The theory of plate tectonics is new, but some of the evidence for the theory of plate tectonics dates back millions if not billions of years. In the 1960s, geologists installed seismometers to monitor nuclear testing in the United States. The seismometers also recorded geologic activity, including earthquakes and volcanoes around the world. Geologists noticed that most of Earth’s geologic activity took place along distinct lines. They hypothesized that these lines represented the boundaries between massive tectonic plates. Thus, the theory of plate tectonics was born.
The Theory of Plate Tectonics
Plate tectonics explains the mechanism behind Alfred Wegener’s continental drift theory. Continents move because tectonic plates move. The rigid tectonic plates of the lithosphere float on the softer asthenosphere, and the plates move because of the convection currents within the asthenosphere.
The First Evidence for the Theory of Plate Tectonics
The lines of geologic activity discovered by the seismometers were the first proof of plate tectonics, but there is more evidence for the theory. First, all of the evidence Alfred Wegener collected to show the continents were moving can be used to support the plate tectonics theory. Next, Wegener explained that similar fossils and geologic formations on different continents suggested that the continents were once connected. He also pointed out that the continents appeared to fit together like puzzle pieces. Finally, he found tropical plant fossils within the Arctic Circle, which suggested the land once sat near the equator.
More Evidence for the Theory of Plate Tectonics
Modern geologists found more evidence of plate tectonics. They discovered the Mid-Ocean Ridge, a massive underwater mountain range where magma forms new oceanic crust. They also found the deep trenches of subduction zones where older oceanic crust is forced back into the mantle. Its young age further supports the recycling of the oceanic crust. The oldest oceanic crust is only 200 million years old, while continental crust is over three billion years old.
Hot spots also prove that the tectonic plates are moving. A hot spot is an especially hot place in the mantle that forms volcanoes in the crust above it. As the tectonic plate moves, the hot spot leaves behind an old, extinct volcano and creates a new one. The island chain of Hawaii is an example of a hot spot.
Finally, geologists use paleomagnetism as proof of plate tectonics. Most oceanic crust is made up of basalt, an igneous rock full of magnetite, a magnetic mineral. When the basalt forms from cooling magma, the magnetite lines up with the Earth’s magnetic field. Deep under the ocean, geologists found bands of oceanic crust with alternating magnetic field alignments. The bands were identical on either side of the Mid-Ocean Ridge. The Earth’s magnetic field reverses every few thousand years, so the alternating bands suggest the ocean floor formed at different times and is moving apart from the Mid-Ocean Ridge. With all of this evidence supporting plate tectonics, it has become the accepted theory for geologists.
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