SECTION 17.2 Seafloor Spreading

Question

In the space at the left, write true if the statement is true: if the statement is false, change the italicized word or phrase to make the statement true.

_________1. Sonar uses sound waves to measure water depth.
___________ 2. Maps made from sonar and magnetometer data led to the discovery of ocean ridges and deep-sea trenches.

___________ 3. Deep-sea trenches are vast, underwater mountain chains.

____________ 4. Rock samples taken near ocean ridges are older than rock samples taken near deep-sea trenches.

_____________ 5. The thickness of ocean-floor sediments decreases with distance from an ocean ridge.

__________6. The oldest ocean floor rocks are about 3.8 billion years old.

____________7. The study of the magnetic record preserved in Earth's rocks is called paleomagnetism.

____________8. An isochron is a change in Earth's magnetic field.

___________ 9. Earthquake activity and volcanism are common along ocean
ridges.

_____________ 10. The magnetic patterns on either side of a deep-sea trench are mirror images of each other.

_____________11. The theory of continental drifi states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches.

___________ 12. As new seafloor is carried away from an ocean ridge, it becomes more dense heats up, expands, and becomes less dense than the
material beneath it.

___________13. The theory of seafloor spreading explains that Earth's continents move because they ride atop ocean crust as it moves away from
ocean ridges.

The statements below describe the steps involved in the process of seafloor spreading. Number these steps in the order in which they occur.

14. Magma fills the gap that is created.
15. Magma hardens to form new ocean crust.
16. Magma is forced upward toward the crust.

Answer 1

Here are the corrected statements:

1. True
2. True
3. False - Deep-sea trenches are vast, underwater valleys.
4. False - Rock samples taken near ocean ridges are younger than rock samples taken near deep-sea trenches.
5. True
6. True
7. True
8. False - An isochron is a line on a map that connects points of equal age of rocks.
9. True
10. False - The magnetic patterns on either side of an ocean ridge are mirror images of each other.
11. False - The theory of continental drift states that continents have moved over geologic time and not specifically about ocean crust formation and destruction.
12. False - As new seafloor is carried away from an ocean ridge, it becomes more dense as it cools, contracts, and becomes denser than the material beneath it.
13. True

Now, ordering the steps involved in the process of seafloor spreading:

1. 16. Magma is forced upward toward the crust.
2. 14. Magma fills the gap that is created.
3. 15. Magma hardens to form new ocean crust.