula:


Echo Time x Speed of Sound  = Depth



Example:  It takes 12.6 seconds for sound to return to the ship traveling.  Calculate the depth to the ocean floor.


12.6 seconds x 1500 meters/sec = 18,900 meters.  Since this is the distance the sound traveled down to the ocean floor and back up, you need to divide it by 2 for the distance just to the ocean floor.


18,900 meters   = 9,450 meters



EXERCISE 1:  For each example on the worksheet, calculate the depth to the ocean floor.  Write your answers on the pre-lab worksheet.



When the data from the fathometer was analyzed, an amazing diversity of landforms common to all of the oceans was discovered.  In lab you will be producing your own ocean floor profile and labeling these features.  Here is a brief introduction to some of the major features:


CONTINENTAL SHELF – This fairly flat surface extends from the beach out towards the open ocean.  When researchers drilled through the sediment down to the bedrock, they discovered granite which is the rock of the continental crust.  This feature is a flooded extension of the continent. 


SHELF BREAK – This is the point where the continental shelf breaks off and the slope steepens abruptly.


CONTINENTAL SLOPE – This relatively steep slope marks the true edge of the continent.


CONTINENTAL RISE – This feature occurs at the base of the continental slope and has a more gradual gradient.  It forms as sediment landslides down the steep continental slope and collects at the bottom, gradually thinning out until it disappears.


ABYSSAL PLAIN – This is the deepest portion of the ocean floor.  It is relatively flat and covered by a thin blanket of sediment that settles out of the ocean water.  Periodically, volcanic seamounts poke up out of the abyssal plain. 



EXERCISE 2 – Identify and label these areas shown on the map of the ocean floor provided on the worksheet:  continental slope, continental shelf, continental rise, shelf break, abyssal plain.  You can use Figure 13.9 on p. 387 for reference.  Use either brackets or arrows with the labels to point out the features on the worksheet.



PART III – Surface Ocean Currents


An ocean current can be defined as the horizontal movement of seawater at the ocean's surface. Ocean currents are driven by the circulation of wind above surface waters. Frictional stress between the ocean surface and the wind causes the water to move in the direction of the wind.  On a global scale, large ocean currents are blocked by the continental masses bordering the three oceanic basins. Continental borders cause these currents to develop an almost closed circular pattern called a gyre.


Surface current temperatures reflect the area where they form.  Currents near the equator typically are warm currents and transfer heat from the equatorial regions towards the higher latitudes in each hemisphere.  Cold currents form near the polar regions and distribute excess cold to lower latitudes as they move towards the equator.


EXERCISE 3 – Label surface ocean currents


Using the surface ocean currents map and Figure 15.2 on page 428 of your textbook, label each of the arrows on the map with the name of the current.  In addition, color code the arrows to reflect each current’s temperature.  Use blue to color in the cold ocean current arrows and red to color in the warm ocean current arrows.