It showed that earthquakes, volcanoes, and other active geologic features for the most part aligned along distinct belts around the world, and those belts defined the edges of tectonic plates. In addition, further paleomagnetic studies revealed a striped pattern of magnetic reversals in the crust of the ocean basins.
Basalt contains a fair amount of magnetic minerals called magnetite. When the lava from spreading centers in the oceans forms and cools, these minerals align to the north pole. The Earth has undegone several magnetic reversals in the past, in which the north and south poles are reversed for a period of time.
When geologists and geophysicists discovered that the crust in the ocean recorded these reversals, it was even more positive proof that the lithosphere had to be in motion, otherwise there would be no "stripes" of normal and reversed polarity crust. Some researchers think it started more than 4 billion years ago, and others say it started only about 1 billion years ago.
That's a big range, and the uncertainty stems from the fact that it's simply hard to find well-preserved ancient rocks. The oldest rocks in Greenland range from 3. They measured hafnium isotopes — atoms of the same element that have different numbers of neutrons in their nuclei — in the rocks to figure out how long each sample had been part of the Earth's crust.
As rocks on the surface are melted and recycled, the proportions of hafnium isotopes change. One of the key observations was that of sea-floor spreading - the process that creates new crust at the ridges from upwelling magma. As the rock cools and moves away from a ridge, it locks into its minerals the direction of Earth's magnetic field. And as the field reverses, as it does every few hundred thousand years, so does the polarity in the rocks, presenting a zebra-like, striped pattern to traversing research ships and their magnetometers.
In , all roads led to the spring meeting of the American Geophysical Union. Some 70 abstracts summaries of research were submitted on sea-floor spreading alone. A heady time, it must have been. The coherent narrative of plate tectonics was about to fall rapidly into place. McKenzie's paper was published in December that year. Concurrently, other researchers were extending the model to describe all the other plates.
As to the mechanism that eluded Wegener, scientists can now see how the weight of underthrusting plates plays such a major role in driving the whole system. Much as the slinky dog needs no encouragement once it has started its journey downstairs, so the descending rock appears to have an unstoppable momentum. Of course, everything is connected in the deep mantle through convection, but trench pull does seem to be key. Nothing is ever done and dusted in science.
There is still a lively debate for example about precisely when and how plate tectonics got going on Earth. Credit: K. Earth did not always have plate tectonics. For millions of years after the planet accreted, its surface roiled with a molten magma ocean.
Once the planet cooled enough for a crust to form, the surface may have looked more like modern-day Venus, with the crust and upper mantle — collectively called the lithosphere — forming a single unbroken plate. Oceanic crust is most commonly formed when basaltic magma rises to the surface at mid-ocean ridges, such as the Mid-Atlantic spreading ridge, and is thinner — typically about 7 kilometers thick — and denser than continental crust.
Today, continental crust is formed mainly along subduction zones, where partial melting of descending slabs forms granitic and andesitic magmas at volcanoes on the overriding plate.
This process produces thicker — up to 70 kilometers thick — and more buoyant crust that is not as easily subducted. But it is not known how continental crust formed in the past.
Subduction zones form where two plates converge and one begins sliding under the other. As old lithosphere is recycled back into the mantle at subduction zones and new lithosphere is formed at spreading centers, the balance of lithosphere on Earth remains relatively constant. Today, the planet has eight major plates defined as those with areas over 20 million square kilometers and dozens of minor plates between 1 million and 20 million square kilometers and microplates less than 1 million square kilometers.
While some plates are composed solely of oceanic or continental crust, most major plates contain portions of both. While continental crust that is billions of years old still exists on Earth's surface, most oceanic crust is less than million years old Ma. Older oceanic crust, which is more dense than continental crust, has long since been recycled in the process of subduction. Credit: U. Geological Survey.
Mantle convection is driven by temperature differences between the hot interior and the gradually cooling outer layers of the planet. Cooler, denser material sinking down into the mantle is thought to be the primary driver of circulation, while hotter, less dense material rising to the surface in the form of mantle plumes and upwellings provides a secondary driver. The forces generated by these vertical movements result in horizontal shifts of the tectonic plates at the surface at rates of about a few centimeters per year.
One of the big questions about the onset of plate tectonics is how subduction got started. Geologists think that the lithosphere of the pretectonics Earth existed as a single plate that covered the whole planet. Massive forces would have been needed to break this single lithosphere into multiple plates and to initiate plates descending into the mantle.
Minerals in lithospheric slabs restructure as slabs descend into the mantle, releasing water and increasing the slabs' densities. The dense, downgoing slabs pull on the parts of the plates still at the surface, driving plate tectonics.
Some subducting slabs stall at the transition zone, while others descend toward the core-mantle boundary. Credit: both: K.
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