New crust forms along the ridge when old crust splits and magma rises to fill the gap. Continued divergence inevitably splits that new crust in its turn, and the broken halves are pushed in opposite directions. Strips of crust form along the ridge, break in half, then spread away from each other. Thus the bands of polarity mirror each other across the ridge.
Additional Information
Albert Einstein called the generation of Earth’s magnetic field one of the greatest mysteries in physics. Scientists are still not sure how the field is generated, though it is clearly related to the movement of molten iron in the liquid outer core. Even more puzzling is why the field switches back and forth between normal and reversed polarity.
The measured strength of the magnetic field has dropped by 5-10% in the last 150 years, and less precise readings suggest it may have weakened by 25-50% over the last 5,000 years. Satellites have detected two areas in the mantle that appear to have reversed polarity. These data have led some to speculate that a magnetic reversal may be imminent.
Critical Thinking
You receive an e-mail offer to get rich quick by investing in an undersea gold mine on top of the Mid-Atlantic Ridge. What scientific issues should you consider before you send a check?
Scientists seek to understand and explain how the natural world works. Many of the questions raised in this endeavor have no absolute answers.
Mid-ocean ridges are rich in mineral deposits. Hydrothermal vents are sometimes encrusted with ores of gold, silver, copper, and zinc. The deposits are very young, so the minerals haven’t been degraded by weathering. In one sense, these rocks are easier to get at than many continental deposits – they’re just sitting out on the seafloor, not buried deep underground.
It is also true that these rocks cannot be mined with a pick and a shovel. Mining operations carried out far from land and kilometers underwater will likely require expensive technologies sure to cut into your profit margin. Mid-ocean ridges are also geologically unstable – a mine site would be in constant danger from earthquakes, landslides, and volcanic eruptions. Hydrothermal vents are home not just to minerals, but to small, delicate ecosystems as well. Refining mining techniques to avoid harming vent animals would be expensive, while destroying a vent fauna could lead to public outcry and governmental regulation.