Chapter 9 - Questions

Assuming the average speed of sound waves in water is 1500 meters per second, determine the water depth if the signal sent out by an echo sounder requires 6 seconds to strike bottom and return to the recorder.

11. Since the velocity of sound waves in water is 1500 meters per second and the total travel time is 6 seconds, the water depth is 4500 meters.

How are mid ocean ridges and deep ocean trenches related to plate tectonics?

17. Mid-ocean ridges are spreading centers; that is, they are divergent plate boundaries where magma wells up from below, creating new oceanic crust. Ocean trenches form where crustal plates plunge back into the mantle. Thus, these long, narrow depressions are associated with convergent plate boundaries.

Why are seafloor sediments useful in studying climates of the past?

19. Most sea-floor sediments contain the remains of organisms that once lived near the sea surface. Since the numbers and types of organisms living near the sea surface vary as climate (especially temperature) fluctuates, sea-floor sediments hold important clues to changes in climate through time.

What is meant by salinity? What is the average salinity of the ocean?

4. Salinity is the proportion of dissolved salts to pure water, usually expressed in parts per thousand. The average salinity of seawater is 3.5% or 35 parts per thousand.

What are the two primary sources for the materials that comprise the dissolved components in seawater?

6. Some of the material is the product of chemical weathering on the continents. These dissolved substances are delivered to the oceans chiefly by streams. Other elements, such as chlorine, bromine, sulfur, and boron come from volcanic outgassing.