List and Describe the Evidence for the Plate Tectonics Theory
By Jason Steele, Ph.D.
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Plate tectonics theory is taught in science classrooms around the world, but just 50 years ago it was a hotly contested theory. The idea that the Earth's crust is made up of huge tectonic plates that are floating on an ocean of molten hot magma seemed too fantastic to be believed. It may be fantastic, but that is exactly what is happening. Inexorably, the continents are floating along at a slow constant motion that has been continuing for hundreds of millions of years. Scientists have gathered many different types of evidence that supports plate tectonics theory.
The Theory of Plate Tectonics
The basic theory of plate tectonics is that along seafloor spreading zones, the continents are separating from one another. As they spread apart, magma comes to the surface and becomes new continental crust. As the tectonic plates move away from spreading zones, they collide with one another. In some cases, the edges of two different plates will grind against each other in a horizontal fashion. These areas, called transform boundaries, experience many earthquakes. A well-known transform boundary is the San Andreas fault in California. In other cases, the plates directly collide, forcing one plate upward while the other plate is forced back into the mantle. These collision areas, called convergent boundaries, create mountain ranges. An active convergent boundary is under the Himalayan Mountains, which are being created by the subduction of the Indian crust sliding under the Tibetan crust.
Sea Floor Magnetism
Stripes of magnetic material in the seafloor provide strong evidence for tectonic theory. The stripes alternate between those with magnetic material orientated toward magnetic north, and those oriented in the opposite direction. Seafloor spreading is the mechanism behind this phenomenon. As new magma forces its way up to the surface, magnetized minerals in the liquid rock orient along the Earth's magnetic field and then harden as the lava cools. As spreading continues, the material moves away from the spreading zone as if on a conveyor belt. The Earth's magnetic field flips every few hundred thousand years, and the stripes on the ocean floor show a record of those changes. By estimating when the flips occurred and pairing that with the distance the strips have moved from the spreading zone, scientists can estimate how fast the continents are moving.
Fossil Evidence
The continents have moved a great deal in the history of the planet, but they carry records of where they've been. Some of this evidence is the fossils of animals and plants. Tropical species found in the Antarctic and similar fossils found in western Africa and eastern South America tell a story of where those land masses used to be. Paleomagnetic evidence is an even stronger piece of evidence. Magnetic strata within the fossil record show how the land masses were oriented at different times during Earth's history. By constructing detailed records of changes in land mass orientation, scientists can reconstruct paths of tectonic movement much further back in history than they can from the magnetic striping on the sea floor.
Direct Measurement
Modern technology gives us a range of ways to directly measure the movement of tectonic plates. These methods are based around the idea of measuring distance between two points on Earth by using some intermediary transmitter in space. For example, SLR (Satellite Laser Ranging) uses two lasers on Earth, each of which fires a laser to a satellite orbiting the planet. The beam is reflected back to each laser and the difference in laser beam travel time is used to estimate the distance between the two lasers on Earth. Another approach using radio-telescopes measuring natural radio signals from deep space called VLBI (Very Long Baseline Interferometry) achieves a similar effect. Both SLR and VLBI allow for repeated measurements of distance between two points. As the continents move, the distances change. Using these measurements, scientists can accurately estimate movement of the tectonic plates today.
USGS: What on Earth is Plate Tectonics?
Smithsonian Museum of Natural History: The Dynamic Earth
NASA: Crustal Dynamics Data Information System
Oregon State University: Scientists Create First Complete Image of Himalayan Fault, Subduction Zone
National Geographic: Plate Tectonics
Smithsonian Magazine: When Continental Drift Was Considered Pseudoscience
Popular Science: Scientists Spot Possible Remains of "Rodinia", Ancient Lost Microcontinent
Based in Washington, D.C., Jason Steele has been writing science articles since 2007. His articles have appeared in a variety of magazines and websites. He received a Bachelor of Science in biology from John's Hopkins and a PhD in biology from the University of Maryland.
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