Even though his name is not as famous as either James Hutton or Charles Lyell, Arthur Holmes ranks as one of the greatest geologists of all time. He is best known for his textbook Principles of Physical Geology. First published in 1944, this book is still used by many geology students today. When it comes to plate tectonic theory, however, Holmes can be thought of as "the man who moved the continents."
Holmes's career as a geologist started soon after his graduation from Imperial College of Science in London in 1910. To help pay his college expenses, he went to work as a prospecting geologist in the country of Mozambique. Unfortunately, he also wound up catching malaria. He returned to England in 1913 just when World War I was starting. As luck would have it, he was too sick to go into the army and fight. Instead, he returned to Imperial College to work as a lab technician and get his Ph.D.
As a graduate student, Holmes read about the work in radioactivity done by Rutherford and Boltwood. He became interested in radioactive dating techniques and, by the end of 1913, he used Boltwood's uranium-lead dating technique to develop the first absolute geologic time scale. For the first time, geologists had actual numbers with which to measure the length of the different geologic periods.
While he was at Imperial College, Holmes had heard about We-gener's book and his theory of continental drift. At the time, he really did not think much about it, preferring to concentrate on better reason: When radioactive elements decay, they also produce heat. Holmes reasoned that the heat from these elements inside Earth would keep the planet from ever cooling completely. If the planet was not cooling, then it could not be contracting, either. To him, continental drift seemed like a more likely answer to the question of where mountains came from.
In 1927, he presented a paper to the Edinburgh Geological Society outlining a new idea. Instead of the continents moving through the rocks of the ocean floor as Wegener had said, Holmes suggested that extremely dense rocks below the outer layer of the Earth were slowly flowing like thick taffy. The continents were like rafts riding along on top of these hot, fluid rocks. He then said that the hot rocks were flowing because of convection currents caused by high concentrations of radioactive minerals found inside Earth.
Convection is the process that transfers heat from one place to another by the movement of heated matter. Convection happens most often in fluids like water, air, or magma. As an example of how a convection current works, think of a pot of thick soup cooking on a stove. The heat from the stove causes the soup at the bottom of the pot to heat up first. As the soup at the bottom gets hot, it begins to expand and becomes less dense. It slowly rises to the top of the pot and the cooler soup near the top begins to sink. Once the hot soup reaches the top of the pot, it begins to cool down again, which causes it to contract and become denser. It then begins to sink, replacing new hot soup that is rising. If you watch the soup on the stove, you can actually see this swirling action.
Convection currents happen in many places in nature. Convection currents in the air are what make the wind blow. Convection in the ocean helps to make currents flow. Most geologists believe that convection currents in the Earth are what make the continents move.
By 1930, Holmes had refined his ideas on how convection currents could be the force behind continental drift. He also described how these currents would have caused Pangaea to break up and form the Atlantic Ocean. He published his ideas in an article in a journal called Transactions of the Geologic Society of Glasgow, but few people actually read it. When he wrote his groundbreaking textbook in 1944, he devoted an entire chapter to the idea of continental drift. While he noted that the basic concept was possible, he also pointed out many flaws in Wegener's data. After explaining his own ideas about how convection currents could be the driving force behind continental drift, he concluded the chapter with the following statement: "It must be clearly recognized, however, that purely speculative ideas of this kind, specially invented to match the requirements, can have no scientific value until they acquire support from independent evidence."
That independent evidence would come 20 years later from the bottom of the sea and be presented by a scientist named Harry Hammond Hess.
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