The following questions refer to the description and figure below.
The figure represents a cross section of the sea floor through a mid-ocean rift valley, with alternating patches of black and white indicating sea floor with reversed magnetic polarities. At the arrow labeled "I" (the rift valley), the igneous rock of the sea floor is so young that it can be accurately dated using carbon-14 dating. At the arrow labeled "III," however, the igneous rock is about 1 million years old, and potassium-40 dating is typically used to date such rocks. Note: The horizontal arrows indicate the direction of sea-floor spreading, away from the rift valley.

Figure 25.1
-Assuming that the rate of sea-floor spreading was constant during the 1-million-year period depicted above, what should be the approximate age of marine fossils found in undisturbed sedimentary rock immediately overlying the igneous rock at the arrow labeled "II"?
A) 10,000 years
B) 250,000 years
C) 400,000 years
D) 1,000,000 years

Question

The following questions refer to the description and figure below.
The figure represents a cross section of the sea floor through a mid-ocean rift valley, with alternating patches of black and white indicating sea floor with reversed magnetic polarities. At the arrow labeled "I" (the rift valley), the igneous rock of the sea floor is so young that it can be accurately dated using carbon-14 dating. At the arrow labeled "III," however, the igneous rock is about 1 million years old, and potassium-40 dating is typically used to date such rocks. Note: The horizontal arrows indicate the direction of sea-floor spreading, away from the rift valley.
  
Figure 25.1
-Assuming that the rate of sea-floor spreading was constant during the 1-million-year period depicted above, what should be the approximate age of marine fossils found in undisturbed sedimentary rock immediately overlying the igneous rock at the arrow labeled "II"?
A) 10,000 years 
B) 250,000 years 
C) 400,000 years 
D) 1,000,000 years

Quizplus · Accepted Answer

The age of the igneous rock at point III is 1 million years old, indicating the rate of sea-floor spreading in the area. If we assume that the rate was constant during the 1-million-year period depicted above, then we can estimate the age of marine fossils in the sedimentary rock at point II to be around 400,000 years old, which is halfway between the ages of the igneous rock at points I and III. Therefore, C is the best choice.

The Following Questions Refer to the Description and Figure Below