(Magnetic inclination vs. latitude graph).
Two igneous rocks crystallized at different times at the same geographic location on a continent. One rock crystallized at 120 Ma and records a magnetic inclination of 68 degrees; the second rock crystallized at 65 Ma and records a magnetic inclination of 50 degrees.
Which of the following best describes the motion of this continent between the time interval from 120 Ma to 65 Ma?
Assume the rocks remained in the northern hemisphere throughout their history.
A. It moved northward by about 28 degrees latitude.
B. It moved southward by about 28 degrees latitude.
C. It moved northward by about 20 degrees latitude.
D. It moved southward by about 20 degrees latitude.
E. It moved northward by about 58 degrees latitude.
F. It moved southward by about 58 degrees latitude.
I have been stuck on this problem for three days and my TA's have not helped. Any help?
To determine the motion of the continent between 120 Ma and 65 Ma based on the magnetic inclinations of the two rocks, we need to understand the relationship between magnetic inclination and latitude.
Magnetic inclination refers to the angle at which the magnetic field lines of the Earth intersect the Earth's surface. It is a valuable tool for studying the movement of continents over time.
In general, the magnetic inclination decreases as we move closer to the magnetic poles (north or south) and increases as we move towards the equator. At the magnetic poles, the inclination is 90 degrees (perpendicular), while at the equator, it is 0 degrees (parallel).
Given that both rocks remained in the northern hemisphere throughout their history, we can infer the following:
- The rock that crystallized at 120 Ma recorded a magnetic inclination of 68 degrees.
- The rock that crystallized at 65 Ma recorded a magnetic inclination of 50 degrees.
Since the inclination decreases as we move towards the magnetic pole, we can deduce that the continent (and consequently, the rock formations) must have moved closer to the magnetic pole between 120 Ma and 65 Ma. This corresponds to a northward motion.
To determine the amount of northward motion, we need to find the difference in magnetic inclination between the two time points.
The difference in inclination is calculated as 68 degrees - 50 degrees = 18 degrees.
This 18-degree difference in inclination represents the change in latitude between the two time points on the continent. However, since each degree of inclination corresponds to approximately 111 km in latitude, we can multiply the difference by this value to obtain the distance in kilometers.
18 degrees * 111 km/degree = 1998 km.
So, the continent moved northward by approximately 1998 km or 1998/111 ≈ 18 degrees of latitude.
Taking the available options into account, the correct answer would be:
A. It moved northward by about 28 degrees latitude. (Since 18 degrees is closest to 20 degrees, which is not an available option).
Therefore, the motion of the continent between 120 Ma and 65 Ma was northward by about 28 degrees latitude.