Conquer It
You have learned to combine close reading of a text with close viewing of visual information. Now, show off your skills by reading an excerpt and combining the information given with information from a visual representation.
Read the excerpt below from the last section of the article “Plate Boundaries”opens in new window, and view diagrams A, B and C on page 3. After reading and viewing the diagrams, answer the question in the activity below by providing a written response that combines information from both sources.
Earth's Magnetic Field
Most of the Earth’s magnetic field is caused by motions of liquid metal in the outer core as the planet rotates. The Earth’s magnetic field is similar to that generated by a simple bar magnet (Fig. A). The field behaves as if the magnet were located through the axis of the Earth’s rotation. Currently, it is inclined about 11 degrees from the geographic axis of rotation. However, magnetic north is not fixed, and the location drifts at rates up to 0.2 degrees per year.
At present, the lines of force of the Earth’s magnetic field are normal polarity (Fig. B). The negative magnetic pole is in the Northern Hemisphere and the positive magnetic pole is in the Southern Hemisphere. However, the Earth’s magnetic field periodically reverses; i.e. the north magnetic pole becomes the South Pole and vice versa. The Earth has experienced periods of this reversed polarity alternating with times of normal polarity.
These magnetic fields are recorded on the ocean floor (Fig. C). Magmas contain iron-bearing minerals such as magnetite which acts like a compass. As these iron-rich magmas cool, they become magnetized in the direction of the surrounding magnetic field. As magma rises to form new ocean floor at a mid-ocean spreading center, it records the polarity of the magnetic field existing at the time magma crystallized. As spreading pulls the new oceanic lithosphere apart, stripes of approximately the same size are carried away from the ridge on each side. Seafloor maps of magnetic anomalies can be used to compare and establish mid-ocean ridge spreading rates.
