Guest “Did you know?” by David Middleton
A rise in rock under the Atlantic can drive continents apart
The mid-Atlantic ridge may play a more active role in plate tectonics than thought
From Maria Temming
FEBRUARY 4, 2021
An upswing of hot rock deep under the Atlantic can drive the continents apart on either side.
America moves a few inches away from Europe and Africa every year as the tectonic plates underlying these continents drift apart. Researchers usually believe that tectonic plates separate when the distant edges of these plates sink into the Earth’s mantle and create a gap (SN: 1/13/21). Material from the upper mantle then seeps through the gap between the plates to fill the ocean floor.
However, new seismic data from the Atlantic floor shows that hot rock rises beneath a sea floor rift called the Mid-Atlantic Ridge from hundreds of kilometers into the mantle. This suggests that material rising from under the ridge is not just a passive response to tectonic plates sliding apart. Rather, deep rock that presses on the earth’s surface could drive a wedge between the plates that separates them, researchers report online on January 27 in Nature.
A better understanding of the plate tectonics that cause earthquakes and volcanic eruptions could help people better prepare for these natural disasters (SN: 9/3/17).
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Science news
Centers of dispersion are driving continents apart? Who could have guessed? Um … Almost everyone since at least the late 1960s.
What drives the plates?
Based on seismic and other geophysical evidence and laboratory experiments, scientists generally agree with Harry Hess’s theory that the driving force of the plate is the slow movement of a hot, softened mantle underlying the rigid plates. This idea was first considered in the 1930s by Arthur Holmes, the English geologist who later influenced Harry Hess’ thoughts on the expansion of the seabed. Holmes speculated that the circular motion of the mantle moved the continents along like a conveyor belt. However, at the time when Wegener proposed his theory of continental drift, most scientists believed that the earth was a solid, motionless body. We know better now. As J. Tuzo Wilson eloquently stated in 1968: “The earth does not appear as an inert statue, but is a living, moving thing.” Both the surface of the earth and its interior are in motion. Below the lithospheric plates, the mantle is partially melted at some depth and can flow, albeit slowly, in response to steady forces exerted over long periods of time. Just as a solid metal such as steel can soften and assume different shapes when exposed to heat and pressure, so can solid rock in the mantle when exposed to heat and pressure in the Earth’s interior for millions of years.
The moving rock beneath the rigid plates is believed to move in a circular motion, much like a saucepan of thick soup, when heated to a boil. The heated soup rises to the surface, spreads and begins to cool. Then it sinks back to the bottom of the pot, where it is heated again and rises again. This cycle is repeated over and over to create what is known as a convection cell or convective flow. While convective flow is easy to observe in a pot of boiling soup, the idea of such a process that stirs up the Earth’s interior is much more difficult to understand. While we know that convective motion on earth is much, much slower than that of boiling soup, many unanswered questions remain: How many convection cells exist? Where and how are they created? How is its structure?
Convection cannot take place without a heat source. The heat in the earth comes from two main sources: radioactive decay and residual heat. Radioactive decay, a spontaneous process that forms the basis of “isotope clocks” used to date rocks, involves the loss of particles from the nucleus of an isotope (the parent) to an isotope of a new element (the daughter) to build. The radioactive decay of naturally occurring chemical elements – especially uranium, thorium and potassium – releases energy in the form of heat that slowly migrates to the earth’s surface. Residual heat is gravitational energy that was left over when the earth was formed 4.6 billion years ago through the “collapse” and compression of cosmic debris. How and why the escape of internal heat is concentrated in certain regions to convection cells remains a mystery.
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USGS “Concept drawing of the assumed convection cells in the jacket (see text). Below a depth of approximately 700 km, the descending plate begins to soften and flow and lose its shape. ”
From my college graduation in 1980 to a few minutes ago (when I read the Science News article), I assumed that every geoscientist knew that mantle convection was pushing the ocean floor, which in turn was driving the continents apart.
However, it appears that this was turned bass-ackwards while I was looking for oil and gas instead of reversing cause and effect. Similar to how atmospheric CO2 suddenly became the geological driver of climate change in 1988, subduction apparently became the driver of the expansion of the sea floor …
Until the 1990s, prevailing explanations of what was driving plate tectonics emphasized mantle convection, and most earth scientists believed that the expansion of the ocean floor was the main mechanism.
Cold, denser material convectively downwards and hotter, lighter material rises due to gravity; This movement of the material is an essential part of convection. In addition to convection forces, some geologists argue that the penetration of magma into the spreading ridge provides an additional force (called “ridge thrust”) to propel and maintain plate motion. Therefore, subduction processes are seen as secondary, a logical but largely passive consequence of the expansion of the sea floor. In recent years, however, the tide has turned. Most scientists today advocate the idea that forces associated with subduction are more important than the expansion of the ocean floor. Professor Seiya Uyeda (Tokai University, Japan), a world-renowned expert on plate tectonics, concluded in his keynote address at a major scientific conference on subduction processes in June 1994 that “Subduction. . . plays a more fundamental role than the expansion of the ocean floor in shaping the surface features of the earth ”and“ in operating the plate tectonic machinery ”. The gravitational sinking of a cold, denser oceanic plate into the subduction zone (known as the “plate pull”) that pulls the rest of the plate with it is now seen as the driving force of plate tectonics.
We know that forces are deep within the motion of the Earth’s inner drive plate, but we may never fully understand the details. At present, none of the proposed mechanisms can explain all facets of disk movement; Because these forces are buried so deep, no mechanism can be directly tested and proven beyond doubt. The fact that tectonic plates moved in the past and are still moving today is beyond dispute, but the details of why and how they move will continue to challenge scientists in the future.
USGS
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