{"id":119,"date":"2018-03-14T15:26:37","date_gmt":"2018-03-14T19:26:37","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/physicalgeologyh5p\/chapter\/3-4-earths-magnetic-field-2\/"},"modified":"2021-07-07T14:57:55","modified_gmt":"2021-07-07T18:57:55","slug":"3-4-earths-magnetic-field-2","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/physicalgeologyh5p\/chapter\/3-4-earths-magnetic-field-2\/","title":{"raw":"3.4 Earth\u2019s Magnetic Field","rendered":"3.4 Earth\u2019s Magnetic Field"},"content":{"raw":"Similar to the mantle, Earth's liquid outer core convects because it's heated from beneath by the inner core. What's different is that because it's made of iron and conducts electricity (even when molten), the motion of the outer core generates a magnetic field.\r\n\r\nEarth\u2019s magnetic field is defined by north and south poles representing lines of magnetic force flowing into Earth in the northern hemisphere, and out of Earth in the southern hemisphere (Figure 3.14). Because of the shape of the field lines, the magnetic force is oriented at different angles to the surface in different locations. The tilt, or inclination<\/strong> of magnetic field lines is represented by the tilt of compass needles in Figure 3.14. At the north and south poles, the force is vertical. The force is horizontal at the equator. Everywhere in between, the magnetic force is at an intermediate angle to the surface.\r\n\r\n \r\n\r\n[caption id=\"attachment_116\" align=\"aligncenter\" width=\"626\"]\"\"<\/a> Figure 3.14<\/strong> Earth\u2019s magnetic field depicted as the field of a bar magnet coinciding with the core. The south pole of the magnet points to Earth\u2019s magnetic north pole. The red and white compass needles represent the orientation of the magnetic field at various locations on Earth\u2019s surface. Source: Karla Panchuk (2018). CC BY-SA 4.0. Modified after Steven Earle (2015; CC BY-SA 4.0, view source<\/a>), and T. Stein (2008; CC BY-SA 3.0 view source<\/a>).[\/caption]\r\n\r\n
\r\n\r\nPractice: Magnetic Inclination<\/strong>\r\n\r\nUse Figure 3.14 as a guide to help you complete this exercise.\r\n\r\n[h5p id=\"146\"]\r\n\r\n<\/div>\r\n

Polarity Reversals<\/h1>\r\n

Instability in Earth's Magnetic Field<\/h2>\r\nEarth\u2019s magnetic field is generated mostly within the outer core by the convective movement of liquid iron, but although convection is continuous, the magnetic field is not stable. Periodically, the magnetic field decays, then re-estabilshes. When it does re-establish, the polarity may have reversed. In other words, your compass needle would point south rather than north.\r\n\r\nChanges in Earth\u2019s magnetic field have been studied using mathematical models that simulate convection in the outer core (Figure 3.15).\u00a0 Reversals happened spontaneously when the model was run to simulate a period of several hundred thousand years. Spontaneous reversals can happen because convection doesn't occur in an orderly way, in spite of what the bar magnet analogy may suggest.\r\n\r\n \r\n\r\n[caption id=\"attachment_1506\" align=\"aligncenter\" width=\"650\"]\"\"<\/a> Figure 3.15<\/strong> Computer simulations showing Earth's \"normal\" magnetic field (top), and the magnetic field as polarity flips from reversed on the left to normal on the right. Notice how the magnetic field lines become disorganized, then converge on a more orderly arrangement. Source: Karla Panchuk (2021) CC BY 4.0. Click for more attributions<\/a>.[\/caption]\r\n\r\nThe solid inner core also convects, with many small-scale variations in convection patterns, but it does so more slowly than the liquid outer core. Yet Earth's magnetic field is the sum of all of those variations\u2014both inner and outer\u2014and for a polarity reversal to \"take,\" a reversal must happen in the magnetic fields of both parts of the core. If the inner core weren't solid, magnetic reversals would happen far more frequently.\r\n

How Often Do Polarity Reversals Happen?<\/h2>\r\nOver the past 250 Ma, there have been hundreds of magnetic field reversals, and their timing has been anything but regular. The shortest ones that geologists have been able to identify lasted only a few thousand years, and the longest one was more than 30 million years, during the Cretaceous Period (Figure 3.16).\r\n\r\n \r\n\r\n[caption id=\"attachment_1821\" align=\"aligncenter\" width=\"1024\"]\"\"<\/a> Figure 3.16<\/strong> Magnetic field reversal chronology for the past 170 Ma. Black stripes mark times when the magnetic field was oriented the same as today. Source: Steven Earle (2015). CC BY 4.0. View source.<\/a> Modified after AnomieX (2010), Public Domain. View source.<\/a>[\/caption]\r\n\r\n
\r\n\r\nConcept Check: Polarity Reversals<\/strong>\r\n\r\n[h5p id=\"147\"]\r\n\r\n<\/div>\r\n

References<\/h4>\r\nBritish Geological Survey, Natural Environment Research Council (n.d.). Reversals: Magnetic Flip<\/em>. Visit website<\/a>\r\n\r\nGlatzmaier, G. A. (n.d.) The Geodynamo<\/em>. Visit website<\/a>\r\n\r\nGlatzmaier, G. A., & Roberts, P.H. (1995). A three-dimensional self-consistent computer simulation of a geomagnetic field reversal. Nature, 377<\/em>, 203-209.","rendered":"

Similar to the mantle, Earth’s liquid outer core convects because it’s heated from beneath by the inner core. What’s different is that because it’s made of iron and conducts electricity (even when molten), the motion of the outer core generates a magnetic field.<\/p>\n

Earth\u2019s magnetic field is defined by north and south poles representing lines of magnetic force flowing into Earth in the northern hemisphere, and out of Earth in the southern hemisphere (Figure 3.14). Because of the shape of the field lines, the magnetic force is oriented at different angles to the surface in different locations. The tilt, or inclination<\/strong> of magnetic field lines is represented by the tilt of compass needles in Figure 3.14. At the north and south poles, the force is vertical. The force is horizontal at the equator. Everywhere in between, the magnetic force is at an intermediate angle to the surface.<\/p>\n

 <\/p>\n

\"\"<\/a>
Figure 3.14<\/strong> Earth\u2019s magnetic field depicted as the field of a bar magnet coinciding with the core. The south pole of the magnet points to Earth\u2019s magnetic north pole. The red and white compass needles represent the orientation of the magnetic field at various locations on Earth\u2019s surface. Source: Karla Panchuk (2018). CC BY-SA 4.0. Modified after Steven Earle (2015; CC BY-SA 4.0, view source<\/a>), and T. Stein (2008; CC BY-SA 3.0 view source<\/a>).<\/figcaption><\/figure>\n
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Practice: Magnetic Inclination<\/strong><\/p>\n

Use Figure 3.14 as a guide to help you complete this exercise.<\/p>\n

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