{"id":447,"date":"2019-06-11T14:50:41","date_gmt":"2019-06-11T14:50:41","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/10-3-geological-renaissance-of-the-mid-20th-century\/"},"modified":"2021-12-08T16:58:15","modified_gmt":"2021-12-08T16:58:15","slug":"10-3-geological-renaissance-of-the-mid-20th-century","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/10-3-geological-renaissance-of-the-mid-20th-century\/","title":{"raw":"10.3 Geological Renaissance of the Mid-20th Century","rendered":"10.3 Geological Renaissance of the Mid-20th Century"},"content":{"raw":"As the mineral magnetite (Fe3<\/sub>O4<\/sub>) crystallizes from magma, it becomes magnetized with an orientation parallel to that of Earth's magnetic field at that time. This is called [pb_glossary id=\"1617\"]remnant magnetism[\/pb_glossary]<\/strong>. Rocks like basalt, which cool from a high temperature and commonly have relatively high levels of magnetite (up to 1 or 2%), are particularly susceptible to being magnetized in this way, but even sediments and sedimentary rocks, as long as they have small amounts of magnetite, will take on remnant magnetism because the magnetite grains gradually become reoriented following deposition. By studying both the horizontal and vertical components of the remnant magnetism, one can tell not only the direction to magnetic north at the time of the rock's formation, but also the latitude where the rock formed relative to magnetic north.\r\n\r\nIn the early 1950s, a group of geologists from Cambridge University, including Keith Runcorn, Ted Irving,[footnote]Ted Irving later set up a paleomagnetic lab at the Geological Survey of Canada in Sidney, B.C., and did a great deal of important work on understanding the geology of western North America.[\/footnote]\u00a0and several others, started looking at the remnant magnetism of Phanerozoic British and European volcanic rocks, and collecting [pb_glossary id=\"1618\"]paleomagnetic[\/pb_glossary]<\/strong> data. They found that rocks of different ages sampled from generally the same area showed quite different apparent magnetic pole positions (Figure 10.3.1). They initially assumed that this meant that Earth's magnetic field had, over time, departed significantly from its present position\u2014which is close to the rotational pole.\r\n\r\n[caption id=\"attachment_433\" align=\"aligncenter\" width=\"700\"]\"\"<\/a> Figure 10.3.1 Apparent polar-wandering paths (APWP) for Eurasia and North America. The view is from the North Pole (black dot) looking down. The outer circle is the equator. In the diagram to the right the curve locations have been corrected taking continental drift into account.[\/caption]\r\n\r\nThe curve defined by the paleomagnetic data was called a [pb_glossary id=\"1619\"]polar wandering path[\/pb_glossary]<\/strong> because Runcorn and his students initially thought that their data represented actual movement of the magnetic poles (since geophysical models of the time suggested that the magnetic poles did not need to be aligned with the rotational poles). We now know that the magnetic data define movement of continents, and not<\/em> of the magnetic poles, so we call it an apparent<\/em> polar wandering path (APWP).\r\n
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What is a polar wandering path?<\/a><\/p>\r\n\r\n<\/header>\r\n

[caption id=\"attachment_434\" align=\"alignright\" width=\"400\"]\"The<\/a> Figure 10.3.2 [Image Description]<\/a>[\/caption]At around 500 Ma, what we now call Europe was south of the equator, and so European rocks formed then would have acquired an upward-pointing magnetic field orientation (see Figure 9.3.2 and Figure 10.3.2). Between then and now, Europe gradually moved north, and the rocks forming at various times acquired steeper and steeper downward-pointing<\/em> magnetic orientations.When researchers evaluated magnetic data in this way in the 1950s, they plotted where the North Pole would have appeared to be based on the magnetic data and assumed that the continent was always where it is now. That means that the 500 Ma \u201capparent\u201d north pole would have been somewhere in the South Pacific, and that over the following 500 million years it would have gradually moved north.Of course we now know that the magnetic poles don\u2019t move around much (although polarity reversals do take place) and that the reason Europe had a magnetic orientation characteristic of the southern hemisphere is that it was in the southern hemisphere at 500 Ma.Runcorn and colleagues soon extended their work to North America, and this also showed apparent<\/em> polar wandering, but the results were not consistent with those from Europe. For example, the 200 Ma pole for North America plotted somewhere in China, while the 200 Ma pole for Europe plotted in the Pacific Ocean. Since there could only have been one pole position at 200 Ma, this evidence strongly supported the idea that North America and Europe had moved relative to each other since 200 Ma. Subsequent paleomagnetic work showed that South America, Africa, India, and Australia also have unique polar wandering curves. In 1956, Runcorn changed his mind and became a proponent of continental drift.This paleomagnetic work of the 1950s was the first new evidence in favour of continental drift, and it led a number of geologists to start thinking that the idea might have some merit. Nevertheless, for a majority of geologists working on global geology at the time, this type of evidence was not sufficiently convincing to get them to change their views.<\/div>\r\n<\/div>\r\nDuring the 20th century, our knowledge and understanding of the ocean basins and their geology increased dramatically. Before 1900, we knew virtually nothing about the bathymetry and geology of the oceans. By the end of the 1960s, we had detailed maps of the topography of the ocean floors, a clear picture of the geology of ocean floor sediments and the solid rocks underneath them, and almost as much information about the geophysical nature of ocean rocks as of continental rocks.\r\n\r\nUp until about the 1920s, ocean depths were measured using weighted lines dropped overboard. In deep water this is a painfully slow process and the number of soundings in the deep oceans was probably fewer than 1,000. That is roughly one depth sounding for every 350,000 square kilometres of the ocean. To put that in perspective, it would be like trying to describe the topography of British Columbia with elevation data for only a half a dozen points! The voyage of the Challenger<\/em> in 1872 and the laying of trans-Atlantic cables had shown that there were mountains beneath the seas, but most geologists and oceanographers still believed that the oceans were essentially vast basins with flat bottoms, filled with thousands of metres of sediments.\r\n\r\nFollowing development of acoustic depth sounders in the 1920s (Figure 10.3.3), the number of depth readings increased by many orders of magnitude, and by the 1930s, it had become apparent that there were major mountain chains in all of the world's oceans. During and after World War II, there was a well-organized campaign to study the oceans, and by 1959, sufficient bathymetric data had been collected to produce detailed maps of all the oceans (Figure 10.3.4).\r\n\r\n[caption id=\"attachment_435\" align=\"aligncenter\" width=\"600\"]\"""\"<\/a> Figure 10.3.3 Depiction of a ship-borne acoustic depth sounder. The instrument emits a sound (black arcs) that bounces off the sea floor and returns to the surface (white arcs). The travel time is proportional to the water depth.[\/caption]\r\n\r\n[caption id=\"attachment_436\" align=\"aligncenter\" width=\"700\"]\"\"<\/a> Figure 10.3.4 Ocean floor bathymetry (and continental topography). Inset (a): the mid-Atlantic ridge, (b): the Newfoundland continental shelf, (c): the Nazca trench adjacent to South America, and (d): the Hawaiian Island chain.[\/caption]\r\n\r\nThe important physical features of the ocean floor are:\r\n